ALBERT

Description: Cichlid fishes’ famous diversity in body coloration is accompanied by a highly diverse and complex visual system. Although cichlids possess an unusually high number of seven cone opsin genes, they express only a subset of these during their ontogeny, accounting for their astonishing interspecific variation in visual sensitivities. Much of this diversity is thought to have been shaped by natural selection as cichlids inhabit a variety of habitats with distinct light environments. Also, sexual selection might have contributed to the observed visual diversity, and sexual dimorphism in coloration potentially co‐evolved with sexual dimorphism in opsin expression. We investigated sex‐specific opsin expression of several cichlids from Africa and the Neotropics and collected and integrated datasets on sex‐specific body coloration, species‐specific visual sensitivities, lens transmission and habitat light properties for some of them. We comparatively analyzed this wide range of molecular and ecological data, illustrating how integrative approaches can address specific questions on the factors and mechanisms driving diversification, and the evolution of cichlid vision in particular. We found that both sexes expressed opsins at the same levels ‐ even in sexually dimorphic cichlid species – which argues against coevolution of sexual dichromatism and differences in sex‐specific visual sensitivity. Rather, a combination of environmental light properties and body coloration shaped the diversity in spectral sensitivities among cichlids. We conclude that although cichlids are particularly colorful and diverse and often sexually dimorphic, it would appear that natural rather than sexual selection is a more powerful force driving visual diversity in this hyper‐diverse lineage.

Description: Sympatric speciation occurs without geographical barriers and is thought to often be driven by ecological specialization of individuals that eventually diverge genetically and phenotypically. Distinct morphologies between sympatric populations occupying different niches have been interpreted as such differentiating adaptive phenotypes, yet differences in performance and thus likely adaptiveness between them were rarely tested. Here, we investigated if divergent body shapes of two sympatric crater lake cichlid species from Nicaragua, one being a shore‐associated (benthic) species while the other prefers the open water zones (limnetic), affect cruising (Ucrit) and sprinting (Usprint) swimming abilities ‐ performances particularly relevant to their respective lifestyles. Furthermore, we investigated species differences in oxygen consumption (MO2) across different swimming speeds and compare gene expression in gills and white muscle at rest and during exercise. We found a superior cruising ability in the limnetic Amphilophus zaliosus compared to the benthic A. astorquii, while sprinting was not different, suggesting that their distinct morphologies affect swimming performance. Increased cruising swimming ability in A. zaliosus was linked to a higher oxygen demand during activity (but not rest), indicating different metabolic rates during exercise ‐ a hypothesis supported by coinciding gene expression patterns of gill transcriptomes. We identified differentially expressed genes linked to swimming physiology, regulation of swimming behaviour and oxygen intake. A combination of physiological and morphological differences may thus underlie adaptations to these species’ distinct niches. This complex ecological specialization likely resulted in morphological and physiological trade‐offs that contributed to the rapid establishment and maintenance of divergence with gene flow.

Description: Evolutionary novelties—derived traits without clear homology found in the ancestors of a lineage—may promote ecological specialization and facilitate adaptive radiations. Examples for such novelties include the wings of bats, pharyngeal jaws of cichlids and flowers of angiosperms. Belonoid fishes (flying fishes, halfbeaks and needlefishes) feature an astonishing diversity of extremely elongated jaw phenotypes with undetermined evolutionary origins. We investigate the development of elongated jaws in a halfbeak (Dermogenys pusilla) and a needlefish (Xenentodon cancila) using morphometrics, transcriptomics and in situ hybridization. We confirm that these fishes' elongated jaws are composed of distinct base and novel ‘extension’ portions. These extensions are morphologically unique to belonoids, and we describe the growth dynamics of both bases and extensions throughout early development in both studied species. From transcriptomic profiling, we deduce that jaw extension outgrowth is guided by populations of multipotent cells originating from the anterior tip of the dentary. These cells are shielded from differentiation, but proliferate and migrate anteriorly during the extension's allometric growth phase. Cells left behind at the tip leave the shielded zone and undergo differentiation into osteoblast-like cells, which deposit extracellular matrix with both bone and cartilage characteristics that mineralizes and thereby provides rigidity. Such bone has characteristics akin to histological observations on the elongated ‘kype’ process on lower jaws of male salmon, which may hint at common conserved regulatory underpinnings. Future studies will evaluate the molecular pathways that govern the anterior migration and proliferation of these multipotent cells underlying the belonoids' evolutionary novel jaw extensions.

Description: The deep-sea is vast, remote, and largely underexplored. However, methodological advances in environmental DNA (eDNA) surveys could aid in the exploration efforts, such as using sponges as natural eDNA filters for studying fish biodiversity. In this study, we analyzed the eDNA from 116 sponge tissue samples and compared these to 18 water eDNA samples and visual surveys obtained on an Arctic seamount. Across survey methods, we revealed approximately 30% of the species presumed to inhabit this area and 11 fish species were detected via sponge derived eDNA alone. These included commercially important fish such as the Greenland halibut and Atlantic mackerel. Fish eDNA detection was highly variable across sponge samples. Highest detection rates were found in sponges with low microbial activity such as those from the class Hexactinellida. The different survey methods also detected alternate fish communities, highlighted by only one species overlap between the visual surveys and the sponge eDNA samples. Therefore, we conclude that sponge eDNA can be a useful tool for surveying deep-sea demersal fish communities and it synergises with visual surveys improving overall biodiversity assessments. Datasets such as this can form comprehensive baselines on fish biodiversity across seamounts, which in turn can inform marine management and conservation practices in the regions where such surveys are undertaken.