Biological traits and identification tools of early life stages of two sympatric species of Chionodraco in the Ross Sea

The genus Chionodraco includes three closely related species, two of them ( C. hamatus and C. myersi ) are sympatric throughout the entire circum-Antarctic distribution. As adults, they exhibit subtle morphological differences, whereas larval stages were considered to be indistinguishable so far. By combining genetics and pigmentation pattern analysis of early life stages of Chionodraco collected in January in eastern and western sectors of the Ross Sea, we attempted for the first time to solve this issue. In addition, microincrement counts on sagittal otoliths allowed us to shed more light on their early life-history and reproductive traits. Based on mitochondrial markers, the larval stages of the two species were genetically distinctive, differing also in the lateral body pigmentation. Spatial and demographic structure of sampled populations differed significantly as well, C. myersi being characterized by a wider distribution and less advanced larval stages than C. hamatus . Daily growth rate and the corresponding percentage change in size were higher in C. myersi than in C. hamatus . Starting from individual age estimates and the date of capture, timing and larval size at hatch were backcalculated. Interestingly, C. myersi hatched later and at a smaller size than C. hamatus , consistently with the spawning season and egg size of gravid females reported in previous studies. Present results provide evidence of an existing premating barrier preventing hybridization between the two sympatric species.