The Ecology of Seashores - Chapter 4 ppsx

Many motile shore animals on both hard and soft shores have evolved behavioral strategies that enable them to both evade extreme environmental conditions and to undertake feeding migrati

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CONTENTS

4.1 Introduction 238

4.2 Ecological Niches on the Shore 238

4.2.1 Introduction 238

4.2.2 The Environment 238

4.2.3 Environmental Stress 239

4.2.3.1 Desiccation 239

4.2.3.2 Thermal Tolerance 243

4.2.4 Ecological Niches 244

4.2.4.1 Introduction 244

4.2.4.2 The “Envirogram” Concept 245

4.2.4.3 Weather 247

4.2.4.4 Resources 249

4.2.4.5 Other Organisms 249

4.2.4.6 Disturbance and Patchiness 251

4.2.4.7 The Importance of Recruitment 251

4.3 The Establishment of Zonation Patterns 251

4.3.1 Reproduction 251

4.3.1.1 Developmental Types in Marine Benthic Invertebrates 251

4.3.1.2 Development Types in Marine Algae 252

4.3.1.3 Reproductive Strategies 252

4.3.1.4 A Model of Nonpelagic Development Co-adaptive with Iteroparity 254

4.3.2 Settlement and Recruitment 255

4.3.2.2 Distinction Between Settlement and Recruitment 256

4.3.3 Settlement 256

4.3.3.2 Settlement Inducers 257

4.3.3.3 Settlement on Rock Surfaces and Algae 258

4.3.3.4 Avoidance of Crowding 259

4.3.3.5 Settlement on Particulate Substrates 260

4.3.3.6 Variation in Settlement 261

4.3.4 Recruitment 261

4.3.4.2 Components of Recruitment 261

4.4 The Maintenance of Zonation Patterns 263

4.4.1 Introduction 263

4.4.2 Elements of Behavior in Littoral Marine Invertebrates 263

4.4.3 Behavior Patterns in Representative Species 265

4.4.3.1 Movement Patterns and Orientation Mechanisms in Intertidal Chitons and Gastropods 265

4.4.3.2 Interaction Between the Siphonarian Limpet Siphonaria Theristes and Its Food Plant Iridacea Corriucopiae 265

4.4.3.3 Maintenance of Shore-Level Size Gradients 266

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238 The Ecology of Seashores

4.4.4 Clock-Controlled Behavior in Intertidal Animals 271

4.4.4.2 Behavior Rhythms, Tidal Oscillations, and Lunar Cycles 273

4.4.4.3 Locomotor Rhythms and Maintenance of Zonation 273

4.1 INTRODUCTION

In previous chapters, it was shown that the plants and animals on the shore occupy distinct zones or habitats in which they can survive and obtain the resources they require for growth and reproduction They thus occupy a specific ecological niche The ecological niche concept will be explored in the succeeding section While a limited number of animal species exhibit direct development in which the juveniles hatch directly from the egg, other species have pelagic larvae that need to settle at an appro-priate level on the shore in order to maintain viable pop-ulations For sessile species, the choice of a settlement site is irreversible Hence, such species have evolved behaviors that will ensure that they will settle at the right level on the shore Other species such as mussels and limpets settle low on the shore and subsequently migrate

to occupy the zone in which the adults are found Most algae reproduce by forming microscopic life cycle stages that are released into the water column and later settle on rocky substrates If they settle at the appropriate level, they will grow to give rise to the adult plant

Many motile shore animals on both hard and soft shores have evolved behavioral strategies that enable them

to both evade extreme environmental conditions and to undertake feeding migrations in order to utilize available food resources These behavioral strategies will be dis-cussed in detail later in this chapter

4.2 ECOLOGICAL NICHES ON THE SHORE

4.2.1 I NTRODUCTION

It is obvious from the preceding chapters that plants and animals on the shore are not randomly distributed, but occupy distinct vertical zones, and are often restricted to microhabitats within these zones Basic to an understand-ing of shore ecology is a knowledge of the ways in which organisms are adapted to the environmental conditions they are subjected to, and the particular functional role or

which they are an integral part Here we will consider the twin concepts of “environment” and “ecological niche” in some detail

The history of the niche concept is well known and documented (see reviews by Whittaker and Levin, 1977;

Diamond and Case, 1986) More recently, Price (1980) has reviewed niche and community concepts in the inshore

benthos with particular reference to macroalgae The niche concept includes the ideas of ecopotential, fundamental niche, and realized niche Ecopotential can be considered

as the unexpressed individual, breeding group, or local population potentiality to occupy a particular role in a community The fundamental niche is the unconstrained expression of that ecopotential in the presence of only those limitations that derive from interactions between the ambient physical environment and the population of the species under consideration The realized niche is the totally constrained living relationships of the population

of a species within its delimited community

4.2.2 T HE E NVIRONMENT

The term “environment” is not an easy one to define since organisms, populations, and communities form interacting systems within their environments For the individual organisms, substrate, physical and chemical conditions, its disease organisms, parasites, symbionts and commensals, its associated organisms, competitors and predators, its food resources and other phenomena, all form part of its environment The environment of a population is more difficult to define, since the individuals within a population

do not all respond in the same way to a particular environ-mental factor However, it is useful to consider the envi-ronment of a population as the sum of all those phenomena

to which the population as a whole and its individuals respond Communities, on the other hand, modify and con-trol the physical and chemical conditions and resources of the areas in which they are found to such an extent that separate consideration of the environment and community

is of little value It is best to view the community and the sum total of the environmental conditions of an area in which it is found as the components of an ecosystem.

It is useful to break down the environment of an organ-ism into its component factors, all of which must remain within tolerable limits if the organism is to survive Any one of these factors may become limiting in the sense that,

if it exceeds the tolerable limits for the individual, it will die, although the other factors remain suitable We will consider this concept of limiting factors in some detail later In addition to limiting environmental factors we also need to consider regulatory factors that control the size of the population, e.g., disease, competition, or predation may prevent the population from expanding, but it does not threaten its continuous existence

The environment, then, is a term used to describe in

an unspecific way, the sum total of all the factors of an

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Adaptations to Shore Life 239

area that influence the lives of the individuals present

There have been numerous attempts to classify the

impor-tant environmental variables, ranging from very general

ones (e.g., biotic vs abiotic), to habitat-specific schemes

(e.g., for a rocky shore mussel community, these include

tidal emersion and immersion, wave action, water

move-ment, water and air temperatures, salinity, substrate,

aspect, etc.) However, a better classification reflecting

causal relationships is needed Perhaps the most useful

one is that of Andrewartha and Birch (1954), who

sepa-rated the environment into four major divisions: weather,

food, organism of the same and different kinds, and a place

to live A modified subdivision of these categories as they

apply to the shore is given in Table 4.1

4.2.3 E NVIRONMENTAL S TRESS

4.2.3.1 Desiccation

There is a considerable body of literature on the responses

of intertidal communities and individual species to

gradi-ents of emersion/submersion (tidal height) and wave

expo-sure Nevertheless, many of the conclusions reached on

the effects of emersion during low tide do not provide

completely satisfactory explanations for littoral zonation,

species distributions, and abundance patterns (Chapman,

1973; Underwood, 1978a,b; 1985; Underwood and

Den-ley, 1984)

During periods of emersion (exposure to air), tion (water loss and temperature stress) may affect the

desicca-photosynthetic capacities of plants (Schonbeck and

Norton, 1979; Dring and Brown, 1982; Smith and Berry,1986), the nutritional performance of algae (Schonbeckand Norton, 1979), and the ability of animals to grow andcarry out the normal functions of feeding and reproduction.The amount of water lost by algae depends on theduration of exposure to air, the atmospheric conditions(solar insolation, temperature, cloud cover, humidity, etc.),and the surface-to-volume evaporation ratio of the plant(Dromgoole, 1980) While a brief exposure of an algawould have little impact, prolonged exposure could besevere The higher up the shore that a species grows, thelonger it is exposed to desiccation effects However, des-iccation can be minimized by growing in favorable habi-tats, e.g., under overhangs, in shade, in rock pools, orbeneath the canopy of larger algae Some algae (e.g.,fucoids) tolerate desiccation rather than having the ability

to avoid stress (i.e., by maintaining a high water potential).Moreover some algae have the ability to harden to droughtconditions (Schonbeck and Norton, 1979)

Emersion from the marine environment exposes roalgae to increased osmotic stress because of tissue waterloss (desiccation), increased irradiances, and elevated thal-lus temperatures as tissues dry Desiccation stress reducesphotosynthetic capacity (Dring and Brown, 1982), as well

mac-as altering respiration rates Incremac-ased thallus temperaturesare typically associated with emersion stress increases,photosynthesis, and dark respiration rates with a Q10 of

ca 2.0 Photosynthetic rates reach temperates above whichthey rapidly decline

Many experiments have tested the recovery of algaefrom emersion, usually by measuring the rates of photo-synthesis or respiration (see review of Gesner andSchramm, 1971) Some representative results shown in

Figure 4.1 illustrate the recovery of Fucus vesciculosus

(mid-intertidal) and Pelvetia canaliculata (high intertidal).The latter, as expected, was able to withstand longer peri-ods of desiccation If relative humidity is experimentallymaintained at a level high enough to prevent desiccation,the photosynthetic rate may be maintained for long peri-ods, as found in Fucus serratus by Dring and Brown(1982) These authors assessed three hypotheses thatmight explain the effects of desiccation on intertidal plantsand zonation: (1) species from the upper shore are able tomaintain active photosynthesis at lower tissue water con-tent than are species lower on the shore (this was refuted

by the experimental data); (2) the rate of recovery ofphotosynthesis after a period of emersion is more rapid inspecies on the upper shore (this was also refuted by theavailable data); and (3) the recovery of photosynthesisafter a period of emersion is more complete in speciesfrom the upper shore (this hypothesis was supported byDring and Brown’s data)

Beach and Smith (1997) have studied the ogy of the Hawaiian high-tidal, turf-forming red alga, Ahn- feltiopsis concinna They found that the capacity to recover

ecophysiol-TABLE 4.1 Classification of Environmental Factors

A Weather

1 Immersion

2 Emersion and water loss

3 Temperature — heat and cold

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photosynthetic activity from emersion stresses varied

between algae from microsites separated by <10 cm Algae

from canopy microsites that were regularly exposed to a

greater range of irradiance, temperature, and osmotic stress

than algae from understory microsites had greater capacity

to recover from these stresses alone or in combination

compared to tissues from understory microsites Net

pho-tosynthesis was enhanced by 20% water loss or exposure

to 2,150 MosM kg–1 media compared to values for algae

that were in a fully immersed state The temperature

optima for net photosynthesis was 33°C, while the upper

performance threshold was 40°C Highly responsive stress

acclimation capacity, coupled with microclimate benefits

of a turf form, substantially contribute to the ecologicalsuccess of A concinna as an ecological dominant at hightidal elevations in the Hawaiian archipelago

The situation concerning the consequences of algaedrying out is further complicated in that there is evidence

it may be accompanied by an increase in the rate ofexudation of organic matter (Siebruth, 1960) The amount

of carbon released in 10 minutes by Fucus vesciculosus

after resubmergence increased in relation to the duration

of exposure (and hence the amount of water lost) Algaefrom higher on the shore lost more water and releasedless carbon

Unless rocky-shore animals have special mechanisms

to combat water loss, they lose water to the air If this occursfor extended periods, they eventually die from desiccation.Death of animals on the shore due to desiccation may

be due to disturbances in the metabolism resulting from

an increasing concentration of the internal body fluids ormore usually from asphyxia For those organisms thatrespire by means of gills, a constant water film must bemaintained over the respiratory surfaces

In addition to water loss by evaporation, animals alsolose water by excretion Most marine animals excreteammonia as their principal nitrogenous waste product, but

it is highly toxic, requiring a very dilute urine and thepassage out of the body of a large volume of water Somelittoral species of gastropods have been able to reducetheir excretory water loss by excreting appreciableamounts of uric acid, which is a soluble and less toxicproduct requiring less water for its excretion In Britishgastropods, those living highest on the shore have thegreatest uric acid concentration in their nephridia.Desiccation stress, of course, varies with position onthe shore in relation to the amount of exposure to air over

a tidal cycle, as well as to the periods of continuous sion Animals on hard shores are much more vulnerable todrying out than those on soft shores, and for the latter theproblem is more acute on sandy than on muddy shores.Mudflats rarely dry out, but the upper regions of sandybeaches can become quite dry However, on sandy beaches,the inhabitants avoid desiccation by burrowing On muddyshores, surface dwellers such as some mudflat snails bur-row into the surface sediments when the tide is out

emer-On hard shores the animals found in the eulittoral canresist desiccation inside an impervious shell or tube thatcan be tightly closed up (barnacles and mussels), sealedoff by a horny membrane (many gastropods), a calcareousoperculum (serpulid tubeworms), or closely pressed to therock surface (limpets) In many of these species there is

a correlation between shell thickness and position on theshore, animals living higher on the shore having, in gen-eral, thicker shells than those lower down Some attachedsoft-bodied forms, such as anemones, produce a copioussecretion of mucus that assists resistance to drying out In

FIGURE 4.1 Recovery of photosynthesis in two intertidal

following desiccation for several days Upper curve in (a) is rate

in a thallus resubmerged immediately after reaching 10 to 12%

is expressed as percentage of the rate in undehydrated control

plants (From Lobban, C.S., Harrison, P.T., and Duncan, M.J.,

Press, Cambridge, 1985, 170 Based on Gessner, F and

Schramm, W., 1971.)

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addition, a number of physiological adaptations have

developed to enable animals to withstand the risk of

des-iccation Since water loss results in an increasing

concen-tration of the body fluids, an efficient osmoregulatory

sys-tem is required Also, since desiccation is usually

accompanied by an increase in body temperature,

toler-ance of high temperatures is also required It must be

pointed out, however, that the latent heat of evaporation

helps to cool an animal that is losing water, and this may

be a significant factor in reducing body temperature

On New Zealand shores, littorinids and trochids form

a useful series (Figure 4.2), with overlapping vertical

ranges and distinctive midpoints, for studying the effects

of desiccation on intertidal animals Rasmussen (1965)

has tested the relative amount of desiccation these four

species can tolerate by determining the 50% mortality

point when they were exposed in sunlight at 35°C The

results are given below:

He also carried out a series of experiments to test whetherthere was a differential susceptibility to desiccation withincreasing age The distribution curve for Melagraphia aethiops is shown in Figure 4.3 It can be seen that thereare four definite size classes and possibly a fifth Mela- graphia spat settle over the entire intertidal range and thenmigrate as they grow toward a central vertical zone Thosethat do not reach this zone perish The first-year classremains well sheltered from desiccation in runnels, pools,and under rocks Older individuals are found on the openrock surface Desiccation experiments (Figure 4.3) indi-cate that there is a definite increase in desiccation toler-ance with size and age

Broekhuysen (1940) studied a series of gastropodsranging, from the upper shore Littorina africana knys-

trgina, and Burnupena cincta to the low shore species O sinensis (Figure 4.4) Broekhuysen (1940) compared therelative tolerance of the six gastropods to desiccation bymeasuring both the percentage water loss and mortality

in the gastropod over a range of temperatures However,

as pointed out by Brown (1960), the water loss wasexpressed as percentage of total wet weight including theshell, whereas most investigators express the rate of des-

FIGURE 4.2 The vertical distribution of littorinid and trochid gastropods on New Zealand shores.

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iccation as water loss per unit dry weight including the

shell Brown repeated Broekhuysen’s experiments to give

the results shown in Table 4.2, where desiccation is

expressed as water loss per unit dry weight including shell

From the table, two general conclusions can be drawn

First, there is a correspondence between zonational level

and the percentage water loss causing 50% mortality

However, some species, such as L africana knysnaensis,

are less tolerant of water loss than would be assumed from

their level on the shore, while others such as Burnupena

cincta are apparently more tolerant than would be

indi-cated by their position on the shore Second, a tolerance

of between 15 and 37% water loss is characteristic of thespecies series before 50% mortality occurs

The reasons for the exceptions in the zonationalsequence are twofold Burnupena cincta lives in a drier sit-uation on the open rock surface compared to Oxystele tig- rina, which is restricted to damp situations and pools Sec-ond, L africana knysnaensis, in common with other hightidal species, can cement the rim of the shell to the substra-tum with mucus and thus limit water loss Other speciesincluding the gastropod Nerita and limpets can retain extra-

FIGURE 4.3 A Size class numbers of the trochid, Melagraphia aethiops B Size classes of the catseye, Turbo smaragda C Percent

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corporeal water under the shell for much of the intertidal

emersion period and thus reduce desiccation effects

4.2.3.2 Thermal Tolerance

The temperature tolerance of an intertidal organism is an

important factor in determining the upper level at which

a particular species can survive when the tide is out

How-ever, the situation is complicated by the interaction of a

large number of variables The magnitude of the

temper-ature stress is dependent on season, the time of day when

emersion occurs, the duration of the exposure to air, and

other factors Its effect may be modified by factors such

as shape and color of the organism, body size, and themagnitude of the water loss Desiccation may modify theeffects of temperature stress in a variety of ways Forexample, each gram of water evaporated from the tissues

at 33°C removes 544 calories of heat, and this valueincreases with temperature so that it represents an impor-tant potential method of facilitating heat loss Many inter-tidal organisms have evolved structural and physiologicaladaptations that minimize the impact of thermal stresssuch as shell shape and the retention of extracorporealwater in the mantle cavities of molluscs

Much of the extensive literature on the thermal ance of intertidal and subtidal organisms has beenreviewed by Kinne (1971), Somero and Hochachka(1976), and Newell (1979) The most detailed of theseearly studies relating the temperature tolerances of inter-tidal animals to their zonational position on the shore wasthat of Broekhuysen (1940) He demonstrated that thesequence of thermal death points of a series of SouthAfrican gastropods showed a general correspondence withtheir zonational position on the shore much as describedfor their desiccation tolerance (see Section 5.1.4.2 above)(Figure 4.5) The highest species on the shore, L africana

(48.6°C), while the lowest species, Oxtstele sinensis, hadthe lowest (39.6°C) Since then, numerous studies haveconfirmed and amplified such sequences in the thermaltolerances for a variety of taxa

FIGURE 4.4 Graph showing the relation between the distribution of gastropods on the shore at False Bay, South Africa, and tidal

TABLE 4.2

The Range of Distribution (Height in Feet above

Datum) and Water Loss Required to Induce 50%

Mortality in a Series of Gastropods from Cape

Peninsula, South Africa

Species

Mean Zonational Level

% Water Loss for 50% Mortality

Littorina africana knysnaensis 12.3 33.17

Source: After Brown, A.C., Porta Acta Zool., 7, 1960.

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4.2.4 E COLOGICAL N ICHES

4.2.4.1 Introduction

As Warren (1971) points out in referring to animal species,

“Each species has evolved as part of an ecosystem: an

ecosystem in which it occupies certain spaces during

cer-tain times; and ecosystem in which it can tolerate the

ranges of physical and chemical conditions; an ecosystem

in which it utilizes some of the species for energy and

material resources and in which it is utilized by other

organisms; an ecosystem in which it has many kinds of

relations with different species, and in which it can satisfy

its shelter and other needs.”

In considering the ecological niche of a species, we

concern ourselves with what a species does, what activities

characterize its life, how and where it carries out these

activities, why or for what purpose they are carried out,

and when they occur The concept of the niche is thus a

functional one

The ecological niche of a species can be described by

considering:

1 The interaction of the species populations with

the environmental factors listed in Table 4.1

2 The structural, physiological, and behavioral

adaptations that enable the species to surviveand reproduce in the environment it inhabits

3 The times at which the interactions occur

4 The effects of the species’ activities on the

eco-system of which it is a part

While a complete description of the ecological niche

of a species is not usually possible, the concept is theless a useful one in that it enables us to gain an under-standing of the role of a particular species in the ecosystem

never-in which it is found

Species can be categorized (Vermeij, 1978) as:

repro-ductive output, a short life history, high ability, reduced long-term competitive abilities,and generally occupy ephemeral or disturbedhabitats

chronic physiological stress, exhibit low rates

of recolonization, tend to be long-lived withslow growth rates and, consequently are gener-ally poor competitors

live in physiologically favorable environments,have long life spans, are good competitors, andhave evolved mechanisms to reduce predation

In the rocky intertidal zone, stress-tolerant forms arecharacteristic of the upper intertidal habitat, whereas biot-ically competent forms are prevalent in the lower inter-tidal Opportunistic forms appear ephemerally on dis-turbed or newly available substrates

Andrewartha and Birch (1984) make three tions concerning the way in which the environment works.The first is that the environment can be considered as a

proposi-FIGURE 4.5 Graph showing the relationship between upper zonational limit (height in feet above chart datum) and upper limit of

3rd ed., Marine Ecological Surveys, Faversham, Kent, 1979 146 Data from Broekhuysen, 1940 With permission.)

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together with a web of indirectly acting components that

affect those in the centrum (Figure 4.6) The second is

that the centrum consists of four divisions here modified

to include (1) resources, with two components (food and

a place to live), (2) other organisms (competitors and

predators), (3) weather, and (4) disturbances (accidental

events that eliminate an organism or population) The third

proposition is that the web is a number of systems of

branching chains; a link in the chain may be a living

organism (or its artifact or residue), or inorganic matter

or energy

4.2.4.2 The “Envirogram” Concept

According to Andrewartha and Birch (1984), activity in

the directly acting components is the proximate cause of

the condition of an individual of a species that affects its

chance to survive and reproduce But the distal cause of

an individual’s condition is to be found in the web, amongthe indirectly acting components that modify the centrum

A modifier may be one or several steps removed from thecentrum, and the pathway from a particular modifier to itstarget in the centrum may be joined by incoming pathwaysfrom other modifiers that may be behind or alongside thefirst one (n steps away from its target in Figure 4.8) Theenvirogram is a graphic representation of these pathways

An example of an envirogram for the food resource

of a limpet, Patelloida latistrigata, is given in Figure 4.7.The food of this limpet on the rocky shores of southernAustralia comprises the spores and young stages of algaethat it scrapes from the rock The envirogram depicts theweb of effects determining the supply of food and, hence,indirectly affecting the limpet Nearby mature algae arethe source of the spores and the water currents are required

to carry them onto the shores Another limpet, Cellana

FIGURE 4.6 The environment comprises everything that might influence an animal’s chance to survive and reproduce Only those

“things” that are the proximate causes of changes in the physiology or behavior of an animal are placed in the centrum and recognized

as “directly acting” components of the environment Everything else acts indirectly, that is, through an intermediary of chain of intermediaries that ultimately influences the activity of one or other of the components of the centrum All these indirectly acting

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FIGURE 4.7 Part of the envirogram of the limpet Patelloida latistrigata on the coast of New South Wales, showing only the

Longman Chesire, Melbourne, 1994, 156 Adapted from Andrewartha and Birch, 1984 With permission.)

FIGURE 4.8 Basic algal life cycles (Redrawn from Hinde, R., in Coastal Marine Ecology of Temperate Australia, Underwood, A.J and Chapman, M.G., Eds., University of New South Wales Press, Sydney, 1995, 127 With permission.)

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tramoserica, which eats the same food as Patelloida, is

an important factor determining the abundance of food

available to Patelloida Other variables that determine the

action of the first-order interactions (1n) lie further out in

the web of the environment of Patelloida Barnacles may

preempt space needed by Cellana (Patelloida can graze

over the tops of the barnacles) so barnacles indirectly

influence the food supply of Patelloida The density of the

barnacles in turn is determined by the suitability of the

substrate for settling, predation by the whelk Morula

Cellana Other 2n to 5n factors affecting the supply of

food for Patelloida are also shown

4.2.4.3 Weather

The various weather factors listed in Table 4.1 will be

considered in turn with reference to the ways in which

selected organisms are adapted to cope with the problems

encountered

sub-mersion and esub-mersion have already been discussed in

Sections 1.3.4 and 2.3.3 Some shore organisms require

periods of emersion in order to function normally and die

if subjected to periods of continuous submergence On the

other hand, as discussed in the sections listed above,

emer-sion poses considerable problems for aquatic organisms

and a variety of mechanisms have evolved (morphological,

physiological and behavioral) to minimize water loss

zonation patterns of rocky shore intertidal organisms has

been considered in Section 2.3.6.2 When the shore is

uncovered by the tide, wide and rapid temperature changes

are encountered by the species living there However, it

needs to be borne in mind that the body temperatures of

intertidal organisms rarely correspond exactly to the

ambi-ent air temperatures In some cases, the body temperatures

may exceed those of the air as has been recorded for

barnacles; in other cases, the body temperature may be

reduced below that of the ambient temperature due to the

cooling effect of evaporative water loss Edney (1951) has

demonstrated the importance of transpiration in the

con-trol of body temperature in the littoral fringe isopod, Ligia

oceanic The thermal sensitivity of most species

popula-tions correlates with both latitudinal distribution and level

occupied on the shore, as well as the topography and

aspect of the shore from which samples have been taken

for experimental studies In such experiments, as Newell

(1979) points out, the duration of exposure at each

tem-perature is important; a long exposure at a lower

temper-ature may cause the same percentage mortality as a brief

exposure at a higher temperature In considering the

tem-perature tolerances of intertidal organisms, care in their

interpretation needs to be taken since the stresses of

tem-perature and desiccation are interdependent, and otherenvironmental factors, such as wind velocity, are involved

In considering thermal tolerances, the phenomenon

of “acclimation” must also be taken into account Thus,animals have the ability to adjust their metabolic ratesover a period of time to the prevailing temperature con-ditions The results of numerous studies have led to gen-eral agreement that in many species, cold acclimationinvolves a compensatory rise in the level of activity insuch a way that the rate remains comparably with that ofwarm-acclimated animals The result of this is that com-parably sized individuals of a species with a wide latitu-dinal range show similar levels of activity at the coldtemperate and warm temperate limits of their ranges As

a measure of metabolic activity, the rates of various tions, such as cirral beat in barnacles, heartbeat, and therate of water filtration in bivalves, are measured Sincethese rate functions all vary with body weight, it is impor-tant that comparisons be made with comparable sizeranges of the species being compared In considering thetemperature tolerances of intertidal organisms, thestresses of temperature and desiccation are interdepen-dent, and other environmental factors, such as wind veloc-ity and humidity, are also involved

the vertical zonation patterns on hard shores has alreadybeen considered in Section 2.3.1 Here we shall examine

in more detail some of the adaptations to combat waveshock found in intertidal plants and animals

Wave action has a considerable effect on the size range

of many species of intertidal animals This is especiallynotable in the case of littorinid snails On New Zealandshores, both Littorina cincta and L unifasciata show adecrease in average size with increasing wave action Smith(1958) found that the degree of wave action has a moder-ating effect on numbers, vertical distribution and range, andsize and shape of both species on the shores of LytteltonHarbour An increase in wave action was accompanied by

an upward shift in the zone of vertical distribution and anincrease in the width of the zone Shading permitted anupward extension of the range of L unifasciata Maximumand mean densities showed a general increase correspond-ing with an increase in exposure (see Section 4.2.3.1) L.

cincta was less tolerant of extreme exposure than L

was least Larger individuals occupied a wider range ofsites than smaller individuals There were also differences

in shell shape that could be correlated with exposure towave action; shells were significantly more elongated inthe more sheltered areas and the rate of increase in thediameter of the whorl, and hence aperture size relative tothe rate of growth of the shell, also differed significantly

The elongated shape would presumably be more vulnerable

to wave action and more liable to dislodge

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Other species of gastropods appear to be able to

mod-ify the form and thickness of the shell in order to withstand

the stresses imposed by strong wave action For British

species of limpets, it has been shown that those living high

on the shore in exposed situations have shells that are

flatter in shape and better adapted to withstand wave action

than those in sheltered situations

On soft shores, especially on sand beaches, wave

action influences the distribution of the animals by the

effect of the waves on the type of sediment, strong wave

action being associated with coarser sediments Beach

profiles and beach stability are also affected, especially

after storm events when the levels may change by a meter

or more Such changes can prevent some species from

establishing themselves

have to cope with fluctuations in the salinity of the

sea-water that covers them during the tidal cycles Exceptions

are those living in high shore tide pools, rock substrates

in estuaries, and near the mouths of rivers A limited

number of species, such as the barnacle Elminius

modes-tus, are able to adjust to salinity changes and consequently

substrates in areas of low or fluctuating salinities are

char-acterized by an impoverished flora and fauna Often their

vertical ranges are extended due to the absence of

com-peting species Organisms high on the shore may be

affected by freshwater seepage and runoff from the land

and in such situations, blue-green algae and filamentous

greens such as Enteromorpha spp are common.On

mud-flats, runoff from the land may dilute the surface salinities

of the sediment

Kinne (1967) has analyzed the responses of intertidal

organisms to environmental stresses such as salinity and

groups them under the following headings: escape,

reduc-tion of contact, regulareduc-tion, and acclimareduc-tion Escape may

be affected by vertical or horizontal migration into an area

where the salinity range is more tolerable Motile animals,

especially rapid movers such as fishes, can respond in this

way as can many organisms in soft deposits by simply

burrowing into the sediments where conditions are more

suitable Many organisms are able to escape temporary

short-term salinity changes by reduction of contact This

may involve the closing of valves (barnacles and bivalves),

operculum (gastropods and tubeworms), or by the

secre-tion of a mucus coating

The most important class of response is that of

regu-lation Space does not permit a detailed discussion of the

ways in which regulation occurs In general, species react

in one of three ways First, there are those species in which

the blood concentration is almost isotonic (i.e., similar in

concentration) throughout the salinity range that is

toler-ated by the organism Such species are poikilosmotic

organisms or conformers Second, there are the species

whose blood is hyperosmotic (higher in concentration) to

that of the medium at reduced salinities and isosmotic at

higher salinities Third, there are those whose blood ishyperosmotic in reduced salinities and hyposmotic (lower

in concentration) at higher salinities Reduced and highersalinities are relative to the salinity in which the animalnormally lives and to which it is acclimatized The lattertwo classes are the regulators or homiosmotic organisms

Some open shore organisms are conformers; others canregulate to a varying extent Most are stenohaline (tolerat-ing a narrow range of salinity variation); a few are eury-haline (tolerating wide variations in salinity) These areoften the open shore organisms that extend into estuaries

shores, apart from the special case of rock pools, oxygenconcentration is rarely a significant factor In tide poolsunder bright light, photosynthesis by dense algal vegeta-tion can sometimes raise the oxygen content appreciablyand at the same time the withdrawal of carbon dioxidefrom the water raises the pH Bacterial degradation ofstranded debris, on the other hand, can lead to decreasedoxygen, increased carbon dioxide, and reduced pH

Since the amount of dissolved oxygen in water is afunction of temperature, the heating of shallow water overmudflats may result in lowered oxygen levels This may

be accentuated by the presence of decaying organic matter

The infauna of mudflats and estuarine intertidal flats oftenhave to cope with reduced oxygen levels at low tide

Burrowing forms have a variety of mechanisms for lating water through their burrows but are unable to do sowhen the tide is out This situation is tolerated by reducingthe metabolic rate to low levels and many burrowing spe-cies can withstand surprisingly long anaerobic periodssuch as the nine days for the lugworm Arenicola marina

circu-and 21 days for the tubeworm, Owenia fusiformis,reported by Dales (1958) If the burrows drain completely,some species, provided the body surface remains wet, canbreathe atmospheric air The meio- and microfauna of softshores, on the other hand, must cope with anaerobic con-ditions if they live below the RPD layer

with the rise and fall of the tide Excessive illuminationcan be damaging due to the ultraviolet and infrared rays,which can be lethal to some organisms Such rays are,however, rapidly absorbed by seawater It is difficult, how-ever, to dissociate the effects of radiation from those ofheat and desiccation stress For many organisms, as dis-cussed below, light is of great significance in controllingthe movements of animals in maintaining themselveswithin optimal environmental conditions or microclimates

The role of light in controlling the photosynthetic rates

of algae and the attenuation of light and its spectral position with water depth have already been covered inSection 2.3.5.1 On sand and mud shores, light is not animportant factor in the lives of burrowing species How-ever, the degree of illumination is an important factor inthe photosynthesis of benthic microalgae

com-0008_frame_C04 Page 248 Monday, November 13, 2000 9:53 AM

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Water currents: Perhaps the most important role of

currents in the life of shore organisms is in the distribution

of larval stages of animals and the sporelings of algae

Much of the patchy distribution patterns that occur, e.g.,

bivalves on sandy beaches, can be explained by the

vagar-ies of current systems during the critical settling period

Special conditions occur where tidal currents, which can

often be termed rapids, occur in narrows where tidal

move-ment is compressed in a funneling effect between islands

or the narrow entrances of some inlets

4.2.4.4 Resources

Resources are those “things” provided by the environment

that enhance the ability of an individual or population to

establish itself, survive, and grow to reproduce The

prin-cipal resources for an animal are “food” and “a place to

live.” For plants, “foods” are nutrients and carbon dioxide

Food and feeding have been considered in Section 2.7 and

will be dealt with in greater detail in Chapter 6 A place

to live is the equivalent of the “habitat” of an organism

For littoral species, this includes the vertical range over

which a species can live and includes all those physical

factors that limit their distribution

Food: In general, for most shore animals, food does

not become a limiting factor It is, however, a limiting

factor for those filter feeders that grow high on the shore

near the upper limits of the species vertical range Here

the limiting effect is the time coverage by water which

may be insufficient to enable an adequate supply of food

to be obtained

A place to live: A place to live is synonymous with

the habitat, or the place you would go to find the species

in question For shore organisms, the types of substratum

(rock, shingle, sand, mud, and various intermixtures) are

of prime importance in determining whether the

appropri-ate habitat is available for a particular organism

On hard shores, rock surfaces at the appropriate level

are necessary for sessile plants and animals, as well as for

relatively sedentary forms such as limpets and some other

herbivores For rock borers, rock of the type that they can

bore into is essential and for those species that live in the

shade and humidity under boulders, the distribution of

such habitats will determine their presence or absence

Other species, such as the epiflora and epifauna that live

in association with seaweeds, depend on the presence of

other living organisms to provide a suitable habitat

On soft shores, many animals construct permanent

bur-rows in substrates of the appropriate grain size Deposit

feeders are restricted to sediments of a particular grain size

composition For the interstitial animals on soft shores, a

place to live is on or in the spaces between sand and silt

particles of the appropriate size Throughout this book

many examples have been given of the importance of

hab-itat in determining the presence or absence of a particular

species As a population regulator, this factor acts in anegative sense since it becomes significant by its absence

4.2.4.5 Other Organisms

The other animals and plants that make up the livingcomponent of an organism’s environment include mem-bers of the population to which it belongs as well members

of the populations of other species On this basis we can

divide the interactions that occur into intraspecific (between members of the same species) and interspecific

(between members of different species)

Intraspecific interactions: Such interactions are most

obvious when the members of the same species competefor some resource that is in short supply, e.g., food orspace In general food is not as limiting as is space

Intraspecific interactions will be dealt with in Sections5.3.2.2, 5.3.2.3, and 5.3.4.2

Interspecific interactions: Populations of two

spe-cies may interact in basic ways that correspond to binations of 0 (no significant interaction), + (positive inter-action), and – (negative interaction) The different kinds

com-of possible interactions are shown in Table 4.3 All com-of theseinteractions are likely to occur in littoral communities For

a given pair of species, the interactions may change underdifferent environmental conditions or during successivestages in their life histories

1 Competition Competition occurs when two

species strive and compete for the same ronmental resource, and is best exemplifiedwhen it is for living space, especially betweensessile species such as barnacles, mussels, andserpulids The various kinds of competition will

envi-be discussed in Sections 6.6.2 and 6.3.4

TABLE 4.3 Analysis of Intraspecific Population Interactions Type of

Interaction

Species

General Nature of the Interaction

1 Neutralism 0 0 Neither population affects the other

2 Competition – – Inhibition of each species by the other

3 Amensalism – 0 Population 1 inhibited, 2 not affected

4 Parasitism + – Population 1 the parasite, generally

smaller than 2, the host

5 Predation + – Population 1, the predator, generally

larger than 2, the prey

6 Commensalism + – Population 1, the commensal, benefits,

while 2, the host, is not affected

7 Mutualism + + Interaction favorable to both populations

Note: 0 indicates no significant interactions; + indicates growth,

sur-vival, or other population attribute benefited; and – indicates population growth, survival, or other attribute inhibited.

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2 Parasitism It would be difficult to find a shore

organism that does not have its quota of asites The most widely studied of the para-sitic animals are the flukes or trematodes thatoccur as larval stages in invertebrates, and theshore mollusc in particular The final hosts forthese parasites are vertebrates, especiallyfishes and birds Estuarine molluscs, whichform the principal food resource of manywading birds, often carry a great variety ofthe larval trematodes Other parasites includeprotozoans, bacteria, and viruses The impact

par-of parasites on the population density par-of seaurchins on the Nova Scotia coast is discussed

in Section 5.2

3 Predation The dense populations of animals on

both hard and soft shores offer ideal nities to predators and on all types of shore,predation is a significant factor in controllingthe densities of many species Filter-feedinganimals can be regarded as indiscriminate pred-ators on small zooplankton as well as the larvalstages of many species of shore animals Otherpredators are more specific and in some casesthe prey may be restricted to a single group ofanimals and even to a single species, e.g.,bivalve molluscs as is the case of some preda-tory gastropods

opportu-Shore animals are exposed to a double set

of predators During submergence they arepreyed upon by other marine animals, but whenuncovered they are subject to terrestrial preda-tors, especially marine birds Most of themarine invertebrate predators are either anem-ones, gastropod molluscs, cards, lobsters, andechinoderms, while fishes are important marinepredators The role of predators will not be dis-cussed here but will be dealt with in detail inSections 6.2.3 and 6.3.5

4 Commensalism and Mutualism There are a

great variety of associations between marineanimals in addition to those of competitor, pred-ator and prey, and host and parasite Broadlyspeaking, these other relationships can begrouped into those of commensalism and mutu-alism In the former association, the two specieslive together in some degree of harmony withone species generally benefiting to a greater orlesser degree from the association In mutual-ism there is a close physiological associationbetween the two species, usually for mutualbenefit It must be remembered, however, thatthere are many transitional examples that do not

fit neatly into either category Commensalism:

Based on the type of commensal relationship,

we can distinguish three subgroups: epizoitism, endoecism, and iniquilism.

• Epizoitism Epizoites are animals that live

attached to the surfaces of other animals,such as barnacles and tubeworms and othersessile species found growing on the shells

of molluscs Epizoites sometimes displaydefinite preferences for particular speciessuch as the New Zealand species, the small

mudflat limpet, Notoacmea helmsi, on the shells of the mudflat snail, Zediloma subros- trata, and the hydroid, Amphisbetia fascic- ulata, which attaches itself to the shell of the bivalve, Paphies donacina, on sand

beaches

• Endoecism There are large numbers of

com-mensals that lurk in the burrows, tubes, ordwellings of various animals Manypolynoid worms are such commensals They

include the polynoid Lepidasthenia aecolus,

found in the burrows of the lugworm,

Abarenicola assimilis; the short, rather broad and flat Lepidastheniella comma liv-

ing in the tubes of terebellids, especially

Thelepus species; and the larger, more der Lepidasthenia sp found in the tubes of the sand beach maldanid, Axiothella quadri- maculata Experimental analyses of host-

slen-commensal relationships have shown thatsome species are attracted to specific chem-icals given off by their copartners

A number of molluscs, especiallybivalves, are commensal with other animals.New Zealand examples include the small

bivalves of the genus Arthritica, A hulmei

which lives under the elytra of the scale

worm Aphtodita australis, A crassiformis living with the large rock borer, Anchomasa similis, and A bifurcata attached to the outer

surface of the head end of the perctinarid

polychaete, Pectinaria australis.

• Iniquilism This term is applied to the cases

where the commensals live in the body ities or internal cavities of their hosts Suchspecies benefit by obtaining access to foodsupplies, by sharing the host’s refuge, or bytaking advantage of the repellent properties

cav-of the host This is probably one cav-of the routes

to parasitism and many inquiline sals are close to being parasites One suchexample are the pinnitherid (pea) crabs,which are found in the mantle cavities ofmussels and a range of other bivalves

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commen-Adaptations to Shore Life 251

4.2.4.6 Disturbance and Patchiness

Two of the salient features of littoral systems are that they

are spatially patchy and that important processes such as

disturbance and recruitment are spatially and temporarily

variable On rocky shores in particular, spatial patchiness

is obvious as a result of differences in substrate, angle of

slope, degree of wave action, amount of shade, degree of

sand coverage and sand scouring, and in the frequency

and intensity of disturbances that result in new substrates

being made available for colonization Aspects of all of

these have already been covered in Chapter 2 and they

will be further considered in Chapter 5 Patchiness can

also be a consequence of random settlement and

recruit-ment in intertidal invertebrates, e.g., barnacles and

mus-sels on rocky shores, and bivalves and polychaetes on

soft shores

4.2.4.7 The Importance of

Recruitment

Many recent field studies have shown that, once

plank-tonic larvae are transported to a substrate suitable for

settlement, variation in recruitment (the proportion of the

settlers that have survived over a time period with the

potential to contribute to the adult population) on a

relatvely small spatial scale (sites of meters to ten meters

apart) can be determined by variation in the supply of

planktonic larvae (Grosberg, 1982; Minchinton and

Scheibling, 1991; Bertness et al., 1992) Further, it has

been shown that variation in recruitment may be directly

proportional to the amount of space on the substratum

available for colonization (Gaines and Roughgarden,

1985; Minchinton and Scheibling, 1993; Chabot and

Bourget, 1988) However, Raimondi (1990) has shown

that under certain conditions this relationship does not

apply In addition, a wealth of studies (mostly

mechanism-orientated and carried out in the laboratory) have

demon-strated that physical (Butman, 1987; Raimondi, 1988a)

and biological (Raimondi, 1988b; Andre et al., 1993)

interactions between the incoming larvae and established

residents and the behavioral responses of the larvae when

selecting a settlement site can influence the distribution

of the larvae at settlement (Crisp, 1984; Pawlik et al.,

1991) A prevalent indicator of the suitability of a habitat

for settlement is the presence of conspecific adults (i.e.,

gregarious behavior: Gabbott and Larman, 1987;

Rai-mondi, 1988b) Resident individuals may exude chemical

attractants that stimulate settlement of conspecific larvae,

or physical contact between conspecific individuals may

be required

The role of settlement and recruitment in the

estab-lishment of intertidal communities will be considered

later in this chapter and will be explored in detail in

Chapter 5

4.3 THE ESTABLISHMENT OF ZONATION PATTERNS

In order to maintain a population at a particular zone onthe shore, a species must reproduce, disperse its larvae,and the larvae must settle at the appropriate level on theshore and survive to reproduce In this section we willconsider these events in the life histories of intertidalorganisms and the various factors that influence them

spec-we adopt a modified version of that proposed by Thorson(1946; 1950)

1 Pelagic (long-life) planktotrophic These larvae

spend a significant proportion of the ment time swimming freely in the surfacewaters and feeding on other planktonic organ-isms, usually phytoplankton Duration of larvallife varies from a few weeks to two or threemonths Eggs are released with little yolk butare produced in great numbers, e.g., up to

develop-85,000 eggs per spawning in the gastropod, torina littorea, (Bingham, 1972), and up to 70million eggs per individual in a single spawning

Lit-of the oyster, Crassostrea virginica Predation,

starvation, and other factors take a tremendoustoll on planktotrophic larvae, with an estimatedmortality exceeding 99% (Thorson, 1950).However, the enormous numbers of larvae thatare produced counterbalance this extremelyhigh larval mortality Scheltema (1967) dividedthe planktonic stage of planktotrophic speciesinto two phases: (1) growth and development(the pre-competent period of Jackson andStrathmann, 1981), followed by (2) a “delayperiod” (“competent period”) in which devel-opment is essentially completed but larvaladaptations for a planktonic existence areretained until a suitable substrate for settlement

is found

2 Short-life planktotrophic These larvae are

dis-criminated chiefly on the basis that their sizeand organization change, hardly, if at all, in thecourse of the week or less spent in the plankton

3 Pelagic lecithotrophic larvae These larvae are

large and provided with much yolk, hatchingfrom a large yolky egg The yolk provides all

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the energy needed by the larva until phosis into a settled juvenile (thus they are non-planktotrophic) Reproductive effort per off-spring is thus much higher than in the plank-totrophic species and larval mortality is muchlower Accordingly, far fewer eggs per parentare produced (4,100 eggs per parent in the

metamor-bivalve, Nucula proxima, and 1,200 in the related species, N annulata).

4 Non-pelagic lecithotrophic To the above

pelagic species must be added those that show(1) the so-called “mixed development,” (2)

“direct development,” and (3) some form ofbrooding Mixed development occurs whenearly developmental stages are encapsulated, butlater stages emerge as free-swimming, pre-metamorphic larvae (Pechenik, 1979; see also

Caswell, 1981) Mixed development is nent in several benthic marine groups, especiallypolychaetes and gastropods Direct developmenttakes place within a encapsulated egg fromwhich a benthic juvenile eventually hatches

promi-Among the higher prosobranch gastropods(Neogastropoda), oviparous species maydeposit, along with viable eggs, a supplementaryfood source in the form of nurse eggs Broodedlarvae, which are characteristic of ovoviviparousspecies, are retained (sometimes encapsulated)within the parent throughout development,emerging as metamorphosed juveniles

4.3.1.2 Developmental Types in Marine Algae

The basic life cycle of an algal macrophyte is depicted in

Figure 4.8 Type A is what is known as a sporic life cycle,

which has a spore-producing phase, the sporophyte, which

is usually diploid (2n); and a usually haploid (n),

gamete-producing phase, the gametophyte The sporophyte and

gametophyte are separate free-living plants, which may

look exactly the same, or be very different The sporophyte

produces spores that are liberated into the water column

(they may be motile, and as such are called zoospores)

The spores give rise to the gametophytes (male and

female), which produce eggs and sperm The fertilized

egg results in the production of the zygote (2n) which

develops into the mature plant (sporophyte), which by

meiosis produced the spores that are the agent for

dis-persal Figure 4.9 illustrates the life cycle of a kelp The

adult sporophyte releases spores into the water These are

washed around by waves and currents, eventually to settle

on the bottom where they develop into gametophytes

In addition, some algae can reproduce vegetatively

through fragmentation It is especially common in

fila-mentous species and appears to play an important role in

maintaining populations in habitats such as estuaries or

salt marshes (Norton and Mathieson, 1983) Other speciescan regenerate from basal structures after the foliosefronds have been abraded or consumed by herbivores

4.3.1.3 Reproductive Strategies

Life history patterns are often referred to as “strategies,”

a viewpoint that has often been criticized However, aspointed out by Grahame and Branch (1985), if the view

is taken that survival and progeny leaving are the outcomes

of a series of features (morphological, physiological, andbehavioral), then these adaptations can be seen as a “strat-egy” assembled by natural selection and ensuring survival.Todd (1985) has reviewed reproductive strategies of rockyshore invertebrates with special reference to northern tem-perate regions He differentiates the terms life historystrategy, life cycle strategy, and larval strategy, and illus-trates their interrelations as shown in Figure 4.10 Repro- ductive strategy is a general term encompassing all three strategies listed above Life history strategy has a dichot-

omous base and refers to organisms as being either parpous (reproducing once and then dying) or interparous

semi-(undergoing repeated breeding periods) (Cole, 1954) val strategy applies to the three fundamental larval types

Lar-— planktotrophic, pelagic lecithotrophic, and non-pelagiclecithotrophic

Energetic considerations also enter into life historystrategies Since the total energy budget of an individualorganism is finite, the proportion allocated to reproductionwill vary depending on age, availability of food resources,and environmental variables Montague et al (1981) con-sidered that the reproductive strategy of an individualorganism is the set of physiological, morphological, andbehavioral traits that dictate the “where,” “when,” “howoften,” “how many” (and for marine invertebrates, “whatkind of”) tactics of propagule production In this contextthere is the concept of reproductive effort (RE, i.e., ameasure of the proportion of somatic effort (in energyterms) allocated specifically to reproduction) (Hirshfieldand Tinkle, 1974)

Life cycle and life history strategies: Most temperate

rocky shore invertebrates, almost without exception, play extended or perennial life cycles This contrasts to softshore invertebrates in which a range of life cycles, espe-cially those of bivalves and gastropods, which are longlived, to some polychaetes which are annuals or less A

dis-distinction needs to be made between potential and realized

longevity in that survivorship of (potentially) long-livedanimals, such as barnacles, may be markedly controlled bythe activities of predators (e.g., Connell, 1970; 1972; 1975),

or abiotic factors such as freezing (e.g., Wethey, 1985) Inaddition, local habitat patches, separated by perhaps only

a few meters, may confer very different survivorship andgrowth probabilities for conspecific individuals (e.g.,

Patella vulgata, see Lewis and Bowman, 1975)

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FIGURE 4.9 The life cycle of a kelp (Redrawn from Kennelly, S.J., in Coastal Marine Ecology of Temperate Australia, Underwood,

A.J and Chapman, M.G., Eds., University of New South Wales Press, Sydney, 1995, 108 With permission.)

FIGURE 4.10 The interrelationships between reproductive energy traits (Redrawn from Todd, C.D., in The Ecology of Rocky Shores,

Moore, P.C and Seed, R., Eds., Hodder & Straughton, London, 1985, 204 With permission.)

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Barnacles, characteristic of rocky shores worldwide, are

all potentially long lived and interoparopus Most species

(e.g., Chthalamus stellatus and Elminius modestus)

com-plete several reproductive cycles at variable intervals

depending on a complex of factors such as natality, mortality,

food availability, and ambient temperatures (Barnes and

Bar-nes, 1968; Crisp and Patel, 1969) Life spans are highly

variable Tetraclita squamosa, which requires ten years to

reach maturity, may attain 14 years of age (Hines, 1979),

while Semibalanus balanoides has a maximum (realized)

longevity of only two years on New England shores (Wethey,

1984) Nevertheless, on British shores S balanoides matures

in its first year and together with Chthalamus spp has a

considerably greater life span than on New England shores

On the North American west coast, the goosenecked

barna-cle, Pollicipes polymerus, has a maximum life span of a least

6 (Paine, 1974) and perhaps up to 20 years

Temperate shore bivalves are also long lived The

cos-mopolitan Mytilus edulis may attain sexual maturity

within only 1 to 2 months of settling and may live as long

as 18 to 24 years (Seed, 1969; Suchanek, 1981) In

con-trast, the much larger, faster-growing M californianus

may require anywhere from 4 months to 3 years to mature

(Suchanek, 1981) but certainly lives for 7 to 30, and

pos-sibly as many as 50 to 100 years (Suchanek, 1981)

On North American Pacific shores, the large (up to

1,200 g) chiton, Cryptochiton stelleri, lives for 16 to 25

years (Branch, 1981) Other chitons are similarly long

lived Branch (1981) in his review of limpet biology found

that they invariably displayed extended perennial life

cycles Of the 20 species for which he had data, all except

one had life cycles ranging from in excess of one year to

approximately 30 years

Among the archaeogastropoda trochid (“topshell”)

prosobranchs, Williamson and Kendell (1981) estimated a

maximum adult life span of more than 10 years for

Mon-odonta lineata and concluded that other major British

top-shells, Gibbula cineraria and G umbilicalis, were similarly

long lived For British littorinids, Hughes and Roberts’

(1980) estimated age at first maturity ranged from 8.5

months (Littorina littorea) to 18 months (L nigrolineata

and L saxatilus) to 3 years (L neritoides), while longevity

ranged from 8 years (L littorea and L nigrolineata), to 8

to 11 years (L saxtilus), to 16 years (L neritoides) The

predatory dogwhelk, Nucella lapillus, matures after 2.5 to

3 years (Hughes, 1972) and individual survivorship does

not exceed 6 years (Hughes and Roberts, 1980) The three

Nucella species on Pacific Northwest shores show variable

life spans from 2 to 4 years (Todd, 1985)

Pisaster ochraceus on the Pacific Northwest coast of

North America is estimated to have a life span of perhaps

34 years (Menge, 1972), while the smaller Leptasterias

hexactis has an estimated longevity of from 4 to 18 years.

Larval dispersal and larval strategies: Pelagic larval

forms generally display “delay” of metamorphosis in the

absence of a specific cue or cues, and as the pause passesthe larvae become less and less discriminating with regard

to the choice of a settlement site (e.g., Crisp, 1974; mann, 1978; Pechenik, 1984; and others in Chia and Rice,1978) In this context there is a distinction between the “pre-competent” and “competent” phases of development (e.g.,Crisp, 1974) During the pre-competent phase, the larva ismorphologically and/or physiologically incapable of settle-ment and metamorphosis; the competent (= delay) phasecommences from the point at which metamorphosis can takeplace upon the reception of the appropriate stimulatory cues.The competent phase is not, however, of indefinite duration(see review by Jackson and Stratham, 1981) and varies for

Strath-a rStrath-ange of tStrath-axStrath-a from Strath-a few dStrath-ays to Strath-a few months of durStrath-ation.All pelagic larvae are subject to dispersal away fromthe potential micro-habitat, and for rocky shore species inparticular this poses considerable risks in subsequentlyfinding a suitable substratum for settlement Larval trans-port is unpredictable and suitable habitats are patchy inspace and time However, there are advantages commen-surate with a pelagic phase; these include the potential toincrease the species’ geographical range, an increase ingene flow, the reduction of local extinctions resulting fromdensity-independent perturbations, and (as a result of thenecessarily high fecundity) the increase in potential juve-nile offspring per unit RE

Timing of reproduction: In many species the release

of eggs and sperm or larvae coincides with particularphases of the tidal cycle; e.g., in the pulmonate snail,

Melampus bidentatus, which inhabits the higher levels of

salt marshes, both hatching of the eggs and settlement oflarvae are synchronized with spring tides (Russel-Hunter

et al., 1972) Both Littorina littorea and L melanostoma

have a lunar-tidal rhythm in which the release of eggscoincides with spring tides

Many species of intertidal organisms reproduce ally, or concentrate their reproduction over a certain period

annu-of the year In many cases, temperature may be the cuefor reproduction However, food availability may, in manyinstances, be more important than temperature Manyinvertebrates spawn so as to coincide with the spring phy-toplankton bloom and thus allow the larvae to capitalize

on a rich but transient food resource In the tropics, ever, many invertebrates may breed continuously

how-4.3.1.4 A Model of Non-pelagic Development

Co-adaptive with Iteroparity

Todd (1985) has developed a flowchart illustrating thepossible reproductive strategy responses of rocky shoreanimals (especially prosobranchs), based largely on theextensive body of research on British littorinids (e.g.,Hughes and Roberts, 1980; Raffaelli, 1982; Atkinson andNewberry, 1984) (Figure 4.11) In the model, increasesand decreases in life span, current fecundity, and juvenile

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survivorship are balanced according to the prevailing

selective regime Selection (predatory) pressure favors an

increase in hatching size (fewer larger individuals, plus

reduced adult longevity and increased reproductive effort)

4.3.2 S ETTLEMENT AND R ECRUITMENT

4.3.2.1 Introduction

The initial settlement of planktonic propagules of benthic

organisms usually varies considerably both in time and

space (e.g., Connell, 1961a; 1985; Hawkins and Hartnoll,1982; Caffey, 1985; Wethey, 1985) As a consequence,over the last few years there has been much interest andresearch on local variation of settlement and/or recruit-ment, and its consequences upon the distribution andabundance of intertidal species (e.g., Grosberg, 1982;Keough and Downes, 1982; Keough, 1983; Underwoodand Denley, 1984; Gaines and Roughgarden, 1985;Bushek, 1988; Fairweather, 1988a; Bertness et al., 1992;Rodriguez et al., 1993) This emphasis has been termed

FIGURE 4.11 Flowchart illustrating possible reproductive strategy responses of rocky shore animals (especially prosobranch

gas-tropods) The responses are to selection pressure favoring an increase in hatching size The diagram should be followed according

to the key (Redrawn from Todd, C.D., in The Ecology of Rocky Shores, Moore, P.C and Seed, T., Eds., Hodder & Straughton,

London, 1985, 211 With permission.)

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