Sclerasterias

Sclerasterias is a genus of starfish in the family Asteriidae. Adult individuals have five arms but small, immature individuals have six. This led to the giving of a separate generic name to the juveniles, Hydrasterias, before it was realised that only one genus was involved. These young individuals often undergo fissiparity. The disc splits into two parts, each bearing three arms, and new arms develop on each part to complete the complement of arms. This sometimes happens repeatedly and may be an adaptation to life in cold, deep seas where most of the species are found.