Sea Urchins: The Marine Hedgehogs

Red Sea Urchins in a kelp forest off the coast of British Columbia  (Photo credit: NatGeo)

The sea urchin is a small echinoderm found in marine ecosystems from the very depths of the ocean to the intertidal zone. Feeding primarily on algae and small invertebrates, these spectacular pin-cushions move along the ocean floor using tubular feet powered by a water vasculature system. In 14th century England, the term "urchin" was used to refer to the hedgehog. Thus, when these spiky marine creatures were first described, they were given the name "sea urchin." From kina in New Zealand, to uni in Japan, sea urchin is served in local cuisine all around the world. But perhaps more importantly, this organism has helped better our understanding of animal embryonic development. A dioecious organism, the male and female urchins produce sperm and eggs, respectively. Both gametes are dispersed into the open ocean in synchrony, in order to increase the likelihood of fertilization.

In a recent laboratory exercise, I had the opportunity to collect gametes from live sea urchins of the species Lytechinus variegatus. After observing the sperm and eggs under a microscope, we mixed the two and watched as the eggs were fertilized, began to cleave, and eventually gave rise to motile, feeding plutei. The video below, of a 48 hour pluteus was created by taking a series of images through the larva on Rhode Island College's new Olympus FluoView 1000 confocal laser scanning microscope. The green coloration is the result of autofluorescence.

Thanks to Dr. Thomas Meedel of Rhode Island College for the excellent lab, and Dr. Eric Roberts for his assistance with the confocal microscopy.