Feeding biology of the flatfish Citharichthys spilopterus (Bothidae) in a tropical estuary of Mexico

The flatfish Citharichthys spilopterus Günther 1862 is the most common bothid in the coastal lagoons of the Gulf of Mexico. The objective of the present study was to describe the trophic biology of this species in the tropical coastal lagoon of Tampamachoco, Mexico. For the diet analysis, we used multivariate discriminant analysis and trophic niche breadth. The morphological analysis showed that the features of the bucco-pharyngeal cavity, the large stomach and the short intestine of this flatfish relate to a carnivorous habit. The overall diet showed that this species is a third-order consumer, feeding mainly on fish (52%, mainly gobies) and crustaceans (36%, mostly decapods). Diet and trophic niche breadth showed no significant differences between sexes (P > 0.1). By contrast, there were significant ontogenetic differences in the diet and trophic niche breadth, where larger C. spilopterus were almost entirely piscivorous and showed the narrowest trophic niche. As the flatfish grew in size there was a trend toward the consumption of larger prey. The importance of copepods and peracarids correlated inversely with flatfish size (P < 0.001), but fish prey correlated directly to flatfish size (P < 0.001). Likewise, there was an inverse significant correlation between niche trophic breadth and flatfish size (P < 0.005). There were also significant seasonal differences in the diet (P < 0.02), related to the availability and vulnerability of prey in the lagoon during the dry and rainy seasons. However, the trophic niche showed no significant differences between seasons. Finally, we discuss the advantages of discriminant analysis applied to evaluate differences among diets of fish groups when compared with other bivariate and multivariate techniques.     

The food and feeding habits of Hydrocynus forskahlii Cuvier and Hydrocynus brevis Günther in Lake Kainji, Nigeria

The diets and modes of feeding of Hydrocynus forskahlii Cuvier and Hydrocynus brevis Günther are described. Both species are mainly piscivorous in Lake Kainji. H. forskahlii feeds largely upon small clupeids though very large specimens prey more heavily upon the characid Alestes baremose. H. brevis preys upon a wide variety of species by swallowing them whole but also attacks large fish and bites pieces from them. Sexual dimorphism in H. brevis is described. The effects of predation upon the fish stock of Lake Kainji by Hydrocynus species are discussed.     

A comparison of the mouth morphology of three co-occurring species of atherinid

The mouth morphology of three species of atherinids, which feed at different levels in the water column (benthos, plankton and water surface) were compared. These three species, which all grow to less than 100 mm in length, inhabit the shallows (<2m) of Wilson Inlet, a temperate south-western Australian estuary. The species could be distinguished primarily on the basis of the extent to which they can protrude their jaws. Thus, whereas Leptatherina presbyteroides feeds highest in the water column, including at the water surface on terrestrial insects, and has the most protrusible jaws, Atherinosonia elongata feeds predominantly at or near the benthos and has the least protrusible jaws. Leptatherina wallacei , which ingests prey from the plankton and near the benthos, is intermediate in the degree to which it can protrude its jaws. Other characters of the three species which are associated with feeding, such as the number of gill rakers and the size of teeth, show consistent trends with the degree of jaw protrusion in relation to the type of prey consumed.     

Experimental evidence for trait utility of gill raker number in adaptive radiation of a north temperate fish

North temperate fish in post‐glacial lakes are textbook examples for rapid parallel adaptive radiation into multiple trophic specialists within individual lakes. Speciation repeatedly proceeded along the benthic–limnetic habitat axis, and benthic–limnetic sister species diverge in the number of gill rakers. Yet, the utility of different numbers of gill rakers for consuming benthic vs. limnetic food has only very rarely been experimentally demonstrated. We bred and raised families of a benthic–limnetic species pair of whitefish under common garden conditions to test whether these species (i) show heritable differentiation in feeding efficiency on zooplankton, and (ii) whether variation in feeding efficiency is predicted by variation in gill raker numbers. We used zooplankton of three different size classes to investigate prey size dependency of divergence in feeding efficiency and to investigate the effect strength of variation in the number of gill rakers. Our results show strong interspecific differences in feeding efficiency. These differences are largest when fish were tested with the smallest zooplankton. Importantly, feeding efficiency is significantly positively correlated with the number of gill rakers when using small zooplankton, also when species identity is statistically controlled for. Our results support the hypothesis that a larger number of gill rakers are of adaptive significance for feeding on zooplankton and provide one of the first experimental demonstrations of trait utility of gill raker number when fish feed on zooplankton. These results are consistent with the suggested importance of divergent selection driven feeding adaptation during adaptive radiation of fish in post‐glacial lakes.     

Specific features of structure and functioning of gill-jaw apparatus of whitefish <Emphasis Type="Italic">Coregonus lavaretus pravdinellus</Emphasis>

The food spectrum of multirakered whitefish Coregonus lavaretus pravdinellus was studied. It was shown that C. lavaretus pravdinellus feeds mainly on large zooplankton; however, it also consumes nauplii whose average sizes are considerably smaller than the intergill distance. This fact counts in favor of the view of the active functioning of gill rakers. The type of feeding of C. lavaretus is related to a set of morphological characters of the gill-jaw apparatus and the head, such as the number of gill rakers, the length of the upper and lower jaws, mouth position, and the shape and slope of the snout tip area. A great number of gill rakers allows coregonids to use as food a wider size range of prey.     

Specialization in the dentition of the southern graylings—genus Prototroctes (Galaxioidei: Prototroctidae)

The southern hemisphere graylings, Prototroctes oxyrhynchus Günther (1870) (New Zealand) and P. maraena Günther (1864) (Australia) have highly specialized dentition. The alimentary canal is longer than in other salmoniform fishes. These characters, and observations on feeding, suggest that Prototroctes is omnivorous.     

Gill raker structure and selective predation on zooplankton by particulate feeding fish

The relationship between the gill raker structure of planktivorous fish (number, distance between gill rakers and length) and selective feeding on different species and size classes of zooplankton was studied. Gill raker structure was measured for brown trout Salmo trutta , Arctic charr Salvelinus alpinus , whitefish Coregonus lavaretus , roach Rutilus rutilus , bleak Alburnus alburnus , and three-spined stickleback Gasterosteus aculeatus . All species are facultative planktivorous fish and occur commonly in Scandinavian lakes. The effect of gill raker structure was studied by comparing prey found in fish stomachs with the availability of zooplankton from several lakes. Gill raker length and distance were significantly correlated with fish length. Although gill raker structure differed among species, all fish species selected the larger zooplankters. The minimum size of cladoceran species found in fish stomachs was much smaller than the distance between gill rakers. Despite great differences in gill raker spacing, the minimum size ingested of Daphnia galeata and Bosmina longispina was similar for all predators. The hypothesis that small zooplankton are strained and retained by the gill rakers in particulate feeding planktivorous fish, particularly in salmonids and roach, is rejected.     

Trophic polymorphism in introduced pumpkinseed (<Emphasis Type="Italic">Lepomis gibbosus</Emphasis>) inhabiting Iberian reservoirs

Introduced pumpkinseed in Iberian reservoirs display marked external morphological differentiation along two simultaneous dimensions of flow and trophic structure. We assessed the degree of internal morphological differentiation using gill rakers, pharyngeal jaws and the levator posterior muscle among pumpkinseeds occupying four different habitats and determined whether prey consumption accounted for any discernible differences in feeding structures among ecomorphs. Results showed significant differentiation by habitat based on pharyngeal muscle and jaw dimensions in all study reservoirs, with pelagic pumpkinseeds having smaller jaws than littoral pumpkinseeds in four of the five reservoirs. Gill rakers, however, differentiated morphs in only three of the five reservoirs, corresponding to differences in zooplankton consumption among pelagic and littoral individuals in those reservoirs. Based on all internal morphological traits, greater divergence was seen along the littoral-pelagic trophic axis in the lacustrine zones of reservoirs compared to the fluvial zone. Overall differences noted in internal morphology are likely the result of phenotypic plasticity; the ability of this species to readily adapt to changing physical environments may explain the success of the pumpkinseed in its introduced range.     

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.     

The feeding ecology of three species of Caribbean angelfishes (family Pomacanthidae)

Synopsis The foraging behavior and associated morphology of the feeding apparatus of three sympatric species of angelfishes, Holacanthus tricolor, Pomacanthus arcuatus and Pomacanthus paru were studied at St. Croix, U.S. Virgin Islands. All three had overlapping diets, consisting of algae and numerous species of sponges. The two Pomacanthus species also fed on gorgonians. The morphology of the dentition, jaws and gill rakers was similar in all three species. Male Holacanthus tricolor defended territories overlapping the foraging areas of two to four females. Within the male's territory, females defended smaller territories against other females of the same size, but tolerated smaller females. In contrast, both Pomacanthus spp. formed pairs which defended intraspecific feeding territories.     

The food, feeding habits and feeding structures of the whiting species Sillago sihama (Forsskål) and Sillago analis Whitley from Townsville, North Queensland, Australia

The diet, feeding habits and feeding structures of 519 Sillago sihama and 493 S. analis specimens collected from beaches and estuaries in the Townsville region are described. The species are carnivores on a wide range of benthic, epibenthic and planktonic prey. Both undergo size related dietary shifts; S. sihama from predominantly planktonic crustaceans in fish of less than 80 mm t.l . to polychaetes, penaeid and brachyuran crustaceans and molluscs at larger sizes; S. analis from a mixture of small Mesodesma eltanae and amphipods at sizes of less than 80 mm t.l . to predominantly M. eltanae and small quantities of penaeids and brachyuran crabs in larger fish. The dietary shift was not associated with migration from a nursery area.
While the species showed considerable overlap in the types of prey utilized, there were significant differences in the relative importance of shared components in their diets. The major difference was in the relative importance of the bivalve mollusc, M. eltanae. This was the dominant food of S. analis but of secondary importance in S. sihama. Associated with this basic difference in diet was a specialization of the pharyngeal dentition in S. analis to molariform crushing plates and a reduction of the gill rakers to vestigial knobs. In S. sihama , the pharyngeal teeth are sharp and pointed and the gill rakers normal.
The feeding activity of S. analis , but not S. sihama , is limited by the tidal cycle, a factor which may be a form of temporal partitioning.     

Ecomorphology of the eyes and skull in zooplanktivorous labrid fishes

Zooplanktivory is one of the most distinct trophic niches in coral reef fishes, and a number of skull traits are widely recognized as being adaptations for feeding in midwater on small planktonic prey. Previous studies have concluded that zooplanktivores have larger eyes for sharper visual acuity, reduced mouth structures to match small prey sizes, and longer gill rakers to help retain captured prey. We tested these three traditional hypotheses plus two novel adaptive hypotheses in labrids, a clade of very diverse coral reef fishes that show multiple independent evolutionary origins of zooplanktivory. Using phylogenetic comparative methods with a data set from 21 species, we failed to find larger eyes in three independent transitions to zooplanktivory. Instead, an impression of large eyes may be caused by a size reduction of the anterior facial region. However, two zooplanktivores (Clepticus parrae and Halichoeres pictus) possess several features interpreted as adaptations to zooplankton feeding, namely large lens diameters relative to eye axial length, round pupil shape, and long gill rakers. The third zooplanktivore in our analysis, Cirrhilabrus solorensis, lacks all above features. It remains unclear whether Cirrhilabrus shows optical specializations for capturing planktonic prey. Our results support the prediction that increased visual acuity is adaptive for zooplanktivory, but in labrids increases in eye size are apparently not part of the evolutionary response.     

Interspecific variations in the surface ultrastructure of the gills of freshwater mullets

SEM studies were made on the gills of freshwater mullets,Rhinomugil corsula andSicamugil cascasia, to correlate surface ultrastructure of various gill units with their probable functions. Two types of lamellated gill rakers of the former fish are suited for plankton feeding and the short, stumpy and transversely beaded gill rakers of the latter reflect the varied food and feeding habit of the fish.R. corsula has numerous mucous glands on the epithelium covering the gill arch and gill filaments,S. cascasia has fewer. In accordance with the differences in the density and distribution of the mucous glands, the microridged epithelial cells also show variations in their architectural plan. In both species the epithelium of the secondary lamellae is smooth, probably an adaptation for better gaseous exchange.     

Morphological evidence of correlational selection and ecological segregation between dextral and sinistral forms in a polymorphic flatfish, Platichthys stellatus

Phenotypic polymorphisms in natural systems are often maintained by ecological selection, but only if niche segregation between morphs exists. Polymorphism for eyed-side direction is rare among the approximately 700 species of flatfish (Pleuronectiformes), and the evolutionary mechanisms that maintain it are unknown. Platichthys stellatus (starry flounder) is a polymorphic pleuronectid flatfish exhibiting large, clinal variation in proportion of left-eyed (sinistral) morphs, from 50% in California to 100% in Japan. Here I examined multiple traits related to swimming and foraging performance between sinistral and dextral morphs of P. stellatus from 12 sites to investigate if the two morphs differ in ways that may affect function and ecology. Direction of body asymmetry was correlated with several other characters: on an average, dextral morphs had longer, wider caudal peduncles, shorter snouts and fewer gill rakers than sinistral morphs. Although the differences were small in magnitude, they were consistent in direction across samples, implying that dextral and sinistral starry flounder may be targeting different prey types. Morphological differences between morphs were greatest in samples where the chances of competitive interactions between them were the greatest. These results suggest that the two morphs are not ecologically identical, may represent a rare example of divergent selection maintaining polymorphism of asymmetric forms, and that correlational selection between body asymmetry and other characters may be driven by competitive interactions between sinistral and dextral flatfish. This study is one of very few that demonstrates the ecological significance of direction in a species with polymorphic asymmetric forms.     

Structure,development and function of the branchial sieve of the common bream,Abramis brama,white bream,Blicca bjoerkna and roach,Rutilus rutilus

Synopsis The filter feeding organ of cyprinid fishes is the branchial sieve, which consists of a mesh formed by gill rakers and tiny channels on the gill arches. In order to establish its possible role during growth we measured the following morphological gill raker parameters over a range of sizes in three cyprinid fishes, bream, white bream and roach: inter raker distance, bony raker length, raker width, cushion length and channel width. At any given standard length common bream has the largest inter raker distance, roach the lowest and white bream is intermediate. In the comb model of filter feeding the inter raker distance is considered to be a direct measure of the mesh size and retention ability (= minimal size of prey that can be retained) of a filter. For the three species under study there is a conflict between the comb model and experimental data on particle retention. Lammens et al. (1987) found that common bream has a large retention ability whereas roach and white bream have a much smaller one. A new model, the channel model (Hoogenboezem et al. 1991) has been developed for common bream; in this model the lateral gill rakers can regulate the mesh size of the medial channels on the other side of the gill slit. The present data indicate that this model is not appropriate for white bream and roach. At any given standard length white bream and roach only reach 70% of the raker length of common bream, which means that in this model the gill slits should to be very narrow during filter feeding. The gill rakers consist of a bony raker and a fleshy cushion. The bony rakers have a rather long needle-like part outside the cushion in bream, but not in white bream and roach which have blunt gill rakers. Blunt gill rakers are not suited to reduce the diameter of the medial channels. The comb model seems more appropriate for white bream and roach, but doubts about the validity of this simple model remain. The sum of the areas of the medial channels is an approximation of the area through which water flows in the filter. This channel area therefore gives an impression of the capacity or flow rate of the filter. With this capacity estimation and an estimation of energy consumption we calculated an energy ratio of filter feeding. The energy ratio decreases with increasing standard length with an exponent close to the expected exponent of -0.40. The energy ratio is highest in bream, intermediate in white bream and lowest in roach.     

Prey capture behavior and kinematics of the Atlantic cownose ray, Rhinoptera bonasus

The structurally reinforced jaws of the cownose ray, Rhinoptera bonasus testify to this species' durophagous diet of mollusks, but seem ill-suited to the behaviors necessary for excavating such prey. This study explores this discordance by investigating the prey excavation and capture kinematics of R. bonasus. Based on the basal suction feeding mechanism in this group of fishes, we hypothesized a hydraulic method of excavation. As expected, prey capture kinematics of R. bonasus show marked differences relative to other elasmobranchs, relating to prey excavation and use of the cephalic lobes (modified anterior pectoral fin extensions unique to derived myliobatiform rays). Prey are excavated by repeated opening and closing of the jaws to fluidize surrounding sand. The food item is then enclosed laterally by the depressed cephalic lobes, which transport it toward the mouth for ingestion by inertial suction. Unlike in most sharks, upper jaw protrusion and mandibular depression are simultaneous. During food capture, the ray's spiracle, mouth, and gill slit movements are timed such that water enters only the mouth (e.g., the spiracle closes prior to prey capture and reopens immediately following). Indigestible parts are then hydraulically winnowed from edible prey portions, by mouth movements similar to those used in excavation, and ejected through the mouth. The unique sensory/manipulatory capabilities of the cephalic lobes, as well as the cownose ray's hydraulic excavation/winnowing behaviors and suction feeding, make this species an effective benthic predator, despite its epibenthic lifestyle.     

Diet overlap among flatfish species in the southern North Sea

The diets of two non-commercial flatfish species (solenette Buglossidium luteum and scaldfish Arnoglossus laterna) and two commercial flatfish species (dab Limanda limanda and plaice Pleuronectes platessa) were compared in a study area in the German Bight (southern North Sea) to investigate prey-resource partitioning between these species. The diets of A. laterna and B. luteum mainly comprised crustaceans (harpacticoids, amphipods, cumaceans and decapods), whereas the diet of L. limanda and P. platessa consisted mainly of polychaetes. The Schoener index, calculated for different fish size classes between these flatfish species, showed a biologically significant diet overlap between small-sized L. limanda and P. platessa and B. luteum and A. laterna, using similar prey resources of smaller prey (e.g. amphipods, harpacticoids and juvenile bivalves). In contrast, with increasing body size, a change in the diet of L. limanda and P. platessa towards larger prey occurred (e.g. polychaetes and decapods), resulting in low diet overlap values with B. luteum and A. laterna. Due to these size-related differences in resource use, it is assumed that there is reduced interspecific competition for prey between larger L. limanda and P. platessa and both non-commercial flatfishes, probably facilitating resource partitioning within the same area. In contrast, smaller L. limanda and P. platessa may compete directly for the same prey resources with B. luteum and A. laterna. Furthermore, prey availability of most important prey items of the studied flatfishes was relatively low in the study area. Therefore, increasing abundances of B. luteum and A. laterna in the southern North Sea since the late 1980s, owing to fishing effects and climate change, might affect the population dynamics of L. limanda and P. platessa.     

Adaptation in the jaws of flatfish (Pleuronectiformes)

The structure and mechanisms of the jaws of 18 species of flatfish have been investigated. Clear adaptations to different modes of feeding were found. The mechanisms of the jaws of Soleidae, Cynoglossidae, and Rhombosoleinae are highly specialized and the representatives of the two latter groups have some interesting jaw muscles of doubtful homology.     

Making a master filterer: Ontogeny of specialized filtering plates in silver carp (Hypophthalmichthys molitrix)

Filter feeding fishes possess several morphological adaptations necessary to capture and concentrate small particulate matter from the water column. Filter feeding teleosts typically employ elongated and tightly packed gill rakers with secondary bony or epithelial modifications that increase filtering efficiency. The gill rakers of Hypophthalmichthys molitrix, silver carp, are anatomically distinct from and more complex than the filtering apparatus of other teleostean fishes. The silver carp filtering apparatus is composed of biserial, fused filtering plates used to capture particles ranging in size from 4 to 80 μm. Early in ontogeny, at 15–25 mm standard length (SL), silver carp gill rakers are reminiscent of other more stereotypical teleostean rakers, characterized by individual lanceolate rakers that are tightly packed along the entirety of the branchial arches. At 30 mm SL, secondary epithelial projections and concomitant dermal ossification begin to stitch together individual gill rakers. During later juvenile stages, dermal bone further modifies the individual gill rakers and creates a bony scaffold that supports the now fully fused and porous epithelium. By adulthood, the stitching of bone and complete fusion of the overlying epithelium creates rigid filtering plates with morphologically distinct faces. The inner face of the plates is organized into a net‐like matrix while the outer face has a sponge‐like appearance comprised of differently sized pores. Here, we present morphological data from an ontogenetic series of the filtering apparatus within silver carp. These data inform hypotheses regarding both how these gill raker plates may have evolved from a more basal condition, as well as how this novel architecture allows this species to feed on exceedingly small phytoplankton, particles that represent a greater filtering challenge to the typical anatomy of the gill rakers of fishes.