The foraging ecology of sympatric marine fish in the genus Embiotoca (Embiotocidae): Importance of foraging behavior in prey size selection

Summary Two locally sympatric temperate marine reef fishes, Embiotoca jacksoni and E. lateralis (Embiotocidae), have high taxonomic similarity in diets. Subdivision of gammarid amphipods, their principal prey, was found. E. jacksoni took more tubicolous gammarid amphipods whereas E. lateralis consumed mostly free-living individuals. The species differed considerably with respect to between-individual variability in taxonomic compositions of their diets. Each E. jacksoni closely resembled other conspecifics in this regard while individual E. lateralis displayed very high between-fish variation. The principal interspecific difference in fish diets concerned the sizes of prey items taken. E. jacksoni ate small but very common items and the mean prey weight in their guts did not differ from random collections of available prey. E. lateralis concentrated on large, rarer sizes such that the average prey weight in their guts was much heavier than available or in the diet of E. jacksoni of the same length. Disparate foraging behaviors was a much better indicator of the relative differences in diets of these two fishes than was external fish morphology. E. jacksoni, which can winnow prey items from unwanted debris, was a relatively indiscriminant forager. E. lateralis did not winnow but actively searched for prey. This species was a much more discriminating forager, but displayed much variability in foraging behavior.     

The foraging ecology of two pairs of congeneric demersal fish species: importance of morphological characteristics in prey selection

The feeding habits of two sympatric species pairs of demersal fish ( Mullus barbatus-Mullus surmuletus, Serranus cabrilla-Serranus hepatus ) which occupy the shallow coastal area (25–30 m) in Iraklion Bay were investigated from samples collected on a monthly basis (August 1990 to August 1992). Stomach content analyses revealed that all of them were carnivores, feeding mainly on benthic invertebrates, and that each species consumed a narrow range of prey species with no significant dietary overlap. The morphology of their feeding apparatus was compared to examine the effect of any morphological differences on food selection and resource partitioning between the fish species. The species could be distinguished on the basis of the size of their mouth gape, the number of gill rakers and the length of their intestine. This study shows that each species pair follows a different strategy segregating along food niche dimensions. In particular, M. barbatus and M. surmuletus segregate their feeding niche consuming different prey taxa with similar sizes whereas S. cabrilla and S. hepatus differ considerably with respect to the degree to which prey species contribute to their diets coupled with differences in mean prey sizes.     

Patterns of food and microhabitat resource use by two benthic gobiid fishes

The mechanisms responsible for different patterns of habitat use by two benthic gobiid fishes, Acentrogobius sp. 1 and A. sp. 2, which displayed identical food use but resided in shallow and deep zones of coarse and fine sediments, respectively, in Lake Hamana, Shizuoka Prefecture, Japan, were investigated by field removal and laboratory sediment-grain size selection experimentation. Following field manipulation, involving removal of both species, the distribution patterns of each were found to be similar in both control and manipulated quadrats, suggesting their differential habitat use patterns may be the outcome not of competition but of differing preferences for habitat characteristics. Results of a sediment-grain size selection experiment in the laboratory suggested a weak preference of A. sp. 2 for fine sediment, while A. sp. 1 showed no grain size preference. The observed difference in the distribution patterns between these two species, therefore, may possibly have resulted from, at least in part, a combination of differences in their preference for sediment grain size and other habitat characteristics such as water depth.     

Chimpanzees' choices of prey between two sympatric species ofMacrotermes in the Campo Animal Reserve,Cameroon

Details are presented relating to chimpanzees' choices between two sympatric species of termites,Macrotermes lillijeborgi andM. vitrialatus, as food in the Campo Animal Reserve, southwest Cameroon, West Africa. An attempt was made to determine the various factors that affected such choices. The two species of termites seemed to have almost the same value in terms of ecological factors. However, chimpanzees fed almost exclusivelyonM. lillijeborgi, using digging sticks and fisching probes, during the study period which extended from the end of August to the middle of January, with their feeding activity showing peak at the beginning of the rainy season. By contrast,M. vitrialatus was rarely eaten in spite of the ease with which such prey could be obtained, namely, by desctruction of termite mounds by hand, without the need for tools. The reason that the chimpanzees discriminated between the two species of termite cannot be explained in terms of ecological factors such as size of prey, seasonal differences in termite activity, etc. Sticks used as tools were fairly uniform in size and character, mainly because of physical constraints related to the structure of termite mounds, and the brush-like ends of sticks seemed to be incidentaly byproducts of the chimpanzee's choice of plant species. Ecological factors could provide chimpanzees with a basis for the use of some kind of tool and help them modify it, while other factors, for example, something akin to leisure or the chimpanzee's interest in use of a tool, could provide an opportunity for inventing some tool-using behavior or for maintaining such behavior. These different factors, not being exclusive of one another, might affect the invention and maintenance of tool using-behavior at different phases. It is possible that chimpanzee's choice of prey may not always be the most efficient or appropriate in a given ecological situation.     

The effect of prey type and density on the foraging efficiency of juvenile perch, (Perca fluviatilis)

The foraging efficiency of juvenile perch (Perca fluviatilis), feeding on two types of prey, was studied in laboratory experiments. Waterfleas (Daphnia magna) and phantom midge larvae (Chaoborus flavicans) were offered in a range of densities, either separately or combined. Perch fed more efficiently on each prey type separately than when both were mixed. Foraging efficiency decreased with an increase of mixed prey density with both prey types present in equal numbers, but also when the proportion of Chaoborus increased. This could be caused by the existence of different hunting techniques, each of which is fully efficient in the presence of one prey type only. In the presence of two prey types, the predator constantly has to switch from one hunting technique to another.     

Does gonad structure reflect sexual pattern in all gobiid fishes?

Synopsis In immature and adult females of protogynous gobies, small distinctive masses of cells associated with the ovarian wall develop into testis-associated glandular structures during sex change. These precursive accessory gonadal structures, or pAGS, have been found in females of known protogynous goby species, but not among gonochoric goby species, suggesting that their presence can be used as a species-specific indicator of protogyny within the family. However, a detailed examination of a developmental series of ovaries in two gonochoric species,Gobiosoma illecebrosum andG. saucrum, revealed the presence of a gonadal feature previously thought to be restricted to protogynous gobies. Among immature females of both species, pAGS-like structures having a similar appearance and placement as functional pAGS of protogynous gobies were found. In femaleG. illecebrosum, the size of these structures among immatures progressively decreased with maturation and were absent in all but the smallest adult females. A similar pattern was evident in a small sample ofG. saucrum. Population demography based on field collections showed thatG. illecebrosum exhibits sex ratios and male and female size-frequency distributions typical of gonochores and laboratory experiments indicated that final sexual identity was unaffected by social environment during the juvenile period. Thus, the presence of pAGS in juvenile femaleG. illecebrosum is not related to an ability to change sex at that ontogenic interval. Whether the transient pAGS observed here are vestiges of an ancestral protogynous condition is unknown. Based on their presence among immatures in two gonochore gobies, however, only the presence of pAGS in adult females should be used to predict protogyny among gobies.     

Optimal foraging and feeding mode shifts in fishes

Synopsis Most optimal foraging models for fishes are based on particulate feeding behavior. But many obligate planktivores also filter zooplankton. I suggest that feeding mode shifts (e.g. from particulate feeding to filtering) may be predictable from the costs and benefits of foraging in various modes. Quantitative examples of feeding mode shifts in three species of fishes (northern anchovy, pacific mackerel and alewife) from 3 different families support this hypothesis. Feeding mode shifts seem to depend on relative profitability of each mode, but improvements in model predictions will need to include the effects of spatial and temporal patchiness on encounter rates of prey of various sizes.     

Swan foraging shapes spatial distribution of two submerged plants,favouring the preferred prey species

Compared to terrestrial environments, grazing intensity on belowground plant parts may be particularly strong in aquatic environments, which may have great effects on plant-community structure. We observed that the submerged macrophyte, Potamogeton pectinatus, which mainly reproduces with tubers, often grows at intermediate water depth and that P. perfoliatus, which mainly reproduces with rhizomes and turions, grows in either shallow or deep water. One mechanism behind this distributional pattern may be that swans prefer to feed on P. pectinatus tubers at intermediate water depths. We hypothesised that when swans feed on tubers in the sediment, P. perfoliatus rhizomes and turions may be damaged by the uprooting, whereas the small round tubers of P. pectinatus that escaped herbivory may be more tolerant to this bioturbation. In spring 2000, we transplanted P. perfoliatus rhizomes into a P. pectinatus stand and followed growth in plots protected and unprotected, respectively, from bird foraging. Although swan foraging reduced tuber biomass in unprotected plots, leading to lower P. pectinatus density in spring 2001, this species grew well both in protected and unprotected plots later that summer. In contrast, swan grazing had a dramatic negative effect on P. perfoliatus that persisted throughout the summer of 2001, with close to no plants in the unprotected plots and high densities in the protected plots. Our results demonstrate that herbivorous waterbirds may play a crucial role in the distribution and prevalence of specific plant species. Furthermore, since their grazing benefitted their preferred food source, the interaction between swans and P. pectinatus may be classified as ecologically mutualistic.     

A new genus and two new species of gobiid fishes (Perciformes) from the Chagos Archipelago,Central Indian Ocean

Synopsis A recent (1979) expedition to the Chagos Archipelago resulted in the collection of about 40 new taxa of fishes. A new genus,Trimmatom, and two new species,T. nanus andT. offucius, are described here. The new genus is characterized by having all pelvic-fin rays simple (unbranched), a scaleless body, no head pores, a wide gill opening extending anteroventrally to below the eye, and hypurals 1 and 2 fused to the complex formed by the fusion of the ural centrum and hypurals 3 and 4.T. nanuss andT. offucius are differentiated on the basis of fin ray counts and colour pattern.T. nanus is the smallest vertebrate yet to be described. Mature females with ovaries full of eggs are 8–10 mm in standard length.     

Seasonal and ontogenetic variations in resource use by two sympatric Arctic charr morphs

The study compares the resource utilization of two sympatric Arctic charr morphs over an annual period in a subarctic lake. The two morphs are reproductively isolated in time and place of spawning, and are referred to as the littoral and profundal morphs (L-morph and P-morph) according to their spawning habitats. Fish were sampled monthly (ice-free season) or bimonthly (winter) using gillnets in the main lake habitats. The spatial range of the P-morph was restricted to the profundal zone throughout the whole annual period. The L-morph in contrast utilized all main habitats, exhibiting distinct seasonal and ontogenetic variations in habitat distribution. In the spring, the whole L-morph population was located along the bottom profile of the lake, in profundal and littoral habitats. During summer and autumn, habitat segregation occurred between different life-stages, juveniles mainly utilizing the profundal, pre-adults the pelagic and adult fishes the littoral zone. During winter the whole population was assembled in the littoral habitat. The L-morph also had large seasonal and ontogenetic variations in their feeding ecology, with littoral zoobenthos, zooplankton and surface insects being important prey. The P-morph had a narrower diet niche mainly consisting of chironomid larvae and other profundal zoobenthos. Hence, the two Arctic charr morphs exhibited a consistent resource differentiation during all annual seasons and throughout their life cycles, except for a dietary overlap between P-morph and juvenile L-morph charr in the profundal during summer. The findings are discussed in relation to resource polymorphism and incipient speciation.     

Species differences in substrate use and diet between sympatric galagos in two Kenyan coastal forests

Two sympatric species of galago,Galago zanzibaricus andG. garnettii, differed significantly in diet (judged mainly by fecal analysis) and substrate use at each of two coastal forest sites in Kenya. The larger-bodiedG. garnettii tended to use branches of a bigger diameter and to be found higher in the canopy than the smallerG. zanzibaricus. The latter species spent most of its time in the thick undergrowth. The larger species ate more fruit than the smaller. There were differences between the species in the type of invertebrate prey remains found in the feces. It is suggested that the differences in habitat use were influenced by the different body sizes of the two galagos, which so enabled these closely related species to coexist. Order of authorship is alphabetical.     

Prey selection by the larvae of three species of littoral fishes on natural zooplankton assemblages

The prey selection of larvae of three common littoral fish species (pike, Esox lucius; roach,Rutilus rutilus; and three-spined stickleback, Gasterosteus aculeatus) was studied experimentally in the laboratory by using natural zooplankton assemblages. Zooplankton prey was offered at four different concentrations to study the functional responses of the planktivores. The diets of pike and sticklebacks were formed mainly of copepod juveniles and adults, which dominated the prey communities, although sticklebacks ate also cladocerans. The diet of roach larvae consisted of rotifers, cladocerans and copepods, without prey selection, in equal proportions indicating a more omnivorous diet. All fish larvae were able to feed selectively although in sticklebacks prey selection was less pronounced. Pike and roach larvae preferred large prey to smaller prey types. Patterns of prey selection are discussed in the context of size-selection theory and apparent vs. true selection.     

Aggregation of polyphagous predators in response to multiple prey: ladybirds (Coleoptera: Coccinellidae) foraging in alfalfa

The spatial distribution of polyphagous predators may often reflect the integration of aggregative responses to local densities of multiple species of prey, and as such may have consequences for the indirect linkages among the prey sharing these predators. In a factorial field experiment in which we manipulated local prey densities within a field of alfalfa in Utah (USA), we tested whether aphidophagous ladybirds would aggregate not only in response to their primary aphid prey, but also in response to an abundant alternative prey, the alfalfa weevil (Hypera postica [Gyllenhal]). Native North American ladybirds (primarily Hippodamia convergens Guerin and H. quinquesignata quinquesignata [Kirby]) responded only to spatial variation in aphid density. In contrast, the introduced ladybird, Coccinella septempunctata L., aggregated also at local concentrations of the weevil late in the experiment when weevil density was high and aphid density was relatively low throughout all experimental plots. The results support the hypothesis that C. septempunctata is more responsive than are native ladybirds to the availability of alternative prey in alfalfa, which may account in part for the displacement of native ladybirds from alfalfa by the introduced species as aphid numbers have declined. The differing responses of the native and introduced ladybirds to spatial patterns of the alternative prey underscore the importance of extending the study of predator aggregation to understand better how polyphagous predators distribute themselves in response to spatial patterns of multiple species of potential prey.     

Diet and prey selection in larval and juvenile Japanese anchovy Engraulisjaponicus in Ariake Bay, Japan

We studied the diet of larval and juvenile Japanese anchovy Engraulis japonicus in the upper Ariake Bay, Japan. Diet was analyzed by examining the digestive tracts; feeding intensity, proportion of empty guts, and prey selectivity were calculated. Anchovy density was negatively influenced by temperature and positively by salinity and prey density. Diet was dominated by Acartia omorii, which was positively selected with two other copepods, Calanus sinicus and Pseudodiaptomus marinus. In contrast, Oithona davisae was highly dominant in the environment but was absent in anchovy guts; thus, this copepod was negatively selected, with two others, Tortanus derjugini and Sinocalanus sinensis. Overall, larger prey were positively selected and smaller ones were negatively selected; value of electivity index correlated negatively with prey size. Larvae [<18 mm of standard length (SL)] showed significantly lower feeding intensities and higher rates of empty guts than juveniles (≥18 mm SL). In juveniles, feeding intensity increased steadily as the fish grew in size, with a corresponding reduction in empty guts. Feeding intensity correlated positively and empty gut correlated negatively with fish size. We suggest that larger prey are important diets for postlarval Japanese anchovy in Ariake Bay.     

Kinematics of swimming in two burrowing anguilliform fishes

Anguilliform or eel-like fishes are typically bottom dwellers, some of which are specialized burrowers. Although specializations for burrowing are predicted to affect the kinematics of swimming, it remains unknown to what extent this is actually the case. Here we examine swimming kinematics and efficiency of two burrowing anguilliform species, Pisodonophis boro and Heteroconger hassi, with different degrees of specialization for burrowing. Our data suggest that differences in the swimming kinematics may indeed be related to the differences in burrowing specialization and style between both species. The resemblance between the swimming kinematics of P. boro and previously published data for Anguilla anguilla and Anguilla rostrata may be linked with the relatively limited burrowing specialization of P. boro and suggests an overall stereotypy in anguilliform forward-swimming patterns. The body of H. hassi, in contrast, is more specialized for tail-first burrowing and backward swimming bears a striking resemblance to the backward burrowing motions observed in this species. These motions differ significantly from backward swimming in Anguilla and in P. boro. The kinematics of forward swimming are, however, comparable across species. Thus, our data suggest that specializations for burrowing may affect swimming kinematics in anguilliform fishes, but also that forward swimming and burrowing are not necessarily incompatible. Future studies comparing the kinematics and mechanics of burrowing in these and other anguilliform fishes are needed to better understand how specializations for burrowing constrain backward swimming in H. hassi.     

The influence of prey behaviour on prey selection of the carnivorous plant Utricularia vulgaris

The mechanisms underlying differential prey selection of two microcrustaceans by the common bladderwort (Utricularia vulgaris) were studied in the laboratory. Functional response experiments with single prey showed that Utricularia had a higher attack rate coefficient and a longer handling time coefficient with the cladoceran Polyphemus pediculus than with the cyclopoid copepod Eucyclops serrulatus. Observation of predation rate, defined as number of prey eaten per unit time, from direct behavioural observation on single prey species, showed a higher predation rate with Polyphemus than on Eucyclops, at low prey densities. The opposite pattern was found at high prey density. When the two prey were presented simultaneously to the predator, Eucyclops was preferred over Polyphemus. Results from the situation with two prey and some of the results from the direct behavioural observations support field data on the diet of Utricularia, which shows that cyclopoid copepods are selected more frequently than Polyphemus.     

Morphological variation in a larval salamander: dietary induction of plasticity in head shape

We examined diet-dependent plasticity in head shape in larvae of the eastern long-toed salamander, Ambystoma macrodactylum columbianum. Larvae in some populations of this species exhibit trophic polymorphism, with some individuals possessing exaggerated trophic features characteristic of a cannibalistic morphology in larval Ambystoma; e.g. a disproportionately broad head and hypertrophied vomerine teeth. We hypothesized that 1) head shape variation results from feeding upon different types of prey and that 2) cannibal morphs are induced by consumption of conspecifics. To induce variation, we fed three groups of larvae different diets: 1) brine shrimp nauplii only; 2) nauplii plus anuran tadpoles; 3) nauplii, tadpoles and conspecific larval salamanders. Comparisons of size (mass)-adjusted means revealed that this manipulation of diet induced significant variation in six measures of head shape, but not in the area of the vomerine tooth patch. For five of the six head traits, larvae that ate tadpoles and brine shrimp nauplii developed significantly broader, longer and deeper heads than did larvae that only ate brine shrimp nauplii. The ingestion of conspecifics, in addition to nauplii and tadpoles, significantly altered two head traits (interocular-width and head depth), compared to larvae only fed nauplii and tadpoles. Canonical discriminant function analysis detected two statistically reliable canonical variables: head depth was most highly associated with the first canonical variable, whereas three measures of head width (at the jaws, gills and eyes) and interocular width were most highly associated with the second canonical variable. Despite this diet-enhanced morphological variation, there was no indication that any of the three types of diet (including conspecific prey) induced the exaggerated trophic features of the cannibal morph in this species. These results illustrate that ingestion of different types of prey contributes to plasticity in head shape, but that some other proximate cue(s), either alone or in combination with diet variation, is essential to induce the extremes of trophic polymorphism in this species.     

Consumption of introduced prey by native predators: Argentine ants and pit-building ant lions

When populations of native predators are subsidized by numerically dominant introduced species, the structure of food webs can be greatly altered. Surprisingly little is known, however, about the general factors that influence whether or not native predators consume introduced species. To learn more about this issue, we examined how native pit-building ant lions (Myrmeleon) are affected by Argentine ant (Linepithema humile) invasions in coastal southern California. Compared to areas without L. humile, invaded areas contained few native ant species and were deficient in medium-sized and large bodied native ants. Based on these differences, we predicted that Argentine ants would negatively affect ant lion larvae. Contrary to this expectation, observational surveys and laboratory growth rate experiments revealed that Myrmeleon were heavier, had longer mandibles, and grew more quickly when their main ant prey were Argentine ants rather than native ants. Moreover, a field transplant experiment indicated that growth rates and pupal weights were not statistically different for larval ant lions reared in invaded areas compared to those reared in uninvaded areas. Argentine ants were also highly susceptible to capture by larval Myrmeleon. The species-level traits that presumably make Argentine ant workers susceptible to capture by larval ant lions—small size and high activity levels—appear to be the same characteristics that make them unsuitable prey for vertebrate predators, such as horned lizards. These results underscore the difficulties in predicting whether or not numerically dominant introduced species serve as prey for native predators.     

Control of foraging behavior of individuals within an ecosystem context: the clam Macoma balthica,flow environment,and siphon-cropping fishes

Macoma balthica (L.), an abundant clam, ubiquitous in temperate estuaries across the North Atlantic, is known to practice both alternative basic modes of feeding available to seafloor invertebrates. It either holds its feeding organ, the siphon, at a fixed position just above the sediment surface to filter out food particles suspended in the overlying water or else extends and moves its siphon around to vacuum up deposited food particles on the sediment surface. Previous laboratory experiments have established an understanding of the role of current flow in dictating the choice of whether suspension or deposit feeding will be used by marine invertebrates with the facultative flexibility to choose. Faster flows imply greater fluxes of suspended particles so that the energetic rewards of suspension feeding are enhanced. Slower flows imply reduced renewal rates of suspended foods in the bottom boundary layers and enhanced deposition of food particles on the seafloor so that a switch to deposit feeding is favored. Like early optimal foraging theory, this understanding is based on energetic considerations alone without incorporation of broader implications of how population interactions such as predation and competition influence individual foraging behavior. Feeding behavior of Macoma balthica is influenced in the Neuse River estuary by both hydrodynamics and siphon-cropping by juvenile demersal fishes. Under conditions of identical concentrations of suspended particulates in the water column and organic contents of surface sediments, Macoma exhibited much higher levels of deposit feeding where currents were slower. In addition, exclosure and fish inclosure experiments demonstrated that juvenile demersal fishes influence feeding behavior of Macoma by cropping exposed siphons and inducing reduction in deposit-feeding activity. Effects of croppers were substantial in early to midsummer, when juvenile fish abundances were greatest in trawl samples from this estuarine nursery and before the growing fish exhibited ontogenetic changes in diet away from early concentration on bivalve siphons. Field experiments in which siphon-cropping fish were caged at varying distances off the bottom failed to detect any effective behavioral avoidance by Macoma of cropping in response to proximity of fish. One might have hypothesized that under high risk of cropping, Macoma would switch to suspension feeding and away from deposit feeding, the feeding method entailing more risk of losses to croppers because of greater siphon activity and greater extension of siphons on the sediment surface. Consequently, partial predation by siphon-cropping fishes greatly reduces deposit-feeding activity by Macoma balthica during summer as an apparent direct effect of disfigurement and reduction of siphons, the organ required for efficient deposit feeding. Information on current flows alone would not suffice to predict feeding behavior of this marine invertebrate: the influence of partial predation must also be included.