Blue linckia (four-armed)

Description:

These sea stars may grow up to 30 cm in diameter, with rounded tips at each of the arms; some individuals may bear lighter or darker spots along each of their arms. Individual specimens are typically firm in texture, possessing the slightly tubular, elongated arms.

Habitat:

A coral lagoon at the south end of the Coral Sea.

Notes:

This, and one other individual I spotted, had only four arms. It doesn't seem to be that one arm was lost to injury, but rather naturally occurring a mutation.