Ecomorphological Hypotheses Research Articles

Ontogenetic shifts in body form in the bull shark Carcharhinus leucas.

Recent studies have uncovered mosaic patterns of allometric and isometric growth underlying ontogenetic shifts in the body form of elasmobranch species (shark and rays). It is thought that shifts in trophic and spatial ecology through ontogeny drive these morphological changes; however, additional hypotheses relating to developmental constraints have also been posed. The bull shark (Carcharhinus leucas) is a large-bodied coastal shark that exhibits strong ontogenetic shifts in trophic and spatial ecology. In this study, we utilise a large data set covering a large number of morphological structures to reveal ontogenetic shifts in the body form of C. leucas, stratifying analyses by sex and size classes to provide fine-scale, more ecomorphologically relevant results. Our results indicate shifts in functional demands across the body through ontogeny, driven by selective pressures relating to trophic and spatial ecology driving the evolution of allometry. We also find significant differences in scaling trends between life stages, and between the sexes, highlighting the importance of utilising large, diverse datasets that can be stratified in this way to improve our understanding of elasmobranch morphological evolution. Ultimately, we discuss the implications of these results for existing ecomorphological hypotheses regarding the evolution of specific morphological structures, and pose novel hypotheses where relevant.

Reconstructing the origin and early evolution of the snake brain.

Snakes represent one-eighth of terrestrial vertebrate diversity, encompassing various lifestyles, ecologies, and morphologies. However, the ecological origins and early evolution of snakes are controversial topics in biology. To address the paucity of well-preserved fossils and the caveats of osteological traits for reconstructing snake evolution, we applied a different ecomorphological hypothesis based on high-definition brain reconstructions of extant Squamata. Our predictive models revealed a burrowing lifestyle with opportunistic behavior at the origin of crown snakes, reflecting a complex ancestral mosaic brain pattern. These findings emphasize the importance of quantitatively tracking the phenotypic diversification of soft tissues-including the accurate definition of intact brain morphological traits such as the cerebellum-in understanding snake evolution and vertebrate paleobiology. Furthermore, our study highlights the power of combining extant and extinct species, soft tissue reconstructions, and osteological traits in tracing the deep evolution of not only snakes but also other groups where fossil data are scarce.

The preorbital depression and recess of antiarch placoderms (jawed stem-gnathostomes) revisited from an ontogenetic (saltatory) point of view

ABSTRACT During fish growth, short periods of rapid changes (morphological, behavioral, physiological, ecological), or thresholds, are interspersed with longer periods of slower development, the steps. This growth pattern is known as saltatory ontogeny. Thresholds delimit main developmental stages (embryo, larva, juvenile, adult) and are periods where modifications can lead to new life-histories, and ultimately to evolutionary novelties. We sought to determine whether saltatory ontogeny could be recognized in a jawed stem-gnathostome, the Late Devonian antiarch placoderm Bothriolepis canadensis, using an extensive size series (220 specimens: 5–220 mm in armor length). The small specimens of this series reveal a previously undocumented immature feature, a preorbital depression on the premedian and lateral plates of the dermal headshield. This depression is a plesiomorphic condition reputed to be absent in highly nested antiarchs such as B. canadensis, which possess instead a preorbital recess. Our objectives were: to describe the ontogenetic morphological changes of the preorbital area in B. canadensis, including the timing of disappearance of the depression using binomial logistic regressions, and to quantify growth allometries of the premedian and lateral plates, with linear and segmented regressions. We found significant segmented allometric patterns in the premedian plate, suggesting saltatory ontogeny in B. canadensis. Moreover, segmented patterns were congruent with the ontogenetic loss of the preorbital depression. An ecomorphological hypothesis is proposed to explain these simultaneous morphological and morphometric changes in B. canadensis. A similar hypothesis is extrapolated to interpret the innovation of the preorbital recess and loss of the preorbital depression during the antiarch phylogeny.

Phylogeny, function and ecology in the deep evolutionary history of the mammalian forelimb.

Mammals are the only living members of the larger clade Synapsida, which has a fossil record spanning 320 Ma. Despite the fact that much of the ecological diversity of mammals has been considered in the light of limb morphology, the ecological comparability of mammals to their fossil forerunners has not been critically assessed. Because of the wide use of limb morphology in testing ecomorphological hypothesis about extinct tetrapods, we sought: (i) to estimate when in synapsid history, modern mammals become analogues for predicting fossil ecologies; (ii) to document examples of ecomorphological convergence; and (iii) to compare the functional solutions of distinct synapsid radiations. We quantitatively compared the forelimb shapes of the multiple fossil synapsid radiations to a broad sample of extant Mammalia representing a variety of divergent locomotor ecologies. Our results indicate that each synapsid radiation explored different areas of morphospace and arrived at functional solutions that reflected their distinctive ancestral morphologies. This work counters the narrative of non-mammalian synapsid forelimb evolution as a linear progression towards more mammalian morphologies. Instead, a disparate array of early-evolving shapes subsequently contracted towards more mammal-like forms.

Incorporating Trace Fossils when Exploring the Interplay of Form and Function in the Crocodyliform Feeding Apparatus

Many crocodylians, both living and extinct, fill the semi‐aquatic ambush predator niche, and their prey preferences roughly can be predicted based on a combination of snout shape, tooth morphology, and body size. Among extant crocodylians, slender‐snouted taxa such as Gavialis gangeticus and Tomistoma schlegelii generally prefer smaller‐bodied prey, whereas the broader‐snouted taxa divide into generalists like Crocodylus niloticus and macro‐generalists like Alligator mississippiensis, who will eat a diversity of prey items including animals that are as big or even bigger than themselves. Large‐bodied members of these generalist ecomorphotypes generate the highest bite forces observed in modern taxa and are often apex predators in their ecosystems. The availability of a robust phylogenetic hypothesis for crocodylians makes them an excellent clade with which to explore these types of ecomorphological hypotheses, which link form and function in the feeding apparatus within an evolutionary framework. The resulting synthesis of extant and extinct datasets has provided both exciting new insights, but also some complications. The addition of extinct taxa into studies of morphological disparity, both within the crown group and deeper in the crocodyliform evolutionary tree, introduce a diverse array of snout shapes and associated niches that are not represented in the surviving members of the clade (e.g. fully marine, fully terrestrial, durophagous, omnivorous, and even possibly herbivorous forms). Though hypotheses regarding diet in these diverse groups are based on craniodental morphology, indirect evidence, in the form of trace fossils, provides an independent means of testing those predictions. Gut contents are extremely rare in this clade because of the highly destructive digestion of crocodyliforms, and coprolites, though comparatively common, contain remains that are so degraded as to be largely unidentifiable and uninformative. Isotopic and dental microwear studies can provide broad patterns of diet. However, for detailed, direct evidence of crocodyliform feeding, bite marks are a particularly useful source of direct evidence with which to explore patterns of diet in this clade. A recent, concentrated effort to characterize modern crocodylian bite marks has resulted in the identification of novel patterns and diagnostic traces associated with feeding behavior in that clade. These trace fossils provide critical insights into diet, feeding strategy, and even intraspecific competition in crocodyliforms and their extinct relatives. By integrating these ichnological data with traditional dietary hypotheses based on morphology and phylogeny of extinct groups, a more complete, and oftentimes more complex and realistic, concept of past ecosystems and trophic structures can be explored.Support or Funding InformationFunding provided by the National Science Foundation, the Leakey Foundation, and National Geographic.

Comparative analysis of the shape and size of the middle ear cavity of turtles reveals no correlation with habitat ecology

The middle ear of turtles differs from other reptiles in being separated into two distinct compartments. Several ideas have been proposed as to why the middle ear is compartmentalized in turtles, most suggesting a relationship with underwater hearing. Extant turtle species span fully marine to strictly terrestrial habitats, and ecomorphological hypotheses of turtle hearing predict that this should correlate with variation in the structure of the middle ear due to differences in the fluid properties of water and air. We investigate the shape and size of the air-filled middle ear cavity of 56 extant turtles using 3D data and phylogenetic comparative analysis to test for correlations between habitat preferences and the shape and size of the middle ear cavity. Only weak correlations are found between middle ear cavity size and ecology, with aquatic taxa having proportionally smaller cavity volumes. The middle ear cavity of turtles exhibits high shape diversity among species, but we found no relationship between this shape variation and ecology. Surprisingly, the estimated acoustic transformer ratio, a key functional parameter of impedance-matching ears in vertebrates, also shows no relation to habitat preferences (aquatic/terrestrial) in turtles. We suggest that middle ear cavity shape may be controlled by factors unrelated to hearing, such as the spatial demands of surrounding cranial structures. A review of the fossil record suggests that the modern turtle ear evolved during the Early to Middle Jurassic in stem turtles broadly adapted to freshwater and terrestrial settings. This, combined with our finding that evolutionary transitions between habitats caused only weak evolutionary changes in middle ear structure, suggests that tympanic hearing in turtles evolved as a compromise between subaerial and underwater hearing.

A gyroscopic advantage: phylogenetic patterns of compensatory movements in frogs

Head and eye compensatory movements known as vestibulo-ocular and vestibulo-cervical reflexes are essential to stay orientated in space while moving. We have used a previously developed methodology focused on the detailed mathematical description of head compensatory movements in frogs without the need for any surgical procedures on the examined specimens. Our comparative study comprising 35 species of frogs from different phylogenetic backgrounds revealed species-specific head compensatory abilities ensuring gaze stabilization. Moreover, we found a strong phylogenetic signal highlighting the great ability of compensatory head movements in families of Pyxicephalidae and Rhacophoridae from the Natatanura group. By contrast, families of Dendrobatidae and Microhylidae exhibited only poor or no head compensatory movements. Contrary to our expectation, the results did not corroborate an ecomorphological hypothesis anticipating a close relationship between ecological parameters and the head compensatory movements. We did not find any positive association between more complex (3D structured, arboreal or aquatic) habitats or more saltatory behavior and elevated abilities of head compensatory movements. Moreover, we found compensatory movements in most basal Archeobatrachia, giving an indication of common ancestry of these abilities in frogs that are variously pronounced in particular families. We hypothesize that the uncovered proper gaze stabilization during locomotion provided by the higher head compensatory abilities can improve or even enable visual perception of the prey. We interpret this completely novel finding as a possible gyroscopic advantage in a foraging context. We discuss putative consequences of such advanced neuromotor skills for diversification and ecological success of the Natatanura group.

2D or Not 2D? Testing the Utility of 2D Vs. 3D Landmark Data in Geometric Morphometrics of the Sculpin Subfamily Oligocottinae (Pisces; Cottoidea).

We contrast 2D vs. 3D landmark-based geometric morphometrics in the fish subfamily Oligocottinae by using 3D landmarks from CT-generated models and comparing the morphospace of the 3D landmarks to one based on 2D landmarks from images. The 2D and 3D shape variables capture common patterns across taxa, such that the pairwise Procrustes distances among taxa correspond and the trends captured by principal component analysis are similar in the xy plane. We use the two sets of landmarks to test several ecomorphological hypotheses from the literature. Both 2D and 3D data reject the hypothesis that head shape correlates significantly with the depth at which a species is commonly found. However, in taxa where shape variation in the z-axis is high, the 2D shape variables show sufficiently strong distortion to influence the outcome of the hypothesis tests regarding the relationship between mouth size and feeding ecology. Only the 3D data support previous studies which showed that large mouth sizes correlate positively with high percentages of elusive prey in the diet. When used to test for morphological divergence, 3D data show no evidence of divergence, while 2D data show that one clade of oligocottines has diverged from all others. This clade shows the greatest degree of z-axis body depth within Oligocottinae, and we conclude that the inability of the 2D approach to capture this lateral body depth causes the incongruence between 2D and 3D analyses. Anat Rec, 301:806-818, 2018. © 2017 Wiley Periodicals, Inc.

Ecomorphological relationships among four Characiformes fish species in a tropical reservoir in South-eastern Brazil

The aim of this study was to assess the ecomorphological patterns and diet of four Characiformes fish species in a poorly physically structured tropical reservoir. We tested the hypothesis that body shape and diet are associated, because environmental pressure acts on the phenotype, selecting traits according to the available resources. Ten ecomorphological attributes of 45 individuals of each species - Astyanax cf. bimaculatus (Linnaeus, 1758), Astyanax parahybae Eigenmann, 1908, Oligosarcus hepsetus (Cuvier, 1829), and Metynnis maculatus (Kner, 1858) - , collected between February and November 2003, were analyzed, and the patterns were assessed using Principal Components Analysis (PCA). Diet similarity among fish species was assessed using cluster analysis on feeding index. The first two axes from PCA explained 61.73% of the total variance, with the first axis being positively correlated with the compression index and relative height, whereas the second axis was positively correlated with the pectoral fin aspect. Two well-defined trophic groups, one herbivorous/specialist (M. maculatus) and the other formed by two omnivorous/generalist (A. cf. bimaculatus, A. parahybae) and one insectivorous-piscivorous (O. hepsetus) were revealed by the cluster analysis. Astyanax. cf. bimaculatus and A. parahybae differed. The first has comparatively greater relative height, relative length of the caudal peduncle and lower caudal peduncle compression index. However, we did not detect a close correspondence between diet and body shape in the reservoir, and inferred that the ecomorphological hypothesis of a close relationship between body shape and diet in altered systems could be not effective.

Habitat partitioning, habits and convergence among coastal nektonic fish species from the São Sebastião Channel, southeastern Brazil

Based on a fish survey and preliminary underwater observations, 17 "morphotypes" were identified that characterize the morphological diversity found within 27 nektonic fish species sampled at São Sebastião Channel. Such "morphotypes" were studied using an ecomorphological approach, with the intention to investigate similarities and differences in shape and habits. Underwater field observations were also performed, to verify if the lifestyle of these species, such as vertical occupation of the water column and the habitat use, are in accordance with their distribution in the morphospace. The results, complemented with data from scientific literature on the taxonomy and phylogenies of these species, allowed discussing some of the typical cases of convergent and divergent evolution. Some of the ecomorphological clusters had no phylogenetic support although this is probably due to the environmental conditions in which theirs members have evolved. The body shape and fins positions of a fish clearly influence its ecological performance and habitat use, corroborating the ecomorphological hypothesis on the intimate link between phenotype and ecology.

Pliocene colonization and adaptive radiations in Australia and New Guinea (Sahul): Multilocus systematics of the old endemic rodents (Muroidea: Murinae)

The old endemic rodents of Australia and New Guinea (Sahul) represent one or more large adaptive radiations including novel morphological adaptations to aquatic, arboreal, hopping, and arid ecologies. Four tribes recognized among the Sahulian old endemics (Hydromini, Conilurini, Anisomyini, and Uromyini) reflect distinct biogeographic and ecomorphological hypotheses about diversification within the Old Endemics. We present the first character-based phylogeny of the Sahulian Old Endemic rodents with broad sampling, nested within a broader phylogeny of the Murinae. We estimated phylogenies from >2500 nucleotides of mtDNA sequence and >9500 nucleotides from six autosomal nuclear loci, for individual genes and for the full concatenated data using parsimony, likelihood, and Bayesian methods. Our results strongly supported monophyly of the group and its sister relationship to the Philippine old endemics of the Chrotomys division. Most striking was the rapid diversification after the Late Miocene or Early Pliocene colonization of New Guinea from the west, consistent with a single colonization of the Sahulian continent. That was followed 2–3 My later by a second adaptive radiation resulting from one or more colonizations of Australia. Monophyly was not supported for the Anisomyini or the Conilurini but was for the Uromyini nested within the Conilurini and for the Hydromyini. Conflict among gene phylogenies was weak, and support for the consensus topology increased with more (even conflicting) data.

Resource Use and Morphology of Two Sympatric Japalura Lizards (Iguania: Agamidae)

We investigated the underlying mechanism of coexistence of two sympatric Japalura lizards (Japalura swinhonis and Japalura polygonata xanthostoma). We examined the differences in resource use along three resource dimensions: time, space, and diet. Specifically we tested two ecomorphological hypotheses. Correspondence exists between: perch habitat and limb morphology, and diet and head morphology. The species with shorter forelimbs would perch on narrower surface. The species with larger heads would consume larger or tougher prey. The results showed that the two species did not divide resources by temporal partitioning. Spatially, the two species used similar macrohabitats. However, female J. swinhonis, perching lower and on narrower surfaces, differed from other groups in perch habitats. However, male J. swinhonis and J. p. xanthostoma, despite size and shape differences, were similar in perch habitat. In addition, high diet overlap was detected both intra- and interspecifically despite the presence of significant differences in head sizes. In conclusion, male J. swinhonis and J. p. xanthostoma used largely overlapping resources and therefore, might be potential competitors if resources are limited. We suggest that there was a trade-off between fighting ability and maneuverability because of morphological differences that allowed the coexistence of the two species. Although male J. swinhonis, being larger, may have better fighting ability, J. p. xanthostoma could maneuver around tree trunks and branches to search for prey and avoid fighting more easily.

Testing the ecomorphological hypothesis in a headwater riffles fish assemblage of the rio São Francisco, southeastern Brazil

The ecomorphology of 14 fish species resident in a headwater riffles area of the São Francisco river, southeastern Brasil, was analyzed and combined with diet and feeding behavior data, previously obtained by us. The three larger species groups formed in the ecomorphological analysis were found to reflect primarily microhabitat occupation in the following manner: a) nektonic characids with compressed bodies, lateral eyes and lateral pectoral fins, with diurnal and opportunistic feeding habits (Astyanax rivularis, Bryconamericus stramineus, and Bryconamericus sp.); b) nektobenthic characiforms and siluriforms with fusiform bodies and expanded pectoral fins, including sit-and-wait characidiins, predators of aquatic insect larvae (Characidium fasciatum and Ch.zebra), as well as the algae grazing parodontids (Apareiodon ibitiensis and Parodon hilarii), and also the heptapterid and trichomycterid catfishes that practice substrate speculation and feed on benthic aquatic insect larvae (Cetopsorhamdia iheringi, Imparfinis minutus, Rhamdia quelen, and Trichomycterus sp.); c) benthic species with depressed bodies, suctorial oral discs, dorsal eyes, and horizontal pectoral fins, represented by the periphytivorous loricariid catfishes (Hisonotus sp., Harttia sp., and Hypostomus garmani). Correlation between diet and general morphology was not significant in our analysis, unless when the analyzed set included only nektonic and benthic species, indicating that the lack of correlation between these factors is most pronounced in the group of nektobenthic species. The unequivocal case of morphological convergence found between the nektobenthic Characidiinae and Parodontidae is a clear example of the integration between phylogenetic information and ecomorphology, and provides a way to objectively identify cases of morphological and adaptive convergence and divergence. Furthermore, the general congruence between the ecomorphological results and the independently obtained ecological data about the analyzed fish species in their natural environment seems to be strong evidence in favor of the proposed predictive capabilities of the ecomorphological hypothesis.

Dietary-morphological relationships of fishes in Liangzi Lake, China

Species in Liangzi Lake were clustered into four trophic groups: Hemiramphus kurumeus and Hemiculter bleekeri bleekeri fed predominantly on terrestrial insects; Carassius auratus auratus and Abbottina rivularis on non-animal food; Hypseleotris swinhonis, Ctenogobius giurinus, Pseudorasbora parva and Toxabramis swinhonis on cladocerans or copepods; Culterichthys erythropterus on decapod shrimps. Gut length, mouth width, mouth height, gill raker length and gill raker spacing, varied widely among species. With the exception of three species pairs (H. swinhonis, C. glurinus; C. erythropterus, H. kurumeus; T. swinhonis, H. bleekeri bleekeri), principal components analysis of morphological variables revealed over-dispersion of species. Canonical correspondence analysis of dietary and morphological data revealed five significant dietary-morphological correlations. The first three roots explained > 85% of the total variance. The first root reflected mainly the relationship of gut length to non-animal feud, with an increase in gut length associated with an increase in non-animal food. The second root was influenced strongly by the relationship of the gill raker spacing to consumption of copepods, with an increase in gill raker spacing associated positively with copepods in the diet. The third root was influenced by the relationship of mouth gape to consumption of fish and decapod shrimps, with an increase in mouth gape associated with more fish and decapod shrimps in the diet. These significant dietary-morphological relationships supported the eco-morphological hypotheses that fish morphology influence food use, and morphological variation is important in determining ecological segregation of co-existing fish species. (C) 2001 The Fisheries Society of the British Isles.

Morphological-dietary relationships within two assemblages of marine demersal fishes

In this study we have used morphological characters related to feeding and prey capture and dietary data to investigate the trophic organization within two assemblages of marine demersal fishes. Morphological and dietary disparities within fish assemblages were estimated from species similarities based on Euclidean distances plus species projections on the principal axes from multivariate analyses. The analyses of the morphological variables indicated that species in each assemblage comprised morphologically distinct groups strongly influenced by trophically linked characters. Stomach content analyses revealed that fish species in each assemblage were classified into three basic feeding groups: polychaete-shrimp feeders, crab feeders and fish feeders. These results indicated that food resource partitioning was operating within each assemblage. However, when morphological and trophic data were compared no significant correlations were found. The results did not particularly support the ecomorphological hypothesis that dietary differences are due to morphological differences, since similar diets do not correspond to similar morphologies. The patterns of trophic organization within the fish assemblages examined, possibly reflect differences among species due not only to the effect of ecological demands on morphology but also by their evolutionary history and constructional constraints imposed by phylogeny.

Morphological evolution and changes in foraging behaviour of island and mainland populations of Blue Tit (Parus caeruleus) ? a test of convergence and ecomorphological hypotheses

We study the leg morphology and feeding postures of two subspecies of the Blue Tit (Parus caeruleus; Tenerife island and the Iberian Peninsula) and the Coal Tit (Parus ater; Iberian Peninsula). We search for evidence supporting the hypothesis of convergent evolution in morphological and ecological traits and we discuss the role of ecomorphological hypotheses as predictors of foraging differences at the intraspecific level. To overcome the problems introduced by environmental characteristics not related to locomotion and competition, we make observations under controlled situations to manage food quality and food access. We determine that the island Blue Tit has a longer tarsometatarsus, larger foot span and a more proximal insertion of the tibialis cranialis muscle (flexor of the tarsometatarsus) than the mainland Blue Tit. These morphological differences are consistent with the more frequent use of hanging and clinging ‘head-up’ postures by the Iberian Blue Tit. Several ecomorphological hypotheses obtained at the interspecific level with other taxa, have proved to be of high predictive value for explaining ecological differences considering morphological evolution. The Tenerife Blue Tit and the Iberian Coal Tit clearly show close convergence in both feeding postures and leg structure, although some differences in morphology were found between these two species. Convergence in foraging methods between the island Blue Tit and the mainland Coal Tit can be explained without considering current interspecific competition as a determinant of niche space.

Ecomorphology, Performance Capability, and Scaling of West Indian Anolis Lizards: An Evolutionary Analysis

Studies of ecomorphology–the relationship among species between morphology and ecology–contain two implicit and rarely tested hypotheses: (1) that morphological differences among species result in differences in performance capability at ecologically relevant tasks, which, in turn, produce differences in behavior and ecology; and (2) that morphology, performance capability, ecology and behavior have evolved synchronously. I tested these hypotheses using the Anolis lizards of Jamaica and Puerto Rico. I measured morphological and performance variables on recently caught lizards. Movement, display rate and microhabitat measurements were made on lizards observed in the field. Body size explained most of the variation in morphology and performance ability, but was not correlated with the ecological or behavioral variables. When the effect of body size is removed from the morphological and performance variables, the ecomorphological hypotheses were confirmed. Species that were similar morphologically were also similar in performance ability, ecology, and behavior. Evolutionary changes in morphology, performance, and ecology and behavior (ecobehavior) were correlated. The morphology–ecobehavior comparison revealed that: long—legged, heavy—bodied lizards jump farther in nature, jump and display more often, walk less often, and use wide perches that are distant from the nearest available perches; and that species with many subdigital lamellae perch lower, use narrower supports, and walk more frequently. Inclusion of performance parameters revealed a relationship, in agreement with biomechanical models, between body proportions and running and jumping capability; in turn, performance capability related to locomotor patterns and microhabitat use. Despite the relationship between lamellae number and ecobehavioral variables, clinging performance did not contribute to the correlations with either morphology or ecology.

Functional morphology of the feeding apparatus of ten species of Pacific butterflyfishes (Perciformes, Chaetodontidae): an ecomorphological approach

The functional morphology of the feeding apparatus and the feeding ecology of an assemblage of ten species of butterflyfishes was investigated using a comparative ecomorphological approach. Behavioral observations in situ and in vivo, morphological measurements of fresh-killed specimens, scanning electron microscopy, and kinematic modeling were utilized. The fishes show varying degrees of morphological and behavioral specialization and generalization. The more specialized species group according to how they feed, rather than what they feed on. The feeding guild concept is therefore not very instructive in understanding the functional significance of the feeding apparatus. Many of the morphologically specialized butterflyfishes demonstrate evolutionary convergence in feeding morphology. Whereas the more morphologically specialized species do generally have more ecologically specialized diets, these data do not particularly support the ecomorphological hypothesis in that similar diets do not correspond to similar morphologies.

Patterns of Resource Use and Age Structure Among Species of Desert Scorpion

(1) The niche patterns of four species of desert scorpion were analysed from 1975 to 1983 in the Coachella Valley, California. There is a marked skew in the relative abundance of the four species with one (Paruroctonus mesaensis) always constituting >95% of all individuals and three other species (Hadrurus arizonensis, Paruroctonus luteolus and Vaejovis confusus) being relatively rare. (2) The size ranking of animals at both birth and maturity is the same with H. arizonensis (adults = 5.5 g) > P. mesaensis (1-5-2.0 g) > P. luteolus (0-2 g) > V. confusus (0.1-0.2 g). However, because newborn animals require 2-4 years to reach maturity, the size relationship among these species is complex with an array of either 3 or 4 year classes per species each interacting with other year classes of their own and other species. (3) These scorpions prey on a variety of insects and arachnids. Average overlap in prey size (0-67) is considerably higher than the overlap in prey taxa (0-43). Different year classes of the same species use substantially different prey from each other and differ in the degree that they overlap with adults of other species. As predicted by the ecomorphological hypothesis, prey use is a general function of scorpion size; scorpions more different in size are less similar in prey size but not prey taxa. (4) There are significant differences among species in temporal patterns and habitat (=substrate) use. In general, P. mesaensis (especially the adults) occur during times (spring; late summer to early fall) and in the habitat (sand) characterized by the highest availability of prey. Only the temporal patterns of P. mesaensis adults were significantly correlated with prey abundance and the feeding rate of P. mesaensis is significantly greater than that of the other species. (5) The success of P. mesaensis is partially due to the fact that it is the only one of these species fully adapted to sand. However, it is also an aggressive species that frequently eats other scorpions and thus possibly restricts the distribution and abundance of the other three species.

Food resource partitioning in a rocky intertidal fish assemblage

A resident intertidal fish assemblage at Dillon Beach, California, U.S.A., was persistent and resilient for a 42‐month period. Data from a previous study indicated that productivity was the best predictor of several assemblage characteristics; hence, food resources may have been limiting in this assemblage. To determine whether food resource partitioning was a potential organizational mechanism for this assemblage, the following aspects of trophic organization were quantified for 1,858 individuals of 16 species, collected over a 31‐month period: (1) annual, seasonal, and size‐related dietary changes, (2) annual and seasonal patterns of interspecific dietary overlap, (3) delineation of the relationship between morphological similarity and dietary similarity and (4) estimates of annual prey consumption rates by fish predators.Fishes were classified into seven basic feeding groups: crab feeders, crab‐shrimp feeders, shrimp‐hermit crab feeders, polychaete‐shrimp feeders, Idotea feeders, algal‐sea anemone feeders, and gammarid feeders. Major prey taxa consumed by these groups were: Cancer spp., Pugettia producta, Heptacarpus taylori, Pagurus hemphilli, Idotea spp., Anthopleura spp., Viva spp., Porphyra spp., gammarids and polychaetes. Based on dietary breadth estimates using Levins' index (Levins, 1968), species possessed either narrow (1 08‐2‐30) or wide (7–62‐9‐71) dietary niches, with only one species displaying an intermediate value. High values were obtained for estimates of intraspecific dietary overlap between seasons (x̄± IS.D. = 0–678 ± 0–197) and among years (x̄± IS.D. = 0–634 ± 0178). Hence, species generally exhibited stable patterns of food resource utilization. Many fishes showed size‐related dietary changes. These generally represented switches from smaller (e.g. gammarids) to larger (e.g. crabs, shrimp, polychaetes) prey.Interspecific dietary overlap was very low and averaged 0–275 ± 0–255 for all pairwise comparisons. Nine of 12 species did not display seasonal differences in interspecific dietary overlap, and the three remaining species exhibited lower overlap during the season of lowest productivity. There was a significantly greater number of low overlap values (i.e. < 0–400) than could be expected by chance alone. In addition, species which displayed the lowest amount of dietary overlap were those for which food limitation was probably most severe (i.e. resident‐resident overlaps < resident‐seasonal overlaps < seasonal‐seasonal overlaps). These data indicate that food resource partitioning probably was operating within this assemblage. This mechanism is the most likely process responsible for assemblage organization because predators, competition for space and environmental disturbances do not seem to have a strong effect on this assemblage.Morphological and dietary similarity were assessed by conducting a multivariate analysis on nine variables associated with prey capture: standard length, head length, head height, jaw length, jaw width, mouth protrusibility, mouth orientation, eye position and the number of gill rakers on the first gill arch. A principal component analysis of these data extracted two components which accounted for 84% of the variance and indicated that species comprised morphologically distinct groups. The first principal component was a general size‐related component, and the second was a trophically linked component on which mouth orientation, eye position and the number of gill rakers loaded most heavily. MANOVA indicated that significant differences existed among species' morphologies and a discriminant function analysis (DFA) correctly assigned individuals to species 96% of the time. To test the ecomorphological hypothesis, Mahalanobis distances derived from the DFA were regressed against interspecific dietary overlap. None of ten regressions, representing all possible combinations of residency pattern and season, yielded correlations which were significantly different from zero. These results are discordant with the predictions of the ecomorphological hypothesis and suggest that behaviour is more important than morphology in determining prey utilization.Estimates of prey consumption rates were quantified by deriving an estimate of the mean number of each prey species consumed per day by each fish species. Annual consumption rates summed over all predators for various prey species ranged from 625 to 13,826 individuals consumed per year in the 19 m by 30 m study site. These data indicate that fish predation may exert a strong selective pressure on intertidal invertebrates.The Dillon Beach rocky intertidal fish assemblage was organized through deterministic processes (i.e. food resource partitioning) over a 42‐month period. This mechanism is different from the organizational mechanism exhibited by intertidal algal and invertebrate assemblages which are primarily affected by stochastic forces. The evolutionary forces responsible for assemblage organization affect different taxa in different ways; hence, habitat‐wide extrapolations regarding community organization are probably unwise.