Invertebrates in the Plankton: Echinodermata

The echinoderms include the familiar sea stars, brittle stars, and sea urchins, as well as the more enigmatic sea cucumbers and crinoids.  All species are marine, and most live in benthic habitats.  Many of the animals have a planktonic larval stage, some of which may live in the plankton for months before settling as adults.  Echinoid larvae are among the smaller meroplankton, approximately 0.01-0.03 mm in length, and are relatively rare to find in the plankton.  Consequently, we borrowed some specimens from other researchers at FHL for these photographs.


Sea stars, or asteroids, develop through several larval stages, including this brachiolaria larva of the ochre seastar, Pisaster ochraceous.  The larva uses its ciliated arms to sweep food into its mouth as it glides through the water column.  The arms can also be used to supplement the larva's cilia-drive locomotion.  Each arm has a glandular tip, with which the larva attaches itself to the substratum as it settles.  The animal is then able to metamorphose into the familiar five-armed adult form.



The brittle stars, or ophioroids, have a distinctive larval form known as the ophiopluteus.  Like all echinoderm larvae, the ophiopluteus uses ciliated bands to feed on particles suspended in the water column.  Brittle star larvae may be found in the plankton throughout the year in this region, and are thought to spend several weeks in the plankton before settling as juveniles.


The echinopluteus larva (1 mm wide) of the green sea urchin uses its extensive ciliated band for swimming and suspension feeding.  As the larva develops, it will add arms, changing from the four-armed animal shown here to a six and then eight-armed individual.  The body form changes dramatically with metamorphosis.  Many of the larval structures used during planktonic life are lost, to be replaced by appendages adapted to the adult's benthic lifestyle.

Young sand dollars have a pluteus larva as well.  This larva captures food along a ciliated band that loops around the larval arms.  Food is then transfered by cilia to the mouth.  The body shape slows the natural tendency of the larva to sink, enabling it to stay in the food-rich upper layers of the coastal ocean.  Echinoids have separate sexes and reproduce by free spawning gametes into open water.



Sea cucumbers, such as this larva of Parastichopus californicus, begin life as a feeding planktonic larvae in the early summer.  They swim in a gliding motion, using a ciliated band, similarly to other echinoderm larva.  The larvae feed in the plankton for one to two months, after which they settle subtidally, in areas with high current flow.

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