Phylogenetic conservatism of abiotic niche in sympatric Southwestern Atlantic skates

Abstract

ABSTRACT From the perspective of phylogenetic niche conservatism (PNC), closely related species must share more aspects of a niche than randomly expected. However, considering the competitive exclusion principle, PNC is questionable for related species in sympatry. This research aimed to test PNC in Riorajini, a tribe of four skates endemic to the Southwestern Atlantic Ocean: Atlantoraja castelnaui, A. cyclophora, A. platana, and Rioraja agassizii. We updated available distribution maps for these species with literature data, reconstructed the phylogenetic relationships of the group with genetic data (mt-nd2), modelled the ecological niche (ENM) of each species, and calculated niche overlap, equivalency and similarity between models. The results recovered Riorajini as a monophyletic group and indicated PNC with bathymetry, distance to shore and concentration of nitrate characterizing the tribe's abiotic niche. However, no linear relationship between phylogenetic proximity and niche similarity was clear since pairs of species more phylogenetically distant showed more similar niches (R. agassizii and A. castelnaui, > 70% niche similarity, P < 0.005) than congeners (A. castelnaui and A. platana, 45%, P < 0.005). ENMs of each species showed different responses to variations in environmental predictors, with A. castelnaui and R. agassizii showing higher probability of occurrence in shallower waters, closer to shore, while A. cyclophora and A. platana explore deeper areas, although still constrained to the continental shelf. We discuss how these differences influence the distribution of each species in the environment and used the phylogeny to argue that R. agassizii and A. castelnaui share a conserved niche from an ancestral lineage.