Temperature-related increase in growth rate in four freshwater lake fish species.

Growth is one of the most direct and common ways fish respond to climate change, as fish growth is intimately linked to the temperature of the environment. Observational studies on the effect of shifts in temperature on fish growth are scarce for freshwater fish, and particularly lacking for lake populations. Here, changes in growth rate of bream (Abramis brama), perch (Perca fluviatilis), pikeperch (Sander lucioperca), and roach (Rutilus rutilus) over three decades were studied and compared with changes in temperature in the two largest lakes of western Europe: Lake IJsselmeer and Lake Markermeer in the Netherlands. In the autumnal survey catches of bream, perch, and roach, the mean length of YOY increased significantly between 1992 and 2021 in both lakes, but for YOY pikeperch, no temporal changes were found. In a length-stratified dataset of age groups of bream, roach, and perch, the relationship between length and age differed significantly between time periods. In the more recent time periods, indications for higher growth rates across multiple ages were found. Temperature during the growth season increased in the same decades and showed significant correlations with the YOY mean length, for bream, perch, and roach in both lakes, and for pikeperch in Lake Markermeer. These results point toward consistent temperature-induced increases in growth over the age groups for bream, roach, and perch. These increases were found despite the simultaneous process of de-eutrophication in this water system and its potential negative effect on food production. For pikeperch, it is hypothesized that the absence of temporal increase in YOY growth rate is related to its necessary switch to piscivory and subsequent food limitation; the lower thermal range of its main prey smelt, Osmerus eperlanus, is hypothesized to have inhibited food availability for YOY pikeperch and its opportunity to achieve higher growth rates.

Strong species structure but weak geographical structure in demersal Lake Victoria cichlids.

Studying phenotypic and genetic differentiation between very young species can be very informative with regard to learning about processes of speciation. Identifying and characterizing genetic species structure and distinguishing it from spatial genetic structure within a species is a prerequisite for this and is often not given sufficient attention. Young radiations of cichlid fish are classical speciation study systems. However, it is only during the past decade that population genomics based on next-generation sequencing has begun to provide the power to resolve species and distinguish speciation from spatial population structure for the youngest of these radiations. The Lake Victoria haplochromine cichlids constitute the youngest large cichlid fish radiation, probably <20,000 years old. Earlier work showed that communities of rocky reef cichlids are composed of many reciprocally monophyletic species despite their very recent origins. Here, we build on this work by studying assemblages of offshore demersal cichlids, adding analyses of within-species spatial structure to the sympatric species structure. We sampled seven multispecies communities along a 6-km-long transect from one side of the Mwanza Gulf to the other side. We investigated whether phenotypically diagnosed putative species are reciprocally monophyletic and whether such monophyly is stable across species geographic ranges. We show that all species are genetically strongly differentiated in sympatry, that they are reciprocally monophyletic, and that monophyly is stable across distribution ranges. We found significant differentiation between geographically distinct populations in two species, but no or weak isolation by distance. We further found subtle but significant morphological differences between all species and a linear relationship between genomic and morphological distance which suggests that differences in morphology begin to accumulate after speciation has already affected genome-wide restrictions of gene flow.

The onset of ecological diversification 50 years after colonization of a crater lake by haplochromine cichlid fishes.

Adaptive radiation research typically relies on the study of evolution in retrospective, leaving the predictive value of the concept hard to evaluate. Several radiations, including the cichlid fishes in the East African Great Lakes, have been studied extensively, yet no study has investigated the onset of the intraspecific processes of niche expansion and differentiation shortly after colonization of an adaptive zone by cichlids. Haplochromine cichlids of one of the two lineages that seeded the Lake Victoria radiation recently arrived in Lake Chala, a lake perfectly suited for within-lake cichlid speciation. Here, we infer the colonization and demographic history, quantify phenotypic, ecological and genomic diversity and diversification, and investigate the selection regime to ask if the population shows signs of diversification resembling the onset of adaptive radiation. We find that since their arrival in the lake, haplochromines have colonized a wide range of depth habitats associated with ecological and morphological expansion and the beginning of phenotypic differentiation and potentially nascent speciation, consistent with the very early onset of an adaptive radiation process. Moreover, we demonstrate evidence of rugged phenotypic fitness surfaces, indicating that current ecological selection may contribute to the phenotypic diversification.

Prevalence of disruptive selection predicts extent of species differentiation in Lake Victoria cichlids.

Theory suggests that speciation with gene flow is most likely when both sexual and ecological selection are divergent or disruptive. Divergent sexual and natural selection on the visual system have been demonstrated before in sympatric, morphologically similar sister species of Lake Victoria cichlids, but this does not explain the subtle morphological differences between them. To investigate the significance of natural selection on morphology during speciation, we here ask whether the prevalence of disruptive ecological selection differs between sympatric sister species that are at different stages of speciation. Some of our species pairs do (Pundamilia) and others do not (Neochromis) differ distinctively in sexually selected male nuptial coloration. We find that (i) evidence for disruptive selection, and for evolutionary response to it, is prevalent in traits that are differentiated between sister species; (ii) prevalence of both predicts the extent of genetic differentiation; and (iii) genetic differentiation is weaker in species pairs with conserved male nuptial coloration. Our results speak to the existence of two different mechanisms of speciation with gene flow: speciation mainly by sexual selection tightly followed by ecological character displacement in some cases and speciation mainly by divergent ecological selection in others.

Climatic variability in combination with eutrophication drives adaptive responses in the gills of Lake Victoria cichlids.

Textbook examples of adaptive radiation often show rapid morphological changes in response to environmental perturbations. East Africa's Lake Victoria, famous for its stunning adaptive radiation of cichlids, has suffered from human-induced eutrophication over the past decades. This cultural eutrophication is thought to be partly responsible for the dramatically reduced cichlid biodiversity, but climatic variability in itself might also have contributed to the eutrophication which resulted in low oxygen levels and decreased water transparency. To determine how recent environmental changes have influenced the lake and its cichlids over the past 50 years, we gathered environmental and meteorological variables and compared these with gill surface area of four cichlid species. We found that during the period of severe eutrophication and temperature increase (1980s), reduced wind speeds coincided with a reduction in oxygen levels and a decrease in both water temperature and transparency. The gill surface area in three out of the four cichlid species increased during this period which is consistent with adaptive change in response to increased hypoxia. During the 2000s, wind speeds, oxygen levels, water transparency and water temperature increased again, while cichlid gill surface area decreased. Our results imply that climatic changes and especially wind speed and direction might play a crucial role in tropical lake dynamics. The changes in Lake Victoria's water quality coincide with fluctuations in cichlid gill surface area, suggesting that these fish can respond rapidly to environmental perturbations, but also that climatic variability, together with continued eutrophication, might be detrimental to the lake's cichlid biodiversity.

Fast adaptive responses in the oral jaw of Lake Victoria cichlids.

Rapid morphological changes in response to fluctuating natural environments are a common phenomenon in species that undergo adaptive radiation. The dramatic ecological changes in Lake Victoria provide a unique opportunity to study environmental effects on cichlid morphology. This study shows how four haplochromine cichlids adapted their premaxilla to a changed diet over the past 30 years. Directly after the diet change toward larger and faster prey in the late 1980s, the premaxilla (upper jaw) changed in a way that is in agreement with a more food manipulating feeding style. During the 2000s, two zooplanktivorous species showed a reversal of morphological changes after returning to their original diet, whereas two other species showed no reversal of diet and morphology. These rapid changes indicate a potential for extremely fast adaptive responses to environmental fluctuations, which are likely inflicted by competition release and increase, and might have a bearing on the ability of haplochromines to cope with environmental changes. These responses could be due to rapid genetic change or phenotypic plasticity, for which there is ample evidence in cichlid fish structures associated with food capture and processing. These versatile adaptive responses are likely to have contributed to the fast adaptive radiation of haplochromines.

Habitat-related birdsong divergence: a multi-level study on the influence of territory density and ambient noise in European blackbirds.

Song plays an important role in avian communication and acoustic variation is important at both the individual and population level. Habitat-related variation between populations in particular can reflect adaptations to the environment accumulated over generations, but this may not always be the case. In this study, we test whether variation between individuals matches local conditions with respect to noise level and territory density to examine whether short-term flexibility could contribute to song divergence at the population level. We conducted a case study on an urban and forest population of the European blackbird and show divergence at the population level (i.e. across habitats) in blackbird song, anthropogenic noise level and territory density. Unlike in several other species, we found a lack of any correlation at the individual level (i.e. across individuals) between song features and ambient noise. This suggests species-specific causal explanations for noise-dependent song differentiation which are likely associated with variation in song-copying behaviour or feedback constraints related to variable singing styles. On the other hand, we found that at the level of individual territories, temporal features, but not spectral ones, are correlated to territory density and seasonality. This suggests that short-term individual variation can indeed contribute to habitat-dependent divergence at the population level. As this may undermine the potential role for song as a population marker, we conclude that more investigations on individual song flexibility are required for a better understanding of the impact of population-level song divergence on hybridisation and speciation.