Astrotoma agassizii is a large brittle star that has a circumpolar Antarctic/subantarctic distribution and occurs throughout the southern part of South America, at depths of 80 to 1200 meters. This species is reportedly viviparous, with brooding occurring in the bursae at the base of the arms. Studies of geographic variation in mitochondrial DNA have revealed that individuals of this species from the Antarctic Peninsula belong to a lineage that is distinct from two South American lineages, although within lineages there is striking genetic continuity across a large geographic range (>500 km), a somewhat surprising finding. (Hunter and Halanych 2008 and references therein)
Heimeier et al. (2010) proposed a possible explanation for the genetic homogeneity of this species in the Antarctic Peninsula. Using mtDNA sequence analysis, they found that a variety of early developmental stages collected from plankton samples in the Ross Sea matched sequences of A. agassizii from the Antarctic Peninsula and that these A. agassizii developmental stages occurred regularly in plankton samples. They proposed that that the Antarctic lineage of this species has a planktonic dispersive stage (explaining the sequence homogeneity over great distances), with brooding restricted to the South American lineages, and that these lineages may represent two or more cryptic species.
