Sea pen

Abstract: Observations were made with scuba on the diet and behavior of 18 species of undisturbed sea stars in their natural habitats along the shores of Washington state through all seasons. Some sea stars are specialists. Hippasteria spinosa feeds almost exclusively on a sea pen; Solaster stimpsoni eats holothurians; full—sized Orthasterias koehleri consume the venerid clam Humilaria; Solaster dawsoni eats its congener S. stimpsoni. Others show remarkably variable diets. In different habitats, Dermasterias imbricata specializes on either anemones, holothurians or sea pens, but within these habitats its diet is consistent throughout the year. The diet of Mediaster aequalis varies with both habitat and season. Pycnopodia helianthoides feeds on sea urchins on rocky substrata but digs clams from sand and cobble. Many other sea stars, including Luidia foliolata, Pteraster tesselatus, Pisater ochraceus, Evasterias troschelii and Leptasterias hexactis, are quite generalized in their diet and, though often demonstrating preferences in laboratory studies, will feed on a variety of prey determined largely by relative abundance of prey species in the particular habitat. The areal and seasonal variation in the diet of sea stars coupled with the reluctance of some to eat any prey in the laboratory makes extension of laboratory observations to the field diet suspect. Laboratory observations can be used to provide a more detailed understanding of field observations. Several previously undescribed behavioral mechanisms of food capture in asterioids and escape or defense responses or prey are described: Orthasterias pulls chips of shell off Humilaria until an opening is made, thus allowing exploitation of a prey species unavailable to other clam—eating asteroids. A number of sea stars, such as Luidia, Hippasteria, Mediaster, Pisaster brevispinus, Orthasterias and Pycnopodia dig into mud, sand or cobble substrata to obtain infaunal prey. The locomotory patterns of S. dawsoni and Crossaster papposus tend to allow encounter with their prey in such a way as to reduce the effectiveness of the escape response. A pushing response by Solaster stimpsoni and autotomy in Pycnopodia and Evasterias are thought to be effective responses to S. dawsoni predation. The similar swimming responses to two anemones, a nudibranch, and a holothurian are discussed in relation to asteroid predation. The avoidance responses that many invertebrates show to asteroids are correlated with predator—prey relationships. Two mechanisms may obscure this correlation. The very success of the response may effectively remove the responding organism from the predator's diet. Thus scallops, although abundant, form a very inconspicuous portion of starfish diet. Also biochemical similarities between the predator and other organisms may make a prey species unable to distinguish only its predator. Thus Pycnopodia will move rapidly away from Crossaster and Solaster stimpsoni as well as from S. dawsoni, although the latter is its only asteroid predator. The similarities may be based either on a close taxonomic relationship with the predator, or on the diet of the other organism including species closely related to the responding prey species.