sea anemones

Phylum: Cnidaria
Class: Anthozoa
Order: Actiniaria

A sea anemone usually attaches itself to rocks or coral. They have a central mouth which is surrounded by tentacles with nematocysts, stinging cells that paralyze and entangle small marine animals. Sea anemones primarily reproduce sexually: most often, their eggs are fertilized in the gastric cavity, and then their young are released through the mouth temporarily as free-swimming larvae--they soon find somewhere to attach themselves to. A few sea anemones have symbiotic relationships with some hermit crabs; they attach themselves on the shell of the hermit crab (or they are attached by the hermit crab). There are theories that this is beneficial to the hermit crab for protection since the stinging cells on certain sea anemones can be quite potent, and the sea anemone may benefit from scraps of the hermit crab's food that it did not eat. They are the relatives of and sea fans. They exist only in the polyp stage and obtain their food by filter feeding.