Digestive enzyme activities in carnivores and herbivores: comparisons among four closely related prickleback fishes (Teleostei: Stichaeidae) from a California rocky intertidal habitat

Three digestive enzymes in four species of closely related prickleback fishes (family Stichaeidae: Cebidichthys violaceus, Xiphister mucosus , Xiphister atropurpureus and Anoplarchus purpurescens ) were analysed to assess whether diet or phylogeny played a larger role in influencing digestive enzyme activity. Cebidichthys violaceus and X. mucosus are primarily herbivorous, whereas X. atropurpureus and A. purpurescens are mainly carnivorous. The two Xiphister species are sister taxa, and A. purpurescens is in a clade adjacent to that of the three other species. Pepsin and trypsin specific activities did not differ significantly among the four species, but α‐amylase activity was significantly higher in the two Xiphister species, followed by C. violaceus , and then A. purpurescens . The wide disparity between the two carnivores, the striking similarity between the two sister taxa, and the significant difference between the two herbivores indicate that activity of α‐amylase follows a pattern influenced more by phylogeny than by diet in these fishes.     

Storage and absorption in the digestive system of carnivorous and herbivorous prickleback fishes (Teleostei: Stichaeidae): ontogenetic, dietary, and phylogenetic effects

The effects of ontogeny, diet, and phylogeny on glycogen storage levels and esterase and alkaline phosphatase activities in four related prickleback fishes were determined in situ using quantitative histochemistry. Of these species, Cebidichthys violaceus and Xiphister mucosus shift from carnivory to herbivory at approximately 45 mm standard length (SL), whereas Xiphister atropurpureus and Anoplarchus purpurescens remain carnivores. Comparisons between small (30-40 mm SL) and larger (60-75 mm SL) wild-caught juveniles showed that glycogen storage levels and alkaline phosphatase activity were unchanged with ontogeny. Comparisons between the larger wild-caught juveniles and juveniles of the same size that had been raised on a high-protein animal diet revealed that glycogen storage level and alkaline phosphatase activity increased in all species in response to this diet. Esterase activity also increased in response to the high-protein animal diet in all four species but increased with ontogeny only in C. violaceus, X. mucosus, and X. atropurpureus, in the xiphisterine clade, and not in A. purpurescens, in the adjacent alectriine clade. Xiphister mucosus and X. atropurpureus showed indistinguishable responses in esterase activity to ontogeny and diet despite their divergent natural diets. Overall, glycogen storage level and alkaline phosphatase activity responded primarily to diet, whereas esterase activity was also influenced by ontogeny and phylogeny and differed between intestinal regions among the species.     

Sequence and expression of an α-amylase gene in four related species of prickleback fishes (Teleostei: Stichaeidae): ontogenetic,dietary, and species-level effects

Partial α-amylase gene sequences were determined and α-amylase gene expression was quantified in four species of carnivorous, omnivorous, and herbivorous prickleback fishes (family Stichaeidae) to assess the effects of ontogeny, diet, and species on expression of this gene. Pairwise comparison of α-amylase nucleotide sequences revealed 96–98 % identity, and comparison of amino acid portions revealed 93–95 % similarity among the four prickleback species. Expression was determined using in situ hybridization and intensity of expression quantified using image analysis. Alpha-amylase expression level was compared in three feeding categories of the four species: (1) small, wild-caught carnivorous juveniles; (2) larger, wild-caught juveniles of the carnivorous species and the three that had shifted to herbivory or omnivory; and (3) larger, juveniles produced by feeding a low-starch artificial diet to small juveniles until they reached the size of the larger wild-caught juveniles. The results showed no dietary effect in any species but significant ontogenetic and species-level effects in Cebidichthys violaceus, as well as in the sister species Xiphister mucosus and X. atropurpureus. Based on a phylogeny for the Stichaeidae produced for this study using two mtDNA genes and one nuclear gene, the ontogenetic dietary shifts to herbivory/omnivory evolved independently in C. violaceus and in the clade containing the two species of Xiphister. All three of these species increased α-amylase gene expression with increase in size and had higher expression than Anoplarchus purpurescens, which is a member of a third, stichaeid clade comprising carnivores. These results show the importance of α-amylase in the herbivores and omnivores.     

Transitional ecological requirements for early juveniles of two sympatric stichaeid fishes,Cebidichthys violaceus andXiphister mucosus

Synopsis The distribution patterns, diets, and substratum (refuge) requirements of early juveniles of two sympatric stichaeid fishesCebidichthys violaceus andXiphister mucosus, were investigated in a rocky intertidal habitat at Diablo Canyon, California. Monthly investigations were conducted at low tide for four consecutive months, to assess ontogenetic differences in distribution, diet, and refuge requirements within and between the two species. Distinct differences in vertical zonation were exhibited by both stichaeids throughout the study. Interspecific zonation patterns were similar to those recorded for adults of both species. Diet analyses showed that early juveniles of both stichaeids were zooplanktivorous, differing markedly from the primarily herbivorous diets of adults. Changes in diet were largely due to the selection of larger prey taxa as both fishes, and their mouth size, grew over the study period. Predation by both fishes on water-column planktors (calanoid copepods, zoea and polychaete larvae) was greatest following initial intertidal settlement and habitat establishment by early juvenile fishes. Greater dependence on substrate-oriented and/or benthic prey (harpacticoid copepods, gammarid amphipods and mysid shrimp) was exhibited by both fishes as they grew in size. Affinities for sand, gravel, and pebbles during monthly field surveys were similar for both species throughout the study. As the fishes grew, their substratum preferences changed in relation to the substrata which provided the best refuge. Results from laboratory experiments indicated that young stichaeids select very specific substrata based on fish age (size) and substratum suitability (i.e. adequate refuge). Similarities in diet and substratum preferences, and changes in those preferences over time, appear to be the result of morphological similarities (body size and shape and mouth gape) for both species at a given age.     

More than one way to be an herbivore: convergent evolution of herbivory using different digestive strategies in prickleback fishes (Stichaeidae)

In fishes, the evolution of herbivory has occured within a spectrum of digestive strategies, with two extremes on opposite ends: (i) a rate-maximization strategy characterized by high intake, rapid throughput of food through the gut, and little reliance on microbial digestion or (ii) a yield-maximization strategy characterized by measured intake, slower transit of food through the gut, and more of a reliance on microbial digestion in the hindgut. One of these strategies tends to be favored within a given clade of fishes. Here, we tested the hypothesis that rate or yield digestive strategies can arise in convergently evolved herbivores within a given lineage. In the family Stichaeidae, convergent evolution of herbivory occured in Cebidichthys violaceus and Xiphister mucosus, and despite nearly identical diets, these two species have different digestive physiologies. We found that C. violaceus has more digesta in its distal intestine than other gut regions, has comparatively high concentrations (>11 mM) of short-chain fatty acids (SCFA, the endpoints of microbial fermentation) in its distal intestine, and a spike in β-glucosidase activity in this gut region, findings that, when coupled to long retention times (>20 h) of food in the guts of C. violaceus, suggest a yield-maximizing strategy in this species. X. mucosus showed none of these features and was more similar to its sister taxon, the omnivorous Xiphister atropurpureus, in terms of digestive enzyme activities, gut content partitioning, and concentrations of SCFA in their distal intestines. We also contrasted these herbivores and omnivores with other sympatric stichaeid fishes, Phytichthys chirus (omnivore) and Anoplarchus purpurescens (carnivore), each of which had digestive physiologies consistent with the consumption of animal material. This study shows that rate- and yield-maximizing strategies can evolve in closely related fishes and suggests that resource partitioning can play out on the level of digestive physiology in sympatric, closely related herbivores.     

Characterization of the ultrastructure of the gastrointestinal tract mucosa, stomach contents and liver enzyme activity of the pinfish during development

Carnivorous juveniles (<16 mm L_S) of pinfish Lagodon rhomboides apparently lacked gastric glands in the stomach while larger fish, intermediates (30-33 mm L_S) and herbivorous adults (>80 mm L_S) had numerous gastric glands. Two cell types were identified in the gastric glands of larger fish: mucous secreting cells and secretory cells which had ultrastructure features typical of digestive enzymes and acid secretion. Lipid absorption occurred throughout the caeca and intestine in all sizes of fish. Microvilli found on the rectal epithelial cells of the intermediate and adult pinfish occurred on stalks and were possibly associated with water reabsorption. Liver enzyme activities changed in small fish (26–39 mm L_S) compared to the adult fish. Alanine amino transferase (ALT) and fructose diphosphotase (FDP) activities declined while pyruvate kinase (PK) activities increased significantly. These changes were consistent with a change in diet from a carnivorous (high protein) diet in juveniles to an omnivorous (lower protein) diet determined by stomach content analyses.     

Contrasting quaternary histories in an ecologically divergent sister pair of low-dispersing intertidal fish (Xiphister) revealed by multilocus DNA analysis

Recurrent glacial advances have shaped community histories across the planet. While biogeographic responses to glaciations likely varied with latitude, the consequences for temperate marine communities histories are less clear. By coalescent analyses of multiloci DNA sequence data (mitochondrial DNA control region, alpha-enolase intron, and alpha-tropomyosin intron) collected from a low-dispersing sister pair of rocky intertidal fishes commonly found from southeastern Alaska to California (Xiphister atropurpureus and X. mucosus), we uncover two very different responses to historical glaciations. A variety of methods that include a simulation analysis, coestimates of migration and divergence times, and estimates of minimum ages of populations sampled up and down the North American Pacific coast all strongly revealed a history of range persistence in X. atropurpureus and extreme range contraction and expansion from a southern refugium in X. mucosus. Furthermore, these conclusions are not sensitive to the independent estimates of the DNA substitution rates we obtain. While gene flow and dispersal are low in both species, the widely different histories are rather likely to have arisen from ecological differences such as diet breadth, generation time, and habitat specificity.     

Enzymatic digestion in stomachless fishes: how a simple gut accommodates both herbivory and carnivory

The lack of a stomach is not uncommon amongst teleost fishes, yet our understanding of this reductive specialisation is lacking. The absence of a stomach does not restrict trophic preference, resulting in fishes with very similar alimentary morphology capable of digesting differing diets. We examined the digestive biochemistry of four beloniform fishes: two herbivorous halfbeaks (Hemiramphidae) and two carnivorous needlefish (Belonidae) to determine how these fishes digest their respective diets with their simple, short gut. We found that although the halfbeaks showed significantly greater α-amylase activity than that of the needlefish (P < 0.01), trypsin, lipase, aminopeptidase and maltase activity were not substantially different between the two families. We also found that habitat (freshwater vs. marine) appears to play a significant role in digestive capability, as the two freshwater taxa and the two marine taxa were significantly different (ANOSIM; dietary Gobal R = 0.544, P = 0.001, habitat Global R = 0.437, P = 0.001), despite their phyletic and dietary similarities. Our findings offer partial support for the adaptive modulation hypothesis, support the Plug-Flow Reactor model of digestion in herbivorous halfbeaks and also support the compartmental model of digestion but suggest that another model is required to describe stomachless carnivorous needlefish.     

Ontogenetic plasticity of food habits in the Mexican spiny-tailed iguana, Ctenosaura pectinata

Ontogenetic shifts from insect consumption by juveniles to plant consumption by adults are rare in the herbivorous lizard family Iguanidae. My investigations on diet and digestive tract anatomy of the iguanid lizard Ctenosaura pectinata show that this species has an ontogenetic diet shift. Insects were rare in adult diets but constituted 86.5% (by volume) of the food eaten by the smallest juveniles. All age classes ate some plant parts from a range of plant types, but flowers and leaves of legumes were a primarily food source. Non-adult lizards had the widest food niche breadths. Arthropods in the diet of juveniles and immatures covaried seasonally with the decline of arthropod abundance. Several hypotheses could explain this ontogenetic plasticity in diet. I rejected hypotheses that gut structure constrained juveniles to an arthropod diet and that insect consumption was purely an artifact of plant consumption because (1) size-adjusted gut morphology and capacity was similar among age classes, and (2) no food plants sampled had an excessive density of arthropods. I supported an alternative hypothesis that juveniles can eat plants but do not because insects provide a more nutritious diet. This conclusion was based on the observation that the juvenile hindgut is similar to that of herbivorous adults, and the propensity for juveniles to consume primarily, but not exclusively, insects when they were most abundant. The hindgut represents the site of fermentative plant fiber breakdown in many herbivorous lizards. Insect foods can compensate for size-related nutritional needs (energy and protein) and digestive limitations in juveniles. Opportunistic feeding to maintain a broad diet might help juvenile and immature lizards through high-predation-risk growth periods by reducing searching costs, increasing nutritional and energetic gains due to associative effects, and increasing new food exposure. Received: 20 January 1999 / Accepted: 25 January 2000     

Interspecific dietary diversity has little influence on pathways of glucose metabolism in liver and heart of piranhas and pacus (family Serrasalmidae)

The Serrasalmidae is a speciose family of Neotropical freshwater fishes with diverse interspecies dietary preferences, from the herbivorous or frugivorous pacus to the omnivorous or carnivorous piranhas. The Serrasalmidae is an ideal comparative model to explore the biochemical correlates of dietary preference. For example, debate exists about whether variation in dietary preference among fishes influences the capacities for de novo glucose synthesis (gluconeogenesis) or glucose use as metabolic fuel (glycolysis). We predicted that carnivory in piranhas would be associated with an enhanced gluconeogenic capacity compared with the herbivorous pacus, because carnivores consume less carbohydrate. As a corollary, we expected similar glycolytic capacities at a major site of glucose use (heart) in fishes with different diets. We measured activities of key enzymes of gluconeogenesis and the opposing reactions of glycolysis in liver (the primary gluconeogenic site) and heart of five serrasalmid species showing an interspecies dietary gradient from herbivory to omnivory to carnivory. The interspecific variation in enzyme activities was, in general, unrelated to dietary preference. Glycolytic capacity in heart was lower in herbivores but this may be related to non-diet factors. Among serrasalmid fishes, the enzymatic capacities of glucose metabolism are not overtly influenced by interspecific divergence of dietary preference.     

Prey use of wetland benthivorous sunfishes: ontogenetic, interspecific and seasonal variation

The intensity of competitive interactions between fishes is partly determined by prey use and ontogenetic niche shifts. In a wetland where distinct habitat shifts are missing we compared prey use of three generalist benthivorous sunfishes to look for evidence of ontogenetic, interspecific, and “seasonal” variation in prey composition. Diet analysis revealed evidence of diet ontogeny in warmouth (Lepomis gulosus, 30–152 mm standard length, SL), but not in bluespotted sunfish (Enneacanthus gloriosus, 30–47 mm SL) or dollar sunfish (Lepomis marginatus, 30–60 mm SL). Bluespotted and dollar sunfishes consumed small dipteran and amphipod prey and had similar diets in both seasons suggesting a potential for strong interspecific competition. In the dry season, warmouth shifted from using smaller insect prey to larger decapod and fish prey with increasing size. This shift to prey types that were little used by the other species reduced dietary niche overlap with the other sunfishes. After drought and re-flooding (in the wet season), decapods and small fish were less abundant in the wetland and the warmouth ontogenetic shift was less distinct. When matched for gape width, prey composition differed between warmouth and both dollar and bluespotted sunfishes in the wet season, suggesting differences in sunfish foraging modes, but prey use differences were less clear in the dry season when prey were abundant. Both warmouth ontogenetic diet shifts and seasonal variation in prey use (probably mediated by prey abundance) had strong influences on diet overlap and therefore the potential for intra- and interspecific competition between sunfishes in this wetland ecosystem.     

The effects of diet and digestive cycle on the gastrointestinal tract pH values in the goldfish, Carassius auratus L., Mozambique tilapia, Oreochromis mossambicus (Peters), and channel catfish, Ictalurus punctatus (Rafinesque)

The digestive tract pH values of the teleost fish Oreochromis mossambicus, Carassius auratus and Ictalurus punctatus were determined in fish that were (1) starved, (2) fed herbivorous, omnivorous and carnivorous diets and (3) fed altered pH diets. The digestive tract pH profile was determined throughout an 8 h cycle. The herbivorous or carnivorous nature of the diet did not affect the pH values in any of the species. Significant differences in gut pH values were observed when the fish ingested diets with altered pHs. There was a difference between the starved and baseline fed values in Carassius auratus which was not observed in the other species. The pH values in the digestive tracts of the three species varied during the digestive cycle.     

Physiological responses to short-term fasting among herbivorous,omnivorous, and carnivorous fishes

We explored the integrated role of dietary specialization and feeding periodicity on the response of the gastrointestinal tract of teleosts fishes to short-term (7–10 days) fasting and refeeding. Fasted and fed herbivorous grass carp (Ctenopharyngodon idella), omnivorous channel catfish (Ictalurus punctatus), and carnivorous largemouth bass (Micropterus salmoides) were compared for digestive organ masses, intestinal morphology, gastrointestinal pH, and the specific activities and total intestinal capacities of the intestinal hydrolases aminopeptidase (APN) and maltase and intestinal nutrient transporters. All three species experience intestinal hypertrophy with feeding as noted by significant increases in enterocyte dimensions. Of the three, only I. punctatus experienced a postprandial increase in intestinal length, and only C. idella experienced significant modulation of intestinal microvillus length. Feeding resulted in acidification of the stomachs of I. punctatus and M. salmoides. Predicted to exhibit a relatively modest set of postprandial responses because of their more frequent feeding habits, C. idella only experienced increases in APN and maltase activity with feeding and no significant regulation of nutrient uptake. Significant regulation of hydrolase activities and nutrient uptake were exhibited by I. punctatus and M. salmoides, with I. punctatus experiencing the most comprehensive set of responses. As predicted by food habits, there was an interspecific gradient in intestinal length and glucose uptake extending from longer intestines and greater glucose uptake for the herbivorous C. idella, intermediate lengths and glucose uptake for the omnivorous I. punctatus, and shorter intestines and reduced glucose uptake for the carnivorous M. salmoides. Among teleosts fishes, short episodes of fasting lead to significant alterations in intestinal form and function that are rapidly restored with feeding.     

Diet and endogenous carbohydrases in the temperate marine herbivorous fish Kyphosus sydneyanus

Stomach content analysis showed that Kyphosus sydneyanus collected by spearing from coastal reefs near Leigh, north-east New Zealand, is herbivorous throughout its post-settlement life. All size classes collected consumed a wide range of macroalgae, predominantly the phaeophytes Carpophyllum maschalocarpum and Ecklonia radiata , and the rhodophytes Gigartina macrocarpa and Caulacanthus ustulatus . Multivariate analysis of variance and discriminant function analysis showed an ontogenetic diet shift, with gut contents dominated by rhodophytes and chlorophytes in juveniles and by phaeophytes in adults. No significant seasonal variation in diet was apparent for any size class. Amylase and amylopectinase activity was highest in the smaller size classes. Laminarinase activity was relatively low throughout all size classes, and did not correlate with the ontogenetic increase in phaeophyte consumption. The activity of endogenous enzymes hydrolysing starch broadly correlated with rhodophyte consumption in adult specimens of three other species of marine herbivorous fishes: Odax pullus, Girella tricuspidata and Parma alboscapularis; laminarin hydrolysis, however, was negatively correlated with phaeophyte consumption. Overall, the diet and enzyme results suggest that juvenile K. sydneyanus rely mainly on the endogenous digestion of rhodophytes and chlorophytes, while exogenous (microbial) digestion is used to digest the phaeophyte-dominated diet of adults.     

Piscivorous behavior of a temperate cone snail, Conus californicus

Most of the more than 500 species of predatory marine snails in the genus Conus are tropical or semitropical, and nearly all are thought to be highly selective regarding type of prey. Conus californicus Hinds, 1844, is unusual in that it is endemic to the North American Pacific coast and preys on a large variety of benthic organisms, primarily worms and other molluscs, and also scavenges. We studied the feeding behavior of C. californicus in captivity and found that it regularly killed and consumed live prickleback fishes (Cebidichthys violaceus and Xiphister spp.). Predation involved two behavioral methods similar to those employed by strictly piscivorous relatives. One method utilized stings delivered by radular teeth; the other involved engulfing the prey without stinging. Both methods were commonly used in combination, and individual snails sometimes employed multiple stings to subdue a fish. During the course of the study, snails became aroused by the presence of live fish more quickly, as evidenced by more rapid initiation of hunting behavior. Despite this apparent adaptation, details of prey-capture techniques and effectiveness of stings remained similar over the same period.     

Spatial and seasonal changes in the diet of Oligosarcus hepsetus (Characiformes, Characidae) in a Brazilian reservoir.

We assessed spatial and seasonal changes in the diet of Oligosarcus hepsetus in order to describe the strategy developed by this species that allows their very high abundance in Lajes reservoir, Rio de Janeiro, Brazil. Fish samplings were carried out using gill nets, deployed during ca. 12 and 24 hours, between April 2001 and May 2002. A total of 289 individuals were examined, of which 97 showed gut contents. We used the index of relative importance (IRI) to compare probable dietary shifts, and the frequency of occurrence (% OC) to analyze possible ontogenetic influences on feeding. O. hepsetus showed carnivorous habits, feeding preferably on fish and insects, the latter of which occurred in 71.0% of the guts presenting contents. O. hepsetus consumed different items along the three reservoir zones: insects (61.0% IRI) and Cichla monoculus (38.9% IRI) in the lower zone; Lepidoptera (57.0% IRI) in the middle zone; and C. monoculus (77.0% IRI) in the upper zone. Food items changed seasonally with C. nonloculus predominating in autumn 2001, and Hymenoptera and Lepidoptera in the winter. In spring almost all food was Lepidoptera (99.8% IRI), while in the summer Hemiptera dominated in the diet. In autumn 2002 Hemiptera (97.0% IRI) was dominant, in significant contrast with the previous autumn. Individuals smaller than 190 mm SL fed heavily on insects, while fishes predominated in the diet of individuals larger than 190 mm SL. Shifts in prey-capture ability among length classes suggest decreasing intraspecific competition. A higher food plasticity seems to be the strategy employed by this opportunist species, which used food resources available in the reservoir.     

Development of digestive secretions in white sturgeon juveniles (Acipenser transmontanus)

Development of the digestive enzyme complement corresponded with this species' carnivorous habits and a natural dietary shift at metamorphosis from larval pelagic zooplanktivory to benthic carnivory. During the first developmental stanza when the digestive tract was differentiating and the larvae were dependent on endogenous nutritional reserves, digestive enzyme concentrations were low. Initiation of gastric secretion (pepsin and acid) was concomitant with onset of exogenous feeding. Highest amylolytic and lipolytic activities were recorded from feeding larvae prior to their metamorphosis to juveniles.     

Do herbivorous minnows have “plug-flow reactor” guts? Evidence from digestive enzyme activities,gastrointestinal fermentation,and luminal nutrient concentrations

Few investigations have empirically analyzed fish gut function in the context of chemical reactor models. In this study, digestive enzyme activities, levels of gastrointestinal fermentation products [short chain fatty acids (SCFA)], luminal nutrient concentrations, and the mass of gut contents were measured along the digestive tract in herbivorous and carnivorous minnows to ascertain whether their guts function as “plug-flow reactors” (PFRs). Four of the species, Campostoma anomalum, C. ornatum, C. oligolepis, and C. pauciradii, are members of a monophyletic herbivorous clade, whereas the fifth species, Nocomis micropogon, is a carnivore from an adjacent carnivorous clade. In the context of a PFR model, the activities of amylase, trypsin and lipase, and the concentrations of glucose, protein, and lipid were predicted to decrease moving from the proximal to the distal intestine. I found support for this as these enzyme activities and nutrient concentrations generally decreased moving distally along the intestine of the four Campostoma species. Furthermore, gut content mass and the low SCFA concentrations did not change (increase or decrease) along the gut of any species. Combined with a previous investigation suggesting that species of Campostoma have rapid gut throughput rates, the data presented here generally support Campostoma as having guts that function as PFRs. The carnivorous N. micropogon showed some differences in the measured parameters, which were interpreted in the contexts of intake and retention time to suggest that PFR function breaks down in this carnivorous species.     

Size-related shifts in dietary composition of Centropomus parallelus (Perciformes: Centropomidae) in an estuarine ecosystem of the southeastern coast of Brazil

Size-related and seasonal evaluation of the dietary composition of fat snook ( Centropomus parallelus Poey 1860) in the upper sector of an estuary of the southeastern coast of Brazil were carried out based on stomach analyses of specimens ranging from 40 to 170 mm standard length. Results reveal that C. parallelus is a carnivorous species feeding mainly on benthic crustaceans. Relatively high stomach replenishment suggests that this environment is an important feeding ground for fat snook juveniles. Multivariate analyses indicated that predator size effect is significantly more important than seasonal variation in determining dietary composition. Predator length was associated with increased consumption of palaemonid shrimps ( Macrobrachium spp.) and grapsid crabs, and decreased foraging on tanaids ( Kalliapseudes schubarti ), thus showing a preference shift from smaller to larger prey. Predator length was also positively associated with an increase in the stomach repletion index. Additionally, allometric growth of both gape and head were consistently correlated with this ontogenetic dietary transition, suggesting that such changes might be related to an individual's ability to capture and consume larger, more elusive prey. The digestive tube is short and grows isometrically, which is in accordance with the carnivorous habit of this estuarine fish and its maintenance through ontogeny.