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     

Coordinated Ontogeny of Food Preference and Responses to Chemical Food Stimuli by a Lizard Ctenosaura pectinata (Reptilia: Iguanidae)

When important ecological factors change predictably during the life of an organism, the ontogeny of related behaviors must be timed to maintain appropriate behavioral responsiveness to current ecological conditions. In the brown iguana, Ctenosaura pectinata , hatchlings in natural populations eat primarily insects, consuming little plant matter, whereas adults eat primarily plants, consuming some insects as well. We conducted laboratory experiments on diet preferences and responses to chemical cues that the lizards sampled by tongue-flicking and used to identify food. All hatchlings ate crickets, but only one of six ate romaine lettuce. They responded strongly to chemical cues from prey, as indicated by elevated tongue-flick rates, but not from romaine lettuce. All older individuals ate both crickets and romaine lettuce. They responded much more strongly to chemical cues from both crickets and romaine lettuce than to control chemicals, as indicated by higher proportions of individuals that bit and higher tongue-flick attack scores.
Thus, an ontogenetic change to increased responsiveness to plant chemical stimuli was coordinated with an ontogenetic change to an herbivorous diet. The mechanisms underlying these ontogenetic changes are unknown, but folivory may be unprofitable before juveniles acquire intestinal flora that degrade cellulose by ingestion of feces of adult conspecifics. Possible mechanisms are discussed, including the detection of chemical cues from appropriate food plants during consumption of feces from older individuals. Studies of other squamate reptiles suggest that exposure to these chemicals might affect both future responsiveness to the chemical cues and a tendency to eat the corresponding plants.     

Diet and nutrition in wild mongoose lemurs (Eulemur mongoz) and their implications for the evolution of female dominance and small group size in lemurs

Data collected on the feeding behavior, food intake, and chemical analyses of plant foods were used to document seasonal variation in diet and nutrition in Eulemur mongoz in northwestern Madagascar. E. mongoz conforms to the general Eulemur dietary pattern, with a predominantly frugivorous diet supplemented mainly by leaves, flowers, and nectar. Phytochemical analysis revealed high water contents in all the main plant foods; mature fruit and flowers contained the most water-soluble carbohydrates; immature leaves were richest in protein and essential amino acids; the limiting amino acids in all plant foods were methionine and cystine; ash (mineral) content was highest in petioles and mature leaves; crude lipid content was highest in seeds; and crude fiber content was indistinguishable between immature and mature fruit and leaves. High-fiber foods were eaten during both seasons; the wet season diet was dominated by high-energy foods (mature fruit, nectar, and seeds), while the dry season diet contained foods high in energy (mature fruit and flowers) and high in protein (immature leaves) and minerals (mature leaves and petioles). However, nutrient intake did not vary between seasons, implying that nutrient requirements are met throughout the year. These results suggest we draw more conservative conclusions when interpreting dietary variability in the absence of chemical analysis, and also draw into question the idea that nutritional stress is a factor in the timing of reproduction in lemurs and, by extension, is linked to the prevalence of female dominance and small group size in lemurs.     

Feeding time strategies of the fringe-toed lizard,Uma inornata,during breeding and non-breeding seasons

Summary I examined the foraging behavior during the breeding and non-breeding seasons, May and July 1986, of the fringe-toed lizard Uma inornata (Iguanidae). During the breeding season males differ from females in their diet and in their foraging time strategy, males exhibiting time minimization and females energy maximization. In May, plant associated foods were selectively eaten. Males concentrated on flowers, a readily available quick energy food, which reduced foraging time and increased time for reproductive activities. Time budgets indicate that males spend over twice as much time in the open and in movement in May than do females. Females at this time restrict their activities to the cover of perennial bushes, and feed primarily on plant foods (flowers and arthropods). Energy maximization appears to be maintained by both sexes in the non-breeding season when food resources diminished to one-half of those in the breeding season. The lizards were less selective in their July feeding habits, broadening their diets to include ground-dwelling arthropods and foliage. Predation by these lizards follows a wait-ambush mode of foraging.     

Proteomic analysis of ontogenetic and diet-related changes in venom composition of juvenile and adult Dusky Pigmy rattlesnakes (Sistrurus miliarius barbouri)

Snake venom proteins show high levels of variation at the level of the individual yet the environmental and molecular mechanisms that generate this diversity remain unclear. Here we report the results of a controlled feeding experiment combined with proteomic analyses of periodically collected venom samples to assess the roles of ontogenetic and diet-related effects on venom composition of captive juvenile and adult Dusky Pigmy rattlesnakes (Sistrurus miliarius barbouri). Juvenile snakes fed from birth with mice, lizards, or frogs showed little evidence for an ontogenetic shift in venom composition from 5 to 26 months in terms of substantial changes in the relative abundance of major classes of venom toxins. However, there were fine-scale changes in the relative abundance of D49-PLA? 15, PI-SVMPs, and PIII-SVMP 28, and a decline in the abundance of other PIII-SVMPs. Although juveniles raised on different prey exhibited distinct relative toxin compositional change rates, at 26 months old, their venoms showed similar patterns of protein composition suggesting little effect of diet on the overall make-up of venom in snakes this age or younger. In contrast, adult females raised on different prey over a 26 month period showed prey-related changes in the relative abundance of major protein families from initial to final samples. Specifically, mouse-fed females showed substantial increases in the relative abundance of total PLA?s and serine proteinases of 95% and >100%, respectively, whereas comparable values for lizard- (42% and -22%) and frog-fed females (2% and 11%) were distinctly smaller in magnitude. Venom from adult snakes fed on different prey also showed distinct changes in the abundance of PLA? molecules 15, 19a, and 19, which were, respectively, (>100%, 33%, 63%), (>100%, 0%, 35%), and (71%, 20%, -4%) for the mice-, lizard-, and frog-diet. Venom from snakes raised on frogs contained a small (1.1%) but consistent amount of a PLA? molecule (15a) not present in snakes fed on mice or lizards. This work provides evidence that venom composition is somewhat plastic in both juvenile and adult S. m. barbouri and that, at least in adults, prey consumed may influence the relative abundance of possibly functionally-distinct classes of venom proteins.     

Food chemical cues elicit general and population-specific effects on lingual and biting behaviors in the lacertid lizard Podarcis lilfordi

Actively foraging lizards are capable of identifying prey using only chemical cues sampled by tongue-flicking, and the relatively few omnivorous and herbivorous lizards tested similarly can identify both animal and plant foods from chemical cues. Whether lizards that eat plants respond to cues specific to preferred plant types and whether there is geographic variability in responses to cues from various plants correlated with the importance of those plants in local diets is unknown. In three populations of an omnivorous lacertid, the Balearic lizard Podarcis lilfordi, we studied chemosensory sampling and feeding responses to chemical cues from plant and animal foods presented on cotton swabs. Each lizard population is endemic to one islet off the coast of Menorca, Balearic Islands, Spain. Lizards in all three populations discriminated chemical cues from plant and animal foods from control substances. Our results extend findings of prey chemical discrimination and plant chemical discrimination in omnivores, increasing confidence that correlated evolution has occurred between plant diet and chemosensory response to palatable plants. There were no consistent differences among populations in tongue-flicking and biting responses to stimuli from flowers of syntopic and allopatric plant species. The lizards may respond to cues indicative of palatability in a wide range of plant species rather than exhibiting strong responses only to locally available plant species. Nevertheless, tongue-flicking and biting frequencies varied among plant species, perhaps indicating food preferences. In addition, there were differences among populations in tongue-flick rates, latency to bite, and licking behavior. Licking was observed in only one lizard population as a response to floral chemicals from only one of the plants species tested, raising the possibility of a population-specific linkage between identification of a particular plant species and performance of an appropriate feeding response.     

Ontogenetic Development of the Gastrointestinal Microbiota in the Marine Herbivorous Fish Kyphosus sydneyanus

Molecular techniques were used to investigate the composition and ontogenetic development of the intestinal bacterial community in the marine herbivorous fish Kyphosus sydneyanus from the north eastern coast of New Zealand. Previous work showed that K. sydneyanus maintains an exclusively algivorous diet throughout post-settlement life and passes through an ontogenetic diet shift from a juvenile diet which is readily digestible to an adult diet high in refractory algal metabolites. Terminal restriction fragment length polymorphism (T-RFLP) analysis was used to investigate the relationship between bacterial community structure and fish size. Bacterial diversity was higher in posterior gut sections than anterior gut sections, and in larger fish than in smaller fish. Partial sequencing of bacterial 16S rDNA genes PCR amplified and cloned from intestine content samples was used to identify the phylogenetic affiliation of dominant gastrointestinal bacteria. Phylogenetic analysis of clones showed that most formed a clade within the genus Clostridium, with one clone associated with the parasitic mycoplasmas. No bacteria were specific to a particular intestinal section or size class of host, though some appeared more dominant than others and were established in smaller fishes. Clones closely related to C. lituseburense were particularly dominant in most intestine content samples. All bacteria identified in the intestinal samples were phylogenetically related to those possessing fermentative type metabolism. Short-chain fatty acids in intestinal fluid samples increased from 15.6 ± 2.1 mM in fish <100 mm to 51.6 ± 5.5 mM in fish >300 mm. The findings of this study support the hypothesis that the ontogenetic diet shift of K. sydneyanus is accompanied by an increase in the diversity of intestinal microbial symbionts capable of degrading refractory algal metabolites into short-chain fatty acids, which can then be assimilated by the host.     

The consequences of lifetime and evolutionary exposure to toxic prey: changes in avoidance behaviour through ontogeny

Responses to novel threats (e.g. invasive species) can involve genetic changes or plastic shifts in phenotype. There is controversy over the relative importance of these processes for species survival of such perturbations, but we are realizing they are not mutually exclusive. Native eastern fence lizards (Sceloporus undulatus) have adapted to top-down predation pressure imposed by the invasive red imported fire ant (Solenopsis invicta) via changes in adult (but not juvenile) lizard antipredator behaviour. Here, we examine the largely ignored, but potentially equally important, bottom-up effect of fire ants as toxic prey for lizards. We test how fire ant consumption (or avoidance) is affected by lifetime (via plasticity) and evolutionary (via natural selection) exposure to fire ants by comparing field-caught and laboratory-reared lizards, respectively, from fire ant-invaded and uninvaded populations. More naive juveniles from invaded populations ate fire ants than did adults, reflecting a natural ontogenetic dietary shift away from ants. Laboratory-reared lizards from the invaded site were less likely to eat fire ants than were those from the uninvaded site, suggesting a potential evolutionary shift in feeding behaviour. Lifetime and evolutionary exposure interacted across ontogeny, however, and field-caught lizards from the invaded site exhibited opposite ontogenetic trends; adults were more likely to eat fire ants than were juveniles. Our results suggest that plastic and evolutionary processes may both play important roles in permitting species survival of novel threats. We further reveal how complex interactions can shape adaptive responses to multimodal impacts imposed by invaders: in our system, fire ants impose stronger bottom-up selection than top-down selection, with each selection regime changing differently across lizard ontogeny.     

Ontogenetic antagonism–mutualism coupling: perspectives on resilience of stage‐structured communities

Organisms typically change their diets ontogenetically. Recent studies have shown that an ontogenetic diet shift undermines the resilience of stage‐structured food webs. Here, we study the integration of stage‐structured food‐web theory into theory of hybrid community (i.e. mixture of different interaction types), considering that not only diet but also interaction type often changes because of ontogenetic niche shift (e.g. the metamorphosis of pollinating insects, in which juveniles and adults are herbivores and pollinators, respectively). We developed and mathematically analysed a one‐consumer two‐resource model in which juvenile and adult consumers utilise different resources as antagonists and mutualists, respectively. Model analyses illustrated that the consumer either goes extinct or coexists with the resources depending on the initial condition when the resources have low carrying capacities while their community dynamics always converge to a single steady state when the resources have high carrying capacities. These dynamic features are different from those of the corresponding purely antagonistic module in previous studies, in which the consumer always goes extinct for low resource carrying capacities while the dynamics converge to either juvenile‐dominated or adult‐dominated state depending on the initial conditions for high resource carrying capacities. Taken together, we can suggest that ontogenetic antagonism–mutualism coupling is stabilising in that it increases the potential for species coexistence in unproductive environments while improving community resilience in productive environments. Further, these effects are generally robust to interaction nonlinearity. Beyond the previous concern of the instability in stage‐structured food‐webs, our results suggest that antagonism–mutualism coupling can play a crucial role in stabilising stage‐structured hybrid (e.g. plant–animal) communities under environmental changes. The present study represents an important first step in understanding how interaction type diversity can mediate the dynamics of stage‐structured communities.     

From antagonistic larvae to mutualistic adults: coevolution of diet niches within life cycles

Population structures largely affect higher levels of organization (community structure, ecosystem functioning), especially when involving ontogenetic changes in habitat or diet. Along life cycles, partners and interaction type may change: for instance Lepidopterans are herbivores as larvae and pollinators as adults. To understand variations in diet niche from larvae to adults, we model a community of two plant species and one stage‐structured insect species consuming plants as juvenile and pollinating them as adult. We model the coevolution of juvenile and adult diet specialization using adaptive dynamics to investigate when one should expect niche partitioning or niche overlap among life stages. We consider ecological and evolutionary implications for the coexistence of species. As predicted based on indirect effects among stages, we find that juvenile diet evolution increases niche overlap and favours the coexistence of plants, while the evolution of the adult diet decreases niche overlap and reduces plant coexistence, because of positive feedbacks emerging from the mutualistic interaction.     

Chemosensory responses to foods by an herbivorous acrodont lizard, <Emphasis Type="Italic">Uromastyx aegyptius</Emphasis>

Actively foraging lizards use the lingual-vomeronasal system to identify prey by chemical cues, but insectivorous ambush foragers do not. The major clade Iguania includes numerous herbivores and omnivores; among them, two iguanid and one agamine species identify plant and animal foods by tongue flicking, and data suggest that the leiolepidine Uromastyx acanthinurus may as well. We conducted experiments on chemosensory response to food by the herbivorous U. aegyptius. When chemical stimuli were presented on cotton balls in experiment 1, the lizards exhibited greater responsiveness (tongue-flick attack scores) to chemical stimuli from crickets and a preferred plant food (dandelion flowers) than from deionized water. When chemical stimuli were on ceramic tiles in experiment 2, the lizards exhibited greater total tongue flicks to cricket stimuli than to any other stimuli, and to dandelion than to deionized water. Lizards bit more frequently in response to cricket and dandelion cues than to stimuli from a nonpreferred plant (carrot) and deionized water. Tongue-flick attack scores were greater in response to cricket and dandelion stimuli than to carrot or water stimuli. These findings are consistent with the hypothesis that herbivores, even those having ambush-foraging ancestors, use chemical cues to identify potential foods. The data support the hypothesis that chemosensory responses correspond to diet. Because most lizards are generalist predators, studies of herbivorous species can provide important information on possible evolutionary adjustment of chemosensory response to dietary shifts. Electronic Publication     

Ontogeny, diet shifts, and nutrient stoichiometry in fish

Most stoichiometric models do not consider the importance of ontogenetic changes in body nutrient composition and excretion rates. We quantified ontogenetic variation in stoichiometry and diet in gizzard shad, Dorosoma cepedianum , an omnivorous fish with a pronounced ontogenetic diet shift; and zebrafish, Danio rerio, grown in the lab with a constant diet. In both species, body stoichiometry varied considerably along the life cycle. Larval gizzard shad and zebrafish had higher molar C:P and N:P ratios than larger fish. Variation in body nutrient ratios was driven mainly by body P, which increased with size. Gizzard shad body calcium content was highly correlated with P content, indicating that ontogenetic P variation is associated with bone formation. Similar trends in body stoichiometry of zebrafish, grown under constant diet in the laboratory, suggest that ontogeny (e.g. bone formation) and not diet shift is the main factor affecting fish body stoichiometry in larval and juvenile stages. The N:P ratio of nutrient excretion also varied ontogenetically in gizzard shad, but the decline from larvae to juveniles appears to be largely associated with variation in the N:P of alternative food resources (zooplankton vs detritus) rather than by fish body N:P. Furthermore, the N:P ratio of larval gizzard shad excretion appears to be driven more by the N:P ratio at which individuals allocate nutrients to growth, more so than static body N:P, further illustrating the need to consider ontogenetic variation. Our results thus show that fish exhibit considerable ontogenetic variation in body stoichiometry, driven by an inherent increase in the relative allocation of P to bones, whereas ontogenetic variation in excretion N:P ratio of gizzard shad is driven more by variation in food N:P than by body N:P.     

Adult Male Density Influences Juvenile Microhabitat Use in a Territorial Lizard

Habitat choice often has strong effects on performance and fitness. For many animals, optimal habitats differ across age or size classes, and individuals shift habitat use through ontogeny. Although many studies document ontogenetic habitat shifts for various taxa, most are observational and do not identify the causal factor of size‐specific habitat variation. Field observations of the brown anole lizard (Anolis sagrei) show that juveniles perch on shorter and thinner vegetation than adults. We hypothesized that this variation is due to adult males forcing smaller juveniles to less preferred habitat. To test this assertion, we manipulated adult male densities in mesh enclosures with artificial trees to examine the response of juvenile microhabitat choice. We found that adult male density had strong effects on juvenile perch height, perch width, and substrate use, suggesting that age‐class competition contributes to the observed ontogenetic differences in habitat choice. We also found that time of day significantly affected juvenile perch height and substrate use. In many cases, our results suggest that juveniles distance themselves from adults using different microhabitats from those used in our control ‘no‐adult’ treatment. However, these findings were often body size dependent and varied depending upon time of day. This study highlights the complexity of juvenile perching behavior and demonstrates the role of intraspecific interactions in shaping habitat use by juvenile animals.     

Profile of Digestive Enzymes Activity During Early Development of Featherback <Emphasis Type="Italic">Chitala chitala</Emphasis> (Hamilton, 1822)

The ontogenetic development of the key digestive enzymes of featherback Chitala chitala was assayed during the early development. Amylase, lipase, trypsin and chymotrypsin activities were detected on 1 days after hatch (DAH), (during endotrophic stage) but pepsin activity was detected on 12 DAH (exotrophic phase) correlated with the improvement of gastric secretion at the beginning of flexion stage. The gradual shift of alkaline protease activity (trypsin and chymotrypsin) to more efficient acidic digestion indicated a change in the digestive physiology as a result of metamorphosis acquiring the juvenile characteristic during the postflexion or extrophic phase. The initial high level of amylase (i.e. from 1 DAH to 9 DAH) during endotrophic and endoexotrophic feeding stage could be better explained as a result of programmed gene expression. But a constant decrease in activity after the 12 DAH i.e. on the onset of flexion and exotrophic stage of featherback fish in the present study may be possibly due to the developmental changes in the gut morphology and increased protein level in the tissue. These fluctuations of the enzymatic activities in featherback larvae reflect the ability of the fish to adapt with the diet during ontogenetic shift. This information can lead to the possibility of developing an age specific formulated feed for intensive farming of this new candidate species.     

Does intraspecific niche partitioning in a native predator influence its response to an invasion by a toxic prey species?

Abstract The introduced and highly toxic cane toad (Bufo marinus) is rapidly spreading across northern Australia where it may affect populations of large terrestrial vertebrate predators. The ecological impact of cane toads will depend upon the diets, foraging modes and habitat use of native predators, and their feeding responses to cane toads. However, intraspecific niche partitioning may influence the degree of vulnerability of predators to toxic prey, as well as the time course of the impact of alien invaders on native species. We studied the diet of the northern death adder Acanthophis praelongus and their feeding responses to cane toads. In the laboratory, death adders from all size classes and sexes readily consumed frogs and cane toads. Diets of free ranging A. praelongus from the Adelaide River floodplain were more heterogeneous. Juvenile snakes ate mainly frogs (39% of prey items) and small scincid lizards (43%). Both sexes displayed an ontogenetic dietary shift from lizards to mammals, but adult males fed on frogs (49%) and mammals (39%) whereas adult females (which grew larger than males) fed mainly on mammals (91%) and occasionally, frogs (9%). Feeding rates and body condition of adult snakes varied temporally and tracked fluctuations in prey availability. These results suggest that cane toads may negatively affect populations of northern death adders in the Darwin region. However, we predict that different size and sex classes of A. praelongus will experience differential mortality rates over different timescales. The initial invasion of large toads may affect adult males, but juveniles may be unaffected until juvenile toads appear the following year, and major affects on adult female death adders may be delayed until annual rainfall fluctuations reduce the availability of alternative (rodent) prey.     

Effects of alternate reef states on coral reef fish habitat associations

The present study describes ontogenetic shifts in habitat use for 15 species of coral reef fish at Rangiroa Atoll, French Polynesia. The distribution of fish in different habitats at three ontogenetic stages (new settler, juvenile, and adult) was investigated in coral-dominated and algal-dominated sites at two reefs (fringing reef and inner reef of motu). Three main ontogenetic patterns in habitat use were identified: (1) species that did not change habitats between new settler and juvenile life stages (60% of species) or between juvenile and adult stages (55% of species—no ontogenetic shift); (2) species that changed habitats at different ontogenetic stages (for the transition “new settler to juvenile stage”: 15% of species; for the transition “juvenile to adult stage”: 20% of species); and (3) species that increased the number of habitats they used over ontogeny (for the transition “new settler to juvenile stage”: 25% of species; for the transition “juvenile to adult stage”: 25% of species). Moreover, the majority of studied species (53%) showed a spatial variability in their ontogenetic pattern of habitat use according to alternate reef states (coral reef vs algal reef), suggesting that reef state can influence the dynamics of habitat associations in coral reef fish.     

Digestion, growth and reproductive performance of the zoophytophagous rove beetle Philonthus quisquiliarius (Coleoptera: Staphylinidae) fed on animal and plant based diets

The zoophytophagous feeding habits of larvae and adults of the rove beetle, Philonthus quisquiliarius (Gyllenhal) (Coleoptera: Staphylinidae), are reported for the first time. This study evaluates the effects of different feeding regimes on its growth and reproductive performance (i.e., larval growth, adult weight gain, consumption, fecundity and fertility) and digestive physiology. Larvae presented similar growth rates when fed on living animal or on green plant material for 48h. However, higher consumption rates and lower efficiencies of conversion of digested matter to body mass were obtained when leaves were consumed. Adults presented also positive weight gains regardless of the food consumed (plant or animal material). Interestingly, the highest weight gain rate and efficiency of digestion resulted when adults fed on a rearing diet containing nutrients from both animals and plants. Moreover, we have found negative effects upon P. quisquiliarius fecundity and fertility when supplemental plant nutrients were removed from the optimum rearing diet. Physiological adaptations to allow trophic switching between predation and phytophagy have been found, such as the higher ratio of α-amylase activity to protease activity to deal with the inverted protein-carbohydrate ratio of plant versus animal tissues. Furthermore, this species has an arsenal of digestive proteases whose activity is affected by the type of diet ingested. All together, our results suggest that P. quisquiliarius needs certain nutrients, which are obtained only from plant material. This knowledge will help to understand the complex trophic interactions that occur in agroecosystems.     

Seasonally varying marine influences on the coastal ecosystem detected through molecular gut analysis

Terrestrial predators on marine shores benefit from the inflow of organisms and matter from the marine ecosystem, often causing very high predator densities and indirectly affecting the abundance of other prey species on shores. This indirect effect may be particularly strong if predators shift diets between seasons. We therefore quantified the seasonal variation in diet of two wolf spider species that dominate the shoreline predator community, using molecular gut content analyses with general primers to detect the full prey range. Across the season, spider diets changed, with predominantly terrestrial prey from May until July and predominantly marine prey (mainly chironomids) from August until October. This pattern coincided with a change in the spider age and size structure, and prey abundance data and resource selection analyses suggest that the higher consumption of chironomids during autumn is due to an ontogenetic diet shift rather than to variation in prey abundance. The analyses suggested that small dipterans with a weak flight capacity, such as Chironomidae, Sphaeroceridae, Scatopsidae and Ephydridae, were overrepresented in the gut of small juvenile spiders during autumn, whereas larger, more robust prey, such as Lepidoptera, Anthomyidae and Dolichopodidae, were overrepresented in the diet of adult spiders during spring. The effect of the inflow may be that the survival and growth of juvenile spiders is higher in areas with high chironomid abundances, leading to higher densities of adult spiders and higher predation rates on the terrestrial prey next spring.