Environmental biology of fishes, tập 90, số 4, 2011

Environ Biol Fish (2011) 90:329–342 DOI 10.1007/s10641-010-9744-4 Spatial and vertical patterns in the tidepool fish assemblage on the island of O`ahu Traci Erin Cox & Erin Baumgartner & Joanna Philippoff & Kelly S Boyle Received: 26 February 2010 / Accepted: November 2010 / Published online: 20 November 2010 # Springer Science+Business Media B.V 2010 Abstract The microtides, wave regimes, and relative isolation of the Hawaiian archipelago may provide unique environmental and biogeographic effects that shape the structure of tidepool fishes We sampled fishes across a narrow gradient at low tide from sites on the island of O`ahu We tested predictions of the hypotheses that environmental conditions (pool depth, volume, macroalgal cover, temperature, and salinity) would result in a vertically structured tidepool fish assemblage unique to basalt or limestone rocky shores 343 fish were recorded from 40 pools, and 19 species from 10 families were identified Tidepool fish diversity (H’: O`ahu=2.4; Sites Average=0.0–0.9) was typical for tropical islands, with members from Gobiidae (5 species), Blenniidae (4 species), Pomacentridae (3 species), Acanthuridae (2 species) and Kuhliidae (2 species) T E Cox (*) Department of Botany, University of Hawai`i at Mānoa, 3190 Maile Way, Room 101, Honolulu, HI 96822, USA e-mail: erincox@hawaii.edu E Baumgartner Department of Biology, Western Oregon University, 345 N Monmouth Ave, Monmouth, OR 97361, USA J Philippoff : K S Boyle Department of Zoology, University of Hawai`i at Mānoa, 2538 McCarthy Mall, Edmondson 152, Honolulu, HI 96822, USA among the most common Endemism (32%) was higher than other well studied assemblages yet similar to Hawaiian reef fishes (25%) Assemblage abundance varied among shores with basalt or limestone substrate, among sites, and vertically among high, mid, and low pools In general, blenniids occurred at higher proportions on limestone shores and gobiids were more common on basalt shores High pools were characterized by an abundance of a small sized (29.0 mm median standard length) blenniid Istiblennius zebra, while the blenniid Entomacrodus marmoratus and wrasses Thalassoma spp were more common in low pools Temperature was the best environmental predictor of assemblages and this relationship warrants further investigation Our findings indicate that assemblages can vary across a narrow geographical range and intertidal shore Keywords Intertidal Assemblages Species richness Tropics Substrate Island Introduction Intertidal fish assemblages are known to vary in composition across latitudes and continents, between regions, and within individual localities (Gibson and Yoshiyama 1999) Geographic patchiness, dispersal abilities, and evolutionary history explain the distribution of species across latitudes and continents, while abiotic factors often contribute to patterns at 330 regional and local scales (Gibson and Yoshiyama 1999) Vertical gradients in temperature, air exposure, wave action, and salinity can occur across the shore As water recedes during low tide, fishes that reside in pools (residents) are more tolerant to these variable conditions and the most physiologically tolerant species occur higher on-shore (Yoshiyama et al 1986; Zander et al 1999) Substrate type also can contribute to patterns in tidepool fish assemblages Examples can be found in central California, U.S.A., where stichaeids and pholids are found in tidal boulder fields while heavy vegetated pools are often dominated by cottids and clinids (Yoshiyama et al 1986) and in the Mediterranean where rock structures affect the species composition (Macpherson 1994) Pool rugosity, volume, and depth can further contribute to tidepool fish community patterns (Griffiths 2003) Isolated oceanic island chains, like the Hawaiian Islands, provide an opportunity to explore the importance of abiotic factors and biogeography in shaping fish assemblage structure in these islands, as has been done for numerous continental shores Tidepool fish assemblages are known to exhibit distinct biogeographic affinities resulting from the dispersal abilities of larvae and the degree of geographic connectedness between populations (Prochazka et al 1999) For example, central California and southern Chile have similar environmental regimes but distinct intertidal fish fauna (Boyle and Horn 2006) Similarly, islands often have different flora and fauna in contrast to nearby mainland populations The marine waters surrounding the Hawaiian archipelago contain many tropical fish species that co-occur throughout the Indo-West Pacific and presumably these islands serve as a stepping stone for dispersal across a vast oceanic barrier (Randall 2007) However, the isolation allows for a high number of endemic fish species; 25% of the Hawaiian island marine fish fauna are endemic (Randall 2007) Therefore, the isolated nature of Hawaiian intertidal zones in combination with the typically harsh environmental conditions may facilitate speciation and result in a unique assemblage of intertidal fishes The tropical location and tidal conditions in the state of Hawaii may influence the vertical and spatial patterns of fishes in the intertidal zone Tides in Hawaii are considered microtidal with an amplitude of less than m (Gosline 1965; Abbott 1999) The islands of Hawaii are located in the trade wind belt Environ Biol Fish (2011) 90:329–342 and seasonally directed winds drive wave height and determine which shores (north, south, east, or west) experience wave swell at different times of the year (Gosline 1965; Abbott 1999; Bird 2006) The combination of microtides and surge limit air exposure for intertidal organisms and the vertical span of the intertidal zone is much reduced in comparison to the extensive vertical span of other well studied intertidal shores (Gosline 1965; Abbott 1999) Nonetheless, pools are abundant along Hawaiian shores (Gosline 1965) and are apparent on spring low tides that occur in summer daylight hours when temperatures are at their peak Additionally, O`ahu has both basalt and carbonate based shores (Gosline 1965; Abbott 1999) Basalt shores are often barren of lush macroalgae unlike rough and porous limestone shores (Abbott 1999), thus these types of shores may provide different habitats best suited for the survival of different species of fishes During the approximately 35 years since the first observational description of Hawaiian intertidal fishes (Gosline 1965), much has changed in the near shore environment Changes include the invasion of palatable and unpalatable alien algae (Stimson et al 2001; Smith et al 2002), increased fishing pressure (Friedlander and DeMartini 2002), and altered temperatures and sea level from global warming (Jokiel and Brown 2004) However, it is not known if these changes have impacted the fish assemblages in the intertidal habitat Additionally, comparable descriptive studies on tropical and temperate intertidal zones focus on quantification of resident fishes found in pools during the low tide (Horn et al 1999) Gosline (1965) detailed observations of fishes on high and low tides in these coastal zones but robust quantification was not provided The aims of this study were to describe the tidepool fish assemblage for the island of O`ahu and examine fish assemblage structure across and among shores We tested the hypothesis that tidepool fishes would be vertically distributed Further, we tested the hypothesis that intertidal fish communities would vary among shores with different substrate type (basalt or limestone) Lastly, the isolation of the Hawaiian archipelago is hypothesized to result in a tidepool fish community for the island of O`ahu with high species abundance but low richness and high endemism Environ Biol Fish (2011) 90:329–342 Materials and methods To describe assemblage patterns and abundances of tidepool fishes on O`ahu, six rocky intertidal sites were chosen for sampling: `Ewa Beach, Turtle Bay, Nānākuli, Sandy Beach, Makapu`u and Diamond Head (Fig 1) These sites were selected to ensure a representative sample of fishes and to test the hypothesis that substrate type correlates with structure Sites are located on the south, east, west, and north shores and included tidal benches composed of basalt or limestone (Table 1) To examine the vertical structure of tidepool fishes across the shore, six to eight pools distributed in an on-offshore direction within the intertidal habitat at each site were chosen haphazardly for collection of fishes Because the Hawaiian Islands have microtides and a limited range of vertical intertidal zonation, tidal height was not obvious, hence pools were sampled as high, mid, or low depending on their location to the water at time of peak low tide Pools located near the water’s edge or subtidal zone and usually covered in fleshy macroalgae were referred to as low, whereas barren pools near terrestrial vegetation and above the gastropod Nerita spp and within the gastropod Littoraria spp zone were referred to as high Any pools found between the high and low Fig Map of the island of O`ahu with the location of six intertidal sites 331 pools were referred to as mid Pools in the high zone were within the intertidal and not supratidal as these pools are submersed on the incoming high tide (personal observations TEC) Fishes were collected from at least two pools in each position (high, mid, low) at each site for a total of 6–8 pools Each site was visited once and sampled for fishes in high, mid, and low pools (Table 1) All sampling occurred during the summer months May–August 2008, on a spring low tide Multiple sites could not be sampled in day as microtidal conditions quickly limits access to pools Summer months were chosen for the sampling period because this is when spring low tides co-occur in daylight hours These spring low tides ranged from -0.12- to -0.06 m The peak low tides during the sampling period occurred in the morning hours 06:00–10:11 Sampling began at least h prior to peak low tide and continued until high tide prevented accessibility of pools High, mid and low pools were haphazardly sampled during each site visit Non-destructive sampling was preferred as it lessens the impact on the tidepool community and studies on methodology in other intertidal habitats have found similar results regardless of techniques (Gibson 1999; Griffiths 2000) A battery operated submersible bilge pump and various sized buckets 332 Environ Biol Fish (2011) 90:329–342 Table Site name, substrate type, location and date, tidal height sampled Sites Abbreviation Substrate Type O`ahu Shore Location Date Sampled in 2008 Diamond Head D Basalt Southeast August −0.06 Makapu`u M Basalt East June 30 −0.12 Sandy Beach S Basalt East May 30 −0.09 `Ewa Beach E Limestone Southwest June −0.15 Nānākuli N Limestone West July −0.12 Turtle Bay T Limestone North June −0.12 were used to drain and bail each pool of seawater Any fishes present were scooped up by hand or with hand-net A chopstick or finger was used to probe gently into holes and crevices to ensure the capture of small cryptic fishes Captured fishes were kept alive and placed in aerated buckets of seawater for identification and measurements To determine the abundance and diversity of fishes each individual collected was counted, identified, and measured prior to release In the field, fishes were anesthetized in buckets of seawater with MS-222 and then identified to the lowest possible taxon using dichotomous keys of Hawaiian Shore Fishes (Randall 2007) A hand lens was used to view any diagnostic features difficult to observe with an unaided eye Once the species was identified and recorded, the standard length of fishes greater than 15 mm SL was measured Each individual was assigned an id number, and its size and locality (both site and pool position) recorded To minimize impact to the tidepool community, fishes were revived in aerated seawater and released to the tidepool from which they were collected after pools were inundated from rising tides or to a nearby location Fishes were kept in buckets until all sampling had concluded to avoid re-sampling On rare occasions, fishes were returned while sampling was ongoing but any sampled pools were >20 meters from release site The statistical software package Primer-E (Clarke and Warwick 2001) was used to analyze the spatial distribution and abundance of fishes among and across shores Because of the difficulty in identifying small Bathygobius spp a conservative approach was taken and in this analysis all Bathygobius spp were grouped into one taxonomic category However, results did not differ when all Bathygobius were grouped by genus or when those identified to species level were considered separately These counts of Low Tide Height (m) fishes were expressed as a proportion of total number of individuals found per pool and each pool was considered a replicate of position (high, mid, or low) nested within a site Abundances were square-root transformed to down-weight common species and account for the patchy nature of tidepool species (Gibson and Yoshiyama 1999) These data were then used to construct Bray-Curtis similarity matrix between sites and pool position Dendrograms were used to visualize the similarity of fishes by site, shore substrate type, and pool position Further, PERMANOVA with pool position nested within sites and sites nested within substrate was used to statistically test the hypothesis that fishes were vertically and spatially distributed A series of one-way SIMPERs were used to analyze which species contributed to the observed similarity patterns To examine if sizes of fishes varied across the shore we compared the standard length (mm) of the most abundant species that occurred on O`ahu: Abudefduf sordidus, Bathygobius cocosensis, Entomacrodus maramoratus, and Istiblennius zebra Sizes of fishes across sites were pooled for each tide pool position and differences between length medians were tested with KruskalWallis or Mann–Whitney tests To describe the assemblage and test diversity hypotheses, species richness (S) and Shannon (H’) indices were computed for each pool position at each shore and for the island of O`ahu For site and position comparisons, each pool was considered a replicate sample and computed values were compared statistically with a two-way ANOVA (sites and position) Prior to testing data were log transformed to meet parametric requirements and alpha values were adjusted to account for multiple comparisons To determine S and H’ for the island of O`ahu all species were pooled from every site and values reported Environ Biol Fish (2011) 90:329–342 333 To test if any of these physical features were related to observed fish assemblage patterns a distance based redundancy ordination analysis (dbRDA) was used in combination with a distance based linear model The distance based linear model (DISTLM function in PRIMER-E) models the relationship between predictor variables and the multivariate data cloud based on a multiple regression This routine finds the linear combination of variables that best explains the greatest variation in the data cloud and the amount of variance each covariate explains separately providing a pseudo-F stastical value dbRDA is an ordination analyses that visualizes these results Predictors that best explain the data cloud are viewed as vectors in a biplot The longer the vector the larger the effect of the predictor (Anderson et al 2008) To characterize conditions experienced by tidepool fishes in O`ahu, a snapshot sample of physical conditions and surroundings were collected from tidepools during the sampling period Prior to fish collection, the maximum pool depth, length, and width was measured with a transect tape and were used to calculate a rough estimate of pool volume Salinity measurements (0/00) were collected with a handheld refractrometer, and a visual estimate was made of algal percent cover within and along the edges of pools The surface water temperature was recorded with 2–3 Hobo temperature loggers placed in sampled and unsampled pools during the low tide window At some sites measurements were not collected because of instrument failure or observer oversight, thus only sites with all measurements were included in analyses Table Proportion and total # of fish species by family (F) that were collected and identified in the high, mid, and low pools at the sites B spp = Bathygobius, E spp = Entomarcodus, T spp = Thalassoma; see Table for other taxonomic abbreviations Taxa F Diamond Head Makapu`u H M H M 0.6 0.0 – L L Sandy Beach `Ewa Beach Nānākuli H H M – M L H M L – – B cocosensis G – 0.6 0.6 – 0.8 0.3 – I zebra B 0.3 – – 0.7 0.1 0.0 0.1 0.2 0.2 0.2 0.4 – A sordidus P 0.8 0.3 0.2 – – – – B spp G – – 0.2 0.3 – 0.1 0.7 – 0.2 0.5 – – 0.1 0.2 – H – – 0.3 0.1 0.3 64 1.0 – – 0.2 0.2 0.4 63 1.0 1.0 – – – – M Total # L 0.1 0.1 – – Turtle Bay L 0.0 – 51 0.3 0.2 0.3 46 E marmoratus B – 0.1 0.0 – – – – – 0.1 0.4 0.7 – – – – A triostegus A – – – – 0.1 0.2 – – 0.3 – – – – – – 0.1 0.1 – K sandvicensis K – – – – – 0.1 – – – – – – – – – – 0.0 – B coalitus G – – – – – – – – – 0.2 0.1 0.0 – – – – 0.0 – E spp B – – – – – – – – – – – – – 0.1 0.1 – – – 0.1 0.1 36 20 K xenura K – – – – – 0.1 – – – – – – – – – – – – T purpureum L – – – – – – – – – – 0.1 – – – – 0.0 – Creediidae C – – – – – 0.1 – – – – – – – – – – – – M cephalus M – – – – – 0.1 – – – – – – – – – – – – B cotticeps G – – – – – 0.0 – – – – – – – – – – – – – T spp L – – – – – – – – – – – – – – – – 0.0 – A abdominalis P – – – – – 0.0 – – – – – – – – – – – – B gibbifrons B – – – – – – – – 0.1 – – – – – – – – – C lunula Ch – – – – – 0.0 – – – – – – – – – – – – C obscurus B – – – – – – – – – – 0.0 – – – – – – – D griessingeri G – – – – – – – – – – – – – – – – 0.0 – G anjerensis G – – – – – – 0.1 – – – – – – – – – – – P imparipennis P – – – – – – – – – – – – – – – 0.1 – – S balteata – – – – 0.1 – – – – – – – – – – – – – 12 15 25 14 12 19 11 17 23 18 18 16 50 16 327 Total # L 45 334 Environ Biol Fish (2011) 90:329–342 A total of 343 fishes were recorded and 327 individuals actually captured from six sites (40 sampled tidepools) on the island of O`ahu Fishes that were observed but not captured were often young-of- the-year gobies or blennies Of the 327 captured, 25 taxa were recorded and 19 species (H’=2.5) identified from 10 families (Tables and 3) Those taxa identified to only the family or genus level were of small size and belonged to the genera Bathygobius, Entomacrodus, Thalassoma, or Family Creediidae The most abundant fishes were from families and include (in order of abundance) Bathygobius cocosensis (Gobiidae), Istiblennius zebra (Blenniidae), Abudefduf sordidus (Pomacentridae), Entomacrodus marmoratus (Blenniidae), and Acanthurus triostegus (Acanthuridae) (Tables and 3) Table Distribution, habitat, and resident status of tidepool species and their families (family abbreviation follows) with a comparison to assemblage determined by Gosline (1965) R = resident species (permanent inhabitants), PR = partial residents (spend part of life in intertidal), T = transients (visitors); + = present - = absent in Gosline (1965) splash zone assemblage Distribution and habitat according to Randall (2007), definitions follow Gibson (1982) Results Family Genus species Biogeographic Distribution Habitat Resident Status Presence/Absence in Gosline (1965) Indo-Pacific, Tropical E Pacific Juveniles in tidepools, adults shallow water PR + Indo-Pacific 1–3 m R – – Acanthuridae (A) Acanthurus triostegus Blenniidae (B) Blenniella gibbifrons Cirripectes obscurus Hawaiian Islands 1–6 m R Entomacrodus marmoratus Hawaiian Islands Tidepools R + Istiblennius zebra Hawaiian Islands High tidepools R + Indo-Pacific 1–158 m on coral reefs T – N/A 15–20 m ? – Bathygobius coalitus Indo-Pacific, W Pacific Intertidal zone R – Bathygobius cocosensis Indo-Pacific Tidepools R + Bathygobius cotticeps Indo-Pacific, W Pacific Rocky tidepools, lower intertidal R – Chaetodontidae (Ch) Chaetodon lunula Creediidae (C) Gobiidae (G) Discordipinna griessingeris Indo-Pacific 1–37 m ? – Gnatholepis anjerensis Indo-Pacific Usually occurs in >2 m, tidepools R – Kuhlia sandvicensis Indo-Pacific Shallow-water PR + Kulia xenura Hawaiian Islands Juveniles occur in tidepools, adults offshore PR – Hawaiian Islands & Johnston Atoll Indo-Pacific Shallow-water to 22 m T – Rocky shores shallow-water T + Circumglobal warm-waters Inshore protected waters T – Hawaiian Islands & Johnston Atoll Indo-Pacific Young often in tidepools, adults inshore Young often in tidepools, adults inshore Reefs usually >4 m PR – PR + R – Kuhliidae (K) Labridae (L) Stethojulis balteata Thalassoma purpureum Mugilidae (M) Mugil cephalus Pomacentridae (P) Abudefduf abdominalis Abudefduf sordidus Plectroglyphidodon imparipennis Indo-Pacific Environ Biol Fish (2011) 90:329–342 335 The nMDS and dendrograms (Fig 2) reveal a large amount of overlap in assemblage similarity among sites and pool position, although the centroid based nMDS plot (Fig 2, top) shows clusters of sites based on substrate type (basalt or limestone) The limestone sites are less clustered than basalt sites as the pool samples from Nānākuli are more distinct Furthermore, site assemblages differed among high, mid, and low pools (Fig 2, bottom) Results from the PERMANOVA support significant differences among pool positions, sites, and sites with different substrate types (Table 4) Different abundances of the common fishes contribute to the dissimilarity among tested groups (Tables and 5) Although the presence of species was similar among basalt and limestone based shores, there were significant differences in the proportion of blennies and gobies Bathygobius spp occurred at higher proportions on basalt shores while the blennies I zebra and E marmoratus occurred at higher proportions at limestone shores (Table 5) Within the basalt shores (Diamond Head, Makapu`u, Sandy Beach) roughly 20% of community dissimilarity was accounted for by the differing proportions of I zebra and Bathygobius spp (Table 2) Abudefduf sordidus was absent from Sandy Beach but abundant at both Diamond Head and Makapu`u This species Distance 60 40 Stress: 0.15 20 B Basalt = B Limestone = L Stress: S M T D E N B L B L L 50 40 30 20 10 L Stress: 0.15 Stress: 0.14 H L M H H Distance Fig Non-metric multidimensional scaling ordinations (nMDS plots) on the basis of Bray-curtis dissimilarity measure of each pool (top left symbols = sites, see Table for abbreviation of site names; bottom left symbols = position) and of centroids (right) of sites (top right) and pool position nested within sites (bottom right symbols = sites) Dendrograms in upper left corners are the similarity distance between the centroids of sites (top left) and pool position (bottom left) and serve as a legend for symbols in nMDS plots Note the differences shown as distance between basalt and limestone based shores, sites, and pool position accounts for 30% difference between Sandy Beach and Diamond Head and 12% of the differences between Sandy Beach and Makapu`u Assemblages also varied among limestone sites Nānākuli assemblage was most dissimilar from other limestone sites (84% dissimilar from Turtle Bay, 88% dissimilar from `Ewa Beach) as it has a higher proportion (0.7) of I zebra (which accounted for ~20% of said dissimilarities) The composition at `Ewa Beach and Turtle Bay were only 65% dissimilar and the abundance of E marmoratus accounted for 20% of this dissimilarity The composition of fishes varied vertically across the shore (Fig 2) Assemblages differences were greatest between high and low pools (Fig 2; Table 5) Istiblennius zebra was more common in high pools and E marmoratus, A triostegus, and Thalassoma spp were more common in low pools (Table 5) Also, out of four common species examined (A sordidus, Bathygobius spp., E marmoratus, 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food resources between five sympatric species of Platycephalidae inhabiting the coastal waters of New South Wales, Australia was investigated Samples were collected monthly between March and November 2007 onboard commercial ocean prawn trawlers based in the ports of Yamba and Newcastle Monthly percentage weight contribution of 12 prey categories was analysed to determine if diet was influenced by the variables: species, location, depth, size and maturity Of the 959 stomachs from the five species examined, 28–54% contained prey All Platycephalid species primarily consumed teleosts, however the diversity of prey and the proportion each prey type contributed to the overall diet varied substantially between species Platycephalus caeruleopunctatus, P longispinis, P richardsoni and Ambiserrula jugosa were generalist carnivores and consumed prey from a wide variety of phyla including teleosts, crustaceans, polychaetes, molluscs and echinoderms In contrast, Ratabulus diversidens were primarily piscivorous L M Barnes (*) : M Leclerc : J E Williamson Marine Ecology Group, Department of Biological Sciences, Macquarie University, Sydney, NSW, Australia e-mail: lachlan.barnes@industry.nsw.gov.au L M Barnes : C A Gray Cronulla Fisheries Research Centre, Cronulla, NSW, Australia Partitioning of prey resources between species was more evident in waters at Yamba than at Newcastle Differences in diet between locations were considered a result of differential prey exploitation rather than shifts in the suite of prey consumed Dietary composition was observed to be influenced by size, maturity status and depth however these differences were not observed for all species Keywords Platycephalus Ratabulus Ambiserrula Diet Resource partitioning Australia Introduction The nature and intensity of competition can vary according to resource characteristics in time and space along with the relative competitive abilities of individual species (Ward et al 2006) The way in which fish assemblages partition food resources is considered evidence of evolution driven by past competitive pressures (Wimberger 1994; Castellanos-Galindo and Giraldo 2008) As such, studying how food is partitioned between species may provide an insight into the functional role of different species within an ecosystem (Hajisamae et al 2003) More than 65 species of the teleost family Platycephalidae occur globally Most species of Platycephalid are found throughout the Indian and Pacific Oceans while one species inhabits the Atlantic Ocean and another species inhabits the Mediterranean 430 Sea (Imamura et al 1996) Platycephalids are demersal fishes (Gosline 1996), typically occurring on soft substrate habitats such as mud, sand and seagrasses across a variety of environments ranging from shallow estuaries to the deep ocean (Douglas and Lanzing 1981a; Kailola et al 1993) Platycephalids are morphologically adapted to burying themselves within soft substrates (Douglas and Lanzing 1981b) facilitating an ambush style feeding behaviour (Brown 1977) Species, such as Platycephalus fuscus, P indicus, P endrachtensis, P arenarius and P bassensis, are considered generalist carnivores feeding on a variety of prey such as teleosts, crustaceans, polychaetes and molluscs including cephalopods (Lewis 1971; Brown 1977; State Pollution Control Commission 1981), while P speculator have been Fig Map of the New South Wales coastline showing the sampling locations Yamba and Newcastle Environ Biol Fish (2011) 90:429–441 observed to have a more specialised diet consisting almost exclusively of teleosts (Brown 1977) Several Platycephalid species including P caeruleopunctatus, P richardsoni, P longispinis, Ratabulus diversidens and Ambiserrula jugosa co-occur throughout the inner continental shelf waters of south-eastern Australia The maximum recorded sizes of these species vary from between 25 cm for A jugosa (Hutchins and Swainston 1986) to 65 cm for P richardsoni (Fairbridge 1951) Platycephalus caeruleopunctatus, P richardsoni and R diversidens are commercially and recreationally harvested throughout their distribution The annual commercial harvest of these species within NSW state waters is estimated to be 350 tonnes equating in value to approximately 2.1 million dollars (Scandol et al 2008), while the annual Environ Biol Fish (2011) 90:429–441 431 Table Frequency of occurrence (%F) and percentage contribution by weight (%W) of prey categories to the overall diet of P caeruleopunctatus, P longispinis, P richardsoni, R diversidens and A jugosa from Newcastle and Yamba Species P caeruleopunctatus Location Yamba R diversidens P longispinis A jugosa Prey categories %F %W %F %F %F Teleosts 53.73 78.83 100.00 99.66 24.49 51.44 64.1 77.84 Decapods 28.36 10.13 – – 36.73 14.05 7.69 1.31 Carids 10.45 0.53 – – 3.06 1.17 35.9 17.90 Isopods – – – – 3.06 0.06 2.56 0.09 Amphipods – – – – 2.04 0.51 2.56 1.10 Euphausiids 4.48 0.85 4.35 0.20 5.10 0.73 5.13 1.57 Molluscs – – 4.35 0.13 1.02 9.67 2.56 0.19 %W %W %W Bivalves – – – – 1.02 0.03 – – Cephalopods 2.99 1.68 – – 1.02 0.66 – – Gastropods – – – – – – – – Polychaetes 16.42 7.98 – – 43.88 20.48 – – Echinoderms – – – – 1.02 1.21 – – Stomachs Examined 169 81 239 74 Number Full 67 23 97 39 Percent Full 40 28 41 53 Size Range (cm) 15.4–47 9.5–40.9 4.1–27.5 8.3–19.7 No per size Class Small;Medium;Large 0;44;23 5;16;2 10;86;1 27;12;0 No per maturity category Immature;Mature 15;52 13;10 17;80 29;10 Species P caeruleopunctatus R diversidens P longispinis P richardsoni Location Newcastle Prey categories %F %W %F %W %F %W %F Teleosts 34.04 88.31 100.00 99.23 15.22 39.76 87.5 82.65 Decapods 36.17 3.95 – – 54.35 14.90 15.63 12.55 Carids 21.28 2.51 7.69 0.77 20.65 10.01 9.38 0.24 Isopods 6.38 0.09 – – 11.96 1.80 – – %W Amphipods 4.26 0.24 – – 6.52 0.86 – – Euphausiids 12.77 2.77 – – 14.13 14.24 9.38 4.56 Molluscs 2.13 0.04 – – 6.52 13.64 – – Bivalves – – – – 3.26 1.43 – – Cephalopods 8.51 1.9 – – – – – – Gastropods 2.13 0.03 – – – – – – Polychaetes 2.13 0.16 – – 4.35 1.00 – – Echinoderms – – – – 3.26 2.37 – – Stomachs Examined 101 45 167 83 Number Full 47 13 91 31 Percent Full 47 29 54 37 Size Range (cm) 20–46.7 9.5–40.9 4.1–27.5 8.3–19.7 No per size Class Small:Medium:Large 0:32:15 0:13:0 6:85:0 5:21:5 No per maturity category Immature;Mature 18;29 13;0 28;63 15;16 432 recreational harvest of these species within NSW is estimated to be 440 tonnes (Henry and Lyle 2003) Furthermore, throughout federally regulated Commonwealth waters along the east coast of Australia, the 2008–2009 total allowable catch of flathead species was set at 2850 tonnes (Wilson et al 2009) Platycephalus longispinis and A jugosa are also caught by commercial and recreational fishing sectors however they are discarded as by-catch (Kailola et al 1993) Previous research on the diets of P richardsoni (Colefax 1938; Fairbridge 1951; Davenport and Bax 2002), P caeruleopunctatus (Brown 1977), P longispinis (Platell and Potter 2001) and A jugosa (Lewis 1971) from locations throughout Australia have shown that they consume a variety of prey items such as, but not limited to, teleosts, crustaceans and molluscs Little, however, is known of the diets of these species along the east coast of Australia in habitats where they co-exist Moreover, to our knowledge, there is nothing known of the diet of R diversidens An understanding of the feeding ecology and the partitioning of food between Platycephalid species is fundamental in determining their functional role within the ecosystem and how these species coexist The aim of this study was to investigate the diets of P caeruleopunctatus, P richardsoni, P longispinis, R diversidens and A jugosa and to determine whether these five sympatric Platycephalid species partition prey resources This was completed by analysing the diets of each species caught at two locations within three depths in the inner continental shelf waters of south-eastern Australia We specifically examined the influence of location, size, maturity status and depth on the diet of each species Environ Biol Fish (2011) 90:429–441 locations, two replicate one hour tows were completed in each of three depth strata: shallow (0–30 m), intermediate (31–60 m) and deep (61–90 m) A total of 96 tows were completed during the study period As the catch from each tow was sorted, up to five haphazardly selected individuals of each species were Materials and methods Sample collection Samples of each Platycephalid species were collected from inner continental shelf waters adjacent to the ports of Newcastle (32°55′S, 151°45′E) and Yamba (29°26′S, 153°20′E) in New South Wales Australia (Fig 1) Sampling was done on traditional “trawl” grounds consisting primarily of sand and mud Samples were collected monthly between April and November 2007 within one week of the full moon onboard chartered commercial ocean prawn trawlers Each month at both Fig Non-metric multidimensional scaling (nMDS) ordination plot of the mean monthly percentage weight contribution of different prey categories to the diets of a) P longispinis, P caeruleopunctatus, R diversidens and A jugosa sampled from Yamba and b) P longispinis, P caeruleopunctatus, R diversidens and P richardsoni sampled from Newcastle Environ Biol Fish (2011) 90:429–441 433 retained and immediately placed into an ice slurry These individuals were then transported to the laboratory where their total length (TL) was measured to the nearest 0.1 cm, body weight recorded to the nearest 0.01 g, and their sex and maturity status recorded Sex was determined by macroscopic examination of the gonads and maturity status was determined according to the macroscopic gonad staging criteria described in Haddy and Pankhurst (1998) Observations of opaque ooctyes visible through the ovarian epithelium and partially spermiated testes were considered evidence of female and male gonad maturation respectively (Haddy and Pankhurst 1998) The stomach of each fish were then removed, slit and stored in 70% ethanol, after which the contents were removed and examined using a stereomicroscope Prey were identified to the lowest practical taxonomic resolution possible and weighed to the nearest 0.01 g As much of the stomach contents were extensively masticated, in many cases prey items could only be assigned to one of 12 major dietary categories (see also Platell and Potter 2001) The 12 major categories that were used to classify and subsequently analyse and compare the stomach contents of each species were teleosts, decapods, carids, isopods, amphipods, euphausiids, molluscs, bivalves, cephalopods, gastropods, polychaetes and echinoderms Data analysis Diet was determined as the contribution of each prey category to the overall diet in terms of percentage weight (%W) and percentage frequency of occurrence (%F) The %W was defined as the percentage weight contribution of each prey category to the overall weight of prey consumed by each species The %F was defined as the number of stomachs in which a prey category occurred as a percentage of the total number of fish stomachs containing prey for each species The %W and %F were both calculated for each month in order to standardise the relative Table Prey items identified by SIMPER analysis contributing the most to typify the dietary composition and distinguish between the diets of flathead species sampled at a) Yamba and b) Newcastle a) Yamba Species P caeruleopunctatus P caeruleopunctatus n=67 Teleosts, Decapods R diversidens n=23 Teleosts, Decapodsa, Polychaetesa Teleostsa, Decapodsa, Polychaetes, Molluscs Teleostsa, Carids, Decapodsa, Polychaetesa P longispinis n=97 A jugosa n=39 R diversidens P longispinis A jugosa Teleostsa, Polychaetes, Decapods, Molluscs Teleostsa, Carids Teleosts, Polychaetes, Decapods Teleosts, Carids, Polychaetesa, Decapodsa, Molluscsa Teleosts, Carids R diversidens P longispinis P richardsoni Teleosts b) Newcastle Species P caeruleopunctatus P caeruleopunctatus n=47 Teleosts, Cephalopods, Decapods R diversidens n=13 Teleosts, Cephalopodsa, Decapodsa, Polychaetesa, Euphausiids a Teleostsa, Decapods, Cephalopodsa, Euphausiids, Carids, Polychaetesa, Molluscs Teleosts, Cephalopodsa, Decapodsa, Euphausiids, Polychaetesa P longispinis n=91 P richardsoni n=31 Teleosts Teleostsa, Decapods, Cephalopods, Euphausiids, Carids, Molluscs Decapods, Teleosts, Euphausiids, Carids Teleostsa, Euphausiids, Decapods Teleosts, Decapodsa, Euphausiidsa, Caridsa, Molluscsa Italicised entries indicate significant difference between the diets of species combinations a denotes a prey category that made the greatest contribution to the diet of the species at the top of the column Teleosts, Euphausiids, Crustaceans, Decapods 434 contributions of prey across all months Monthly average %W was compared, when or more samples were collected for each factor using non-metric multivariate analyses to investigate the effect of location, size class, maturity and depth on intra- and inter-specific differences in the diets of each species Non-metric multidimensional scaling (nMDS) ordinations were performed on Bray-Curtis similarity matrices to determine the extent to which individual factors influenced dietary composition through the calculation of a Global R statistic The R-statistic typically ranges from if the dietary compositions of all samples within a group are more similar to each other than to samples from other groups, to if the average similarities of samples between and within groups are the same (Clarke 1993) Bray-Curtis similarity matrices were constructed from square root transformed data in order to reduce the influence of skewed data (Platell and Potter 2001) Analysis of similarities (ANOSIM) were used to test the null hypothesis that there were no significant differences between factors such as season, species, location, size class, maturity status and depth with a significance level of 0.05 To investigate the influence of size class on the diet of individual species, fish were categorised into three size classes; small (0–15 cm), medium (16– 30 cm) and large (>30 cm) Where significant differences in diets were detected between factors using ANOSIM, similarity percentage analysis (SIMPER) was used to identify the prey categories primarily responsible for distinguishing between factors (Clarke 1993) (nMDS) ordination outputs visually depict the similarity and dissimilarity of factors to one another through their relative position on the plot (Clarke and Warwick 2001) Stress values of less than 0.2 indicate that the MDS plot is an accurate 2-dimensional visual representation of the data (Clarke and Warwick 2001) All multivariate analyses were conducted using PRIMER (Plymouth Routines In Multivariate Research) software version Environ Biol Fish (2011) 90:429–441 from Yamba and 83 P richardsoni from Newcastle The proportion of full stomachs varied among species ranging between 28 and 54% for R diversidens and P longispinis respectively (Table 1) By weight, teleost prey dominated the diets of each species and was also the most frequently consumed prey by P caeruleopunctatus, R diversidens and A jugosa from Yamba as well as by R diversidens and P richardsoni from Results Dietary composition A total of 959 stomachs were examined: 406 P longispinis, 270 P caeruleopunctatus and 126 R diversidens from Yamba and Newcastle, 74 A jugosa Fig Non-metric multidimensional scaling (nMDS) ordination plot of the mean monthly percentage weight contribution of different prey categories to the diets of (a) P caeruleopunctatus and (b) P longispinis between Newcastle and Yamba Environ Biol Fish (2011) 90:429–441 435 Table Prey items identified by SIMPER analysis contributing the most to typify the dietary composition and distinguish between the diets of a) P caeruleopunctatus and b) P longispinis at Newcastle and Yamba Location Yamba Newcastle a) P caeruleopunctatus Yamba n=67 Teleosts, Decapods Newcastle n=47 Teleostsa, Cephalopods, Decapodsa, Polychaetes, Euphausiids Teleosts, Cephalopods, Decapods Location Yamba Newcastle Yamba n=97 Teleosts, Polychaetes, Decapods Newcastle n=91 Teleostsa, Decapods, Polychaetesa, Euphausiids, Molluscs, Carids b) P longispinis Decapods, Teleosts, Euphausiids, Carids Italicised entries indicate significant difference between the diets of species combinations a denotes a prey category that made the greatest contribution to the diet of P caeruleopunctatus and P longispinis at the location listed at the top of the column Newcastle (Table 1) Polychaetes were the most frequently consumed prey by P longispinis from Yamba occurring in 44% of stomachs and contributing 20% to the overall dietary weight Decapods were the most frequently consumed prey by P longispinis and P caeruleopunctatus from Newcastle, occurring in 54% and 36% respectively of stomachs examined Decapods also represented 15% and 3% of overall dietary weight of P longispinis and P caeruleopunctatus respectively from Newcastle Teleosts were found in every R diversidens stomach that contained prey, with carids, molluscs and euphausiids consumed occasionally only by individuals from medium size classes P longispinis consumed the most diverse range of prey whereas P richardsoni and R diversidens consumed the least number of prey types (Table 1) Ambiserrula jugosa or P richardsoni were only sampled from Yamba and Newcastle respectively ispinis, R diversidens and A jugosa (Global R=0.391, P=0.001; Fig 2a) Pairwise comparisons revealed a significant difference between the diets of R diversidens and A jugosa (ANOSIM, P=0.02), while the diets of P caeruleopunctatus and P longispinis were not significantly different (ANOSIM, P= 0.173) although distinct from the diets of both R diversidens and A jugosa (ANOSIM, all P0.05) between samples collected in autumn (April–May), winter (June–August) and spring (September–November) of 2007 for any species examined from either location or depth range Consequently, data was pooled across seasons for all subsequent analyses Species comparisons At Yamba, ANOSIM revealed a significant difference between the diets of P caeruleopunctatus, P long- Fig Non-metric multidimensional scaling (nMDS) ordination plot of the mean monthly percentage weight contribution of different prey categories to the diets of small and medium sized P longispinis from Yamba 436 Environ Biol Fish (2011) 90:429–441 influenced by the difference in the importance of decapods and carids to the diet of A jugosa compared to the importance of teleosts to the diet of R diversidens while teleosts, decapods and polychaetes were the most important prey for P caeruleopunctatus and P longispinis (Table 2a) At Newcastle, ANOSIM identified that the diets of P caeruleopunctatus, P longispinis, P richardsoni and R diversidens were significantly different (Global R=0.176, P=0.008; Fig 2b) Pairwise comparisons determined that this difference was driven by the separation of the diet of P longispinis compared with the diets of P richardsoni and R diversidens (ANOSIM, P=0.01 and P=0.009 respectively) These differences were due to the importance of decapods, euphausiids and carids to the diet of P longispinis compared to the importance of teleosts to the diets of both P richardsoni and R diversidens (Fig 2b) As at Yamba, the diets of P caeruleopunctatus and P longispinis were not significantly different (ANOSIM, P=0.136) No significant differences were found between the diets of P caeruleopunctatus, P richardsoni and R diversidens (ANOSIM, all P> 0.05; Table 2b) Spatial comparisons No significant difference was found in the diet of R diversidens between Yamba and Newcastle (Global R=−0.041, P=0.682) In contrast, the diets of P caeruleopunctatus and P longispinis differed significantly between Yamba and Newcastle (Global R=0.166, P=0.018 and Global R=0.252, P=0.021, respectively) (Fig 3a, b) SIMPER analysis revealed that for P caeruleopunctatus this dietary difference between locations was due to the importance of teleosts and decapods to their diet at Yamba compared to the greater dietary contribution of cephalopods, polychaetes and euphausiids to their diet at Newcastle (Table 3a) Teleosts and polychaetes were identified by SIMPER as important contributors to the diet of P longispinis at Yamba compared to the greater relative dietary contribution of decapods, euphausiids, molluscs and carids to the diets of P longispinis at Newcastle (Table 3b) Size class comparisons Dietary comparisons were made between each size class at each location except for large P longispinis and large R diversidens from Yamba and small P richardsoni from Newcastle which had fewer than samples as well as between size classes where no individuals were sampled (Table 1) Significant differences were detected between small and medium P longispinis from Yamba (Global R=0.521, P-value 0.01 Fig 4) SIMPER analysis identified that crustaceans, euphausiids and isopods made a greater contribution to the dietary composition of small individuals while teleosts and polychaetes made a greater contribution to the dietary composition of medium sized individuals (Table 4) Teleosts were not observed to be consumed by any small sized P longispinis No other species examined at either location was observed to have significantly different dietary composition between size classes Maturity comparisons No significant differences (ANOSIM, all P>0.05) in dietary composition were detected between mature and immature individuals of any species from either location at the depths in which they were found to co-occur for more than one sample period Consequently data was pooled across depth for each species at each location Dietary comparisons were not Table Prey items identified by SIMPER analysis contributing the most to typify the dietary composition and distinguish between the diets of small and medium sized P longispinis from Yamba Size class Small Small n=10 Crustaceans, Euphausiids, Isopods Medium n=86 Teleosts, Polychaetes, Crustaceansa, Isopodsa, Euphausiidsa Medium Teleosts, Polychaetes, Crustaceans, Isopods, Euphausiids Italicised entries indicate significant difference between the diets of species combinations a denotes a prey category that made the greatest contribution to the diet of small sized P longispinis Environ Biol Fish (2011) 90:429–441 437 increase in the relative contribution of teleosts, polychaetes and molluscs in the diet of mature P longispinis and the greater contribution of crustaceans and euphausiids to the diet of immature P longispinis was responsible for the dietary differences between mature and immature individuals (Table 5a) Similarly, SIMPER analysis identified an increase in the relative contribution of teleosts in the diet of mature P caeruleopunctatus and the greater contribution of polychaetes, crustaceans and euphausiids to the diet of immature P caeruleopunctatus was responsible for the dietary differences between mature and immature individuals (Table 5b) No significant differences in the diets of mature and immature individuals from any other species or location were observed Depth comparisons Fig Non-metric multidimensional scaling (nMDS) ordination plot of the mean monthly percentage weight contribution of different prey categories to the diets of mature and immature a) P longispinis and b) P caeruleopunctatus from Yamba completed between immature and immature R diversidens from Newcastle as no mature individuals were sampled (Table 1) Significant differences were detected between the diets of mature and immature P longispinis (Global R=0.34, P-value 0.01) and P caeruleopunctatus (Global R=0.272, P-value 0.012) at Yamba (Fig 5a, b) SIMPER analysis identified an The dietary influence of the three depth ranges on each size class was conducted between large and medium P caeruleopunctatus from Yamba, medium P longispinis from Yamba and Newcastle, medium R diversidens from Yamba in deep and medium depth ranges as well as small A jugosa from intermediate and shallow depths Other combinations were not investigated as fewer than three samples were collected Depth significantly influenced the diet of medium-sized (16–30 cm) P longispinis at Yamba (Global R=0.221, P=0.025) (Fig 6) Pairwise comparison revealed that this was because the diet of medium-sized P longispinis in the shallow depth strata (60 m) (ANOSIM, P=0.049) No significant difference was detected between the diet of medium-sized P longispinis from shallow or deep strata (ANOSIM, P=0.175) SIMPER analysis identified an increase in the relative contribution of teleosts in the diet of medium sized P longispinis at both shallow and deep strata and the greater contribution of decapods and molluscs to the diet of medium sized P longispinis at intermediate strata was responsible for depth related dietary differences (Table 6) Depth was not observed to significantly influence the diet of small or large P longispinis at Yamba or be a significant factor influencing the diets 438 Environ Biol Fish (2011) 90:429–441 Table Prey items identified by SIMPER analysis contributing the most to typify the dietary composition and distinguish between the diets of mature and immature a) P longispinis and b) P caeruleopunctatus at Yamba Maturity status Immature Mature a) P longispinis Immature n=17 Crustaceans, Teleosts, Polychaetes, Euphausiids Mature n=80 Teleosts, Polychaetes, Crustaceansa, Molluscs, Euphausiidsa Teleosts, Polychaetes, Crustaceans, Molluscs, Euphausiids Immature Mature b) P caeruleopunctatus Maturity Status Immature n=15 Teleosts, Polychaetes, Crustaceans, Euphausiids Mature n=52 Teleosts, Polychaetesa, Crustaceansa, Euphausiidsa Teleosts, Crustaceans, Polychaetes, Euphausiids Italicised entries indicate significant difference between the diets of species combinations a denotes a prey category that made the greatest contribution to the diet of immature fishes of any other size class of each species that was analysed (all tests P>0.05) Discussion The five Platycephalid species studied were all exclusively carnivorous Platycephalus longispinis, P caeruleopunctatus, A jugosa and P richardsoni had diverse diets consisting of multiple prey items Fig Non-metric multidimensional scaling (nMDS) ordination plot of the mean monthly percentage weight contribution of different prey categories to the diets of medium sized P longispinis from Yamba in shallow, mid and deep depth strata and can be considered generalist carnivores In contrast, R diversidens are primarily piscivores with non-teleost prey contributing little to their overall diet The diets of P longispinis, A jugosa and P richardsoni in the current study are comparable to those previously reported for these species (Fairbridge 1951; Lewis 1971; Platell and Potter 2001; Davenport and Bax 2002) The diverse diet of P caeruleopunctatus observed in this study contrasts however, with the previous paradigm that this species has a relatively narrow trophic niche with a diet consisting almost exclusively of teleosts (Brown 1977) Although teleosts did dominate the diet of P caeruleopunctatus by weight in this study, decapods, polychaetes and a suite of other prey items were found to substantially contribute to the total weight of prey consumed by this species The variation in the proportion of full stomachs we observed among the species studied here is comparable to that for a range of other Platycephalids including Suggrundus harrisii, P indicus, P endrachtensis, P fuscus, P arenarius, P laevigatus and A jugosus (Lewis 1971; Brown 1977; State Pollution Control Commission 1981; Klumpp and Nichols 1983; Hadwen et al 2007) The high proportion of empty stomachs recorded for R diversidens compared with the lower proportion of empty stomachs observed for each of the other species studied is consistent with the hypothesis that primarily piscivorous species more frequently have empty stomachs compared to generalist carnivores (Beaudoin and Tonn 1999) Regurgitation of stomach contents can be problematic when studying the diets of demersal Environ Biol Fish (2011) 90:429–441 439 Table Prey items identified by SIMPER analysis contributing the most to typify the dietary composition and distinguish between the diets of medium sized P longispinis from Yamba between shallow, mid and deep depth strata Depth strata Shallow Shallow n=8 Teleosts, Decapods Intermediate n=43 Teleostsa, Decapods, Polychaetes, Amphipodsa, Molluscs Teleostsa, Polychaetes, Decapodsa, Amphipodsa Deep n=35 Intermediate Deep Decapods, Polychaetes Teleosts, Polychaetes, Decapodsa, Molluscsa Teleosts, Polychaetes Italicised entries indicate a significant difference between the diets of P longispinis between depths a denotes a prey category that made the greatest contribution to the diet of medium-sized P longispinis in the depth strata listed at the top of the column fish species potentially confounding dietary observations (Fairbridge 1951) No individuals were, however, observed to regurgitate their stomach contents in the current study and therefore it was assumed that regurgitation of stomach contents did not influence the results presented here Food resources were partitioned among species although there is evidence of dietary overlap between some species Dietary overlap was primarily driven by the contribution of teleost prey to the diet of each species It is clear however, that each species, with the possible exception of R diversidens, consumes a suite of prey items and the contribution of these prey items to the diet of each species varies considerably The observed spatial variability in the diets of P caeruleopunctatus and P longispinis as well as the depth related dietary variability of P longispinis suggests these two species, and potentially each of the species studied, have flexible dietary preferences This dietary plasticity would enable these species to exploit spatially and temporally abundant food resources (Feyrer et al 2003; Bethea et al 2007; Weitkamp and Sturdevant 2008) which, when surveyed in 1990 by Graham et al (1993) were shown to be variable between Newcastle and Yamba Dietary plasticity is commonly observed among generalist predators (King 2005; Beatty 2006; Jackson and Rundle 2008) and may mitigate competitive interactions between species for available resources if the level of resource exploitation does not exceed the ability of the resource to maintain itself (Feyrer et al 2003; Raborn et al 2004) In addition, as within family dietary variation has been observed to be less than between family dietary variation (Platell and Potter 2001; Kwak et al 2004), the diets of the Platycephalid species studied may be distinct from the diets of other co-existing organisms further reducing potential competitive interactions for available food resources Ontogenetic dietary shifts from benthic, relatively sessile prey, to fast moving teleost prey, provides evidence that the behaviour and physiological ability of Platycephalid species to capture, handle and successfully consume prey items changes throughout their lives (Castellanos-Galindo and Giraldo 2008) Chizinski et al (2007) suggested that changes in dietary preferences due to ontogeny may reduce competition for resources between different life history stages In addition, switching from a crustacean to teleost based diet may be more energetically profitable for larger and reproductively mature individuals, maximising energy intake (Brown 1985; Juanes et al 2001; Pope et al 2001) Although not all species from each location were observed to exhibit ontogenetic changes in dietary composition, a result which, may have been influenced by small sample sizes, ontogenetic dietary shifts can not be ruled out for each species studied It must also be acknowledged that positively correlated shifts in the size range of prey consumed with overall body size rather than a shift in the composition of prey consumed throughout ontogeny may have also occurred (Taylor et al 2006; Potier et al 2007; Griffiths et al 2009) The current sampling protocol was, however, unable to detect such potential changes Unique dental morphological traits such as the recurved canine teeth of P richardsoni (Colefax 1938), the depressible teeth of R diversidens (Gosline 1996) and the small viliform teeth of P caeruleopunctatus (Brown 1977) may be functional traits that allow each Platycephalid species to differentially target and handle prey facilitating partitioning of prey resources The physiological and behavioural charac- 440 teristics of predators (Bolnick et al 2002; Pusineri et al 2008) as well as prey behaviour, (Bolnick et al 2002; Castellanos-Galindo and Giraldo 2008), abundance (Zekeria et al 2002; Collins et al 2007; Castellanos-Galindo and Giraldo 2008), and the influence of habitat on each of these factors (Bethea et al 2007; Collins et al 2007; Weitkamp and Sturdevant 2008) are additional mechanisms by which food resources can be partitioned between species As prey resources were only partially partitioned between the Platycephalid species presented here, the interaction of all the above factors are likely to play an important role in the way in which these species partition food resources and successfully co-exist In conclusion, each Platycephalid species studied is exclusively carnivorous, however the combination of observed dietary flexibility seen in some of the species studied here (e.g., P longispinis and P caeruleopunctatus) in addition to the partitioning of prey resources between others (e.g., R diversidens and A jugosa) may provide mechanisms to reduce competition for food between species in south-eastern Australian coastal waters, thus facilitating co-existence The dietary plasticity and behaviour of generalist carnivores to switch between abundant prey suggest these species may be able to survive periods of variable prey abundances (Sternberg et al 2008) In 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