marine biology - Which poison makes seastars inedible to possible predators?

In the new citizen science project (see: Sea Floor Explorer), numbers of seastars, scallops, crustaceans and other animals are counted. Already one can see a heavy bias in favor of seastars, both the fat and brittle kind.

I would be interested in why this creature is so succesful, especially if it is poisonous to e.g., crustaceans, and what poisons exactly are responsible.