195 The Chemical Ecology of Invertebrate Meroplankton and Holoplankton James B. McClintock,* Bill J. Baker, and Deborah K. Steinberg CONTENTS I. Introduction 196 A. The Problem 196 B. Role in Regulation of Material and Energy Flux 197 C. The Paradox of the Plankton 197 II. Laboratory Studies of Chemical Defenses 198 A. Meroplankton 198 B. Holoplankton 206 III. Field Studies of Chemical Defenses 210 A. Meroplankton 210 B. Holoplankton 210 IV. Chemistry of Meroplankton and Holoplankton 210 V. Other Modes of Predator Avoidance 212 A. Size 213 B. Transparency and Other Forms of Crypsis 213 C. Vertical Migration 214 D. Exploitation of Sea Surface or Surfaces of Other Organisms and Particles 214 E. Structural Defense 215 F. Aposematism 215 G. Other Considerations 216 1. Speed/Swimming Behaviors 216 2. Nutritional Content 216 3. Time in the Plankton 216 VI. Symbioses 216 VII. Potential Antifoulants 218 VIII. Summary and Future Directions 218 Acknowledgments 219 References 219 * Corresponding author. 5 9064_ch05/fm Page 195 Tuesday, April 24, 2001 5:18 AM © 2001 by CRC Press LLC 196 Marine Chemical Ecology I. INTRODUCTION A. T HE P ROBLEM Meroplankton is comprised of organisms that spend some component of their life history in the plankton, usually the eggs and larvae of benthic or nektonic adults. There are a few examples of adult benthic marine invertebrates that spend brief periods of time in the plankton. These include the adult reproductive phase (epitoke) of some marine polychaetes, whose benthic life history is interrupted at reproductive maturity by a dramatic ontogeny of swimming appendages followed by swimming behaviors that result in the swarming and bursting of a pelagic reproductive phase. Some deep sea holothuroids (sea cucumbers) spend some period of their adult life swimming in the plankton. 1 Moreover, there are a number of demersal marine invertebrates such as copepods and amphipods that migrate up into the water column at night. 2,3 Nonetheless, the vast majority of meroplankton in the world’s oceans are comprised of the propagules of algae or the eggs and larvae of benthic invertebrates and fish. Little is known of the chemical defenses of the propagules of algae or the eggs and larvae of fish. Therefore, the present review and discussion of the chemical ecology of meroplankton will focus primarily on feeding-deterrent properties of marine invertebrate eggs and larvae. While all marine invertebrate groups have the potential of producing defensive chemistry in their larval offspring, studies to date have focused specifically on the eggs and larvae of sponges, cnidarians, molluscs, echinoderms, and ascidians, groups of organisms that are well known to possess chemical defenses in their adult stages. Benthic marine invertebrates possess a relatively discrete repertoire of reproductive modes. 4–7 Typically, they fall into two categories: those that broadcast large numbers of small eggs that are fertilized and develop into small feeding planktotrophic larvae, or those that produce small numbers of large eggs that are fertilized and develop into large, often conspicuous, nonfeeding lecithotrophic larvae that are subsequently brooded (protected by the parent) or released into the plankton. 5 Eggs or larvae that are released into the plankton, while in some cases having limited mobility generated by ciliary beating, are extremely sluggish and generally lack protective skeletization, and evidence would suggest that they are exposed to considerable predatory pressure from planktivores. 8,9 More- over, an extensive literature indicates that eggs and larvae face a formidable array of predators (reviewed by Rumrill 10 ). One example of planktivory was aptly described by Emery 11 for plank- tivorous pomacentrid fish when he offered that they constituted a “wall of mouths” facing the plankton. Therefore, one might expect strong evolutionary pressure for selection of defensive chemicals that would decrease the likelihood of predation. This is particularly the case for those benthic marine invertebrates that produce very small numbers of large, conspicuously colored, nutrient-rich lecithotrophic larvae that are released into the plankton, where loss of even small numbers of larval progeny would have strong negative effects on the probability of successful recruitment. 6,12 In summary, information on the chemical defenses of eggs, embryos, and larvae of marine invertebrates is important because models of evolutionary selection of life history patterns make assumptions about patterns of mortality of offspring. 5,13–18 These models generally assume that eggs, embryos, and larvae are vulnerable to predators, and have primarily considered marine invertebrates with planktotrophic modes of development. Unlike meroplankton, holoplankton is comprised of organisms that spend their entire life cycles in the plankton. The holoplankton contain representatives of nearly every algal and animal group. Over 10,000 species of copepods (crustacea) alone are known, and these can reach abundances of 70,000 per square meter of water in the surface waters of the North Sea. 19 A large variety of gelatinous zooplankton inhabit the sea, with prominent members including medusae, siphono- phores, ctenophores, pelagic molluscs, and pelagic ascidians (e.g., salps, larvaceans). The ubiquitous salps, for example, are periodically encountered in swarms extending hundreds of kilometers, 20,21 and, although patchy, can reach densities of 1000 animals per cubic meter. 21 The planktonic protozoa, unicellular, and colonial animals such as acantharia, foraminifera, and radiolaria, are also 9064_ch05/fm Page 196 Tuesday, April 24, 2001 5:18 AM © 2001 by CRC Press LLC The Chemical Ecology of Invertebrate Meroplankton and Holoplankton 197 numerous and widely distributed. 22,23 Similar to meroplankton, marine holoplanktonic algae and invertebrates are likely subject to intense predation pressure, primarily from crustaceans and fish. Cyanobacteria (formerly known as blue-green algae), radiolarians, foraminiferans, and larvaceans move passively with the currents, while some gelatinous zooplankton such as salps, cnidarians, ctenophores, pteropods, and heteropods are generally sluggish swimmers. It is unlikely that they can swim rapidly enough to avoid predatory crustaceans and fish. One might expect that holoplank- tonic marine organisms have evolved secondary metabolites to deal with the problem of predation, particularly those species that are conspicuous. Moreover, organisms symbiotically associated with chemically defended holoplanktonic organisms may derive some protection by simply associating with holoplankton. B. R OLE IN R EGULATION OF M ATERIAL AND E NERGY F LUX Holoplankton, and to some extent meroplankton, are responsible for the regulation of material and energy flow in oceanic food webs. 24 Zooplankton grazing plays a key role in the recycling of all biogenic elements, and the community structure of the pelagic food web determines the export of elements from the upper water column. The abundance of particular taxa as influenced by ecological processes like chemical defense provides a mechanism to affect this structure. The size distribution of pelagic producers (phytoplankton) and trophic position of consumers (zooplankton and micron- ekton) determines the proportion of primary production that is lost and the composition and sedimentation rate of sinking particles from surface communities, and has a significant impact on nutrient cycling. 24 For example, the production in most oceanic food webs tends to be dominated by microbial processes, with protozoan and small crustacean grazers in complex food webs. Much of the carbon and nutrients are recycled in the surface waters with little export, due to loss of energy at each of the many trophic levels. Alternatively, copepods or other large grazers feeding directly on large diatoms in coastal upwelling areas may contribute directly to flux, and a larger fraction of the phytoplankton production is exported in this short food web. There are also generalist consumers, such as the pelagic tunicates that feed with mucous food webs with which they can filter the smallest size particles. 25,26 When abundant, pelagic tunicates can account for large exports of material from the surface waters to the deep sea 21,27–29 through the flux of their fecal pellets or discarded feeding webs. Since they feed at the base of the microbial food web, even in an oceanic ecosystem, they will short circuit the normal paradigm for material and energy flow within these communities. Clearly, there is a need to begin to understand how chemical deterrents may mediate these important patterns of material and energy flow in oceanic systems. C. T HE P ARADOX OF THE P LANKTON Chemically mediated defenses among holoplankton and meroplankton may help resolve why there is a great diversity of co-existing species, all competing for the same resources, in a seemingly homogeneous habitat such as the open sea — “the paradox of the plankton.” 30 According to the theory of competitive exclusion, one species should out-compete them all. However, this uniform environment is characterized by small-scale spatial and temporal heterogeneity, such as development of microhabitats in low-turbulence situations, 31 and, thus, offers a variety of niches. Factors such as selective zooplankton grazing are important as well. For example, if one species is consumed by a predator, while another species is chemically defended, this will result in diversification of both predator and prey and make co-existence possible. An excellent example of niche diversifi- cation in the pelagic environment is the association of crustaceans with gelatinous zooplankton, such as copepods associated with salps 32 and the mucus feeding webs of “houses” of larvaceans. 33,34 Interestingly, virtually all hyperiid amphipods are associates of gelatinous zooplankton such as salps, ctenophores, siphonophores, medusae, or radiolarian colonies for at least part of their lives. 35–38 Amphipods may use gelatinous zooplankton as a feeding platform, food source, or 9064_ch05/fm Page 197 Tuesday, April 24, 2001 5:18 AM © 2001 by CRC Press LLC 198 Marine Chemical Ecology brooding chamber. 35–37 Hyperiids are thought to be descendants of benthic amphipods that have evolved to live in a benthic-like habitat in midwater that is provided by the gelatinous zooplank- ton. 37 Virtually nothing is known about the use of gelatinous zooplankton as a refuge from predation in the pelagic zone. However, a benthic marine amphipod has been shown to build a “domicile” from algal material in which it resides. 39 The amphipods selectively chose for their domiciles algal species with secondary metabolites that deter predation by reef fishes, and are thus chemically defended against predation. Whether pelagic amphipods might reduce their chance of being consumed by associating with chemically defended gelatinous zooplankton in the pelagic zone is unknown. II. LABORATORY STUDIES OF CHEMICAL DEFENSES The vast majority of work conducted to date on the chemical ecology of meroplankton and holoplankton has employed laboratory-based approaches. There are both pros and cons associated with laboratory studies. Bioassays conducted in the laboratory can be carefully controlled, and small organisms or their eggs and larvae can be observed directly, whereas field observations would be much more difficult or even impossible. Nonetheless, it is fair to say that the field of marine chemical ecology has been moving ideologically towards increasingly ecologically relevant approaches to hypothesis testing. 40 The goldfish toxicity assays employed to evaluate chemical defenses in the earlier years 41–43 have given way to sympatric marine predator models, the use of extracts rather than homogenates, employment of ecologically relevant concentrations of extracts or pure compounds, and increasing numbers of studies that couple laboratory and field assays. A. M EROPLANKTON To date, we are aware of no laboratory studies of the chemical feeding-deterrent properties of meroplankton that have an adult planktonic phase, nor of meroplankton comprised of the propagules of marine macroalgae. Lucas et al. 44 were the first known researchers to experimentally examine the feeding-deterrent properties of the eggs and larvae of a marine invertebrate. The focus of their study was on the coralivorous sea star Acanthaster planci, a species known to produce copious numbers of small eggs that develop as planktotrophic larvae. Lucas and others had noted earlier 45,46 that some species of fish discriminated against the larvae of A. planci . Knowing that saponins occurred in the adult body wall and eggs of A. planci, 47 Lucas et al. 44 examined the potential role of saponins as feeding deterrents in eggs and larvae. Gelatin pellets were prepared with yeast extract as a feeding stimulant and contained ecologically conservative concentrations of crude saponin extracts. Four species of sympatric pomacentrid fish were employed as potential predators. In almost all cases, fish rejected experimental gelatin pellets containing saponins while readily consuming control pellets. Interestingly, Lucas et al. 44 noted that with decreasing hunger level, fish increased their discrimination against pellets containing saponins, indicating that the nutritional condition or hunger level of fish predators can influence the ability of chemicals to effectively deter predators. Moreover, Lucas et al. 44 noted that the amount of feeding stimulant added to the experimental pellets influenced acceptability. While their study did not involve an accurate modeling of the nutritional or energetic content of eggs or larvae, they extended this observation to a comparison of planktotrophic and lecithotrophic eggs and larvae, arguing that planktotrophic larvae may be nutritionally less acceptable to fish than yolky eggs and yolky lecithotrophic larvae. Lucas et al. 44 concluded that saponins sequestered in eggs and larvae appear to be effective deterrents against fish, and they offered some preliminary qualitative observations that the larvae of A. planci may also be rejected by planktivorous invertebrate carnivores and benthic polychaetes. The logical extension of this work, posed in a question by Lucas et al. 44 — “Do larvae of other sea stars contain saponins as chemical defenses?” — still remains unanswered. 9064_ch05/fm Page 198 Tuesday, April 24, 2001 5:18 AM © 2001 by CRC Press LLC The Chemical Ecology of Invertebrate Meroplankton and Holoplankton 199 While no bioassays were conducted on discrete gametes, embryos, or larvae, De Vore and Brodie 48 examined the palatability of gravid ovaries of the temperate sea cucumber Thyone briareus to the common killifish Fundulus diaphanus . Fish were held on a maintenance diet and presented pieces of ovaries or control food over an 11 day period, with the first day consisting of a feeding trial on control food (mussel tissue) and the subsequent 10 days consisting of a randomized presentation of either experimental gravid ovary or control food . Fish demonstrated a strong and significant rejection of the gravid ovaries as compared to the controls, and investigators suggest that this may reflect the concentration of a toxin within the ova to protect the progeny. As this species possesses a vitellarian larva that develops in the water column, it is possible that these larvae may possess a chemical defense. However, further work is needed to verify that the deterrent properties of the gravid ovary are indeed related to feeding deterrents sequestered in the ova and not simply in the supporting ovarian tissues. Young and Bingham 49 addressed both issues of chemical defenses and aposematism (warning coloration) in their study of the large, brightly colored larvae of the subtropical colonial ascidian Ecteinascidia turbinata . Larvae were presented to sympatric pinfish ( Lagodon rhomboides ) and acceptance or rejection noted. Importantly, these investigators examined whether rejected larvae were still capable of normal swimming behaviors, an indication that fish mouthing did not cause subsequent mortality. Swimming larvae were rejected by pinfish. The researchers further demon- strated that rejection was chemically based by observing rejection of agar pellets containing larval homogenates; defensive chemicals were effective at a concentration approximately 170 times lower than they occur in the larva. The identity of the defensive chemicals was not determined but shown to have a molecular weight of less than 14,000 Da, and to unlikely be proteins since boiling did not inhibit bioactivity. Employing a rather ingenious approach, Young and Bingham 49 further examined the question of larval aposematism in E. turbinata . Utilizing the palatable larvae of the ascidian Clavulina oblonga , they dyed these larvae a color similar to that of the unpalatable larvae of E. turbinata . Fish that had been conditioned on larvae of E. turbinata generally ignored the dyed larvae of C. oblonga . Indeed, these fish appeared to learn how to avoid colored larvae rapidly, whereas fish that had not been conditioned on the larvae of E. turbinata consumed the larvae of dyed or undyed larvae of C. oblonga in equal frequency. Their observations provide a unique test of whether warning coloration operates at the group or kin level, since larval prey die if sampled during the learning process. 50 Their results argue against group selection since the majority of larvae survive the learning process, and fish retain their recognition of unpalatable larvae for only a very short period of time (Young and Bingham, unpublished). Instead, they argue that if larval survival decreases with successive strikes, then individual selection 51 should be invoked as an explanation for the evolution of aposematic coloration in the larvae of E. turbinata , even given the short memory of the pinfish. This study raises intriguing questions about how widespread aposematism might be among marine invertebrates or even fish eggs, embryos, or larvae. Certainly, visual predators such as fish can comprise a significant component of the planktonic predator population, and other investigators have indicated that colored and thus conspicuous larvae may be more likely to possess chemical defenses. 52,53 A larger data base from carefully controlled studies of aposematism in marine invertebrate larvae is needed before a general pattern can be adequately evaluated. Although the chemical deterrent properties of the eggs and planktotrophic larvae of the tropical nudibranch Hexabranchus sanguineus were not investigated, Pawlik et al. 54 demonstrated that the egg ribbons of this nudibranch are defended from fish predation by macrolides (see kabiramide, Figure 5.2). Sequestering these compounds from its sponge diet, H. sanguineus incorporates the compounds into the mantle tissues and egg cases after chemical modification. The presence of defensive macrolides in the egg cases raises intriguing questions about whether these defensive compounds might also be provisioned in the eggs and subsequently serve a defensive role in meroplanktonic larvae. McClintock and Vernon 52 furthered the study of chemical defenses of echinoderm offspring by examining feeding-deterrent properties of the eggs and embryos of Antarctic sea stars, sea urchins, 9064_ch05/fm Page 199 Tuesday, April 24, 2001 5:18 AM © 2001 by CRC Press LLC 200 Marine Chemical Ecology and a sea cucumber. They chose to use an allopatric fish model as a predator (the marine killifish Fundulus grandis ), arguing that by using a temperate/subtropical fish, this model predator has had no exposure to Antarctic echinoderm eggs or embryos over evolutionary time and would not be expected to have co-evolved adaptations of resistance to toxins. Other chemical ecologists have argued that it is more important to employ sympatric predators in order to effectively evaluate whether chemical deterrents are truly effective against ecologically relevant predators. 40 Both sides of this argument likely have merit, and perhaps a dualistic approach is best if possible, with both sympatric and allopatric models tested. Because many Antarctic echinoderms have been shown to release their eggs into the water column, in contrast to Thorson’s Rule, 7 and have extremely slow rates of development that expose them to long periods of predation, McClintock and Vernon 52 predicted that chemical defenses may be relatively common in Antarctic echinoderm eggs and embryos. They found that lyophilized egg and embryo tissues of four species of sea stars embedded at ecologically relevant concentrations in agar pellets deterred feeding in killifish. Three of these species produced large yolky eggs or embryos (lecithotrophs), and one of these, Perknaster fuscus , did not brood but rather released large yolky eggs into the water column where they developed as pelagic lecithotrophic larvae. A fourth species, Porania antarctica , produced intermediate-sized eggs that developed as relatively large planktotrophic larvae. The nature of the chemical com- pound(s) was not determined, but they are likely to be saponins 44 such as steroid oligoglycosides and polyhydroxysteroids. 55 The results indicate that Antarctic echinoderms that produce small numbers of lecithotrophic or large numbers of planktotrophic eggs and embryos can employ chemicals to defend their offspring, but that chemical defenses may be somewhat more common in lecithotrophic species. Interestingly, both lecithotrophic species that brood and broadcast their eggs and embryos were found to be chemically defended. One obvious question that arises from these observations is why brooding species should invest defensive chemistry in their offspring when they are presumably protected from predation by the adult. The answer may be associated with the especially vulnerable period of time when the juvenile leaves the protection of the adult, presumably provisioned with defensive chemicals until some refuge in size is attained. This may be particularly important in polar marine environments where slow growth rates in sea stars and other marine invertebrates may result in periods of years spent in the vulnerable juvenile phase. 56 McClintock et al. 57 extended the analysis of potential chemical defenses to another phylum of Antarctic marine invertebrates in their analysis of the biochemical and energetic composition and chemical defenses of the common Antarctic ascidian Cnemidocarpa verrucosa . Their chemical studies were limited to an examination of the palatability of the gonad to a sympatric planktonic fish ( Pagothenia borchgrevinki ). Unable to trigger a spawning response in order to collect eggs or embryos or raise larvae, they were only able to test small pieces of intact ovitestes using similarly sized pieces of cod muscle as controls. They found significant rejection of ovitestes by Antarctic fish. While homogenates or extracts were not tested in feeding pellets, it is unlikely that deterrence was related to structural defenses (no skeletal material) or low nutritional content (ovitestes were found to be very high in energy 57 ). Their findings, nonetheless, must be interpreted with caution since feeding deterrence could be attributable to chemicals in the sperm (although unlikely) or the nongametic gonadal tissues. The data do suggest that the bright orange planktotrophic eggs, embryos, and larvae of C. verrucosa may possess a chemical defense. The nature of a potential chemical deterrent was not investigated, but the ovitestes was determined to be mildly acidic (pH = 5.86), a factor that may have contributed to their rejection by fish. Feeding deterrence in the outer tunic of some ascidians has been attributed to sulfuric acid sequestered in small bladders. 58 Using the eggs and larvae of marine ascidians as a model system, Lindquist et al. 53 examined the question, “Why are embryos so tasty?” posed by Orians and Janzen, 59 who had pointed out that birds, reptiles, amphibians, fish, and insects all seem to lose large proportions of their eggs and larvae to predators and that evolution should strongly favor those organisms that produce distasteful eggs. Orians and Janzen 59 speculated that (1) actively developing tissues such as those in eggs and embryos are incompatible with toxic chemicals (autotoxicity), (2) there are energetic constraints 9064_ch05/fm Page 200 Tuesday, April 24, 2001 5:18 AM © 2001 by CRC Press LLC The Chemical Ecology of Invertebrate Meroplankton and Holoplankton 201 that limit the ability of eggs and embryos to produce toxins, and (3) there may be tradeoffs between the production of deterrents and potential development rates. Lindquist et al. 53 noted that the focus of much of this speculation revolved around vertebrates, and that there was a need to extend the evaluation to marine invertebrates before generalizing about any apparent lack of chemical defenses in eggs and embryos. Lindquist et al. 53 selected ascidians as a model group of organisms for such a study because they have large conspicuous eggs and embryos that are amenable to chemical evaluation, and their larval ecology is generally well known. 49,60–62 Shipboard laboratory-based bioassays included an investigation of chemical defenses of both living larvae and larval crude extracts of the Caribbean ascidian Trididemnum solidum exposed to predation by the bluehead wrasse Thalassoma bifascia- tum found in sympatry with this ascidian. Groups of wrasses were held in small aquaria and maintained on a diet sufficient to prevent starvation. Using a rather innovative approach, Lindquist et al. 53 employed the eyes of freeze-dried krill as a larval mimic since they were similar in size and color to larvae and were readily consumed by these fish. Replicate groups of fish were presented a krill eye, then a T. bifasciatum larva, and then yet another krill eye to ensure that a rejection response to the larva was not due to satiation. Crude extracts of single larvae were impregnated into single krill eyes and were presented to fish along with controls and fish ingestion experiments conducted in a similar fashion. The researchers found the tadpole larvae and the krill eyes treated with crude extract of T. bifasciatum to be highly unpalatable to the groups of bluehead wrasse. Coupled with field observations of ascidian larval chemical defenses (see below) and a review of the literature on the unpalatability of ascidian larvae conducted to date (Table 5.1), Lindquist et al. 53 argue that brooding ascidians that produce large conspicuous larvae, and often release their larvae over short durations and distances during daylight hours to ensure that larvae can employ photic cues to enhance settlement, are under strong selection pressure to evolve chemical defenses. Lindquist et al. also point out that many predatory reef fish have limited home ranges, 63 and that clumping of unpalatable larval prey may increase the likelihood that fish will learn to avoid ingesting 49 ). They also propose that chemical defenses among larvae that require several weeks or more to develop in the plankton may be less common because pelagic eggs and larvae are likely to be transported offshore where predation levels are lower. 63,64 However, it would seem that while predation may indeed be lower in these pelagic offshore habitats, the longer duration of exposure may offset any benefit attributable to habitat-specific differences in predation level. Importantly, Lindquist et al. 53 also document that ascidians can exhibit chemical differences between defensive secondary metabolites among adults and larvae. For example, larvae from colonies of Sigillina cf. s ignifera contained more tambjamine C, less tambjamine E, and no tambjamine F as compared to adults. 65 Moreover, larvae of Trididemnum solidum contain only four of the six didemnins found in adults. 53 This could be the result of different selective pressures during planktonic vs. benthic life history phases. In contrast, Lucas et al. 44 found no differences in the saponin chemical defenses of the embryos, larvae, and adults of the sea star Acanthaster planci. Clearly, additional studies are needed to expand the evaluation of ontogenetic shifts in defensive chemistry in marine organisms. Based on their findings as well as those of others for ascidians, Lindquist et al. 53 question the adequacy of the autotoxicity, energetic, or developmental constraints suggested by Orians and Janzen 59 to explain a presumed lack of chemical defenses in the eggs and embryos of animals. Coupled with other reports of chemical defenses in the eggs and embryos of amphibians, 66 insects, 67 and additional marine invertebrates, 44,48,52,54,68–70 there appears to be ample evidence to question the validity of these presumed constraints. However, Slattery et al. 70 recently suggested that the lack of chemical defenses in the larvae of the soft coral Sinularia polydactyla may be attributable to autotoxicity constraints. In yet another study focusing on the larvae of a colonial ascidian, Lindquist and Hay 71 evaluated not only whether secondary metabolites in the large brooded larvae of Trididemnum solidum cause 9064_ch05/fm Page 201 Tuesday, April 24, 2001 5:18 AM © 2001 by CRC Press LLC chemically defended eggs and larvae (also see Young and Bingham 202 Marine Chemical Ecology TABLE 5.1 Chemically Defended or Distasteful Eggs, Embryos, and Larvae of Benthic Marine Invertebrates Taxon Reproductive Phase Reproductive Mode Predator(s) Deterred References Sponges Callyspongia vaginalis Larva Lecithotrophic Coral, fish 68, 72 Monanchora unguifera Larva Lecithotrophic Coral, fish 68, 72 Niphates digitalis Larva Lecithotrophic Coral, fish 68, 72 Pseudoceratina crassa Larva Lecithotrophic Coral, fish 68, 72 Ptilocaulis spiculifera Larva Lecithotrophic Coral, fish 68, 72 Tedania ignis Larva Lecithotrophic Coral, fish 68, 72 Calyx pedatypa Larva Lecithotrophic Fish 72 Mycale laxissima Larva Lecithotrophic Fish 72 Ulosa ruetzleri Larva Lecithotrophic Fish 72 Ectyoplasia ferox Larva Lecithotrophic Fish 72 Xestospongia muta Larva Lecithotrophic Fish 72 Isodictya setifera Egg Lecithotrophic Sea anemone, sea star, amphipod 113 Soft Corals Briareum asbestinum Larva Lecithotrophic Coral, fish 68, 72 Eunicea mammosa Larva Lecithotrophic Coral, fish 68, 72 Erythropodium caribaeorum Larva Lecithotrophic Coral, fish 68, 72 Plexaura flexuosa Larva Lecithotrophic Coral 68, 72 Pseudoplexaura porosa Larva Lecithotrophic Coral, fish 68, 72 Sinularia polydactyla Larva Lecithotrophic Fish 70 Hard Corals Agaricia agaricites Larva Lecithotrophic Coral 68 Siderastrea radians Larva Lecithotrophic Coral 68 Porites asteroides Larva Lecithotrophic Coral 68 Hydroids Eudendrium carneum Larva Lecithotrophic Sea anemone, fish 68, 72 Corydendrium parasticuml Larva Lecithotrophic Sea anemone, fish 68, 72 Bryozoan Bugula neritina Larva Lecithotrophic Sea anemone, fish 68, 72 Polychaetes Streblospio benedicti Larva Planktotrophic, lecithotrophic Crab, fish 78 Capitella sp. Larva Lecithotrophic Crab, fish 78 Nudibranchs Hexabranchus sanguineus Egg ribbon — Fish 54 Tritoniella belli Egg ribbon — Sea star 113 Echinoderms — Sea Stars Porania antarctica Egg Lecithotrophic Fish 52 Diplasteria brucei Embryo Lecithotrophic Fish 52 Notasterias armata Embryo Lecithotrophic Fish 52 9064_ch05/fm Page 202 Tuesday, April 24, 2001 5:18 AM © 2001 by CRC Press LLC The Chemical Ecology of Invertebrate Meroplankton and Holoplankton 203 feeding deterrence, but rather extended their evaluation to measure whether changes in consumer growth or survivorship result from physiological effects of the ingested noxious compounds. Such an evaluation is appropriate considering that many consumers ingest small amounts of noxious compounds when sampling prey, often with minimal apparent detrimental effects. First, they presented the pinfish Lagodon rhomboides with alginate pellets (larval mimics) containing ecolog- ically relevant concentrations of larval didemnin cyclic peptides 53 and squid puree as a feeding stimulant, or, alternatively, control alginate pellets containing only squid. They found that fish rapidly learned to avoid the larval mimics and consequently found it impossible to evaluate long- term impacts of the ingestion of noxious secondary metabolites on fitness. Nonetheless, Lindquist and Hay 71 found that the sea anemone Aptasia pallida provided an excellent model to evaluate long-term effects on fitness. Sea anemones were presented experimental and control pellets daily for a period of 32 days. At the end of this time period, it became clear that while didemnins were capable of causing an emetic response, sea anemones did not become conditioned to avoid ingestion of the experimental pellets. Therefore, the sea anemones consumed some minimal level of didemnins over the experimental period. The results of this minimal consumption of noxious compounds were profound. Growth of the adult sea anemones was reduced by 82% in the exper- imental group. Moreover, the production of asexual clones was reduced by 44%, and the average mass of a clonal offspring was reduced by 41% as compared to clones produced by control sea anemones that had not ingested didemnins. These findings clearly demonstrate that ingestion of ecologically relevant concentrations of noxious secondary metabolites can cause a significant reduction in consumer fitness. This was the first demonstration of an effect on fitness resulting from the consumption of secondary metabolites from larval meroplankton. Lindquist and Hay 71 argue that such dramatic decreases in fitness could clearly select for consumers that recognize and reject prey containing defensive chemicals, even when they form a very small portion of a generalist’s diet. In the case of the ascidian Trididemnum solidum, larvae are released year round, during daylight hours, and remain in the vicinity of the adult. It is likely that predators would repeatedly encounter such larvae. As hypothesized by Young and Bingham, 49 because rejected ascidian larvae are not killed by predators, group or kin selection need not be invoked to explain the evolution of chemical defenses in these larvae. In summary, Lindquist and Hay 71 provide additional support for the hypothesis that there should be strong evolutionary selection of chemical Diplasteria brucei Embryo Lecithotrophic Sea star, sea anemone 113 Perknaster fuscus Larva Lecithotrophic Sea star, sea anemone, amphipod, fish 52, 113 Psilaster charcoti Larva Lecithotrophic Sea star, sea anemone, amphipod 113 Acanthaster planci Larva Planktotrophic Fish 44 Ascidians Ecteinascidia turbinata Larva Lecithotrophic Fish 49 Trididemnum solidum Larva Lecithotrophic Fish 53 Sigillina cf. signifera Larva Lecithotrophic Fish 53 TABLE 5.1 (CONTINUED) Chemically Defended or Distasteful Eggs, Embryos, and Larvae of Benthic Marine Invertebrates Taxon Reproductive Phase Reproductive Mode Predator(s) Deterred References 9064_ch05/fm Page 203 Tuesday, April 24, 2001 5:18 AM © 2001 by CRC Press LLC 204 Marine Chemical Ecology defenses in the eggs, embryos, and larvae of meroplanktonic marine invertebrates, particularly those that have lecithotrophic reproductive modes. 12,49,52,53 Lindquist 68 extended the analysis of the palatability of lecithotrophic marine invertebrate larvae in his comparative investigation of the larvae of a variety of temperate and tropical sponges (nine species), gorgonians (nine species), corals (three species), hydroids (two species), and bryozoans (one species). Noting that while larvae of benthic invertebrates search for an appropriate settlement site they encounter a variety of sessile invertebrate predators, he selected three species of corals and a species of sea anemone as model larval predators. The methodological approach involved placing predators held on a maintenance diet individually in containers and presenting them first with a palatable control food, comprised of a sodium alginate pellet containing squid mantle flesh to ensure they were feeding, and then subsequently presenting them a larva. All larvae presented to corals or sea anemones were monitored to see if they were rejected, ingested, or ingested and regurgitated. Larvae that were regurgitated were followed to ensure that they developed and metamorphosed to the juvenile stage normally. Feeding assays were done across a period of several days to prevent predators from becoming satiated during feeding trials. Lindquist found that many of the species tested were unpalatable to these predators; indeed, only the larvae of three of nine species of sponges and two of nine species of gorgonians were consumed. Importantly, both larval survival and metamorphosis were not significantly different in regurgitated larvae and control larvae that were never attacked. Although Lindquist 68 did not evaluate the basis of rejection, it is likely that this was chemically based, since the larvae had no potential skeletal or behavioral defenses. In yet another survey that focused on fish rather than invertebrate predators, Lindquist and Hay 72 demonstrated that the brooded larvae of 11 species of Caribbean sponges and 3 species of gorgonians, in addition to the brooded larvae of 2 species of temperate hydroids and a bryozoan, were unpalatable to fish. In contrast, brooded larvae of three species of temperate ascidians, a temperate sponge, and three species of Caribbean hard corals were consumed. Larval laboratory assays were conducted by first presenting a single brine shrimp to a species of sympatric fish that had been held on a maintenance diet. Only fish that consumed the brine shrimp were presented larvae, and only larvae that had been sampled by fish and either ingested or rejected were considered in the experimental design. As seen by Lindquist, 68 larvae that had been mouthed and rejected showed no significant decrease in metamorphic competence. Of the species with unpalatable larvae, five were further examined to determine whether noxious chemicals were responsible for deterrence. In all five cases, fish rejected alginate pellets containing a feeding stimulant and ecologically relevant concentrations of larvae extracts, while consuming control pellets containing only feeding stimulant, indicating a chemically based deterrence. While not directly tested, it is likely that deterrent chemistry is responsible for the unpalatable nature of many, if not all, of the larvae tested. Inter- estingly, brooded larvae were most likely to be unpalatable, while broadcasted larvae (both leci- thotrophic and planktotrophic) were generally consumed. Providing additional and more broadly based evidence to their conjecture 71 that larvae of the tropical ascidian Trididemnum solidum are chemically defended in part because they release conspicuous larvae during daylight hours, Lindquist and Hay later found 72 that unpalatable larvae were almost always released during the day (89% of total species investigated), while palatable larvae were seldom found to do so (23% of total species investigated). Many of the unpalatable larvae were brightly colored (60% of total species investigated), while all palatable larvae lacked coloration, supporting earlier predictions that aposematism may operate in chemically defended lecithotrophic marine invertebrate larvae. 49 Extending the analysis of the palatability of marine invertebrate lecithotrophic and plank- totrophic eggs, embryos, and larvae to the polar regions, McClintock and Baker 12 examined a suite of Antarctic marine invertebrates with contrasting modes of reproduction. These included the spawned eggs and larvae of a sea urchin and the intraovarian eggs of a sea star, both with planktotrophic larvae, and the lecithotrophic embryos and larvae of three sea stars with either brooding or broadcasting modes of reproduction. Moreover, a nudibranch and sponge with egg ribbons and brooded lecithotrophic embryos, respectively, were examined. Gravid ovaries, spawned 9064_ch05/fm Page 204 Tuesday, April 24, 2001 5:18 AM © 2001 by CRC Press LLC [...]... Chemistry, Scheuer, P.J., Ed., Springer-Verlag, Berlin, 1987, 1 158 Pawlik, J.R., Chemical ecology of the settlement of benthic marine invertebrates, Oceanogr Mar Biol Ann Rev., 30, 273, 1992 159 Pawlik, J.R., Marine invertebrate chemical defense, Chem Rev., 93, 1911, 1993 160 Hay, M.E and Fenical, W., Marine plant-herbivore-predator interactions: the ecology of chemical defense, Ann Rev Ecol Syst.,... predation, Mar Biol., 84, 253 , 19 85 4 Thorson, G., Reproductive and larval ecology of marine bottom invertebrates, Biol Rev., 25, 1, 1 950 5 Vance, R.R., On reproductive strategies in marine benthic invertebrates, Am Nat., 107, 339, 1973 6 Strathmann, R.R., Feeding and nonfeeding larval development and life-history evolution in marine invertebrates, Ann Rev Ecol Syst., 16, 339, 19 85 7 Pearse, J.S., McClintock,... Press LLC 9064_ch 05/ fm Page 222 Tuesday, April 24, 2001 5: 18 AM 222 Marine Chemical Ecology 69 McClintock, J.B and Baker, B.J., Chemical ecology in Antarctic seas, Am Sci., 86, 254 , 1998 70 Slattery, M., Hines, G.A., Starmer, J., and Paul, V.J., Chemical signals in gametogenesis, spawning, and larval settlement and defense of the soft coral Sinularia polydactyla, Coral Reefs, 18, 75, 1999 71 Lindquist,... 19 85 © 2001 by CRC Press LLC 9064_ch 05/ fm Page 220 Tuesday, April 24, 2001 5: 18 AM 220 Marine Chemical Ecology 15 Emlet, R.R., McEdward, L.R., and Strathmann, R.R., Echinoderm larval biology viewed from the egg, in Echinoderm Studies, Jangoux, M and Lawrence, J.M., Eds., A.A Balkema Press, Rotterdam, 1987, 55 16 Roughgarden, J., Gaines, S., and Possingham, H., Recruitment dynamics in complex life-cycles,... ultimately survive As evidenced by this study, it is likely that marine microbial chemical ecology is perhaps the most understudied, and potentially fruitful, avenue of research in marine chemical ecology A second study documenting a symbiotic relationship involving chemical defenses among marine organisms is that of McClintock and Janssen, 85 who worked with the Antarctic pteropod (sea butterfly) Clione... schooling, and molting, Science, 220, 433, 1983 © 2001 by CRC Press LLC 9064_ch 05/ fm Page 224 Tuesday, April 24, 2001 5: 18 AM 224 Marine Chemical Ecology 119 Shaw, B.A., Harrison, P.J., and Anderson, R.J., Feeding deterrent properties of apo-fucoxanthinoids from marine diatoms II Physiology of production of apo-fucoxanthinoids by the marine diatoms Phaeodactylum tricornutum and Thalssiosira pseudonana, and... and Hall, New York, 1993, 363 © 2001 by CRC Press LLC 9064_ch 05/ fm Page 2 25 Tuesday, April 24, 2001 5: 18 AM The Chemical Ecology of Invertebrate Meroplankton and Holoplankton 2 25 147 Peterson, W.T and Dam, H.G., The influence of copepod swimmers on pigment fluxes in brine-filled vs ambient seawater-filled sediment traps, Limnol Oceanogr., 35, 448, 1990 148 Gilmer, R.W and Harbison, G.R., Morphology and... J.E., and Fenical, W., Host-plant specialization decreases predation on marine amphipods: an herbivore in plant’s clothing, Ecology, 71, 733, 1990 40 Hay, M.E., Marine chemical ecology: what’s known and what’s next?, J Exp Mar Biol Ecol., 200, 103, 1996 41 Bakus, G.J and Green, G., Toxicity in sponges and holothurians: a geographic pattern, Science, 1 85, 951 , 1974 42 Bakus,G.J., Chemical defense mechanisms... Targett, N.M., and Schulte, B., Chemical ecology of marine organisms: an overview, J Chem Ecol., 12, 951 , 1986 162 Paul, V.J., Ed., Ecological Roles of Marine Natural Products, Comstock Press, Ithaca, NY, 1992 163 Amsler, C.D., Iken, K.B., McClintock, J.B., and Baker, B.J., Secondary metabolites from Antarctic marine organisms and their ecological implications, in Marine Chemical Ecology, McClintock, J.B... Press, Boca Raton, FL, 2001 164 Conde-Porcuna, J.M., Chemical interference by Daphnia on Keratella: a lifetable experiment, J Plankton Res., 20, 1637, 1998 1 65 Wolfe, G.V., The chemical defense ecology of marine unicellular plankton: constraints, mechanisms and impacts, Biol Bull., 198, 2 25, 2000 166 Zimmer, R.K and Butman, C.A., Chemical signaling processes in the marine environment, Biol Bull., 198, . cause 9064_ch 05/ fm Page 201 Tuesday, April 24, 2001 5: 18 AM © 2001 by CRC Press LLC chemically defended eggs and larvae (also see Young and Bingham 202 Marine Chemical Ecology TABLE 5. 1 Chemically. 219 References 219 * Corresponding author. 5 9064_ch 05/ fm Page 1 95 Tuesday, April 24, 2001 5: 18 AM © 2001 by CRC Press LLC 196 Marine Chemical Ecology I. INTRODUCTION A. T HE P ROBLEM . Test; P < 0. 05) . Unless otherwise noted, n = 10. ૾ ૾ 9064_ch 05/ fm Page 209 Tuesday, April 24, 2001 5: 18 AM © 2001 by CRC Press LLC 210 Marine Chemical Ecology III. FIELD STUDIES OF CHEMICAL DEFENSES A.