Green Sea Urchin (Strongylocentrotus droebachiensis)
Like the sea star, this echinoderm is radially symmetrical and has an oral surface with a mouth in the center. Shaped like a squashed sphere, its hard outer covering is called a test. Unlike the sea star, the tube feet cover almost the entire surface of the urchin. In addition, it has spines that are jointed at the base, and the urchin can move by moving both its very long, suckered, tube feet and its lower spines in a concerted fashion. It moves slowly but can cover surprising ground.
The anus of sea urchins is in the center of the aboral (upper) side. It is surrounded by a series of plates, five genital plates alternating with five ocular plates. One of the genital plates is porous and serves as the madreporite, where water enters the water-vascular system.
Sea urchins are herbivores, marine algae of many kinds their primary diet. Bull Kelp is one of their major foods in our area. Research has shown that when Sea Otters disappear from an area, urchins (a common otter prey) can reach an abundance sufficient to completely wipe out algae over large areas.
Urchins have a complex feeding structure called Aristotle’s lantern. It features support structures, extensive musculature, and five sets of triangular plates that function as teeth while the urchin grazes on the substrate. The entire lantern is protracted (pushed out) and the plates opened with one set of muscles, then brought together with another set. The urchins can scrape off encrusting material or bite off chunks of algae with these plates.
Sea urchins can be either male or female. They reproduce by shedding gametes into the water, and mass spawning in spring assures that many of the eggs will be fertilized. The eggs develop into echinopluteus larvae that look vaguely like upside-down jellyfish but are bilaterally symmetrical. At a very small size, they settle out of the plankton and undergo metamorphosis into a tiny urchin as small as 2-3 mm in diameter, usually hidden away under something, presumably to avoid predators.
Sea urchins have long been used as food by humans. The gonads are a rich source of protein and are relished, salty as they are, by some groups of people. They are usually eaten raw on the “half shell” of an opened urchin