THE PACIFIC SEA STAR
by Michael J. Ellis
Imagine sitting down to a fine repast at your favorite French restaurant. Your entree of roast duck and pommes de terre francais is steaming hot. Suddenly your stomach comes rising out of your mouth and plops down right on the plate. Blobs of white flesh ooze digestive enzymes and gradually the food dissolves into a slimy paste, which is then absorbed by the lining of your stomach. After an hour or so you suck your stomach back in. Hmmm, not quite satisfied you ask the waiter for chocolate mousse.
Thank goodness this is not how humans eat. But for most sea stars it’s normal procedure. Along the outer coast of central and northern California the commonest sea star is Pisaster ochraceus. We will call it the Pacific seastar. Like human beings this species comes in several color forms including brown, purple and orange. And also like humans these varieties can interbreed. The five rigid arms and the entire top surface are covered with hundreds of tiny white spines. This seastar can tolerate exposure to air for up to eight hours. It may not move for weeks and when it does its motion is almost imperceptible. This is one tough critter; it inhabits an area that routinely gets smashed by giant waves.
Just off center on the top of the animal is a whitish plate. This is a sieve that filters ocean water. This filter is so efficient that even microscopic bacteria cannot get through it. The siphoned water is part of the hydovascular system that enables the animal to maintain its shape and to move. Thousands of tiny tube feet run out along canals in the arms, each tubefoot is capable of exerting suction against the substrate. Under nervous control the tubefeet enable the animal to slide along. This is definitely movement by committee!
Most slow moving objects in the sea are soon covered with fouling organisms such as barnacles and sponges. The Pacific seastar is slow but its surface remains free of organisms. For seastars this is very important because they breathe through their skin. On the skin are tiny pincers called pedicellariae. These snapping jaws remove any animal that attempts to homestead on the seastar’s back. The power of the pincers can be demonstrated by placing the back of a seastar on your forearm. Soon your hairs are firmly in the grasp of the pedicellariae. Ouch!
The Pacific seastar has few predators but birds will sometimes eat one. It’s a comical sight to watch a gull try to ingest a seastar. It may take hours of gulping before the bird can finally swallow it. The seastar’s stiff arms still protrude out the gull’s mouth and stretch its neck. Surely the taste cannot be worth the discomfort. Sea otters will also eat seastars but usually only bite off an arm or two. And since seastars can regenerate body parts the otters do no long-term damage to the population.
Seastars reproduce by shedding sperm and eggs into the ocean via pores in their armpits. Fertilization is external. The microscopic free-swimming larvae are bilaterally symmetrical. The larvae soon change into bottom-dwelling juveniles that grow into radially symmetrical adults. Growth is very slow and depends upon food supply and water temperature. The Pacific sea star can live for over twenty years.
Along the central coast the primary food of the Pisaster ochraceus is mussels, barnacles, limpets, and chitons – in that order. Using their powerful tube feet Pacific seastars only have to open a mussel’s shell .1 mm. Then they evert their stomach, put it into the mussel’s shell and begin eating. The overall effect on mussels is profound. This seastar is solely responsible for determining the lower limit of the mussel beds.
Like most of the other marine invertebrates, seastars are protected by law and may not be collected. What looks beautiful in a tidepool soon loses its color and shape at home and begins to stink. The seastar is then discarded in the garbage can – an ignoble end to such a tenacious survivor.