Livebearing fishes are a standard model for studying the effect of predation on prey biology. Numerous studies have found differences in life history, sexual selection, behavior, and morphology between populations of the same species that co-occur with predators and those that do not. Alfaro cultratus is a livebearing fish with populations in different predation environments, but unlike other livebearers, this species also has an extreme body shape that is laterally compressed. Given this unusual morphology, we asked if predation environment would still predict overall body shape, as has been documented in other species. We collected specimens from both predator and no-predator sites in Costa Rica. We used a geometric morphometrics analysis to determine if body shape is affected by the predation environment while controlling for size and river gradient. Body shape does indeed differ between predation environments; however, the observed differences contrast with the patterns found in other livebearer systems. Alfaro cultratus in predation environments had deeper and shorter bodies and deeper caudal peduncles than those found in environments without dominant fish predators.

Despite belonging to the most abundant and widespread genus of freshwater fishes in the region, the carp gudgeons of eastern Australia (genus Hypseleotris) have proved taxonomically and ecologically problematic to science since the 19th century. Several molecular studies and a recent taxonomic revision have now shed light on the complex biology and evolutionary history that underlies this group. These studies have demonstrated that carp gudgeons include a sexual/unisexual complex (five sexual species plus an assortment of hemiclonal lineages), many members of which also co-occur with an independent sexual relative, the western carp gudgeon (H. klunzingeri). Here we fill yet another knowledge gap for this important group by presenting a detailed molecular phylogeographic assessment of the western carp gudgeon across its entire and extensive geographic range. We use a suite of nuclear genetic markers (SNPs and allozymes) plus a matrilineal genealogy (cytb) to demonstrate that H. klunzingeri s.l. also displays considerable taxonomic and phylogeographic complexity. All molecular datasets concur in recognizing the presence of multiple candidate species, two instances of historic between-species admixture, and the existence of a natural hybrid zone between two of the three candidate species found in the Murray Darling Basin. We also discuss the major phylogeographic patterns evident within each taxon. Together these analyses provide a robust molecular, taxonomic, and distributional framework to underpin future morphological and ecological investigations on this prominent member of regional freshwater ecosystems in eastern Australia.