Trophic divergence of Lake Kivu cichlid fishes along a pelagic versus littoral habitat axis

Local adaptation to the littoral and pelagic zones in two cichlid haplochromine fish species from Lake Kivu was investigated using morphometrics. Cranial variation and inferred jaw mechanics in both sexes of the two species across the two habitat types were quantified and compared. Comparisons of littoral versus pelagic populations revealed habitat‐specific differences in the shape of the feeding apparatus. Also, kinematic transmission of the anterior jaw four‐bar linkage that promotes greater jaw protrusion was higher in the pelagic zone than in the littoral zone for both species. Inferred bite force was likewise higher in pelagic zone fish. There were also sex‐specific differences in craniofacial morphology as males exhibited longer heads than females in both habitats. As has been described for other cichlids in the East African Great Lakes, local adaptation to trophic resources in the littoral and pelagic habitats characterizes these two Lake Kivu cichlids. Similar studies involving other types of the Lake Kivu fishes are recommended to test the evidence of the observed trophic patterns and their genetic basis of divergences.     

Morphological and life-history differentiation between littoral and pelagic forms of pumpkinseed

Trophically dimorphic pumpkinseed populations were investigated in five lakes in Ontario, Canada to determine: (1) whether the morphological traits that distinguish littoral from pelagic forms are consistent among populations; (2) whether the pharyngeal jaw apparatus is diminished in pelagic pumpkinseeds because of a lower proportion of hard-bodied prey in their diets; (3) whether there is life-history differentiation between littoral and pelagic forms. Pumpkinseeds captured from the littoral and pelagic zones differed significantly in morphology in four of the five lakes, but the number of external measures that differed significantly within the differentiated populations ranged from zero to six. Littoral pumpkinseeds generally had longer heads, more rearward placement of dorsal and pectoral fins, longer pectoral fins and deeper bodies than pelagic pumpkinseeds. Littoral and pelagic pumpkinseeds were more readily differentiated by internal morphometric measures, with littoral individuals having larger molars and wider spacing between gill rakers than pelagic individuals. Littoral and pelagic differences in age at maturity, size at maturity and gonado-somatic index were present only in one of three populations assessed for these traits, suggesting that morphological divergence is not necessarily accompanied by life-history differentiation.     

Trophic polymorphism in brook charr revealed by diet, parasites and morphometrics

Stomach contents, parasite assemblages and morphometrics were compared in brook charr Salvelinus fontinalis from the littoral and pelagic zone of two adjacent lakes on the Canadian Shield. In lac Baie des Onze Îles, fish from the littoral zone had greater abundance of benthic prey in their stomach and were more heavily infected by parasites that use intermediate hosts associated with the littoral zone than fish captured in the pelagic zone. Littoral and pelagic brook charr from this lake also differed in regard to body shape and fin length, with each group being anatomically adapted to exploit their respective habitats. The highly significant correlation between morphometric and parasite canonical scores supports the hypothesis of functional diversification of individuals within lac Baie des Onze Îles. While fish from littoral and pelagic zones of lac Caribou did not differ in terms of diet, parasite assemblages or morphometrics, they were different to fish from lac Baie des Onze Îles in that they were less frequently infected with parasites that use gastropods as intermediate hosts, and had shorter pectoral fins. The inter-lake comparisons suggested that parasite assemblages and morphometrics of brook charr reflected the dominance of the limnetic and littoral habitats in lacs Caribou and lac Baie des Onze Îles, respectively.     

Individual variations in habitat use and morphology in brook charr

The specific objectives of this study were to determine if there is individual specialization in habitat use by lacustrine brook charr Salvelinus fontinalis and if so, if specialization is related to fish morphology. Localizations of 28 brook charr equipped with thermosensitive radiotransmitters were recorded during three summers (1991, 1992, and 1993) in two lakes of the Mastigouche Reserve (Québec, Canada). Fifty per cent of the fish were found mainly in the benthic zone (hereafter benthic individuals), 18% in the pelagic zone (pelagic individuals), and 32% travelled regularly between the two zones (generalist individuals). The observed interindividual differences in habitat preference were related to differences in body morphology and coloration: (i) the pectoral fins of benthic and generalist individuals were significantly longer than those of pelagic ones; and (ii) the coloration of the lower flank of benthic and generalist individuals was silver-grey while that of pelagic individuals was red. The results of this study suggest that brook charr inhabiting oligotrophic lakes of the Canadian Shield exhibit trophic polymorphisms, where some individuals are specialists better adapted to feeding in the littoral zone whereas others are specialists better adapted to feeding in the pelagic zone. The potential for reproductive isolation between the two morphs is discussed.     

Effects of habitat and food resources on morphology and ontogenetic growth trajectories in perch

Studies on resource polymorphism have mainly been considered at the end stage of ontogeny, whereas many species undergo diet changes as they grow. We conducted a field survey to analyze the role of adaptive variation during ontogeny in Eurasian perch (Perca fluviatilis). We caught perch from the littoral and pelagic zones of a lake to investigate whether perch differ in morphology and diet between these habitats. We also investigated whether there were any differences in morphological trajectories during the ontogeny of perch collected from the two habitats. We found that perch caught in the littoral habitat, independently of size, had a deeper body, larger head and mouth and longer fins than perch caught in the pelagic zone. Macroinvertebrates and fish dominated the diet of littoral perch, whereas the diet of the pelagic perch consisted mainly of zooplankton and to some extent fish. Independently of size, the more streamlined individuals had a larger proportion of zooplankton and a smaller proportion of macroinvertebrates in their diet than the deeper-bodied individuals, indicating a relation between diet and morphology. Some morphological characters followed different ontogenetic trajectories in the two habitats; e.g. the changes to a deeper body and a larger head were faster in the littoral than in the pelagic perch. The relationship between the length of perch and the size of the mouth and fins also differed between perch from the two habitats, where the increase in the length of the pelvic fin and the area of the mouth increased faster with size in the littoral perch. Our findings show that variation in morphology between habitats differs during ontogeny in a way that corresponds to functional expectations for fish species that occupy these habitats.     

A new approach to quantifying morphological variation in bluegill Lepomis macrochirus

Bluegill Lepomis macrochirus showed intraspecific morphological and behavioural differences dependent on the environment. Pelagic L. macrochirus had more fusiform bodies, a higher pectoral fin aspect ratio, a larger spiny dorsal fin area and pectoral fins located farther from the centre of mass than littoral L. macrochirus (P < 0·05). The shape of the body and pectoral fins, in particular, were suggestive of adaptation for sustained high-speed and economical labriform swimming. Littoral L. macrochirus had a deeper and wider body, deeper caudal fins and wider mouths than pelagic L. macrochirus (P < 0·05). Additionally, the soft dorsal, pelvic, anal and caudal fins of littoral L. macrochirus were positioned farther from the centre of mass (P < 0·05). The size and placement of these fins suggested that they will be effective in creating turning moments to facilitate manoeuvring in the macrophyte-dense littoral habitat.     

Habitat shift in roach (Rutilus rutilus) induced by pikeperch (Stizostedion lucioperca) introduction: predation risk versus pelagic behaviour

Changes in the fish community structure and habitat use were followed after the introduction of pikeperch (Stizostedion lucioperca) to the roach-dominated Lake Gjersjøen. Quantitative echosounding showed that the density of juvenile roach (Rutilus rutilus) was dramatically reduced in pelagic areas, from 12 000–15 000 fish/ha to 250 fish/ha, while total fish density remained unchanged in littoral areas. At the same time, the habitat segregation between different size groups of roach was altered as larger roach utilized the pelagic zone after pikeperch introduction. The loss of the pelagic refuge for juvenile roach increased the availability of juvenile roach to littoral predators, notably perch. In littoral areas, the fish community changed from one dominated by roach (> 95%) to one dominated by perch (> 50%).     

Divergent walleye (Sander vitreus)-mediated inducible defenses in the centrarchid pumpkinseed sunfish (Lepomis gibbosus)

Predation has important ecological and evolutionary consequences. Evolutionary responses to diversifying selection include genetic differentiation, the evolution of adaptive phenotypic plasticity, and the genetic differentiation of plastic responses between populations. We tested if pumpkinseed sunfish ( Lepomis gibbosus ) respond to predation cues by changing their external body form in functionally sensible ways. We then asked whether predation has influenced the divergence of coexisting littoral and pelagic ecomorphs, by testing for divergent predator-induced responses. Juvenile L. gibbosus of both ecomorphs were reared with and without predation cues supplied by walleye ( Sander vitreus ) feeding on L. gibbosus . Predation cues stimulated increased body depth and dorsal spine length, but no increase in anal spine length or pectoral fin size. The dorsal spines of pelagic ecomorphs also grew longer than did those of littoral ecomorphs, while positive body depth responses were similar in both ecomorphs. This is the second fish taxa in which predator-induced morphological responses have been found, and the first in which divergent responses have been detected between ecomorphs. This suggests that the developmental systems of L. gibbosus ecomorphs have diverged under selection related to predation. We propose that other 'resource polymorphisms' in fishes have evolved under selection arising from a variety of factors, including predation, and not just selection related to resource use.  © 2007 The Linnean Society of London, Biological Journal of the Linnean Society , 2007, 90 , 25–36.     

Lake size,macrophytes, and omnivory contribute to food web linkage in temperate shallow eutrophic lakes

Physical, chemical and biological processes facilitate cross-habitat connections in lakes, prompting food webs to be supported by different subsidies. We tested the hypothesis that the pelagic food web is subsidized by littoral resources and fish foraging behaviour plays a major role in carbon flux and on food web structure in shallow hypereutrophic lakes. We performed a fish diet and carbon and nitrogen isotope analyses to predict the linkage between littoral and pelagic habitats in three shallow hypereutrophic lakes. Lakes differed in morphology, fetch, macrophyte composition and width of the littoral zone. δ¹³C signals of seston differed among lakes, but were similar to other producers. Macroinvertebrates and fish carbon signatures were more enriched in the lake co-dominated by emergent and submerged vegetation. Fish foraging behaviour indicates that more than the 80% of the carbon that sustain adult fish was channelled from the littoral. In conclusion, littoral carbon were relevant and sustain, in part, food web in these shallow lakes. Factors like the extension of the littoral zone, lake morphometry, and the dominance of multi-chain omnivorous fish facilitate the connection among lake compartments and the transference of littoral carbon to lake food web.     

Genetic variation and phenotypic plasticity in a trophically polymorphic population of pumpkinseed sunfish (Lepomis gibbosus)

Summary Adaptive variation can exist at a variety of scales in biological systems, including among species, among local populations of a single species and among individuals within a single population. Trophic or resource polymorphisms in fishes are a good example of the lowest level of this hierarchy. In lakes without bluegill sunfish (Lepomis macrochirus), pumpkinseed sunfish (Lepomis gibbosus) can be trophically polymorphic, including a planktivorous limnetic form found in the pelagic habitat, in addition to the usual benthic form found in the littoral zone. In this paper we examine the degree to which morphological differences between the two forms are caused by genetic differences versus phenotypic plasticity. Adults from pelagic and littoral sites in Paradox Lake, NY, were bred separately and their progeny were raised in cages both in the open water and shallow water habitats of an artificial pond. The experimental design permitted two tests of genetic differences between the breeding stocks (in open and shallow water cages, respectively) and two tests of phenotypic plasticity (in the limnetic and benthic offspring, respectively). Limnetic progeny were more fusiform than benthic progeny raised in the same habitat. In addition, progeny of both stocks displayed limnetic-type characteristics when raised in the open water and benthic-type characteristics in the shallow water. Thus, genetic differences and phenotypic plasticity both contributed to the trophic polymorphism. Phenotypic plasticity and genetic differentiation accounted for 53 and 14%, respectively, of the variation in morphology. This study addresses the nature of subtle phenotypic differences among individuals from a single population that is embedded within a complex community, a condition that is likely to be the norm for most natural populations, as opposed to very large differences that have evolved in relatively few populations that reside in species-poor environments.     

The effect of core compression on the measurement of zinc concentrations and anthropogenic burdens in lake sediments

The whitefish (Coregonus lavaretus) in Lake Tyrifjorden, southeastern Norway, is monomorphic with regard to gill raker number: (x=32), and exhibits a bimodal length distribution. The small whitefish (<27 cm) were distributed mainly in the littoral zone and the deeper layers of the pelagic zone. The larger whitefish (>27 cm) were mainly distributed in the upper layers of the pelagic zone. Both small and large whitefish fed on zooplankton in the pelagic zone. In the littoral zone the diet of the two size groups was significantly different; the small whitefish fed mainly on zooplankton whereas the large whitefish fed mainly on zoobenthos.     

Genetic and morphological studies of Nothobranchius (Cyprinodontiformes) from Malawi with description of Nothobranchius wattersi sp. nov.

Molecular and morphological data were used to explore evolutionary differentiation among populations of Nothobranchius in the Lake Malawi–upper Shire River and the Lakes Chilwa–Chiuta drainage systems in Malawi. The aim of the study was to test the hypothesis that Nothobranchius of the Malawi–Shire system constitute a separate evolutionary group from Nothobranchius kirki. Mitochondrial and nuclear sequence data show a strongly supported phylogenetic split into two monophyletic groups separating the Lake Malawi basin fish from N. kirki. Unlike N. kirki, Lake Malawi–Shire fish do not deviate from neutrality and express an excess of rare haplotypes and mutations in terminal branches, characteristic of recently expanded populations. Further, the two groups significantly differ in morphology. Two body characters (dorsal‐fin base length and pre‐pelvic–pre‐anal distance) are significantly different between the two species in both sexes. Several other characters are significantly different in either male or female comparisons with respect to both standard and head lengths, and robust morphological differentiation is detected by multivariate analysis. The two groups are readily distinguished on the basis of male colouration, especially in scale centres and the caudal fin. On the basis of this differentiation at the molecular and morphological levels, in addition to colouration, the Lake Malawi–Shire fish are hereby formally recognized as constituting a new species, Nothobranchius wattersi. This distinction is in agreement with the geomorphologic and recent climatic history in the region.     

Colour plasticity and background matching in a threespine stickleback species pair

Examining differences in colour plasticity between closely‐related species in relation to the heterogeneity of background colours found in their respective habitats may offer important insight into how cryptic colour change evolves in natural populations. In the present study, we examined whether nonbreeding dorsal body coloration has diverged between sympatric species of stickleback along with changes in habitat‐specific background colours. The small, limnetic species primarily occupies the pelagic zone and the large, benthic species inhabits the littoral zone. We placed benthic and limnetic sticklebacks against extremes of habitat background colours and measured their degree of background matching and colour plasticity. Benthics matched the littoral background colour more closely than did the limnetics, although there was no difference between species in their resemblance to the pelagic background colour. Benthics were able to resemble both background colours by exhibiting greater directional colour plasticity in their dorsal body coloration than limnetics, which may be an adaptive response to the greater spectral heterogeneity of the littoral zone. The present study highlights how habitat‐specific spectral characteristics may shape cryptic coloration differences between stickleback species. © 2011 The Linnean Society of London, Biological Journal of the Linnean Society, 2011, 102 , 902–914.     

Escaping malnutrition by shifting habitats: A driver of three-spined stickleback invasion in Lake Constance

Fatty acids, and especially long-chain polyunsaturated fatty acids, are biologically important components in the metabolism of vertebrates, including fish. Essential fatty acids (EFA) are those that in a given animal cannot be synthesized or modified from precursors and must therefore be acquired via the diet. Because EFAs are often unevenly distributed in nature, this requirement may drive species to make behavioral or ecological adaptations to avoid malnutrition. This is especially true for fish like the three-spined stickleback (Gasterosteus aculeatus L.) of Upper Lake Constance (ULC), whose recent marine ancestors evolved with access to EFA-rich prey, but which found themselves in an EFA-deficient habitat. An unexpected and unprecedented ecological shift in the ULC stickleback population from the littoral to pelagic zones in 2012 might be linked to EFA availability, triggering ecological release and enabling them to build a hyperabundant population while displacing the former keystone species, the pelagic whitefish Coregonus wartmanni. To test this hypothesis, sticklebacks from the littoral and pelagic zones of ULC were sampled seasonally in two consecutive years, and their stomach contents and fatty acid profiles were analysed. Pelagic sticklebacks were found to possess significantly higher values of an important EFA, docosahexaenoic acid (DHA), especially during autumn. Evaluation of the DHA supply suggests that sticklebacks feeding in the littoral zone during autumn could not meet their DHA requirement, whereas DHA availability in the pelagic zone was surplus to demand. During autumn, pelagic sticklebacks consumed large amounts of DHA-rich prey, that is, copepods, whereas littoral sticklebacks relied mainly mostly on cladocerans, which provide much lower quantities of DHA. Access to pelagic zooplankton in 2012 was possibly facilitated by low densities of previously dominant zooplanktivorous whitefish. The present study offers a convincing physiological explanation for the observed expansion of invasive sticklebacks from the littoral to the pelagic zones of Lake Constance, contributing to a phase shift with severe consequences for fisheries.     

Evidence for genetic differences in the offspring of two sympatric morphs of Arctic charr

The sub-arctic Lake Fjellfrøsvatn, northern Norway, has two morphs of Arctic charr that are reproductively isolated because they spawn 5 months apart. The smaller morph (≤14 cm L_F ) is confined to the profundal zone of the lake and the larger morph is mainly littoral. Three hypotheses were tested: (i) the offspring of the profundal Arctic charr grow slower than the offspring of the littoral Arctic charr under identical conditions, thus indicating a genetic basis for the slow growth of the profundal Arctic charr in the wild; (ii) the wild phenotypes of the two morphs are morphometrically different and the differences are persistent in the offspring; (iii) the offspring of the two morphs have different behaviour traits under similar treatments. The first hypothesis was rejected; offspring of the profundal morph grew slightly better than offspring of the littoral morph at 10° C in the laboratory. The second and third hypotheses were supported by the data. Wild-caught fish of the two morphs differed in several morphometric characters and most of the differences persisted in the offspring. In the laboratory, offspring of the littoral morph were more active, more aggressive and more pelagic than offspring of the profundal morph and naive offspring of the profundal morph were more effective in eating live chironomid larvae than were offspring of the littoral morph. The data for morphometry and behaviour, but not growth, provide evidence for genetic differences between the two Arctic charr morphs of Fjellfrøsvatn.     

Stable isotopes indicate that zebra mussels (Dreissena polymorpha) increase dependence of lake food webs on littoral energy sources

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Adaptation of rainbow fish to lake and stream habitats

Fish occupy a range of hydrological habitats that exert different demands on locomotor performance. We examined replicate natural populations of the rainbow fishes Melanotaenia eachamensis and M. duboulayi to determine if colonization of low-velocity (lake) habitats by fish from high-velocity (stream) habitats resulted in adaptation of locomotor morphology and performance. Relative to stream conspecifics, lake fish had more posteriorly positioned first dorsal and pelvic fins, and shorter second dorsal fin bases. Habitat dimorphism observed between wild-caught fish was determined to be heritable as it was retained in M. eachamensis offspring raised in a common garden. Repeated evolution of the same heritable phenotype in independently derived populations indicated body shape divergence was a consequence of natural selection. Morphological divergence between hydrological habitats did not support a priori expectations of deeper bodies and caudal peduncles in lake fish. However, observed divergence in fin positioning was consistent with a family-wide association between habitat and morphology, and with empirical studies on other fish species. As predicted, decreased demand for sustained swimming in lakes resulted in a reduction in caudal red muscle area of lake fish relative to their stream counterparts. Melanotaenia duboulayi lake fish also had slower sustained swimming speeds (Ucrit) than stream conspecifics. In M. eachamensis, habitat affected Ucrit of males and females differently. Specifically, females exhibited the pattern observed in M. duboulayi (lake fish had faster Ucrit than stream fish), but the opposite association was observed in males (stream males had slower Ucrit than lake males). Stream M. eachamensis also exhibited a reversed pattern of sexual dimorphism in Ucrit (males slower than females) relative to all other groups (males faster than females). We suggest that M. eachamensis males from streams responded to factors other than water velocity. Although replication of muscle and Ucrit phenotypes across same habitat populations within and/or among species was suggestive of adaptation, the common garden experiment did not confirm a genetic basis to these associations. Kinematic studies should consider the effect of the position and base length of dorsal fins.     

Depth gradients in food‐web processes linking habitats in large lakes: Lake Superior as an exemplar ecosystem

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Differential time budgets of two forms of juvenile brook charr in the open-water zone

No significant differences were found in the time budget (time spent in feeding, moving and stationary), attack rate (number of feeding bouts min⁻¹), and microhabitat use of juvenile (1+ years) littoral and pelagic brook charr Salvelinus fontinalis at 2 and 4 m depth, when restricted to feeding in pelagic enclosures. In contrast, fish of the littoral form allocated significantly more time than pelagic ones to feeding, moving and in total activity at 3 m depth. No significant differences were found in attack rate between the two forms at any given depth. Based on the mean for the water column (all depths pooled), however, fish of the littoral form executed a significantly higher attack rate than fish of the pelagic one. In multiple regressions analyses, the best predictors of time allocated to feeding and attack rate were the dummy variable 'form' (littoral individuals spend significantly more time in feeding than pelagic ones), light intensity at the surface (negative) and water temperature (positive), and explained 48 and 55% of these variations, respectively. Time allocated to moving was only explained by water temperature (negative) and explained 43% of the variation. Time in a stationary position was best explained by water temperature (negative) and light intensity at the surface (positive), explaining 52% of the variation. The results of this study support the hypothesis that littoral brook charr spend more energy in foraging than pelagic ones when restricted to feeding in the pelagic habitat, and thus that trophic diversification is adaptive in this species.