Fatty acid trophic markers elucidate resource partitioning within the demersal fish community of South Georgia and Shag Rocks (Southern Ocean)

Fatty acid analysis was used to study the trophic ecology of 10 demersal fish species in the South Georgia region. Principal component analysis grouped the species into three general clusters, revealing resource partitioning between species. Two groups were characterised by large proportions of either monounsaturated or polyunsaturated fatty acids, separating species according to their predominant feeding habitat. The third group showed fatty acid signatures overlapping with either or both of the previous two groups, suggesting a more opportunistic feeding behaviour for these species. Intraspecific comparisons furthermore revealed dietary variability with size, year and geographical location in several species. Mackerel icefish (Champsocephalus gunnari) in particular showed inter-annual differences in muscle lipid concentrations closely linked to prey availability with low lipid contents found in years of low krill (Euphausia superba) abundance. Despite the intraspecific differences the majority of species could be easily distinguished from each other, which indicates the utility of this method in the dietary analysis of higher predators.     

Lipid composition of yellowtail flounder (<Emphasis Type="Italic">Limanda ferruginea</Emphasis>) in relation to dietary lipid intake

Condition indices (CI), hepatosomatic indices (HSI) and proximate, lipid and fatty acid composition of wild and laboratory-reared yellowtail flounder (Limanda ferruginea) (Storer, 1839) were compared from data taken throughout spring and summer 1996–1998. Cluster analysis was performed on the fatty acid signatures of these two groups along with a commercial diet and several invertebrate species to determine possible feeding patterns in Conception Bay, Newfoundland. HSI and levels of storage fat were significantly higher in the muscle and liver of laboratory-reared yellowtail flounder, indicating an efficient absorption of dietary lipid and an increase in fat deposition. Fatty acid analysis of the liver showed that wild fish contained significantly higher proportions of the essential fatty acids 20:46, 20:53 and 22:63; whereas proportions of 18:1 and 18:26 were significantly higher in all fractions of lipid examined from laboratory-reared fish than they were from wild fish. Polar fractions of lipid were more similar than the neutral fraction of lipid when comparing wild and laboratory-reared fish. Taken together, the differences in CI, HSI, proximate and fatty acid composition suggest that feeding commercial diets to L. ferruginea can cause changes in patterns of lipid deposition and metabolism. Cluster analysis of marine plankton, sedimenting particulate matter, wild invertebrates, the commercial diet and fish tissues showed that the fatty acid signatures of both wild and laboratory-reared yellowtail flounder closely resembled their respective food items. Fatty acid signatures from wild fish were more closely related to plankton and settling particulate matter, suggesting relatively few steps in the food web leading to yellowtail flounder. In addition to the resemblance between fatty acids in the commercial diet and the tissues of laboratory-reared yellowtail flounder, these fish had similar fatty acid signatures to those of wild invertebrates.Communicated by J.P. Grassle, New Brunswick     

Characterizing dietary variability and trophic positions of coastal calanoid copepods: insight from stable isotopes and fatty acids

The spring zooplankton community in the Strait of Georgia (British Columbia, Canada) is characterized by the presence of several calanoid copepod species which collectively make up ~90% of the mezozooplankton biomass. Here, we investigate interspecific, interannual, and geographic variability in the diets and trophic positions of these copepods using a combination of fatty acids and stable isotopes. To characterize geographic variability in diet, we compare our findings from the Strait of Georgia with similar data from Ocean Station P in the subarctic northeast Pacific. Both fatty acid and stable isotope signatures indicate the existence of three trophic levels, even within the limited size range of these copepods: Neocalanus plumchrus and Calanus marshallae are primarily omnivorous, while Euchaeta elongata is carnivorous and Eucalanus bungii is herbivorous. Fatty acid markers of trophic position (e.g., DHA/EPA, 18:1n-9/18:1n-7) correlate significantly with δ¹⁵N, while markers indicating the proportion of diatoms to flagellates in the diet (e.g., 16PUFA/18PUFA and DHA/EPA) correlate significantly with δ¹³C, after the effect of lipid concentration on δ¹³C is accounted for. Despite the general correlation between stable isotopes and fatty acids, the former are not sensitive enough to capture the range of interannual variability observed in the latter, and can only capture substantial shifts in the diet over geographic scales. However, regardless of variability in food quality, the relative trophic positions of these copepods do not change significantly either spatially or temporally.     

Using biochemical and isotopic tracers to characterise organic matter sources and their incorporation into estuarine food webs (Rufiji delta,Tanzania)

Fatty acid biomarkers and stable isotope signatures were used to identify the sources of particulate and sedimentary organic matter and its input into the food web through the dominant consumer within the mangrove-dominated Rufiji estuary, Tanzania. Specific fatty acids were used to identify the preferred basal sources of dominant fauna (i.e. filter feeder bivalves, snails, crabs, shrimps, and three fish species), and their presence in the water and sediment samples in the estuary. Both fatty acid and stable isotope results revealed that food web in the Rufiji estuary depended on a variety of carbon sources (mangroves, allochthonous terrestrial inputs, macroalgae, and phytoplankton), contributing to a different degree into the diets of primary consumers and members of near-shore fish, but none of them were obligatory for the survival of these species. The δ¹⁵N values of major primary producers and consumers/predators revealed a trend for δ¹⁵N enrichment with increasing trophic level. The ratio of docosahexaenoic acid to eicosapentaenoic acid (DHA:EPA) decreased from pelagic to benthic feeding fish. This indicated that fish with different feeding modes derived their fatty acids from different primary sources of nutrition, and suggested that the DHA:EPA ratio may be a useful indicator of feeding mode.     

Temporal shifts in the fatty acid profiles of rocky intertidal invertebrates

Fatty acid profiles were determined in rocky intertidal suspension-feeders (mussels and polychaetes) and grazers (limpets and sea urchins) on a monthly basis over 1 year to assess potential dietary overlap between consumers occupying the same trophic guild, and any temporal shifts in diets. Both reproductive and non-reproductive tissues were assessed in an attempt to separate influences of food quality with those of life cycle. Relative variability in fatty acids over time could not be predicted from the feeding guild occupied by a consumer, and influential factors of the temporal shifts included both dietary and reproductive dynamics (even in muscle tissues). Species in the same trophic guild occupied separate trophic niches throughout the year, hence minimising competitive interactions regarding food acquisition. Based on overall variation in fatty acid profiles of muscle tissues, the suspension-feeders Perna perna and Gunnarea gaimardi and the grazing limpet Cymbula oculus occupied narrower feeding niches relative to the grazing sea urchin Parechinus angulosus. Our results provide compelling evidence for potentially large changes in the lipid composition of intertidal invertebrate populations over relatively small temporal scales (i.e. month to month), and these have important implications for short-term field collections intended for assessing invertebrate diets.     

Larval growth and settlement of the European oyster (Ostrea edulis) as a function of food quality measured as fatty acid composition

Larval growth rate and settlement of the European flat oyster Ostrea edulis were experimentally studied as a function of the composition of dietary fatty acids. Diets differing in fatty acid composition were composed by mixtures of the microalgae Isochrysis galbana, Pavlova lutheri and Chaetoceros calcitrans. Fatty acid content in the tissue of the feeding larvae, analyzed by gas chromatography and mass spectrometry, reflected the composition in the diet. Larval growth rate was significantly correlated to the three omega-3 polyunsaturated fatty acids (PUFA) C18:3, C18:4 and C22:6, with minor differences for neutral and polar lipids. No relation between growth rate and the omega-3 PUFA C20:5 was detected, a PUFA often implied as essential for bivalves. It is suggested that naturally occurring variability in fatty acid composition may constrain larval growth. In settlement experiments in both still water and flume flow little substrate selectivity was found for some contrasting substrates. It is concluded that differences in dietary fatty acids may explain as much of settlement success as the variability of substrates. Received: 12 October 1998 / Accepted: 6 April 1999     

Fatty acid evidence for the importance of myctophid fishes in the diet of king penguins, Aptenodytes patagonicus

We tested the usefulness of the fatty acid signature-method in investigating the diet of seabirds in conjunction with the conventional technique of stomach-content analysis. We compared the fatty acid composition of subcutaneous white adipose tissue (SWAT) of king penguin chicks (Aptenodytes patagonicus) during fattening periods to that of total lipids from their food. In both spring and autumn, the fatty acid composition of chick SWAT was identical to that of the dietary lipids. Because the diet of adult king penguins feeding for self-maintenance (i.e. not for their chicks) was essentially unknown, we subsequently analysed their SWAT fatty acid patterns after premolting and prebreeding foraging trips (during which they build up large energy reserves). The fatty acid composition of SWAT from adults was identical to that of chick adipose tissue and food. King penguin diet and SWAT were characterized by high levels of very long-chain mono-unsaturated fatty acids (20 to 24 carbon atoms, 16 to 23% by mass) and (n-3) poly-unsaturated fatty acids (19 to 27%); these consisted mainly of 20:1n-9 (5 to 8%) and 22:1n-11 (5 to 8%), and 22:6n-3 (10 to 13%) and 20:5n-3 (3 to 9%), respectively. Prey items identified from chick stomach contents indicated that the bulk of the food was oceanic myctophid fishes, mainly Electrona carlbergi, Krefftichthys anderssoni and Protomyctophum tenisoni. The fatty acid composition of four other species of myctophid fishes was similar to that of penguin diet and SWAT, but markedly different from that measured for a squid species and that reported for crustaceans. These findings indicate that adult king penguins prey on myctophid fish not only to feed their chicks but also for their own nutrition. The fatty acid signature-technique is therefore a reliable method to gain information on the food and feeding ecology of seabirds when more conventional techniques are of limited value. Such information is important to the understanding of trophic relationships between key species of the ecosystems, and also to provide insight into the nature of avian adaptations to the marine environment. Received: 11 December 1997 / Accepted: 25 July 1998     

Increased docosahexaenoic acid levels in total and polar lipid of European sea bass (Dicentrarchus labrax) postlarvae fed vegetable or animal phospholipids

A 6-week feeding trial was conducted with 44-d-old European sea bass (Dicentrarchus labrax L.) in order to examine the effect of various dietary phospholipid (PL) sources on the incorporation of n-3 highly unsaturated fatty acids (HUFA) in tissue lipids. From weaning onwards the fish received diets prepared by coating different lipid fractions (7.5% diet) on an extruded basal diet (92.5% diet). The two PL-free control diets contained 0.5 and 2% of an emulsifier blend, respectively. Seven other diets contained 2% PL, differing by their purity and origin (vegetable or animal). All diets were rendered isolipidic by the addition of hydrogenated coconut oil. Feeding the PL-supplemented diets, except the diet containing hydrolyzed soybean PL (lyso PL), resulted in a higher survival and a 10 to 30% better growth as compared to the PL-free diets. No difference according to the PL origin was observed. The sea bass final lipid content increased with increasing body weight. Also the lipid class composition of the fish was clearly correlated with the final weight gain. Total neutral lipid increased from 51% of total lipid (initial fish) to 76% for fish fed the PL-free diets, and up to 88% for fish fed the sunflower PL. Weaning the fish on the experimental diets induced important changes in their fatty acid profiles characterized by a decrease in 18:3n-3, 20:5n-3 and 20:4n-6 and an increase in saturated fatty acids and 22:6n-3 (DHA). According to the fatty acid composition of both total and polar lipid, the weaned fish could be divided into three groups reflecting the dietary fatty acids: a group fed the vegetable PL, a group fed the animal PL and a PL-deprived group. An effect of dietary PL on the incorporation of dietary n-3 HUFA, more particularly DHA, was noticed. For a similar supply of DHA through the neutral lipids in the diet, fish fed PL-supplemented diets (except for the lyso PL diet) had 10 to 25% higher DHA levels in total and polar lipid than PL-deprived fish. This PL effect was already clear at the end of the weaning and was not related to the presence of n-3 HUFA in the PL source, as suspected in a previous study when feeding egg yolk PL. A better incorporation of DHA was not obtained by replacing the PL by an emulsifier or by lyso PL with higher emulsifying properties. Present results confirm a role of dietary PL in the absorption of dietary neutral lipids, by a mechanism other than emulsification. Received: 27 May 1997 / Accepted: 30 June 1997     

Diets of fishes of the upper continental slope of eastern Tasmania: content,calorific values,dietary overlap and trophic relationships

Diets of 15 species of demersal and pelagic fishes on the upper continental slope (420 to 550 m) were determined, based on samples taken every two months over 13 mo (April 1984 to April 1985) off eastern Tasmania. The calorific contribution of each prey item to the diets was determined. The fish could be divided into four trophic categories: pelagic piscivores, epibenthic piscivores, epibenthic invertebrate feeders and benthopelagic omnivores. Dietary overlap between the groups was low. The pelagic piscivores Apogonops anomalus, Trachurus declivis, Brama brama, Lepidopus caudatus and Macruronus novaezelandiae primarily consume the shelf-break myctophid Lampanyctodes hectoris; their diet is narrow, with a large overlap between species. The epibenthic piscivores Deania calcea and Genypterus blacodes both take a greater variety of prey, but have little dietary overlap. The fish feeding on epibenthic invertebrates, Coelorinchus sp. 2 and Centriscops humerosus, obtain most of their energy from benthic Crustacea and Ophiuroidea, supplemented with Lampanyctodes hectoris; the diet is broad, with little overlap. Among the benthopelagic omnivores (Cyttus traversi, Coelorinchus sp. 4, Lepidorhynchus denticulatus, Neocyttus rhomboidalis, Helicolenus percoides, Epigonus denticulatus and E. lenimen), most diets are broad and show slight overlap. All but E. denticulatus consume significant quantities of Lampanyctodes hectoris as well as Crustacea, particularly Polychaeta, Euphausiacea and Pyrosoma atlanticum. Seasonal changes in diet occurred in G. blacodes, T. declivis, Lepidopus caudatus, Coelorinchus sp. 4, Lepidorhynchus denticulatus, H. percoides, E. denticulatus and E. lenimen; these were related to changes in abundance of particular prey species, not to alterations in feeding habits. Only three species, Lepidopus caudatus, Coelorinchus sp. 2 and H. percoides, showed significant diel feeding periodicity. Ontogenetic dietary changes were evident in Cyttus traversi, Coelorinchus sp. 2, Lepidorhynchus denticulatus and H. percoides. Cyttus traversi and H. percoides progressively changed from crustaceans to fish as their size increased. The diets of size classes within species showed little overlap, except for Lepidorhynchus denticulatus, which eats chiefly euphausiids and Lampanyctodes hectoris at all sizes. In addition to describing the diets and trophic relationships of 90% of the fish biomass, the results emphasize the importance to the entire fish community of mesopelagic food resources, particularly Lampanyctodes hectoris. Many benthopelagic species undertake extensive vertical migrations in search of prey, thus playing a major role in the transport of energy from midwater regions to the benthos of the continental slope.     

An experimental study of the effect of diet on the fatty acid profiles of the European Cuttlefish (<Emphasis Type="Italic">Sepia officinalis</Emphasis>)

Fatty acid analysis is an alternative dietary investigation tool that complements the more traditional techniques of stomach content and faecal analysis that are often subject to a wide range of biases. In applying fatty acid analysis to ecosystem studies, it is important to have an understanding of the effect diet has on the fatty acid profile of the predator. A feeding experiment, using crustacean and fish as prey for the European cuttlefish Sepia officinalis, was conducted to evaluate the effect of prey fatty acids on the fatty acid profile of this marine predator. Cuttlefish were fed on a fish diet for the first 29 days, and then changed to a crustacean diet for a further 28 days. Another group of cuttlefish was fed on a crustacean diet for the first 29 days, and then changed to a fish diet for a further 28 days. An analysis of the cuttlefish digestive gland showed that the fatty acid profile reflected that of the prey, with cuttlefish on a crustacean diet being clearly distinguishable from the cuttlefish on a fish diet. Cuttlefish fed on a fish diet for 29 days prior to the switch in diet were comparatively higher in 16:0, AA, 20:1ω9, DPA6, DHA, 22:4ω6 and DPA3 than those fed on crustaceans. Cuttlefish fed on a crustacean diet for 29 days prior to the switch in diet were comparatively higher in 17:1ω8, 18:1ω9, 18:2ω6, 18:1ω7, EPA and 20:2ω6 than those fed on fish. Following a change in diet, the fatty acid profile of the cuttlefish digestive gland reflected that of the new diet within 14 days. The results confirm that the fatty acid profile of the cuttlefish digestive gland clearly reflects the profile of its recent diet. It also shows that the digestive gland may not be an organ that accumulates dietary lipids for long-term storage, but rather is an organ where lipids are rapidly being turned over and potentially excreted.     

The dual functions of sea urchin gonads are reflected in the temporal variations of their biochemistry

Fatty acid analyses are emerging as a powerful technique to probe trophic interactions between organisms. In this paper, the application of both this procedure and gonad index (GI) determination on two populations (intertidal and subtidal) of the echinoid Psammechinus miliaris is reported. The investigation spanned the 3-month spawning period of Scottish west coast populations. In both populations a progressive decrease in the GI was found, coupled with an increasing maturity stage (from mature to spent). Sexual maturation and decrease in GI was synchronous between the two populations. In conjunction, there were distinct changes in gonad biochemistry. Differences in the fatty acid composition of the gonad reflected the changes in sexual maturation. Mature males and females had significant differences in the fatty acid composition of their gonads, whereas post-spawned individuals showed no gender differences. Male urchins had higher levels of polyunsaturated fatty acids (PUFAs) compared to females, and there was a dramatic reduction in the fatty acids 22:6(n−3) and 20:5(n−3) with increasing maturity stage. Using multivariate statistical techniques, these changes in the fatty acid composition of the sea urchin gonad were linked to habitat related diet differences combined with gender differences. These changes in the fatty acid signatures clearly reflect the dual function of the gonad as both a nutrient store and a reproductive organ.     

Use of isotopic analysis of vertebrae in reconstructing ontogenetic feeding ecology in white sharks

We conducted stable 13C and 15N analysis on white shark vertebrae and demonstrated that incremental analysis of isotopes along the radius of a vertebral centrum produces a chronological record of dietary information, allowing for reconstruction of an individual's trophic history. Isotopic data showed significant enrichments in 15N with increasing sampling distance from the centrum center, indicating a correlation between body size and trophic level. Additionally, isotopic values verified two distinct ontogenetic trophic shifts in the white shark: one following parturition, marking a dietary switch from yolk to fish; and one at a total length of >341 cm, representing a known diet shift from fish to marine mammals. Retrospective trophic-level reconstruction using vertebral tissue will have broad applications in future studies on the ecology of threatened, endangered, or extinct species to determine life-long feeding patterns, which would be impossible through other methods.     

Ontogenic variations in fatty acid and alcohol composition of the pelagic amphipod <Emphasis Type="Italic">Themisto libellula</Emphasis> in Kongsfjorden (Svalbard)

The fatty acid and alcohol composition of the pelagic amphipod, Themisto libellula, was monitored during the 5 first months of its life cycle (4–20 mm length) in an Arctic fjord, Kongsfjorden, Svalbard. Fatty acids of the three major lipid classes, polar lipids (PL), triacylglycerol (TAG), and wax esters (WE), were analyzed to highlight ontogenic changes in their diet and metabolism. The PL composition of T. libellula did not show any strong variations along their growth except during the first month where an important increase of 20:5(n-3) (EPA) and 22:6(n-3) (DHA) was observed. The TAG composition revealed a clear gradient corresponding to a diet shift from omnivorous juveniles toward carnivorous sub-adults and adults. Indeed, fatty acid trophic markers of diatoms were dominant in the juveniles, whereas 20:1(n-9) and 22:1(n-11), the Calanus sp. trophic markers, overwhelmed in the older stages. The WE composition highlighted the same general trend, however, differences were found with the TAG and are discussed as a result of differences in turnover rates and assimilation pathways between the two lipid classes.     

Fatty acid composition as a dietary indicator of the invasive caprellid, Caprella mutica (Crustacea: Amphipoda)

The invasive caprellid amphipod Caprella mutica is one of the most widely dispersed marine non-native species globally. Originating in sub-boreal north-east Asia, it has now been found in both the northern and the southern hemispheres. One potential reason why this species is such a successful invader is its ability to utilise a wide variety of food sources. The contribution of different food sources to the diet of C. mutica was estimated using fatty acids as biomarkers. Caprella mutica was collected from three field sites, including sea cages stocked with Atlantic salmon Salmo salar, shellfish longlines stocked with the blue mussel Mytilus edulis and mooring lines marking the Loch Linnhe Artificial Reef (>2 km from caged finfish aquaculture), where established populations of this species are known to occur. In addition, the fatty acid compositions of C. mutica held in aquaria and either fed the microalga, Dunaliella tertiolecta, or the diatom, Phaeodactylum tricornutum, for a period of 21 days were investigated. The fatty acid composition of the diatom and the microalgal diets was also examined. The results showed that C. mutica contained high levels of polyunsaturated fatty acids, particularly 20:5(n-3); other dominant fatty acids included 18:1(n-9), 22:6(n-3) and 16:0 (in decreasing order based on abundance). Significant differences in the fatty acid profiles between caprellids fed on the microalgae and the diatom diets and between C. mutica collected from the field sites were observed. These results provide evidence that lipid biomarkers can be successfully used to provide evidence of feeding strategy for C. mutica and that the flexibility observed in this strategy may play an important role in its invasion success.     

Trophic ecology of coral reef gobies: interspecific,ontogenetic, and seasonal comparison of diet and feeding intensity

In fishes, a small body size may facilitate cost-effective exploitation of various primary and secondary food resources, but may pose difficulties associated with digestion of plant material and finding sufficient food in a foraging area potentially restricted by a high risk of predation. We examined the trophic ecology of five common, small-bodied coral reef fish from the family Gobiidae. For each species, we determined diet composition, feeding bite rate, foraging substrate, and feeding behaviour, and examined whether diet composition and feeding bite rate changed ontogenetically and seasonally. The five species showed a diverse range of trophic modes: Amblygobius bynoensis and Amblygobius phalaena were herbivores, Valenciennea muralis was a carnivore, Asterropteryx semipunctatus a detritivore, and Istigobius goldmanni an omnivore. Both the herbivores and detritivore supplemented their diet with animal material. The consumption of a wide range of dietary resources by the two smallest species with the most restricted mobility (A. semipunctatus and I. goldmanni) may ensure energy requirements are met within a restricted foraging area. There was a significant difference in mean feeding bite rate among species, with carnivore > herbivore > omnivore > detritivore. None of the species exhibited an ontogenetic shift in diet composition or increase in feeding bite rate, indicating that (1) postmaturation growth is not facilitated by a higher quality diet or increased feeding intensity following maturation, and (2) their small body size does not preclude herbivory. The herbivores had the highest gut:fish length ratio, which may facilitate plant digestion. While diet did not change seasonally, the mean feeding bite rate was significantly lower in winter than summer for four of the study species.     

Alimentary niche partitioning in the Galapagos sea lion, Zalophus wollebaeki

Sea lions are generally considered opportunistic feeders. However, studies from different areas suggest their diet consists mostly of four to five types of prey. Previous studies in Galapagos sea lions have identified at least three feeding strategies for this species, suggesting diversification of their diet. Diet diversification is favored in organisms with relatively high trophic position and subject to high intra-specific and low inter-specific competition. Zalophus wollebaeki meet these criteria as the only pinniped on San Cristobal Island, where three sea lion rookeries are located within 11 km: a distance considerably shorter than their 41 km foraging range. To measure the degree of diet diversification, we used scats and stable isotope analyses. A total of 270 scat samples from lactating females and 142 fur samples from sea lion pups were collected during the breeding season 2006. The scat analysis identified distinct diets among rookeries, with minimal trophic overlap (Cλ = 0.19), a trophic level TL = 4.5 (secondary–tertiary carnivore), and trophic breadth of a specialist predator (B _i  = 0.37). The mean δ¹⁵N and δ¹³C values were 13.07 ± 0.52 and −16.34 ± 0.37, respectively. No significant difference was found in the δ¹⁵N values from the sea lion rookeries, but differences were found inter- and intra-population in δ¹³C values for pups from different groups (ANOVA P < 0.05). Our results indicate that diet diversification is present in the Galapagos sea lion and may play important role to the survival of the species in a habitat where pinniped populations are limited.     

Food resource utilization among five species of embiotocids at King Harbor,California, with preliminary estimates of caloric intake

The diets of 5 species of embiotocids (Rhacochilus vacca, Embiotoca jacksoni, Hypsurus caryi ryi, Phanerodon furcatus and Micrometrus minimus) from King Harbor, Redondo Beach, California, USA, are examined by means of a dietary survey, field feeding observations and bomb calorimetry of prey items. Each species is shown to have statistically different diets from all others, with the exception of H. caryi which apparently is a dietary intermediate between E. jacksoni and P. furcatus. Dietary overlap between E. jacksoni and H. caryi appears to have resulted in some competitive exclusion along other niche parameters. Estimated caloric intakes suggest that R. vacca has the highest caloric yield per bite and the lowest feeding rate of the species studied. Possible energetic advantages are found for diet specialization in R. vacca.     

Potential dietary effects on the fatty acid composition of the common jellyfish Aurelia aurita

Fatty acid composition of the natural and aquarium-reared common jellyfish Aurelia aurita was investigated. Fatty acid composition of the aquarium-reared A. aurita clearly reflected that of the diet, brine shrimp (Artemia). In the same way, fatty acid composition of the natural A. aurita was assumed to reflect those of natural diets. Samples of natural A. aurita were collected from April 1995 to September 1995 in the Seto Inland Sea, Japan, and their fatty acids were analyzed by gas chromatography and mass spectrometry. Variation of fatty acid compositions was seasonal rather than dependent on body size. Two major seasonal groups were divided by the cluster analysis of the A. aurita fatty acid composition: the April–June and the August–September clusters. The April–June cluster was characterized by high contents of the (n − 3)-fatty acids of diatom origin, accumulated via the grazing food chain. By contrast, the August–September cluster was characterized by an increase in (n − 6)-fatty acids of macroalgal origin, probably transferred via the detritus food chain. These results suggest that the diet of natural A. aurita may shift between the diatom-based food chain and the detritus-based food chain. Received: 12 April 2000 / Accepted: 1 December 2000     

The trophic status of herbivorous fishes on coral reefs

Dietary analyses of 17 species of nominally herbivorous fishes on the northern Great Barrier Reef were carried out to resolve the questions to what extent diet in nominally herbivorous reef fishes is dominated by living plant material and whether short-chain fatty acid profiles in the alimentary tract predict diet. The fishes included members of the families Acanthuridae, Scaridae and Kyphosidae. The analyses revealed consumption of a wide range of food resources including macroscopic algae, turfing algae, pelagic and planktonic animal matter, organic detritus and sediments. In only 7 of the 17 species was living algae the dominant dietary item. Gastrointestinal short-chain fatty acids (SCFA) profiles reflecting the fermentative activities of gut symbionts were used as a framework for the initial analysis of dietary patterns. The feeding patterns implied by the SCFA profiles were validated by direct observation of food material through gut content analysis. We identified four main trophic groupings differentiated by fermentation profiles and diet. These were: (1) species with low levels of SCFA in gut samples (mean mM 9.2ǂ.8) but a high proportion of isovalerate (mean 15.4%dž.7): Acanthurus nigricauda, A. olivaceus, Ctenochaetus striatus, Chlorurus microrhinos, C.sordidus, Scarus schlegeli; (2) species that fed on turfing, filamentous and small thallate algae and displayed moderate levels of SCFA (mean mM 21.9DŽ.9): Acanthurus nigricans, A.lineatus, Naso tuberosus; (3) species that fed on large thallate (macroscopic) algae with high levels of SCFA (mean mM 41.2Dž.5) with very low proportions of isovalerate (mean 0.5%ǂ.1): Kyphosus cinerascens, K.vaigiensis, Naso unicornis, Zebrasoma scopas; and (4) species with diets dominated by planktonic animal material with moderate levels of SCFA (mean mM 21.3ǃ.5): Naso annulatus, N. brevirostris, N. hexacanthus, N. vlamingii. We conclude that a wide range of dietary items are taken by the 17 nominal herbivores, and that dietary groupings do not reflect taxonomic relationships. There was evidence of convergence in feeding modes and diet between phylogenetically distinct taxa and divergence within particular lineages.     

Quantification of arginine requirements of juvenile marine shrimp,Penaeus monodon,using microencapsulated arginine

Using microencapsulated L-arginine, the quantitative requirement for amino acid has been determined for the first time for a shrimp species, which can not effectively utilize crystalline amino acids. In an 8 wk feeding trial (1990), juvenile Penaeus monodon were fed casein-based purified diets containing one of six levels (13.1, 17.7, 22.3, 26.9, 31.5 and 36.1 g/kg diet) of arginine. In addition to the protein-bound arginine already present in the casein of the test diets, pure arginine was supplemented by L-arginine microencapsulated in cellulose acetate phthalate, which is easily assimilated. The arginine level required for optimal growth was determined by brokenline model analysis of weight gain data to be 25.0 g/kg diet (=54.7 g/kg protein). Examination of the hemolymph 3 h after feeding revealed that the free arginine level in the hemolymph had not increased as a result of increasing levels of the dietary arginine. There was an abrupt increase of urea in the hemolymph when the arginine requirement of the shrimp had been met.