News From The World Of Fishes

Niche overlap in sympatric cichlids, including an incredibly enigmatic species. The cichlid genus Rocio includes a well-known species to many who enjoy keeping cichlids or other fishes in aquariums: Rocio octofasciata, aka the Jack Dempsey Cichlid, which are distributed from southern Mexico south into the Caribbean slope of Honduras. Another species in that genus, Rocio spinosissima, is endemic to the Río Dulce basin in eastern Guatemala and has long been considered a rare and fairly enigmatic species largely due to its scarce representation in collections despite fieldwork efforts. In fact, efforts by Cesar, Diego, and Caleb as part of the IUCN working group for Central American freshwater fishes, assessed the conservation status of R. spinosissima as endangered. Río Dulce is an important area to study to better understand the population status and ecology of this enigmatic cichlid, but given that both species of Rocio occur here, it is also an interesting system to test hypotheses about niche overlap between two sympatric closely related fishes. With Resident Graduate Student César Fuentes, Windsor Aguirre (DePaul University and FMNH Associate), and Christian Barrientos (Wildlife Conservation Society), and Caleb and Diego recently published a paper in the journal Ecology of Freshwater Fish aiming to answer these questions. Environmental data based on their fieldwork showed similarities in habitats for the two species, but a larger range of suitable area for the widespread Jack Dempsey Cichlid compared to the endemic R. spinosissima. Additionally, analysis of morphological data to study patterns of variation in body shape uncovered interesting results trends, not only in very clear differences between the two species, but differences that become more pronounced as the cichlids grow from juveniles to adults. One of the hopeful outcomes of this study was that while R. spinosissima warrants attention to habitat needs and availability, our ichthyologists were excited to find this species might not be quite as rare as initially thought. The results of this work set up new ideas and questions to be studied for these cichlids as well as other fishes in the area. This basin is the study site for a new NSF grant awarded to Caleb to study historical shifts in fish communities and populations.Disentangling historical relationships within livebearing fishes using genomic data.Livebearing fishes (family Poeciliidae) are widely distributed across the Americas, with over 270 species across 27 genera. FMNH ichthyologists Diego Elias (Bass Postdoctoral Fellow) and Caleb McMahan (Collections Manager of Fishes) are authors on a recent study in Molecular Phylogenetics & Evolution that used data based on sequences of ultraconserved elements in the first attempt to use genomic data to study evolutionary relationships and diversification in this family of fishes. Additionally, the authors used the large dataset to assess the influence of various analyses and evolutionary processes on our understanding of relationships across livebearing fishes. While a number of genera or major groups of livebearers were found to be monophyletic (descended from a common ancestor), there were several that were not, and their evolutionary history will be the basis of ongoing and future projects on these fishes—currently under study at the Field Museum.
April 26. 2024