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, IV-V: South Georgia). Two of these clades demonstrated strong size dimorphism when in sympatry and can be linked to differing developmental modes (Clade V: dwarf brooder versus Clade I: giant broadcaster). Based on their restricted geographic distributions and on previous studies, it is likely that Clades III-VI-VII are brooders. Clade II is composed of different morphological species, A. agassizii and A. drachi, the latter originally used as the outgroup. By integrating morphology, reproductive and molecular data we conclude that the variation identified in A. agassizii is best described as crypticism rather than poecilogony.