The pluteus larva (pictured here) is an intermediate stage in the life cycle of echinoderms (e.g. sea stars and sea urchins). The larva glides like a spaceship amongst the plankton in the ocean, and feeds on microscopic algae that it captures using its long ciliated arms that are supported by a skeleton of calcite rods (visible here as white lines in the image). The pluteus larva nurtures a developing sea biscuit, which eventually becomes too large for it. At this time the pluteus larva sinks to the bottom, and in one rapid movement, it opens its arms, and the body of the tiny sea biscuit is deposited onto the sand. The young sea biscuit slowly resorbs the remains of the larval tissues, and begins its new life between the grains of sand where it will grow into an adult sea biscuit.
Adult sea biscuits were collected from sand beds of São Sebastião Channel (São Sebastião, Brazil). The release of eggs and sperm were induced in the laboratory, and after in vitro fertilization, embryonic development was observed using dark field light microscopy. Embryos became swimming pluteus larvae, approximately 0.2 mm wide, which were fed with microalgae until they settled onto the sand on the bottom of the aquarium, and formed young sea biscuits.
Bruno C. Vellutini, PhD Centro de Biologia Marinha, University of Sao Paulo, Brazil