Comb sea stars, Astropecten sp., are common in our sandy inshore areas. Sea stars have no bones for their muscles to work against: instead they are pretty much hydraulic animals. They can draw in water to inflate, through a sieve like plate on their upper body. Coordinated muscle contractions then push the fluid about, moving their body parts. In this shot you can see the ‘tube feet’ which it uses to walk around, inflated and searching for something to walk on.
They can be scavengers or predators, eating pretty much whatever they can envelop and digest, dead or alive. It’s stomach can evert (turn inside out) through the mouth at the centre to digest food externally.
The tiny snails on the sea star are from the family Eulimnidae. They are said to be parasites on sea stars and sea urchins, and are thought to float to the surface and drift along to locate new animals to settle on.