Ontogenetic Diet Shifts Research Articles

Trophic guild structure and dietary patterns of a juvenile-dominated demersal fish community in a tropical mangrove estuarine system

Community-wide understanding of food web dynamics in species-rich tropical estuaries are generally lacking, exacerbating the challenges of managing these valuable coastal habitats. To address this gap, trophic guilds and dietary patterns were elucidated for 39 species of a juvenile-dominated demersal estuarine fish community in the Matang Mangrove Forest Reserve, an Important Marine Mammal Area. Fishes were sampled monthly for a year using otter trawl, and stomach contents were analyzed. Of the seven trophic guilds identified from clustering analysis, the three most speciose ones were shrimp feeders (12 species), zooplanktivores (9), and generalist crustacivores (7). Copepods occurred in diets of 50% of all fish species and were the most important food item volumetrically for specialist zooplanktivores. Diverse shrimp and prawn taxa, including commercially valuable Acetes and juvenile penaeids, showed highest frequency of occurrence in stomachs and supported the shrimp feeder and crustacivore guilds that contained relatively more diet generalists. Biomass-dominant annelidivore guild members are known to be a vital food resource for highly threatened coastal cetaceans. Ontogenetic diet shift, wide intra-guild diet breadth, seasonality in major resource use, and prevalence of detritus in diet were feeding strategies that reduced niche overlap and competition among fish species sharing common food resources. Future fisheries management must consider the bottom-up trophic impacts from excessive removal of valuable prey resources such as shrimps on equally valuable fish consumers that depend on them.

Feeding strategy and diet overlap in major fish stocks of Ayima and Gelegu Rivers, northwestern Ethiopia

AbstractRiverine ecosystems contribute considerable fisheries production. However, in Ethiopia, there is a lack of comprehensive biological studies on such ecosystems. This study assessed the feeding habits of commercially important fishes of the Ayima and Gelegu Rivers. Fishes' guts were sampled seasonally from April 2017 to November 2018 using gill nets, cast nets and electrofishing. Four hundred and forty‐nine stomachs were analysed. Niche breadth and overlap indices were estimated using Levin and Schoener indices, respectively. The data indicated that Clarias gariepinus (Burchell, 1822), Bagrus docmak (Forsskål, 1775) and Hydrocynus forskahlii (Cuvier, 1819) mostly preyed on fish, whereas Heterotis niloticus (Cuvier, 1829) and Labeobarbus bynni (Forsskål, 1775) consumed detritus and macrophytes. H. niloticus, L. bynni and H. forskahlii were generalist feeders. There was a slight ontogenetic dietary shift observed in some predator fishes. In smaller sized L. bynni (6–20 cm, TL), insects and ostracods were important preys and their contribution differed significantly (p < 0.05). In H. forskahlii, the volumetric contribution of bivalves and detritus differed significantly amongst the different size classes. Generally, resource availability had little effect on the feeding behaviour of most fishes. This implies that fishes in floodplain rivers can forage any available prey without preference and accustom to a general feeding strategy to form biologically insignificant dietary overlap.

Dietary partitioning of six co‐occurring Ponto‐Caspian gobies in the Dnieper River basin

AbstractThe diets and resource partitioning of six co‐occurring gobiids (Caucasian dwarf goby Knipowitschia caucasica; monkey goby, Neogobius fluviatilis; racer goby, Babka gymnotrachelus; round goby, Neogobius melanostomus; bighead goby, Ponticola kessleri; and western tubenose goby, Proterorhinus semilunaris) have been investigated in freshwater of the Dnieper River basin. The studied gobies were characterised by relatively similar generalised feeding strategies with the dominance of chironomid larvae in their diets, except Caucasian dwarf goby, the diet of which was dominated by copepods and cladocerans. The ontogenetic diet shift was clearly observed in all studied gobiids, except Caucasian dwarf gobies, adults of which had almost the same food spectrum as their juveniles. The highest diet overlaps were observed between monkey and racer gobies, and between monkey and round gobies. The lowest values of the diet overlap indices were recorded between Caucasian dwarf and round gobies, and between Caucasian dwarf and bighead gobies. Each studied goby species can be characterised by a unique set of features resulting in dietary partitioning such as consuming prey organisms inhabiting different microhabitats, different sizes, different spawning periods and ontogenetic diet shifts.

Comparison of Three Methods for Measuring Dietary Composition of Plains Hog-nosed Snakes

Wild animal diets are challenging to quantify, and the various methods for doing so have strengths and weaknesses. Combining multiple methods can allow ecologists to assess their level of confidence in particular results, increase sample size, and investigate diet over varying time scales. The biases of traditional gut content–based methods are mostly well understood. Newer methods may have important biases that can only be worked out through comparison to established ones. We collected data on the diet of wild Plains Hog-nosed Snakes (Heterodon nasicus) using multiple, fundamentally dissimilar methods, combined analytically using a Bayesian framework to describe an ontogenetic dietary shift. Gut contents were the most straightforward, but yielded a small sample size that fell below any reasonable threshold for making generalizations. Stable isotopes indicated an obvious ontogenetic dietary shift, but were labor-intensive, and conclusions are limited by multiple methodological caveats including similarity among prey groups, maternal carryover effects, and uncertainty in trophic enrichment factors. Fecal environmental DNA (eDNA) was intermediate in terms of effort, yielding results congruent with the other two methods, but the interpretation of which would likely have been confounded by contaminants had we not used all three methods in tandem. Several apparent artifacts are discussed. There are some reassuring similarities among methods. There are also several differences. The most complete picture uses data from all methods taken together. Future studies should attempt to compare the biases, expense, and potential drawbacks of these and other methods in greater detail.

Interpopulational variation and ontogenetic shift in the venom composition of Lataste's viper (Vipera latastei, Boscá 1878) from northern Portugal

Lataste's viper (Vipera latastei) is a venomous European viper endemic to the Iberian Peninsula, recognised as medically important by the World Health Organization. To date, no comprehensive characterisation of this species' venom has been reported. Here, we analysed the venoms of juvenile and adult specimens of V. latastei from two environmentally different populations from northern Portugal. Using bottom-up venomics, we produced six venom proteomes (three per population) from vipers belonging to both age classes (i.e., two juveniles and four adults), and RP-HPLC profiles of 54 venoms collected from wild specimens. Venoms from juveniles and adults differed in their chromatographic profiles and relative abundances of their toxins, suggesting the occurrence of ontogenetic changes in venom composition. Specifically, snake venom metalloproteinase (SVMP) was the most abundant toxin family in juvenile venoms, while snake venom serine proteinases (SVSPs), phospholipases A2 (PLA2s), and C-type lectin-like (CTLs) proteins were the main toxins comprising adult venoms. The RP-HPLC venom profiles were found to vary significantly between the two sampled localities, indicating geographic variability. Furthermore, the presence/absence of certain peaks in the venom chromatographic profiles appeared to be significantly correlated also to factors like body size and sex of the vipers. Our findings show that V. latastei venom is a variable phenotype. The intraspecific differences we detected in its composition likely mirror changes in the feeding ecology of this species, taking place during different life stages and under different environmental pressures. SignificanceLataste's viper (Vipera latastei) is a medically important viper endemic to the Iberian Peninsula, inhabiting different habitats and undergoing a marked ontogenetic dietary shift. In the current study, we report the first proteomic analysis of V. latastei venom from two environmentally different localities in northern Portugal. Our bottom-up venomic analyses show that snake venom serine proteinases (SVSPs), phospholipases A2 (PLA2s), and C-type lectin-like (CTLs) proteins are the major components of adult V. latastei venom. The comparative analysis of young and adult venoms suggests the occurrence of ontogenetic shift in toxin abundances, with snake venom metalloproteinases (SVMPs) being the predominant toxins in juvenile venoms. Moreover, geographic venom variation between the two studied populations is also detected, with our statistical analyses suggesting that factors like body size and sex of the vipers are possibly at play in its determination.Our work represents the first assessment of the composition of V. latastei venom, and the first step towards a better understanding of the drivers behind its variability.

Competition among small individuals hinders adaptive radiation despite ecological opportunity.

Ontogenetic diet shifts, where individuals change their resource use during development, are the rule rather than the exception in the animal world. Here, we aim to understand how such changes in diet during development affect the conditions for an adaptive radiation in the presence of ecological opportunity. We use a size-structured consumer–resource model and the adaptive dynamics approach to study the ecological conditions for speciation. We assume that small individuals all feed on a shared resource. Large individuals, on the other hand, have access to multiple food sources on which they can specialize. We find that competition among small individuals can hinder an adaptive radiation to unfold, despite plenty of ecological opportunity for large individuals. When small individuals experience strong competition for food, they grow slowly and only a few individuals are recruited to the larger size classes. Hence, competition for food among large individuals is weak and there is therefore no disruptive selection. In addition, initial conditions determine if an adaptive radiation occurs or not. A consumer population initially dominated by small individuals will not radiate. On the other hand, a population initially dominated by large individuals may undergo adaptive radiation and diversify into multiple species.

Ontogenetic shift in diet of a large elapid snake is facilitated by allometric change in skull morphology

As snakes are limbless, gape-limited predators, their skull is the main feeding structure involved in prey handling, manipulation and feeding. Ontogenetic changes in prey type and size are likely to be associated with distinct morphological changes in the skull during growth. We investigated ontogenetic variation in diet from stomach contents of 161 Dugite specimens (Pseudonaja affinis, Elapidae) representing the full range of body size for the species, and skull morphology of 46 specimens (range 0.25–1.64 m snout-vent-length; SVL). We hypothesised that changes in prey type throughout postnatal ontogeny would coincide with distinct changes in skull shape. Dugites demonstrate a distinct size-related shift in diet: the smallest individuals ate autotomised reptile tails and reptiles, medium-sized individuals predominantly ate mammals, and the largest individuals had the most diverse diet, including large reptiles. Morphometric analysis revealed that ~40% of the variation in skull shape was associated with body size (SVL). Through ontogeny, skulls changed from a smooth, bulbous cranium with relatively small trophic bones (upper and lower jaws and their attachments), to more rugose bones (as a likely reflection of muscle attachment) and relatively longer trophic bones that would extend gape. Individual shape variation in trophic bone dimensions was greater in larger adults and this likely reflects natural plasticity of individuals feeding on different prey sizes/types. Rather than a distinct morphological shift with diet, the ontogenetic changes were gradual, but positive allometry of individual trophic bones resulted in disproportionate growth of the skull, reflected in increased gape size and mobility of jaw bones in adults to aid the ingestion of larger prey and improve manipulation and processing ability. These results indicate that allometric scaling is an important mechanism by which snakes can change their dietary niche.

Growth, maturity, and diet of the pearl whipray (Fontitrygon margaritella) from the Bijagós Archipelago, Guinea-Bissau.

The pearl whipray Fontitrygon margaritella (Compagno & Roberts, 1984) is a common elasmobranch in coastal western African waters. However, knowledge on their life-history and trophic ecology remains limited. Therefore, we aimed to determine the growth, maturity and diet of F. margaritella from the Bijagós Archipelago in Guinea-Bissau. Growth was modelled with: von Bertalanffy, Gompertz and logistic functions. Model selection revealed no model significantly outperformed another. The sampled age ranged from less than 1 to 7 years (1.8 ± 1.9 cm, mean ± standard deviation) and size (disc width) ranged from 12.2 to 30.6 cm (18.7 ± 5.2 cm). Size-at-maturity was estimated at 20.3 cm (95% CI [18.8–21.8 cm]) for males and 24.3 cm for females (95% CI [21.9–26.5 cm]), corresponding ages of 2.2 and 3.9 years. The diet differed significantly among young-of-the-year (YOY), juveniles and adults (p = 0.001). Diet of all life stages consisted mainly of crustaceans (27.4%, 28.5%, 33.3%) and polychaetes (12.5%, 26.7%, 20.3%), for YOY, juveniles and adults respectively. This study shows that F. margaritella is relatively fast-growing, matures early and experiences ontogenetic diet shifts. These results contribute to status assessments and conservation efforts of F. margaritella and closely related species.

Ontogenetic allometry underlies trophic diversity in sea turtles (Chelonioidea)

Despite only comprising seven species, extant sea turtles (Cheloniidae and Dermochelyidae) display great ecological diversity, with most species inhabiting a unique dietary niche as adults. This adult diversity is remarkable given that all species share the same dietary niche as juveniles. These ontogenetic shifts in diet, as well as a dramatic increase in body size, make sea turtles an excellent group to examine how morphological diversity arises by allometric processes and life habit specialisation. Using three-dimensional geometric morphometrics, we characterise ontogenetic allometry in the skulls of all seven species and evaluate variation in the context of phylogenetic history and diet. Among the sample, the olive ridley (Lepidochelys olivacea) has a seemingly average sea turtle skull shape and generalised diet, whereas the green (Chelonia mydas) and hawksbill (Eretmochelys imbricata) show different extremes of snout shape associated with their modes of food gathering (grazing vs. grasping, respectively). Our ontogenetic findings corroborate previous suggestions that the skull of the leatherback (Dermochelys coriacea) is paedomorphic, having similar skull proportions to hatchlings of other sea turtle species and retaining a hatchling-like diet of relatively soft bodied organisms. The flatback sea turtle (Natator depressus) shows a similar but less extreme pattern. By contrast, the loggerhead sea turtle (Caretta caretta) shows a peramorphic signal associated with increased jaw muscle volumes that allow predation on hard shelled prey. The Kemp’s ridley (Lepidochelys kempii) has a peramorphic skull shape compared to its sister species the olive ridley, and a diet that includes harder prey items such as crabs. We suggest that diet may be a significant factor in driving skull shape differences among species. Although the small number of species limits statistical power, differences among skull shape, size, and diet are consistent with the hypothesis that shifts in allometric trajectory facilitated diversification in skull shape as observed in an increasing number of vertebrate groups.

Ontogenetic shift and feeding habits of the European hake (Merluccius merluccius L., 1758) in Central and Southern Tyrrhenian Sea (Western Mediterranean Sea): A comparison between past and present data.

The present paper aims to investigate the ecological role of Merluccius merluccius, Linnaeus, 1758, in southern and central Tyrrhenian Sea (GSA 10, Resolution GFCM/33/2009/2 General Fisheries Commission for the Mediterranean), analyzing ontogenetic diet shifts, geographical variations on prey composition, and feeding habits. A total of 734 hake specimens ranging in size between 6 cm and 73 cm (Total Length, TL) were collected in 2018. In order to evaluate ontogenetic shifts in prey composition, samples were divided into five size classes and for each class the quantitative feeding indices have been calculated. The statistical analysis, based on index of relative importance percentage (%IRI), resulted in three trophic groups. The most abundant prey found in the immature hake specimens (size class I) were the Euphausiids, Stylocheiron longicorne and Mysidacea, while for samples with a total length over 10.5 cm were crustaceans and fish. Engraulis encrasicolus was the most abundant fish prey identified, followed by Boops boops and Myctophids. The high presence of Euphausiids, Mysids, Myctophidae, and Sternoptychidae in classes I, II, II, and IV (6–23 cm) showed the relevant role of mesopelagic fauna in hake diets, with an essential organic matter and energy flow from the mesopelagic to the epipelagic environment. Additionally, decapod crustaceans were found in the stomach contents of hakes belonging to class V (with size over 36 cm TL), which is notable considering that our study area includes an important decapod crustacean fishing area.

Predator-Prey Interactions between Nonnative Juvenile Largemouth Bass (Micropterus salmoides) and Local Candidate Prey Species in the Pearl River Delta: Predation Capacity, Preference and Growth Performance.

An ontogenetic dietary shift is crucial for the survival and growth of piscivorous largemouth bass (LB). However, there is much to learn about the predator-prey interaction during the switching process. We carried out a series of indoor experiments to examine the predation capacity, predation preference, and growth performance of exotic juvenile LB feeding on candidate prey species in the Pearl River Delta. The widely distributed oriental river prawn (Macrobranchium nipponense), barcheek goby (Ctenogobius giurinus), western mosquitofish (Gambusia affinis), silver carp (Hypophthalmichthys molitrix), and mud carp (Cirrhinus molitorella), with relatively similar total lengths, were selected as potential prey based on their availability and habitat use. Our results show that predation capacity and preference varied quantitatively and qualitatively among prey species. The number of oriental river prawns killed was significantly less than that of fish species, comparing the 1st hour with the 24th hour (p < 0.01). The feeding rhythm of LB varied significantly from crayfish to fish. Numerically, Jacobs’ selection index reinforced LB’s special preference for predating G. affinis. Although there were obvious variations in predation capacity and feed selection, no statistically significant growth differences were detected among LB groups feeding on live M. nipponense, G. affinis, H. molitrix, and C. molitorella (p < 0.05). These findings suggest that the successful ontogenetic dietary shift of juvenile LB may depend on the availability and vulnerability of local fish species. Further study on the reproductive phenology of potential fish prey may help to predict LB’s establishment.

Trophic Niche Dynamics and Diet Partitioning of King Crab Lithodes santolla in Chile’s Sub-Antarctic Water

The southern king crab Lithodes santolla is one of the most economically important fishery species in the southern waters of the Atlantic and Pacific Oceans. A combination of stomach content and stable isotope analyses was used to reveal the potential dietary characteristics, isotopic niche, overlap among maturity stages and sexes, and trophic relationships of an L. santolla population in the Nassau Bay, Cape Horn region. Stable isotope analyses indicated that L. santolla assimilated energy from a basal carbon source, the giant kelp Macrocystis pyrifera, forming the trophic baseline of the benthic food web. Moreover, the trophic position of L. santolla varied among late juveniles and adults, suggesting that the southern king crab does undergo an ontogenetic diet shift. L. santolla exhibited intraspecific isotopic niche variation, reflecting niche differentiation which allows the species to partition resources. The trophic relationships of L. santolla with the associated fauna suggested some potential interactions for food resources/habitat use when they are limited. This study is the first attempt to characterize the trophic dynamics of the southern king crab in the Cape Horn area and, by generating more data, contributes to the conservation of the king crab population and the long-term management of local fisheries that rely on this resource.

Spatial Heterogeneity in Prey Availability, Feeding Success, and Dietary Selectivity for the Threatened Longfin Smelt

Food availability is a key determinant of the nursery value of a given habitat for larval and juvenile fishes. Growth, survival, and recruitment success are often inter-correlated and influenced by prey availability and associated feeding success. This is likely true for the threatened population of Longfin Smelt (Spirinchus thaleichthys) in the San Francisco Estuary (SFE) which has collapsed in recent decades along with its preferred prey. In years with high precipitation and freshwater outflow, larval Longfin Smelt are found in shallow wetland habitats throughout the SFE, but variation in the availability of food and feeding success in these habitats remains unexplored. To examine spatial variation in the trophic value of different rearing habitats, we quantified variation in prey availability, feeding success, and prey selection for larval and juvenile Longfin Smelt captured in restored tidal marshes, sloughs, and open-water habitats in the northern and southern SFE. Prey abundance varied spatially, with densities approximately tenfold greater in southern sloughs and restored tidal ponds relative to northern and open-water habitats. Feeding success of larval Longfin Smelt was positively correlated with both fish length and prey density. Larval Longfin Smelt fed selectively on the copepod Eurytemora affinis, with larger individuals (> 25 mm total length) exhibiting an ontogenetic diet shift to larger mysid shrimps. Our results suggest that wetland habitats across the SFE vary greatly in their trophic value, with previously unexplored habitats exhibiting the highest densities of prey and the highest foraging success for larval Longfin Smelt.

Recreational fishing fight times are not correlated with physiological status of blue sharks (Prionace glauca) in the Northwestern Atlantic

Sharks, like other fish, react to capture and handling stress with more exaggerated physiological responses than most other vertebrates, and the potential consequences of their capture include both sub-lethal and lethal effects. Blue sharks, Prionace glauca, are one of the most heavily exploited species of sharks in the world, both in commercial and recreational fisheries, and while the capture response of blue sharks in commercial fisheries has been comparably well-studied, there is a relative lack of information regarding the influence of handling and capture on the species in the recreational setting. Our analysis of blood-based biomarkers, such as glucose, lactate, pH, and plasma electrolytes, sampled from twenty blue sharks captured in the recreational fishery suggests that over the short hook times (time on the line) characteristic of the fishery, blue sharks do not fight to the point of blood acidosis and are likely able to withstand capture and handling in a catch-and-release setting. We did note an inverse relationship between shark total length and blood glucose levels, suggesting the possibility that smaller individuals may be more susceptible to capture stress, but this variation may also reflect other metabolic factors or an ontogenetic shift in diet. We discuss these findings within the context of the recreational fishery, including priorities for angler education.

The effect of ontogenetic diet shifts on sagittal otolith shape of European perch, Perca fluviatilis (Actinopterygii: Percidae) from Lake Ladik, Turkey

European perch undergoes several ontogenetic dietary shifts during its lifetime. The present study focused on the variability in otolith shape caused by ontogenetic dietary changes in this species. A total of 172 specimens (7.5 to 27.5 cm TL) were caught by local fishermen in Lake Ladik from April to November 2019. Shape indices (SIs) and elliptic Fourier coefficients (EFCs) were calculated for each otolith. To detect ontogenetic changes in otolith shape, SIs and EFCs were compared among the three defined size classes (I, II, and III). Size-related groups were distinguished by both SIs and EFCs. Otolith shape analysis was more effective than morphometric analysis in distinguishing ontogenetic groups. Simultaneous use of both methods improved the classification success of each method alone. This study demonstrated that otolith shape of European perch is influenced by ontogenetic changes in diet. However, sexual maturity of the fish is also responsible for these variations in otolith shape.

Feeding strategy of the introduced Astronotus crassipinnis (Cichlidae) in upper Paraná river floodplain

ABSTRACT The cichlid Astronotus crassipinnis (Heckel, 1840) known as “oscar” or “apaiari” is native to the Amazon basin and presents an omnivorous feeding habit, with a tendency to carnivory. The species was introduced in the upper Paraná River floodplain, where the trophic strategies adopted by the species are not well-known. The objective of this study was to characterize the diet of A. crassipinnis in different size classes to identify ontogenetic differences in diet composition and to determine its feeding behavior in the invaded floodplain. In addition, we experimentally compared the feeding behavior of A. crassipinnis with Hoplias sp. 2, a native piscivorous. We evaluated the predation rate of the two species with different prey (fish and shrimp). Astronotus crassipinnis consumed items from several trophic levels in the juvenile stage, characterizing an omnivore feeding. In the adult stage, A. crassipinnis displayed a specialist strategy in which fish and mollusks dominated the diet. Under controlled conditions, A. crassipinnis consumed more shrimp than Hoplias sp. 2. The preference for shrimp during the experiment contrasted to its feeding in natural environments, showing the potential trophic plasticity of A. crassipinnis. The piscivorous habit in the invaded environment can be considered a trophic strategy of A. crassipinnis, which feeds on an abundant resource in the upper Paraná River floodplain. In conclusion, the ontogenetic diet shifts and the trophic opportunism of the carnivore adults can be the strategies used by the species to invade and settle the floodplain. The piscivorous strategy of A. crassipinnis may impact fish biodiversity, altering ecosystem functioning.

Trophic niche partitioning of three larval Sciaenidae species in the north-western Gulf of Mexico

AbstractThe ability of larval fish to find prey items in sufficient quantities plays an important role in determining survivorship. To understand trophic niche partitioning and the potential for competition, we analyzed morphology and digestive tract content in larvae of three co-occurring, sciaenid fish species: Cynoscion nothus, Larimus fasciatus and Micropogonias undulatus. Larvae were collected in the north-western Gulf of Mexico off Galveston, TX, at seven stations in October/November 2017. Intra- and interspecific diet composition was analyzed between three ontogenetic development stages and two sampling zones (nearshore vs mid-shelf). M. undulatus had a significantly smaller mouth size than the other two species and consumed smaller prey items compared with C. nothus. Copepoda dominated all larval diets. The highly abundant M. undulatus primarily ate Calanoida following typical Sciaenidae feeding preferences. The less abundant C. nothus and L. fasciatus shared Cyclopoida as a primary diet item but differed in the consumption of less important prey taxa (i.e. Ostracoda, Chaetognatha). Ontogenetic dietary shifts (ODSs) were observed for all species, with the timing and number of ODS differing for C. nothus between sampling zones. The inclusion of alternative food sources such as Cyclopoida and non-copepod prey taxa likely alleviated potential interspecific competition allowing for larval co-occurrence.

Elevated methylmercury concentration and trophic position of the non-native bloody red shrimp (Hemimysis anomala) increase biomagnification risk in nearshore food webs

The establishment of non-native species can result in complex shifts in food-web structure and ecosystem function that alter the bioaccumulation, transfer, and biomagnification of contaminants. Hemimysis anomala (the bloody red shrimp), a mysid native to the Ponto-Caspian region that is now established throughout the Laurentian Great Lakes region (North America) and Europe, is characterized by high densities and a trophic ecology that may be consequential to mercury fluxes and bioaccumulation. We combined methylmercury (MeHg) and stable isotope (δ15N and δ13C) analyses of invertebrates from Seneca Lake (NY, USA) to test the hypotheses that mercury concentrations differ among (1) H. anomala and native or naturalized analogs due to food-web position and (2) stages of H. anomala due to ontogenetic diet shifts. The MeHg concentration and δ15N of H. anomala exceeded other littoral invertebrates (such as amphipods, dreissenid mussels, and zooplankton >153 μm) and were as high as the pelagic, native Mysis diluviana. These taxa-specific patterns indicate intensified biomagnification of MeHg is possible in nearshore and reef-spawning fish following the establishment of H. anomala, an energy-dense prey item for economically and ecologically important fish. Larger, adult H. anomala had higher MeHg concentrations and more enriched δ15N than juveniles, and H. anomala collected at a littoral site had higher MeHg than a canal site. These intraspecies contrasts are consistent with the shift toward zooplanktivory in adult H. anomala. As both putative prey for fish and competitors for shared zooplankton resources, H. anomala may impact fish in compounding ways.

Effects of Dietary Shifts on Ontogenetic Development of Metabolic Rates in Age 0 Lake Sturgeon (Acipenser fulvescens).

AbstractIn many fish species, ontogenetic dietary shifts cause changes in both quantitative and qualitative intake of energy, and these transitions can act as significant bottlenecks in survival within a given year class. In the present study, we estimated routine metabolic rate (RMR) and forced maximum metabolic rate (FMR) in age 0 lake sturgeon (Acipenser fulvescens) on a weekly basis from 6 to 76 days posthatch (dph) within the same cohort of fish. We were particularly interested in the period of dietary transition from yolk to exogenous feeding between 6 and 17 dph and as the fish transitioned from an artemia-based diet to a predominantly bloodworm diet between 49 and 67 dph. Measurement of growth rate and energy density throughout indicated that there was a brief period of growth arrest during the transition from artemia to bloodworm. The highest mass-specific RMR (mg O2 kg-1 h-1) recorded throughout the first 76 d of development occurred during the yolk sac phase and during transition from artemia to bloodworm. Similarly, diet transition from artemia to bloodworm-when growth arrest was observed-increased scaled RMR (i.e., mg O2 kg-0.89 h-1), and it did not significantly differ from scaled FMR. Log-log relationships between non-mass-specific RMR or FMR (i.e., mg O2 h-1) and body mass significantly changed as the growing fish adapted to the nutritional differences of their primary diet. We demonstrate that dietary change during early ontogeny has consequences for growth that may reflect altered metabolic performance. Results have implications for understanding cohort and population dynamics during early life and effective management for conservation fish hatcheries.

Effects of Warming on Intraguild Predator Communities with Ontogenetic Diet Shifts.

AbstractSpecies interactions mediate how warming affects community composition via individual growth and population size structure. While predictions on how warming affects composition of size- or stage-structured communities have so far focused on linear (food chain) communities, mixed competition-predation interactions, such as intraguild predation, are common. Intraguild predation often results from changes in diet over ontogeny ("ontogenetic diet shifts") and strongly affects community composition and dynamics. Here, we study how warming affects a community of intraguild predators with ontogenetic diet shifts, consumers, and shared prey by analyzing a stage-structured bioenergetics multispecies model with temperature- and body size-dependent individual-level rates. We find that warming can strengthen competition and decrease predation, leading to a loss of a cultivation mechanism (the feedback between predation on and competition with consumers exerted by predators) and ultimately predator collapse. Furthermore, we show that the effect of warming on community composition depends on the extent of the ontogenetic diet shift and that warming can cause a sequence of community reconfigurations in species with partial diet shifts. Our findings contrast previous predictions concerning individual growth of predators and the mechanisms behind predator loss in warmer environments and highlight how feedbacks between temperature and intraspecific size structure are important for understanding such effects on community composition.