No. 24–Echinoderms, Part II
With 1,500 living species in the class Asteroidea, sea stars make up fully one quarter of the echinoderm phylum.*
With most species sporting five appendages and a horny aboral surface, sea stars are easily recognizable as echinoderms. The species most often found on our sandy beaches is the gray sea star, Luidia clathrata. While none are currently in residence at the Estuarium, I have had the pleasure of accommodating these interesting creatures in the past. Another species known as the common sea star, Asterias forbesi, which has an orange to reddish coloration prefers rocky inshore surfaces like jetties and off shore structure and is occasionally found on local beaches after storms. I have not had the good fortune of finding a live specimen to date.
It’s been my experience that the gray sea star is easily stressed both during collection and in captivity and do not take well to being handled by humans. The first specimen I attempted to collect fell apart in my fingers. After that I adopted the practice of scooping up the creature with the sand beneath it. I also soon learned that even moderately active substrate tank mates can stress the creatures and lead to spontaneous self-dismemberment. One small longwristed hermit crab was pestersome enough to bring on such an incident.
Early on I observed that the tube feet of Luidia lacked the suckers most people associate with sea stars and therefore lacked the ability to pry bivalves open. Close observation of the animals movement with a hand lens revealed an intricate choreography of the little pointy feet digging into and propelling their master lightly and with some speed across the surface of the sand. Once, as an experiment, I removed the sand from a portion of the sea star tank and watched as the inhabitant spun its wheels, as it were, on the slick glass bottom.
The issue of what to feed Luidia in captivity was resolved the first time I dropped a mud snail in its tank. Very soon after doing so the sea star became active and began to move about the tank eventually zeroing in on the snail’s location. This action indicated the ability to detect and find prey species by water bourn scent. Upon contact the sea star positioned its central disc above the snail, pressed down and then engulfed it producing a noticeable bulge within the central disc. Approximately twenty minutes later the empty snail shell was expelled and the hunt began for a second introduced snail.
Sea stars have remarkable powers of regeneration which I have observed first-hand with the gray sea star. I recall an anecdotal story of abalone fishermen of the past having meat cleavers on board their boats to chop up a species of sea star in competition for the delectable gastropod. In their ignorance of the regenerative powers of the species they were only making matters worse by creating more sea stars. The source sited below documents one species, Asterias vulgaris, that is able to regenerate an entire creature beginning with as little as one arm and a fifth of the central disc.
Due to my lack of experience with the species, I have consulted Sea Shore Animals of the Southeast by Edward E. Rupert and Richard S. Fox for information on the afore mentioned common sea star, Asterias forbesi.
Asterias dose have sucker tube feet which it uses for both climbing and feeding on a wide range of sessile (anchored in place) prey species as well as mobile mollusks. While tightly grasping the prey animal the stomach everts through the mouth to envelop and digest the prey. When feeding on bivalves, the stomach membrane may be insinuated through an opening between the valves of as little as 0.1 mm. As much as 12 lb. of force may be applied by the sucker arms to open quahog clams.
The common sea star is a major threat to the sub tidal oyster fisheries of the Northeast as it has been known to consume quantities equal to half the annual commercial harvest. This is not a problem locally due to the fact that the vast majority of our oyster beds are intertidal.
I had hoped to address the self-healing abilities of the key hole urchin as stated in the previous column but an internet search has proven fruitless thus far and queries to experts in the field as yet go unanswered.
*Invertebrate Zoology, Seventh Edition – Rupert, Fox and Barnes