Video-monitored predation by Caribbean reef fishes on an array of mangrove and reef sponges

Although predation by fishes is thought to structure benthic invertebrate communities on coral reefs, evidence to support this claim has been difficult to obtain. We deployed an array of eight sponge species on Conch Reef (16 m depth) off Key Largo, Florida, USA, and used a remote video-camera to record fish activity near the array continuously during five daylight periods (6 h for 1 d, at least 11.5 h for 4 d) and one night period (11 h). Of the eight sponge species, four were from adjacent reefs (Agelas wiedenmayeri, Geodia neptuni, Aplysina fistularis, and Pseudaxinella lunaecharta), and four were from a nearby mangrove habitat (Chondrosia collectrix, Geodia gibberosa, Halichondria sp., andTedania ignis). Each species of reef sponge was chosen to match the corresponding mangrove species in form and color (black, brown, yellow, and red, respectively). Predation events only occurred during daylight hours. Tallies of the number of times fishes bit sponges revealed intense feeding by the expected species of sponge-eating fishes, such as the angelfishHolacanthus bermudensis, H. tricolor, andPomacanthus arcuatus, the cowfishLactophrys quadricornis, and the filefishCantherhines pullus, but surprisingly also by the parrotfishSparisoma aurofrenatum andS. chrysopterum. Of 35 301 bites recorded, 50.8% were taken by angelfish, 34.8% by parrotfish, and 13.7% by trunkfish and filefish. Mangrove sponges were preferred by all reef fishes; 96% of bites were taken from mangrove species, with angelfish preferringChondrosia collectrix and parrotfish preferringGeodia gibberosa. Fishes often bit the same sponge repetitively, and frequently consumed entire samples within 30 min of their deployment. Sponge color did not influence fish feeding. Two of the four mangrove sponge-species deployed on the array were also found living in cryptic habitats on adjacent reefs and were rapidly consumed by fishes when exposed. Our results demonstrate the importance of fish predation in controlling the distribution of sponges on Caribbean reefs.     

Parrotfish predation on cryptic sponges of Caribbean coral reefs

Some sponge species that live in crevices in the reef frame appear to be restricted to their cryptic habitat by predation. When cryptic sponges were excavated, on Guigalotupo reef, San Blas, Panama, exposing them to potential predators, they were eaten by fishes that are generally considered to be herbivores, primarily parrotfishes of the genus Sparisoma: S.aurofrenatum (Cuvier & Valenciennes), S.viride (Bonnaterre), and S.chrysopterum (Bloch & Schneider). Of the 9150 bites observed to be taken by these species during paired (i.e., with sponges versus without sponges) trials conducted in defined feeding areas during 1986, 1987, and 1988, 72% (i.e. 6581 bites) were on cryptic sponges, even though these were only offered during half of the total observation time and never constituted more than 7% of the cover of the feeding observation areas. Individual parrotfish returned over and over to take bites of the exposed cryptic sponges until they were entirely consumed. They vigorously chased each other away from the sponges, but exhibited no such defense of their usual algal foods. A total of 18 sponge species were tested. Of the cryptic and semi-cryptic sponge species tested, only one of six was rejected by the parrotfish. Two of these six sponge species were consistently consumed entirely, and two were consumed entirely whenever their surfaces were sliced off with a razor blade, demonstrating that these sponges concentrate defenses against predators in their surfaces. One semi-cryptic species and one semi-exposed species were fed upon, but not entirely consumed. By contrast, 11 of 12 of the exposed and semi-exposed species were rejected. Cryptic sponges grew out of their cavities in the reef only when protected by seasonally thick mats of macroalgae or by cages that excluded fish. Received: 10 January 1997 / Accepted: 4 February 1997     

Hawksbill sea turtles in seagrass pastures: success in a peripheral habitat

Hawksbill sea turtles, Eretmochelys imbricata, are closely associated with coral reef and other hard-bottom habitats. Seagrass pastures are peripheral habitats for Caribbean hawksbills. With the decline in quality and quantity of coral reefs, seagrass habitats may become more important for hawksbills. We use data from a 30-year mark-recapture study of hawksbills and green turtles, Chelonia mydas, in the southern Bahamas to assess the quality of a seagrass habitat for hawksbills. Size distribution, residence times, and body condition index for the seagrass hawksbill aggregation are similar to those of hawksbill aggregations over Caribbean reefs. Somatic growth rates of seagrass hawksbills are in the upper range of those reported for reef hawksbills. Based on these parameters, peripheral seagrass habitats can support healthy, productive hawksbill aggregations. During the 30-year study, a sixfold variation in green turtle density in the study area did not affect the productivity or body condition of hawksbills.     

Variability in the chemical defense of the sponge Chondrilla nucula against predatory reef fishes

Chondrilla nucula is a common Caribbean demosponge that grows in a range of habitats, from coral reefs to mangrove swamps. On reefs, C. nucula grows as a thinly encrusting sheet, while in mangrove habitats it surrounds submerged mangrove roots as fleshy, lobate clumps. Previous feeding experiments using predatory reef fish revealed a high degree of variability in the chemical defenses of C. nucula. The present study was undertaken to determine whether a relationship exists between habitat, growth form, and chemical defense of C. nucula. Both laboratory and field feeding-assays of crude extracts confirmed that C. nucula possesses a chemical defense with high intercolony variability, but there was no significant variation in feeding deterrency between reef and mangrove habitats at either geographic location (Bahamas and Florida). Extracts of C. nucula collected during September and October 1994 from the Bahamas were significantly more deterrent than those collected during August 1993, May 1994, and May 1995 from Florida, and extracts of these spring and summer Florida collections were more deterrent than extracts of C. nucula collected in December 1994 and February 1995 in the same locations. There was no evidence that deterrent compounds were concentrated in the surface tissues of the sponge, or that chemical defense could be induced by simulated predation. Laboratory and field assays of the fractionated crude extract revealed that feeding deterrency was confined to the most polar metabolites in the extract. Field transplants were used to determine whether predation influenced the growth form of C. nucula. Uncaged sponges transplanted from the mangrove to the reef were readily consumed by spongivorous reef fishes. Lobate mangrove sponges became thinner after being caged on the reef for 3 mo, but encrusting reef sponges did not become thicker after being caged in the mangroves for the same period of time. Reef sponges that were caged for 3 to 15 mo thickened by only a small amount (<1 mm) compared to uncaged and open-caged (i.e. in cages lacking tops) sponges. Simulated bite marks on both reef and mangrove sponges were repaired at a rapid rate (0.8 to 1.6 mm d⁻¹). Fish predation has an important impact on the distribution and abundance of C. nucula, but the thin growth form common to reef environments may be more the result of hydrodynamics than of grazing by spongivorous fishes. Received: 6 October 1997 / Accepted: 19 March 1998     

A test of the senses: fish select novel habitats by responding to multiple cues

Habitat-specific cues play an important role in orientation for animals that move through a mosaic of habitats. Environmental cues can be imprinted upon during early life stages to guide later return to adult habitats, yet many species must orient toward suitable habitats without previous experience of the habitat. It is hypothesized that multiple sensory cues may enable animals to differentiate between habitats in a sequential order relevant to the spatial scales over which the different types of information are conveyed, but previous research, especially for marine organisms, has mainly focused on the use of single cues in isolation. In this study, we investigated novel habitat selection through the use of three different sensory modalities (hearing, vision, and olfaction). Our model species, the French grunt, Haemulon flavolineatum, is a mangrove/seagrass-associated reef fish species that makes several habitat transitions during early life. Using several in situ and ex situ experiments, we tested the response of fish toward auditory, olfactory, and visual cues from four different habitats (seagrass beds, mangroves, rubble, and coral reef). We identified receptivity to multiple sensory cues during the same life phase, and found that different cues induced different reactions toward the same habitat. For example, early-juvenile fish only responded to sound from coral reefs and to chemical cues from mangroves/seagrass beds, while visual cues of conspecifics overruled olfactory cues from mangrove/seagrass water. Mapping these preferences to the ecology of ontogenetic movements, our results suggest sequential cue use would indeed aid successful orientation to novel key habitats in early life.     

Variable effects of symbiotic snapping shrimps on their sponge hosts

Mutualistic relationships are ubiquitous in tropical coral reefs, but the costs and benefits to partner species are often poorly known. In Caribbean coral reefs, several species of snapping shrimp (Synalpheus spp.) dwell exclusively in marine sponges, which serve as both habitat and food source. A paired experimental design was used to examine the effects of Synalpheus occupancy on predation, morphology, and growth of their sponge host Lissodendoryx colombiensis in Bocas del Toro, Panama (9.351°N, 82.258°W) in June 2009. Shrimp occupancy significantly decreased consumption of sponges by a predatory sea star (Oreaster reticulatus) and also affected sponge morphology; sponges grown without shrimps decreased in canal size, in both the laboratory and the field. Shrimp occupancy had more ambiguous effects on sponge growth. In laboratory experiments, shrimp occupancy benefited sponge growth, although all sponges experienced overall decreases in mass. In field experiments, there were no significant differences in growth between occupied and empty sponges. However, the benefits of shrimp occupancy on sponge growth were negatively correlated with overall increases in sponge size; sponges that decreased in mass during the experiment benefited more from shrimp occupancy than sponges that increased in mass. These costs and benefits suggest that Synalpheus has variable effects on sponges: positive effects on sponges in the presence of predators, and/or when sponges are decreasing in mass (e.g., during periods of physical stress), but a negative effect on sponges during periods of active sponge growth.     

Estuarine Vegetated Habitats as Corridors for Predator Movements

Abstract: The spatial proximity of one habitat to another can strongly influence population and community dynamics. We investigated whether the proximity of intertidal oyster reefs to vegetated estuarine habitats, salt marshes, and seagrass beds, affects the abundance and community structure of benthic macroinvertebrates on reefs and predator-prey interactions between mobile predators and bivalves living on reefs. Benthic macroinvertebrate abundance was highest on reefs spatially separated from salt marshes. Macroinvertebrate species richness was highest on reefs separated from both salt marshes and seagrass beds. Comparisons of predation on juvenile bivalves transplanted to reefs for 7–12 days indicated that survivorship of clams was greatest on reefs spatially separated from both salt marshes and seagrass beds, whereas reef proximity to vegetated habitats did not affect the survivorship of oysters. The foraging behavior of blue crabs may explain patterns of macroinvertebrate abundance and clam survivorship among reefs with different proximity to vegetated habitats. In experiments conducted in 30-m² field enclosures, blue crabs had higher predation rates on hard clams transplanted onto artificial reefs adjacent to salt marshes or seagrass beds than on reefs separated from both habitats by unvegetated sand bottom. Thus, vegetated habitats appeared to act as corridors by facilitating the access of blue crabs to oyster reefs and enhancing the intensity of blue crab predation. Such an understanding of the effects of landscape characteristics of estuarine habitats on their value as habitats for estuarine organisms can be used to predict the consequences of habitat fragmentation on ecosystem function and to improve strategies for habitat and species conservation and restoration.     

Ichthyofauna of seagrass meadows along the Caribbean coast of Panamá and in the Gulf of Mexico: Composition,structure and community ecology

Collections of fishes from seagrass meadows along the coast of the Republic of Panamá and the Canal Zone during 1974–1975 revealed that juveniles of reefassociated predators are common in this habitat. There are also important diurnal changes in species composition: members of the families Pomadasyidae, Lutjanidae, Sciaenidae, Apogonidae and Muraenidae were noticeably more abundant in the seagrass meadows at night. Although there were significant differences in species composition and abundance among sampling sites, seasonal differences within sites appeared to be insignificant. The ichthyofauna of the Panamanian seagrass beds is significantly richer than that of similar habitats which have been studied in the Gulf of Mexico. This difference in species richness appears to be due in part to the influence of nearby coral reefs, which add an additional structural dimension to the habitat.     

Differences in feeding ecology among three co-occurring species of wrasse (Teleostei: Labridae) on rocky reefs of temperate Australia

The foraging behaviours and dietary compositions of three co-occurring labrids (Ophthalmolepis lineolatus, Notolabrus gymnogenis and Pictilabrus laticlavius), which are conspicuous on rocky reefs in temperate south-eastern Australia, were investigated between 2003 and 2005. SCUBA observations at two locations showed that the feeding intensity, and hence the associated effects of these fishes on rocky reef invertebrate prey, was temporally consistent. Relative differences in the contributions of ingested prey and use of different feeding microhabitats demonstrated that the feeding ecology differed significantly among the three species. Thus, O. lineolatus fed on proportionately higher volumes of polychaetes, polyplacophorans, marginellid gastropods (especially Austroginella sp.), bivalves and echinoids, which were sighted opportunistically in a wide selection of microhabitats, but particularly in sand/rubble. Ambush hunting was used regularly by smaller N. gymnogenis and all sizes of P. laticlavius to forage on amphipods, small decapods and small gastropods at algal bases or fronds and Diopatra dentata tubes. Amphipods were similarly important in the diet of smaller O. lineolatus. Larger N. gymnogenis foraged opportunistically over an increased reef area and made greater use of microhabitats that offered minimal prey refuge (e.g. sand/rubble, bare rock/steel) from which common prey, in particular decapods, were obtained. The significant intra- and inter-specific differences in dietary compositions, allied with differences in the use of feeding microhabitats, would facilitate co-occurrence of these three conspicuous species and contribute to maintaining high richness of labrid species in reef systems. Echinoids were regularly consumed by each species but they made a moderate contribution to the diet of only O. lineolatus, which suggests that only one of the three labrids is likely to play a significant role in regulation of echinoid densities in these rocky reef habitats. However, the broad diets and diverse forging strategies employed by these labrid species imply that they have a system-wide influence on invertebrate prey on rocky reefs.     

Differences in the structures of fish assemblages inThalassia testudinum beds in Guadeloupe,French West Indies,and their ecological significance

The structures of fish assemblages in twoThalassia testudinum beds in Guadeloupe, French West Indies, one adjacent to mangroves and the other adjacent to coral reefs, were compared between January 1983 and May 1984. The aim of the study was to compare the influences of mangroves and coral reefs on the utilization of seagrass beds by fishes through examination of species composition, catch rate, size of fishes and temporal changes. The two fish assemblages were similar in terms of the number of species they had in common (nearly 44% of the total number of species collected) and the great abundance of juveniles. They both comprised species that usually inhabit other habitats, i.e., estuaries, open waters or coral reefs. Estuary-associated species (e.g. Gerreidae) were the most abundant species in the seagrass bed near the mangroves, while small pelagic species (e.g. Clupeidae) were the most abundant species in the seagrass bed near the coral reefs. The seagrass bed near the mangroves was preferentially utilized as a nursery area by small juveniles of various species (e.g. Clupeidae, Sparidae, Gerreidae, and at least one coral reef species,Ocyurus chrysurus). The abundance of these species varied frequently, suggesting successive arrivals and departures of juveniles over time. The seagrass bed near the coral reefs was characteristically utilized by fishes that are more able to avoid predation, i.e., fishes that forage over seagrass beds at night and shelter in or near the coral reefs during the day (large juveniles of coral reef species and adults of schooling pelagic species, respectively). The constant migrations of these fishes between the coral reefs and seagrass beds explained the relative stability of the structure of the fish assemblage in the seagrass bed over time. Thus, the two seagrass beds were not equivalent habitats for fishes. The distinct ecological influences of the mangroves (as a nursery for small juveniles) and coral reefs (as a shelter for larger fishes) on the nearby seagrass beds was clearly reflected by the distinct utilizations of these seagrass beds by fishes.     

Biodiversity and rockfish recruitment in sponge gardens and bioherms of southern British Columbia,Canada

In the Strait of Georgia and Howe Sound, British Columbia, colonies of individual cloud sponges, growing on rock (known as sponge gardens) receive resource subsidies from the high biodiversity of epifauna on adjacent rock habitats. Bioherms are reefs of glass sponges living on layers of dead sponges. In the same area as the sponge gardens, newly discovered bioherms in Howe Sound, BC (49.34.67 N, 123.16.26 W) at depths of 28- to 35-m are constructed exclusively by Aphrocallistes vastus, the cloud sponge. The sponge gardens had much higher taxon richness than the bioherms. The sponge garden had 106 species from 10 phyla, whereas the bioherm had only 15 species from 5 phyla. For recruiting juvenile rockfish (quillback, Sebastes maliger), the food subsidy of sponge gardens appears to be missing on bioherms of cloud sponge, where biodiversity is relatively low. While adult and subadult rockfishes (S. maliger, S. ruberrimus, S. proriger, and S. elongatus) were present on bioherms, no evidence for nursery recruitment of inshore rockfishes to bioherms was observed, whereas the sponge gardens supported high densities of newly recruited S. maliger, perhaps owing to the combination of both refuge and feeding opportunities. These results indicate that sponge gardens form a habitat for early stages of inshore S. maliger, whereas A. vastus bioherms are associated only with older juvenile and adult rockfishes.     

Abundance,spatial distribution,and size structure of populations of Oreaster reticulatus (Echinodermata: Asteroidea) on sand bottoms

Three populations of Oreaster reticulatus (Linnaeus, 1758) inhabiting shallow-water (<4 m) seagrass habitats in the Grenadines (West Indies, Caribbean Sea) were associated predominantly with beds of Halodule wrightii. Occupation of fringing inshore areas of bare sand was inversely related to wave action; even where sandy patches occurred offshore, the preferred substratum was H. wrightii. The association of O. reticulatus with H. wrightii is related to the asteroid's microphagous feeding habit and the availability of food resources associated with the seagrass. O. reticulatus rarely occurred on dense beds of Thalassia testudinum, but was moderately abundant in areas of sparse cover. Differences in the occurrence of O. reticulatus among seagrass types may be related to factors afdecting foraging effort, such as the tractability of the substratum and mobility upon it. Populations of O. reticulatus exhibited an aggregated dispersion within beds of H. wrightii, possibly attributable to local substratum heterogeneity and/or reproductive behavior. Increased turbulence induced migration to deeper water and markedly increased aggregation along offshore boundaries. The populations were primarily adults, with some late juvenile stages. The paucity of juveniles and their cryptic behavior and coloration suggest that settlement and early postmetamorphic development occurs in alternate habitats, such as dense beds of T. testudinum. Interpopulation differences in size structure may be associated with differences in the quality and availability of food sources.     

Variability in nursery function of tropical seagrass beds during fish ontogeny: timing of ontogenetic habitat shift

Seagrass beds are often considered to be important nurseries for coral reef fish, yet the effectiveness of these nursery functions (refuge and food availability) at different juvenile stages is poorly understood. To understand how the demands of juvenile fish on seagrass nursery functions determines the timing of ontogenetic habitat shifts from seagrass beds to coral reefs, we conducted visual transect survey and field tethering and caging experiments on three different sizes of the coral reef fish Pacific yellowtail emperor (Lethrinus atkinsoni) during its juvenile tenure in seagrass beds at Ishigaki Island, southern Japan. The study showed that although the number of individual L. atkinsoni juveniles decreased by >90 % during their stay in the seagrass nursery, the shelter and/or food availability functions of the nursery, at least for a juvenile size of approximately 5 cm total length (TL), provided the best survival and growth option. The timing of ontogenetic migration to coral reefs of larger fish (>8 cm TL) was attributed to foraging efficiency for larger food items in different habitats. Overall, the function of the seagrass bed nursery changed with juvenile body size, with marginally higher survival and significantly greater growth rates during early juvenile stages in seagrass beds compared to coral reefs. This would contribute to the enhancement in the number of individuals eventually recruited to adult populations.     

Comparative species richness,composition, and abundance of invertebrates in Caribbean seagrass (Thalassia testudinum) meadows (Panamá)

The results of a year-long study in which epibenthic invertebrates were collected monthly from seagrass (Thalassia testudinum) meadows along the Caribbean coast of Panamá and the Panamá Canal Zone are described in this paper. Differences in species composition and abundance among sites were primarily due to the proximity of surrounding habitats, especially coral reefs, which contain a number of species that utilize the seagrass meadows. In contrast to many previous characterizations of tropical marine habitats, important seasonal fluctuations in both species number and abundance took place at each of the sites. Data on breeding activity among several species of decapod crustaceans indicate year-round reproduction, although considerable seasonal differences occur in the percentage of ovigerous females. These interspecific differences in observed reproductive output may be explained by differences in life-cycle length, a factor not often considered in discussions of seasonal breeding patterns in tropical marine invertebrates. Overall species composition was qualitatively similar to that reported in comparable studies of tropical and subtropical seagrass meadows elsewhere, although caridean shrimp and xanthid orab species were reduced in number and total abundance were much lower than in previous studies.     

Resource use by five sympatric parrotfishes in the San Blas Archipelago,Panama

Resource use by five sympatric species of parrotfish was quantified in the San Blas Archipelago of the Republic of Panama from March to August 1987. Detailed observations of parrotfishes on patch reefs and surrounding seagrass beds showed that they partition resources with respect to habitat, food and size, but not time. Although parrotfishes shared resources, the proportions of each resource used differed significantly among species. Scarus iserti (Bloch) scraped filamentous microalgae that grew from eroded coral pavement on lower slopes of patch reefs and in halos, the area of sparse vegetation surrounding reefs. Sparisoma viride (Bonnaterre) foraged on upper slopes of patch reefs where they mostly took bites from dead coral and associated algae. S. aurofrenatum (Cuvier and Valenciennes) had the broadest diet, which consisted mostly of seagrasses and macro- and microalgae that were attached to dead coral on lower reef slopes and in halos. Although S. chrysopterum (Bloch and Schneider) commonly occurred on patch reefs, it primarily foraged in seagrass beds that surround them. S. rubripinne (Cuvier and Valenciennes) was distributed most widely, ranging from seagrass beds to reef crests, where it took bites from seagrasses, dead coral and macroalgae. Juveniles of all species occurred on lower slopes or in halos where they scraped filamentous microalgae from coral pavement. As they matured, parrotfishes moved into other habitats changing access to different types of food. All of these parrotfishes fed throughout the daytime, and resource use did not differ between morning and afternoon.     

Variability in reproductive output across a water quality gradient for a tropical marine sponge

To establish a complete understanding of reproductive variability, larval supply and ultimately population demographics of a species it is important to determine reproduction across a broad spectrum of environmental conditions. This study quantified sexual reproduction of the brooding, gonochoristic sponge Rhopaloeides odorabile from populations across the shelf reefs of the central Great Barrier Reef (GBR). Histological sections of reproductive sponges collected at increasing distances from the coast were used to determine if numbers of reproductive sponges, reproductive output (using a reproductive output index), size at sexual maturity, and sex ratios varied according to their location (distance) from the coastline and therefore from influences of terrigenous/riverine discharge. Significantly higher proportions of reproductive sponges occurred with increasing distance from the coast. The proportion of all reproductive sponges (both male and female) on offshore reefs ranged from 77 to 90%, during November and December, the peak reproductive months of this sponge, compared to 47 to 50% for sponges occurring on coastal reefs. Levels of female reproduction increased with increasing distance from the coastline on two levels. First, oocytes from offshore sponges were significantly larger than oocytes from coastal sponges. Second, sponges from offshore reefs showed a reproductive index (proportions of oocytes, embryos and larvae mm⁻²) approximately 15 times higher than coastal reef sponges. Therefore, both numbers of oocytes, embryos and larvae in conjunction with larger oocytes contribute to a higher reproductive output index for offshore sponges. The production of spermatic cysts in males was consistent across the GBR. Sex ratios for coastal reef sponges showed a male bias while offshore sponges showed approximate equal sex ratios. The effect of terrigenous riverine input from coastal fluvial plains to the inner GBR is well established and is likely to contribute to the lower levels of reproduction associated with female sponges inhabiting coastal reefs of the central GBR.     

Spatial variability in habitat associations of pre- and post-settlement stages of coral reef fishes at Ishigaki Island,Japan

Successful settlement of pelagic fish larvae into benthic juvenile habitats may be enhanced by a shortened settlement period, since it limits larval exposure to predation in the new habitat. Because the spatial distribution of marine fish larvae immediately prior to settlement versus during settlement was unknown, field experiments were conducted at Ishigaki Island (Japan) using light trap sampling and underwater visual belt transect surveys to investigate the spatial distribution patterns of selected pre- and post-settlement fishes (Acanthuridae, Pomacentridae, Chaetodonidae and Lethrinidae) among four habitats (seagrass bed, coral rubble, branching coral and tabular coral). The results highlighted two patterns: patterns 1, pre- and post-settlement individuals showing a ubiquitous distribution among the four habitats (Acanthuridae) and pattern 2, pre-settlement individuals distributed in all habitats, but post-settlement individuals restricted to coral (most species of Pomacentridae and Chaetodontidae) or seagrass habitats (Lethrinidae). The first pattern minimizes the transition time between the larval pelagic stage and acquisition of a benthic reef habitat, the latter leading immediately to a juvenile lifestyle. In contrast, the second pattern is characterized by high settlement habitat selectivity by larvae and/or differential mortality immediately after settlement.     

Cytotoxic haplosclerid sponges preferred: a field study on the diet of the dotted sea slug<Emphasis Type="Italic"> Peltodoris atromaculata</Emphasis> (Doridoidea: Nudibranchia)

Trophic specializations are widespread among opisthobranch molluscs. One purported example from the Mediterranean Sea is the dotted sea slug Peltodoris atromaculata. It has been hypothesized that this species is strongly monophagous on the sponge Petrosia ficiformis. However, the small amount of evidence that has been found for this hypothesis is based just on laboratory tests. Here we study the feeding habits and the diet of Peltodoris atromaculata in its natural habitat. We observed and videotaped 161 individuals together with the organisms on which they were found (their living substrata). Feeding scars were identified and videotaped as well. Individuals and their living substrata were sampled for further analysis in the laboratory. The composition of faeces of Peltodoris, especially undigested sponge spicules, was analyzed by light and scanning electron microscopy and compared to the composition of the living substrata. Most of the faecal samples consisted of undigested sponge spicules. Although Peltodoris was found on 11 species of sponges, only 2 of them, Petrosia spp. and Haliclona fulva, form its diet (76% out of n=121 samples). In accordance with this, feeding scars in the habitats were exclusively observed on these two sponges. Estimation of electivity indices suggests that Haliclona is preferred over Petrosia. One remarkable feature of the exclusive feeding of Peltodoris on Petrosia and Haliclona is that both sponges share specific fulvinol-like polyacetylenes that show cytotoxic activity in bioassays. Potential benefits and evolutionary aspects of this trophic specialization are discussed. Besides sponge-containing faeces, we found spicule-free faeces (24%, n= 29). These were very small in volume compared to sponge-containing faeces, and only few distinct structures were present. However, the use of food other than sponges is not necessarily indicated by this, because the spicule-free faeces might also represent left-overs from the stomach and digestive gland after sponge spicules have been released.Communicated by O. Kinne, Oldendorf/Luhe     

Influence of habitat on abundance and size structure of a large temperate-reef fish, Achoerodus viridis (Pisces: Labridae)

The relationship between densities of Achoerodus viridis (Pisces: Labridae) and reef habitats at various localities within New South Wales (NSW), Australia was examined. Types of habitats were quantified at inner, mid and outer estuarine locations in each of two estuaries (Botany Bay and Port Jackson) to determine whether purported patterns of movement from estuaries could be related to differences in habitat. Although the same types of habitat were generally found at all locations, differences in the proportion of habitat types were found between shallow and deep reefs and among inner, mid and outer estuarine locations for both estuaries. Shallow habitats were usually dominated by Ecklonia radiata, turf and/or fringe habitat in Botany Bay, whereas deep sites were generally dominated by urchin-grazed barrens habitat and, sometimes, sponge- and ascidian-dominated deep reef. Shallow sites in Port Jackson were dominated by a mixture of habitats, as were deep reefs at mid-estuarine locations. Other deep reefs in Port Jackson were dominated by E. radiata (inner estuarine) or barrens (outer estuarine) habitat. Thus, patterns of habitat cover were not consistent between estuaries and numbers of fish could not be related to proportional representation of habitat on reefs along estuarine gradients. Univariate and multivariate analyses showed that there was little evidence that any size class of fish was correlated with the proportional representation of a particular habitat or group of habitats. Counts of fish that focused on barrens and E. radiata forest habitats over a period of 10 yr showed that similar numbers and all sizes of fish were found in the two types of habitat. Greater numbers of small fish were, however, found in the E. radiata forest habitat than in the barrens habitat. Estimates of abundance along the coast of NSW (100s to 1000 km) in a range of habitats (e.g. ascidian-dominated reef, kelp forest, urchin-grazed barrens) showed that there was no indication that a particular habitat consistently had greater numbers of A. viridis than other habitats. Therefore, A. viridis of a range of sizes appears to be flexible in its use of habitats on reefs. Received: 24 December 1997 / Accepted: 23 June 1998     

Variation in emergence of parasitic and predatory isopods among habitats at Lizard Island, Great Barrier Reef

Gnathiid isopods are one of the most abundant groups of ectoparasites on coral reef fishes. They, and other isopods, have been shown to significantly affect the health and behaviour of many reef fish. Whether isopod emergence differs among habitats on coral reefs is not known. In this study, we measured emergence rates of parasitic isopods (Gnathiidea and Flabellifera) in six habitats at two sites at Lizard Island during new moon periods in March and December 2004. Isopods were collected from the periphery and centres of micro-reefs, patch reefs, continuous reefs, and from inter-reefal habitats (sand or rubble) with 1 m² emergence traps. Sites (Casuarina and Coconut Beach) were located on opposite sides of Lizard Island. Live gnathiids were collected with light traps in November 2005 to investigate species differences between sites. At both sites, the most abundant gnathiid species was exclusive to that site. More gnathiid larvae emerged at night, and emergence of fed gnathiids (pranizae) and flabelliferan isopods was almost exclusively nocturnal. Diurnal emergence was greater at Coconut Beach than Casuarina Beach. Although emergence counts were not consistently affected by parameters such as habitat, site, or sampling period, gnathiid size and feeding state were. Where significant differences existed, gnathiids were larger and more often fed over reef borders than centrally. We suggest first stage larvae (Z1) have the largest influence on total abundance and are patchily distributed in accordance with adults from which they have recently hatched. As later stage larvae depend on fish, more successful (fed) and older larvae are found on the edges of reefs where appropriate hosts may be more abundant, or predation is lower. Gnathiids were over-dispersed in all habitats investigated, including apparently homogeneous beds of coral rubble and sand. This indicates that their distributions may be better predicted by very fine scale differences in substrate or that aggregations are the result of gregariousness and may be difficult to predict on the basis of substrate. Emergence traps collected comparatively few parasitic flabelliferan isopods. This community differed greatly from the previously described community of scavenging isopods at Lizard Island. These differences are probably the result of differences in trapping methodology.