Convergence in a mechanically complex phenotype: detecting structural adaptations for crushing in cichlid fish

Morphological convergence provides strong evidence that evolution is adaptive. However, putatively convergent morphology is often examined in two dimensions with no explicit model of function. In this study, we investigated structural and mechanical similarities of the lower pharyngeal jaw (LPJ) in cichlid fish that have evolved the ability to crush hard-shelled molluscs. Using a novel phylogeny, we demonstrated molluscivory has been gained and/or been lost numerous times in Heroine cichlids. Within this comparative framework, we produced three-dimensional computed tomography (CT) scans for LPJs of both morphotypes in the trophically polymorphic Herichthys minckleyi and six evolutionarily independent pairs of closely related species. Like H. minckleyi , these species exhibit divergence between a molluscivore and a nonmolluscivore. Using the CT scans, we generated finite element models of papilliform H. minckleyi LPJs to determine where stress would concentrate in a jaw not modified to crush molluscs. Then, we examined whether stress in the papilliform LPJ predicted structural modifications in molariform H. minckleyi and other molluscivorous species. Despite potential constraints, stresses imposed during prey processing explain 40% of LPJ variation in mollusc crushing species. The structural and mechanical analyses also suggest divergence found in polymorphic species could provide the substrate for trophic differences found in reproductively isolated cichlids.     

Micro- and macroevolutionary decoupling of cichlid jaws: a test of Liem's key innovation hypothesis

The extent to which elements of functional systems can change independently (modularity) likely influences the diversification of lineages. Major innovations in organismal design, like the pharyngeal jaw in cichlid fishes, may be key to a group's success when they relax constraints on diversification by increasing phenotypic modularity. In cichlid fishes, pharyngeal jaw modifications that enhanced the ability to breakdown prey may have freed their oral jaws from serving their ancestral dual role as a site of both prey capture and prey processing. This functional decoupling that allowed the oral jaws to become devoted solely to prey capture has been hypothesized to have permitted the two sets of cichlid jaws to evolve independently. We tested the hypothesis that oral and pharyngeal jaw mechanics are evolutionarily decoupled both within and among Neotropical Heroine cichlids. In the trophically polymorphic species Herichthys minckleyi, molariforms that exhibit enlarged molarlike pharyngeal jaw teeth were found to have approximately 400% greater lower jaw mass compared to H. minckleyi with the alternative papilliform pharyngeal morphology. However, oral jaw gape, lower jaw velocity ratios, anterior jaw linkage mechanics, and jaw protrusion did not differ between the morphotypes. In 40 other Heroine species, there was a weak correlation between oral jaw mechanics and pharyngeal jaw mass when phylogenetic history was ignored. Yet, after expansion of the cytochrome b phylogeny for Heroines, change in oral jaw mechanics was found to be independent of evolutionary change in pharyngeal jaw mass based on independent contrasts. Evolutionary decoupling of oral and pharyngeal jaw mechanics has likely played a critical role in the unparalleled trophic diversification of cichlid fishes.     

Ecological and evolutionary consequences of the trophic polymorphism in Cichlasoma citrinellum (Pisces: Cichlidae)

The neotropical cichlid fish Cichlasoma citrinellum is polymorphic in the structure of its pharyngeal jaw apparatus and external morphology. The pharyngeal jaws are either gracile and bear slender, pointed teeth (papilliform) or robust with strong, rounded teeth (molariform). Molariform morphs have a ‘benthic’, and papilliform morphs a ‘limnetic’ body form. Furthermore, this species is also polychromatic, with yellow and black morphs. The molariform morphology of the pharyngeal jaw apparatus adapts the fish for cracking and feeding on snails. Based on analysis of stomach contents, 94% of the molariform morph ate snails whereas only 19%, of the papilliform morph did so. This result suggests that the morphs occupy different ecological niches. The morphology of the pharyngeal jaw apparatus does not correlate significantly with sex, but it does with body colouration (P<0.005). Cichlasoma citrinellum mate assortatively with their own colour; therefore a mating preference for colour may lead to genetic isolation of trophic morphs. The frequency of the molariform morph differs strikingly among populations of five Nicaraguan lakes and its abundance is correlated with the abundance of snails, the fishes' principal prey item. Among populations the frequency of molariform morphs decreases in the dry season. Morphology possibly changes reversibly within particular individuals between seasons. These results suggest that phenotypic plasticity and polymorphisms may be an adaptive characteristic of cichlid fishes. Patterns of intraspecific morphological variation match patterns of interspecific morphological diversification which suggests that universal developmental mechanisms canalize the possible expressions of morphology. The ability to respond morphologically to environmental shifts, in conjunction with genetically determined trophic polymorphisms and sexual selection via mate choice, could be the basis for speciation through intermediate stages of polymorphism of the impressive adaptive radiation of cichlid fishes.     

Prey selection by molluscivorous cichlids foraging on a schistosomiasis vector snail,Biomphalaria glabrata

Summary This paper considers prey size selection by four molluscivorous cichlids feeding on the intermediate host snail of Schistosoma parasites, Biomphalaria glabrata. Haplochromis ishmaeli obtains its prey by crushing the snails between the pharyngeal jaws, whereas H. xenognathus, H. sauvagei and Macropleurodus bicolor apply both pharyngeal crushing and oral shelling. The fishes crushed significantly more snails with the highest reward in biomass per second of crushing. Oral shelling occurred far less often than pharyngeal crushing. Encounter rates with prey showed significant variations between different size classes of prey. The fish have no overall knowledge of snail availability in a tank. The probability that a snail will be eaten at encounter, calculated from the number encountered and the number eaten, reflects the prey size preference of the fish. Those snails with the highest biomass/crushing-time ratio had the highest probability of being crushed; observed and predicted prey size preferences corresponded well. Although for oral shelling the potential reward in biomass per second is of the same magnitude as for crushing, the probability of successful shelling is very low. Apparently the fish prefer prey with lowest risks.     

Gape morphology of cod Gadus morhua L., haddock Melanogrammus aeglefinus (L.) and whiting Merlangius merlangus (L.) through metamorphosis from larvae to juveniles in the western Irish Sea

Variations in standard length ( L _S), gape size ( S _G) and jaw length ( L _J) were studied in larval and juvenile gadoids (cod Gadus morhua , haddock Melanogrammus aeglefinus and whiting Merlangius merlangus ) from 4 to 70 mm. The increase in S _G and L _J was not linear with respect to L _S. The relationship was best described by segmented regression lines in all three species, with an inflection point at c . 10·5 mm. The S _G and L _J increased more rapidly in relation to larval L _S for individuals smaller than this inflection point size. The rates of increase slowed significantly post-inflection, an effect more noticeable in S _G data compared to L _J data. In each case, the inflection point fell in the intermediate period of development between the larval and juvenile stages, which could be considered as metamorphosis. Published equations that have been used to predict S _G from L _J lead to the overestimation of gape. New relationships are presented, which may be used to predict S _G from measurements of either L _S or upper jaw length in cod, haddock and whiting.     

Cost of morphological specialization: feeding performance of the two morphs in the trophically polymorphic cichlid fish,Cichlasoma citrinellum

Summary The feeding performance on soft and hard prey of two morphs of the trophically polymorphic Neotropical cichlid fish, Cichlasoma citrinellum, was investigated in the laboratory. The molariform morphs, specialized to feed on hard prey, are able to crack snail shells that are twice as hard as those cracked by the papilliform morphs. During ecological bottlenecks in food resources this ability should allow molariform morphs to exploit alternate, less preferred prey sources that are not available to papilliform morphs. Analysis of stomach contents revealed that molariform morphs feed significantly more often on hard snails than do papilliform morphs (Meyer 1989a). The performance advantage of the trophically specialized morphs when feeding on hard prey is countered by their less efficient performance on soft diets. The morphologically generalized papilliform morph feeds more efficiently on soft prey. The abundance of preferred soft prey, seasonal fluctuations in prey availability and the frequency of ecological bottlenecks may determine the relative abundance of these two morphs in natural populations in Nicaraguan lakes.     

Spatial geographic mosaic in an aquatic predator-prey network

The geographic mosaic theory of coevolution predicts 1) spatial variation in predatory structures as well as prey defensive traits, and 2) trait matching in some areas and trait mismatching in others mediated by gene flow. We examined gene flow and documented spatial variation in crushing resistance in the freshwater snails Mexipyrgus churinceanus, Mexithauma quadripaludium, Nymphophilus minckleyi, and its relationship to the relative frequency of the crushing morphotype in the trophically polymorphic fish Herichthys minckleyi. Crushing resistance and the frequency of the crushing morphotype did show spatial variation among 11 naturally replicated communities in the Cuatro Ciénegas valley in Mexico where these species are all endemic. The variation in crushing resistance among populations was not explained by geographic proximity or by genetic similarity in any species. We detected clear phylogeographic patterns and limited gene flow for the snails but not for the fish. Gene flow among snail populations in Cuatro Ciénegas could explain the mosaic of local divergence in shell strength and be preventing the fixation of the crushing morphotype in Herichthys minckleyi. Finally, consistent with trait matching across the mosaic, the frequency of the fish morphotype was negatively correlated with shell crushing resistance likely reflecting the relative disadvantage of the crushing morphotype in communities where the snails exhibit relatively high crushing resistance.     

Functional morphology of durophagy in black carp,Mylopharyngodon piceus

The black carp, Mylopharyngodon piceus (Osteichthyes: Cyprinidae), crushes its snail and other molluscan prey with robust pharyngeal jaws and strong bite forces. Using gross morphology, histological sectioning, and X‐ray reconstruction of moving morphology (XROMM), we investigated structural, behavioral, and mechanical aspects of pharyngeal jaw function in black carp. Strut‐like trabeculae in their pharyngeal jaws support large, molariform teeth. The teeth occlude with a hypertrophied basioccipital process that is also reinforced with stout trabeculae. A keratinous chewing pad is firmly connected to the basioccipital process by a series of small bony projections from the base of the pedestal. The pharyngeal jaws have no bony articulations with the skull, and their position is controlled by five paired muscles and one unpaired median muscle. Black carp can crush large molluscs, so we used XROMM to compare pharyngeal jaw postures as fish crushed ceramic tubes of increasing sizes. We found that black carp increase pharyngeal jaw gape primarily by ventral translation of the jaws, with ventral rotation and lateral flaring of the jaws also increasing the space available to accommodate large prey items. A stout, robust ligament connects left and right jaws together firmly, but allows some rotation of the jaws relative to each other. Contrasting with the pharyngeal jaw mechanism of durophagous perciforms with fused left and right lower pharyngeal jaws, we hypothesize that this ligamentous connection may serve to decouple tensile and compressive forces, with the tensile forces borne by the ligament and the compressive forces transferred to the prey. J. Morphol. 276:1422–1432, 2015. © 2015 Wiley Periodicals, Inc.     

Development of the pectoral, pelvic, dorsal and anal fins in cultured sea bream

The pectoral fin girdle was the first element of the fins to develop in Sparus aurata. By 3·1mm L _N (notochord length) the cleithrum was ossified and the cartilaginous caracoid-scapula was present. The fin was fully developed at 11·6 mm L _S (standard length) and by 16·0 mm L _S most elements of the fin were ossified. The pelvic fins were the last pair to develop and rudiments of these were first detected at 7·9 mm L _S. The pelvic fin and girdle were completely formed and ossified at 16·0 mm L _S. The development of dorsal and anal fins began at c. 6·5–7·0 mm L _S with the formation of 10 cartilaginous dorsal proximal radials and eight cartilaginous ventral proximal radials. The three cartilaginous predorsals (supraneurals) appeared at 7·7 mm L _S and the ossification of dorsal and anal proximal and distal radials began, respectively, at 10·5 mm L _S and 11·3 mm L _S. Ossified structures in the fins were also classified according to their origin, as being either dermal or endochondral. Finally the chronology of appearance of fin structures in S. aurata was compared with that reported for other Sparidae, Engraulidae and Haemulidae.     

Larval development of five species of blenny (Teleostei: Blenniidae) from the western central North Atlantic, with a synopsis of blennioid family characters

Light trap collections on oil and gas platforms in the northern Gulf of Mexico off Louisiana from 1995 to 1997 contained young of tessellated blenny Hypsoblennius invemar , freckled blenny Hypsoblennius ionthas , featherduster blenny Hypleurochilus multifilis , molly miller Scartella cristata and seaweed blenny Parablennius marmoreus , which are described. Interspecific differences were: number of dorsal, anal, pelvic and caudal fin elements; number of mandibular pores; presence or absence of canine teeth, hypural 5 and pectoral fin pigment; width of gill openings; length of preopercular spines (in larvae). Size at settlement differed among some taxa, but all five species settled within a narrow size window of c . 1·5 mm standard length ( L _S). Hypsoblennius invemar , H. ionthas , H. multifilis and S. cristata settled at mean sizes between 11·3 and 12·1 mm L _S, whereas P. marmoreus settled at a mean size of 19·3 mm L _S. Sexually dimorphic differences were consistently evident by 17–18 mm L _S in all species but P. marmoreus . The two smallest blennies with external characters normally associated with sexual maturity were a 20 mm male and 21 mm female H. multifilis . Primary caudal fin rays began to bifurcate between 17·0 and 18·3 mm L _S in H. invemar , H. ionthas , H. multifilis and S. cristata . Hypleurochilus multifilis displayed the external characteristics of being sexually mature at 20–21 mm L _S. Thus, bifurcation of primary caudal fin rays was an indicator of approaching sexual maturity in H. multifilis and this may also be true in the other species studied.     

Shrinkage correction and length conversion equations for Theragra chalcogramma, Mallotus villosus and Thaleichthys pacificus

Preservation of walleye pollock Theragra chalcogramma, capelin Mallotus villosus and eulachon Thaleichthys pacificus by freezing decreased fork length ( L _F) up to 1·8, 5·6 and 2·7% and reduced mass by up to 8·4, 3·5 and 1·1%, respectively. Shrinkage of walleye pollock standard length ( L _S) was greater for fish in 95% ethanol v . 5% formalin and for fish in 10% formalin v . frozen. Equations describing the shrinkage and loss in mass for these species are presented as well as conversions between different length measurements ( L _S, L _F and total length, L _T) for fishes that were frozen.     

Consumption and gut evacuation rate of laboratory‐reared spotted seatrout (Sciaenidae) larvae and juveniles

The temperature and mass dependence of maximum consumption rate was measured for larval and early juvenile spotted seatrout Cynoscion nebulosus . Maximum consumption ( C _MAX) estimates were obtained from feeding and gut evacuation experiments on larvae (3·8–19 mm standard length, L _S) at three temperatures (24, 28 and 32° C), and maximum consumption experiments on juveniles at three temperatures (20, 26 and 32° C). Feeding levels were determined for larvae fed live prey ( Brachionus plicatilis and Artemia salina ) ad libitum . The midgut and total evacuation times were estimated for fish feeding continuously and discontinuously using alternate meals of tagged and untagged live prey. Temperature and fish size had significant effects on gut evacuation and consumption. The gut evacuation time increased with increasing fish size, and decreased with increasing temperatures. Mass‐specific midgut contents increased for small larvae <0·156 mg dry mass ( M _D)( c . 4 mm L _S), and decreased for larger larvae and juveniles. Maximum consumption was modelled by fitting a polynomial function to a reduced dataset of individuals feeding at high levels. The C _MAX model predicted an initial increase in specific feeding rate from 70 to 155% M _D day⁻¹ for small larvae, before declining for larger larvae and juveniles.     

Does hardness of food affect the development of pharyngeal teeth of the black carp,Mylopharyngodon piceus (Pisces: Cyprinidae)?

We investigated whether food type influences development of the pharyngeal crushing apparatus of black carp (Mylopharyngodon piceus, Cyprinidae). Fish fed a hard diet had average standard lengths and weights larger than those fed a soft diet; these observations in part could be related to differences in nutritional value of the two food types. The lower pharyngeal bones, which bear molariform teeth were examined using three dimensional computed tomography (CT) scans. After adjusting for differences in the standard length of the fish, the total volume and exterior surface areas of the pharyngeal teeth were greater in fish fed hard diets than in those fed soft diets.Total weights of the pharyngeal arches were less in the fish fed a soft diet than in those fed a hard diet. These results indicated that food type affects development of the pharyngeal crushing mill of black carp and therefore if black carp are produced for snail control, a hard diet should be provided from an as early time as possible in the production cycle.     

Ontogenetic allometric coefficient changes: implications of diet shift and morphometric traits in Hoplias malabaricus (Bloch) (Characiforme, Erythrinidae)

This study evaluated the relationship between body size and digestive tract characteristics of the important predatory freshwater fish Hoplias malabaricus , which is widely distributed in South America. The allometric coefficients were calculated for the mass and standard length ( L _S) relationships for two different L _S groups: (1) between 20 and 100 mm (characterized as insectivores) and (2) >100 mm (characterized as piscivores). Differential growth measured from the allometric coefficient, b , between the insectivore ( b < 3) and the piscivore ( b > 3) groups was detected. Anterior intestine length and pyloric caeca zone length showed significant differences between groups. Two complementary hypotheses were developed to explain the differential growth: (1) H. malabaricus has a digestive tract adapted to a piscivorous diet, which is independent of its ontogenetic stage of development, and (2) the negative allometry observed in group 1 individuals agrees with a general behavioural strategy, allowing individuals to grow in L _S during a shorter period of time.     

Resolving within- and between-population variation in feeding ecology with a biomechanical model

Studies of phenotypic plasticity have emphasized the effect of the environment on the phenotype, but plasticity can also be used as a tool to study the functional significance of key traits. By inducing variation in phenotypes and testing quantitative models that predict performance based on biological mechanisms, we can develop functionally general models of performance. Pumpkinseed sunfish from lakes with high snail availability have large levator posterior muscles (which are used to crush snail shells), whereas fish from lakes with few snails have relatively small muscles. Here we: (1) quantify differences in the feeding ability of an ontogenetic series of pumpkinseed from two populations; and (2) evaluate whether a biomechanical model can resolve the observed ontogenetic and between-population variation in feeding ecology. Mass, but not length, of the levator posterior muscle in fish from Three Lakes (a lake rich in snails) was greater than for comparably sized fish from Wintergreen Lake (a lake with few snails). Handling times were shorter, crushing strengths were 71% greater, and foraging rate (snail tissue mass consumed per time) and the fraction of thick-shelled snails in the diet were approximately 100% greater for fish from Three Lakes compared to comparably sized fish from Wintergreen. These between-lake differences were not significant after adjusting for variation in pharyngeal morphology, suggesting that the biomechanical model of snail crushing resolved observed ontogenetic and population-level variation in the feeding ecology of pumpkinseed.     

Direct yolk sac volume manipulation of zebrafish embryos and the relationship between offspring size and yolk sac volume

Through direct manipulation of yolk sac volume ( V _y), in the zebrafish Danio rerio , V _y and offspring size was positively correlated and the relationship was independent of geographical location, parental size or age and, most importantly, parental genetic factors. Larval survivorship, under a non-feeding regime and up until hatch, was not significantly affected by the manipulation. Decreased V _y significantly decreased maximum standard length ( L _S), maximum body surface area ( A _B), time to yolk sac absorption, L _S and A _B at yolk sac absorption, and the L _S and A _B at starvation. The methodology of V _y adjustment will be useful for the studies of the interaction between early life-history traits and offspring size, egg quality variables and early vertebrate development.     

Effects of temperature on prey consumption and growth in mass of juvenile trahira Hoplias aff. malabaricus (Bloch, 1794)

The influence of temperature on prey consumption and growth in mass of juvenile trahira Hoplias aff. malabaricus were investigated. Consumption of small-sized lambari Astyanax altiparanae (mean standard length, L _S, 5·43 cm) varied from zero to 65 over a period of 30 days. Temperatures ranged from 14 to 34° C and the size of trahiras ranged from 17·5 to 24·7 cm L _S. Prey consumption differed significantly among temperatures. Trahiras at 18° C consumed significantly less than those at 30° C. A linear multiple regression model including temperature, prey consumption and L _S explained 89·4% of the variability in growth in mass. Some caution is suggested when inferring the impact of H. aff. malabaricus piscivory on assemblage structures in systems that, despite their location in tropical regions, are subjected to seasonal thermal variations.     

Morphological development and allometric growth patterns in the juvenile seahorse Hippocampus kuda Bleeker

The morphological development and allometric growth patterns in the juvenile spotted seahorse Hippocampus kuda were studied under hatchery rearing conditions. Newborn spotted seahorses [mean ± s.d . standard length ( L _S) 9·33 ± 0·79 mm] were raised till the age of 124 days (119·35 ± 6·04 mm). Growth was characterized by three stages with two inflexion points occurring at day 21 and 76. The mean growth rates in the first, second and third stages were 0·68, 1·16 and 0·71 mm day⁻¹, respectively. The growth rate was most rapid in the second stage and was probably influenced by a behavioural shift from pelagic to benthic form. The mass ( M ) and L _S relationship was exponential ( M = 7·14 × 10⁻⁶ L _S^2·76), but the slope, b = 2·76, reflected negative allometric growth. Sexes could be distinguished at c. 110 days, and the sex ratio was unbiased. The L _S in males and females did not differ significantly. Morphological stageing series is proposed, which divides H. kuda juvenile development into eight stages based on the development of coronet, cheek and eye spines, keel and pigmentation. The morphometric ratios for all the body parts, except trunk length, showed considerable changes at a transition point occurring at c. 25 mm L _S. The high proportional growth in head length, head depth, pectoral fin base length, dorsal fin base length, snout length, snout depth and eye diameter at the initial stages, and the abrupt increase in tail length only after the first 2 weeks, possibly reflect development priorities during early development where important organs are being developed first for the enhancement of juvenile survival.     

The feeding ecology of Morone americana larvae in the Chesapeake Bay estuarine turbidity maximum: the influence of physical conditions and prey concentrations

Feeding ecology of white perch Morone americana larvae, a major component of the ichthyoplankton community in the estuarine turbidity maximum (ETM) region of upper Chesapeake Bay, was evaluated in years of contrasting physical conditions. In 1998, high river flow led to an ETM with high turbidity and pronounced salinity stratification. In 1999, under low river flow conditions, turbidities in the ETM region were lower and the salt front was located 15 km up‐stream of its location in 1998. Copepodites and adults of Eurytemora affinis were the predominant prey in guts of all length classes of larvae (3·2–9·8 mm standard length, L _S) that were examined. Repeated‐measures, multiple regression analyses were conducted to evaluate the influence of several factors on feeding success by determining if physical conditions (temperature, current velocity, salinity, turbidity and light) and prey concentrations explained variability in mean gut fullness (gut contents mass per body mass) of small (<5 mm L _S) and large (>5 mm L _S) white perch larvae. Eurytemora affinis concentrations were significant in the statistical models for both small and large larvae. High concentrations of E. affinis , which enhanced encounter rates of white perch larvae with prey, may have been the most important factor determining feeding success in the ETM region. Larval feeding incidence (percentage of guts with food) was higher in 1998, suggesting that annual variability in river flow influenced larval feeding success by controlling physical and biological conditions in the ETM region.     

Response to conspecific and heterospecific alarm cues by pumpkinseeds in simple and complex habitats: field verification of an ontogenetic shift

Three focal size classes [small (<40 mm, standard length, L _S), medium (40–80 mm L _S) and large (>80 mm L _S)] of pumpkinseed Lepomis gibbosus were exposed to conspecific or one of three heterospecific chemical alarm cues under one of three levels of habitat complexity (low, intermediate and high). Under low and intermediate habitat complexities, pumpkinseeds <80 mm L _S exhibited a significant antipredator response when exposed to the chemical alarm cues of heterospecific prey guild members (largemouth bass Micropterus salmonides and hypoxanthine‐3‐ N ‐oxide, the putative Ostariophysan alarm 'pheromone'), while pumpkinseeds >80 mm L _S exhibited a foraging response. Under highly complex habitats, pumpkinseeds of all three size classes exhibited an antipredator response. Moreover, under all three habitat conditions, pumpkinseeds of all size classes exhibited an antipredator response to conspecific alarm cues. There was no change in behaviour under any treatment combination in response to an allopatric heterospecific control (swordtails Xiphophorus helleri ). These results provide the first field verification of the size dependent use of chemical alarm cues by centrarchids and demonstrate that microhabitat complexity significantly influences the threat‐sensitive use of these cues.