Echinodermata: Life History and Ecology

All echinoderms are marine; none can live in fresh water or on land. Echinoderms are also not microscopic, except for their larvae; they range from a few millimeters to a few decimeters in size, although the stalks of some crinoids could reach a length of over a meter. With a few exceptions, echinoderms are all benthic (bottom-dwellers); most Paleozoic echinoderms were sessile, while most living echinoderms can creep from place to place. Few can swim or float.

Within these limits, echinoderms have diversified into a number of life styles. Some, like many starfish, are predators; holothurians, sand dollars, and ophiuroids often feed on detritus; crinoids are filter-feeders; sea urchins scrape algae from rocks. Starfish and sea urchins may be common in very shallow water, while the floor of the deep sea may swarm with ophiuroids or holothurians.

Reproduction in echinoderms is typically by external fertilization; eggs and sperm are freely discharged into the water. A few sea urchins brood their eggs in special pouches, but most provide no parental care. Most echinoderms go through several planktonic larval stages before settling down.