A bipinnaria is the first stage in the
larva
A larva (; plural larvae ) is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle.
The ...
l development of most
starfish, and is usually followed by a
brachiolaria stage. Movement and feeding is accomplished by the bands of
cilia. Starfish that brood their young generally lack a bipinnaria stage, with the eggs developing directly into miniature adults
The bipinnaria is free-living, swimming as part of the
zooplankton
Zooplankton are the animal component of the planktonic community ("zoo" comes from the Greek word for ''animal''). Plankton are aquatic organisms that are unable to swim effectively against currents, and consequently drift or are carried along by ...
. When it initially forms, the entire body is covered by cilia, but as it grows, these become confined to a narrow band forming a number of loops over the body surface. A pair of short, stubby arms soon develop on the body, with the ciliated bands extending into them.
In addition to propelling the larva through the water, the cilia also catch suspended food particles, and deliver them to the mouth (more correctly called a ''stomodeum'').
Eventually, three additional arms develop at the front end of the larva; at this point it becomes a
brachiolaria. In some species, including the common starfish ''
Asterias
''Asterias'' is a genus of the Asteriidae family of sea stars. It includes several of the best-known species of sea stars, including the (Atlantic) common starfish, ''Asterias rubens'', and the northern Pacific seastar, ''Asterias amurensis''. Th ...
'', the bipinnaria develops directly into an adult.
It is very similar in appearance to the
tornaria larvae of some
Hemichordata, reflecting the descent of the
Ambulacraria
Ambulacraria , or Coelomopora , is a clade of invertebrate phyla that includes echinoderms and hemichordates; a member of this group is called an ambulacrarian. Phylogenetic analysis suggests the echinoderms and hemichordates separated around 533 ...
from a common ancestor.
References
Echinoderm biology
Larvae
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