When we look up at the stars our imagination is captured by their power, great distance, and, of course, stellar size. But equally impressive in a different way are the stars that crawl along the sea bottom here in the Antarctic and around the world – sea stars!
Sea stars are marine animals that belong to the echinoderm phylum. This makes them relatives of other marine invertebrates such as feather stars, brittle stars, sea cucumbers, sea urchin, and sand dollars. Sea stars, along with their echinoderm relatives, may not resemble us very much at first glance. But they in fact sit on our side of the branch, as what would be the ancestor to both of our lineages split from the rest of the animal kingdom around the Cambrian explosion. Adult sea stars exhibit radial symmetry, meaning their appendages are arranged around a central axis, contributing to that distinctive star-like look. Their relationship to us and other animals is more easily seen by looking at their larval development. Sea star larvae develop certain early embryonic structures in the same order that we do, and exhibit bilateral symmetry, the left side mirroring the right side.
Sea stars belong to the class Asteroidea, which roughly translates to star-like. Many sea stars are commonly seen with five arms, but some species can have up to 40 arms. Sea stars can readily regenerate lost arms. In many circumstances, species can re-grow an entire body from a lost arm. In addition to their distinctive shape and varied sizes, sea stars come in a variety of colors, as you see from the photos. This Ice Inverts expedition has given us a wonderful opportunity to see their colorful diversity firsthand.
As we move across the Southern Ocean, each sample site presents its own interesting and colorful species of sea stars, many of them endemic to Antarctica. Some of them, even within the same species, vary wildly in color. “I am not sure if these are the same thing,” is quite a frequently heard comment during our sorting process. In most cases, it is not clear why these organisms have such stunning colors or how it helps them survive down here. But whether it is for camouflage, plays into a chemical defense system, or simply a byproduct of their diet, these asteroids ‘shine’ bright in the dark depths of the Antarctic.