ROSENTHAL. TROPHIC INTERACTION BETWEEN SEA STAR AND GASTROPOD 



42 were feeding on dead L. opalescens. The P. 

 giganteus population had temporarily switched 

 from active predators to scavengers with the 

 availability of this abundant, though temporary, 

 food source. I returned to the study site on 

 April 10, 1970, and noted 6 out of 41 P. giganteus 

 to be feeding equally on C. pellucida, B. tintin- 

 nahulum. and H'mnites multirugosiw. With the 

 dead squid no longer present, the sea star pop- 

 ulation returned to feeding on the available prey 

 organisms of the reef. 



Feeding preference experiments by Mauzey 

 et al. (1968) and Landenberger (1968) indi- 

 cated that the sea star P. ochmceus preferred 

 mussels to alternative food items offered the 

 sea stars. Landenberger (1968) also showed 

 that P. giganteus preferred the mussels Mytihis 

 edulis and M. calif oi-nianus to five other mollus- 

 kan species. Additional observations by Paine 

 (1969) suggest that few other prey are con- 

 sumed as long as mussels are readily available 

 to P. ochraceus. However, mussels were rarely 

 available to P. giganteus in the four subtidal 

 locations examined. If P. giganteus has a spe- 

 cific food preference in these subtidal locations, 

 then it must be adaptable or capable of change, 

 since each habitat or microhabitat varies some- 

 what in prey availability. 



P. giganteus appears to exhibit a feeding 

 preference for prey that is either immobilized 

 or sedentary in habit, since these organisms 

 were eaten more frequently than motile forms. 

 Attached or boring bivalves, balanoid barnacles, 

 and sessile tube-dwelling mollusks were "pre- 

 ferred" or eaten most often by P. giganteus. 



P. giganteus preys on K. kelletii more often 

 than any other motile gastropod, and yet the 

 whelks do not appear to be eaten in proportion 

 to their abundance or accessibility in these lo- 

 cations. Abundance and distribution of K. kel- 

 letii was determined by random sampling along 

 150 m transect lines in two of the locations. 

 The following densities were determined: Del 

 Mar 1.82/m2, and Point Loma O.TS/m^. Distri- 

 bution and movement of K. kelletii is, at least, 

 sometimes nonrandom, for the whelks were 

 found to be in an aggregated distribution pattern 

 in each of these areas during late spring of 1970. 

 The distribution pattern of a motile organism 



may reflect spawning or feeding interactions; 

 therefore, these results were compared with and 

 found to coincide with earlier data obtained 

 from the Del Mar kelp bed during the fall of 

 1968. During the reproductive season (Rosen- 

 thal, 1970) , numbers of K. kelletii occur in com- 

 munal spawning groups, and yet predation by 

 P. giganteus did not appear to increase with the 

 availability of the whelks in these locations. 



FEEDING BEHAVIOR OF 



Kelletia kelletii 



K. kelletii is basically a carnivorous scavenger, 

 although it has been observed feeding on live 

 sedentary polychaetes. As a scavenger, it ap- 

 pears to be attracted to almost any injured or 

 dead animal occurring on the sea floor (Table 1) . 

 Often, large numbers of K. kelletii have been 

 observed moving towards and/or feeding upon 

 a common food item in subtidal areas. The 

 food-finding ability of K. kelletii by distance 

 chemoreception has, on more than one occasion, 

 been a nuisance to spiny lobster fishermen in 

 some areas off southern California. These fish- 

 ermen usually bait traps with dead fish to attract 

 the spiny lobster Panulirus interruptus. Many 

 times, however, a single lobster trap may con- 

 tain dozens of K. kelletii which were attracted 

 to the trap by the "scent" of the bait. 



K. kelletii feeds with an extensible muscular 

 proboscis which can be extended from the head 

 region during feeding. Food is ingested by a 

 muscular sucking action of the proboscis and a 

 rasping of the radula. The proboscis is capable 

 of extending approximately twice the length of 

 the whelk's shell; it is this extension which al- 

 lows K. kelletii to reach food items in depressions 

 or within the substratum. Pearce and Thorson 

 (1967) found that the proboscis of the gastropod 

 Neptunea antiqua can be everted and may be 

 extended to 21/4 times the length of the snail's 

 own shell. 



The proportion of the whelk's diet which re- 

 sults from preying on live animals versus scav- 

 enging on carrion or dying organisms was not 

 determined. An aggregation of K. kelletii feed- 

 ing on a dead fish or mollusk attracts the atten- 

 tion of an underwater observer more often than 



673 



