In sea stars, what is the free-swimming larval stage called?

In sea stars, the free-swimming larval stage is called a bipinnaria.

A bipinnaria is a bilateral, planktonic larva that hatches from a sea star egg. Unlike the adult, which has radial symmetry, the bipinnaria has a distinct left-right body plan. Bands of cilia along its arms create feeding currents and help it move through the water.

The larva generally feeds on microscopic plankton before developing additional structures and transforming into a later stage called a brachiolaria. During metamorphosis, the juvenile sea star develops the adult's characteristic radial organization. This dramatic change is one reason sea stars are useful examples in marine developmental biology.

Bipinnaria should not be confused with a nauplius, the common early larva of many crustaceans, or a veliger, the larval stage of many molluscs. Some sea star species have modified development and may skip a feeding bipinnaria stage, but the term remains the standard name for this typical larval form.