Dwarf brooder versus giant broadcaster: combining genetic and reproductive data to unravel cryptic diversity in an Antarctic brittle star

Poecilogony, or multiple developmental modes in a single species, is exceedingly rare. Several species described as poecilogenous were later demonstrated to be multiple (cryptic) species with a single developmental mode. The Southern Ocean is known to harbor a high proportion of brooders (Thorson's Rule) but with an increasing number of counter examples over recent years. Here we evaluated poecilogony versus crypticism in the brittle star Astrotoma agassizii across the Southern Ocean. This species was initially described from South America as a brooder before some pelagic stages were identified in Antarctica. Reproductive and mitochondrial data were combined to unravel geographic and genetic variation of developmental modes. Our results indicate that A. agassizii is composed of seven well supported and deeply divergent clades (I: Antarctica and South Georgia; II: South Georgia and Subantarctic locations including Kerguelen, Patagonian shelf, and New Zealand; III-VI-VII: Patagonian shelf...