TROPHIC INTERACTION BETWEEN THE SEA STAR Pisaster giganieus 

 AND THE GASTROPOD Kelletia kelletii 



Richard J. Rosenthal^ 



ABSTRACT 



The sea star Pisaster giganteus and the gastropod Kelletia kelletii are conspicuous inhabitants of the 

 sublittoral zone off San Diego, Calif. Diving observations over a period of 2'/2 years indicate that the 

 two species are trophically interrelated. P. giganteus, an opportunistic predator, and K. kelletii, a car- 

 nivorous scavenger, have been observed feeding together on common food items. The sea star appears 

 to be a major predator of the whelk, even though K. kelletii made up less than 10% of the diet of the 

 sea star. The whelk does not display an avoidance response in the presence of P. giganteus. Coexis- 

 tence between the two species is believed possible as long as K. kelletii does not become more preferred 

 prey of the asteroid. 



Available information on the behavioral respons- 

 es of marine miollusks in the presence of preda- 

 tory sea stars has increased markedly within the 

 past few decades (Bullock, 1953; Feder, 1963, 

 1967; Margolin, 1964a, 1964b; Feder and 

 Christensen, 1966; Montgomery, 1967). How- 

 ever, most of these investigations have been lim- 

 ited to laboratory or intertidal observations. 

 Except for a recent study by Mauzey, Birkeland, 

 and Da>i;on (1968), the interactions between 

 mollusks and sea stars in the eastern Pacific 

 subtidal waters have not been investigated. Di- 

 rect sublittoral behavioral observations off the 

 west coast of north America have been hampered 

 by cold water and limited underwater observa- 

 tion time. 



Assessment of predator-prey relationships be- 

 tween subtidal organisms has been limited 

 mainly to recording interactions between two 

 organisms under laboratory conditions. A spe- 

 cies-specific avoidance reaction or escape re- 

 sponse by a mollusk to a sea star is considered 

 one indication of a predator-prey relationship. 

 The evolution of such responses, and the recog- 

 nition of chemical stimuli emanating from either 



' Westinghouse Ocean Research Laboratory, Annap- 

 olis, Md.; present address: Scripps Institution of Ocean- 

 ography, tjniversity of California, San Diego, Calif. 

 92037. 



organism, suggests a long standing predator- 

 prey association. Mauzey et al. (1968) found, 

 however, that biochemical similarities between 

 a predator and other organisms could cause a 

 prey species to avoid the nonpredatory species 

 as well as the predator. 



Gastropods which displayed no avoidance re- 

 sponses in the presence of specific sea stars have 

 been observed by Bullock (1953), Margolin 

 (1964b), and Feder (1967). Bullock (1953) 

 even suggested that nonresponsive mollusks are 

 characteristic of ecological situations where star- 

 fish predation on these species must be rare. 



This paper examines laboratory and field data 

 obtained on the behavioral interactions between 

 the sea star Pisaster giganteus (Stimpson) and 

 the gastropod Kelletia kelletii (Forbes). In- 

 cluded are observations on the feeding, species- 

 specific responses, and predator-prey interaction 

 between the two species. 



P. giganteus is reported from Vancouver 

 Island, British Columbia, to northern Baja Cal- 

 ifornia, Mexico (Fisher, 1930), while K. kelletii 

 has been found from Santa Barbara, Calif., to 

 San Quintin Bay, Baja California, Mexico (Ab- 

 bott, 1954). Both species are conspicuous and 

 abundant inhabitants of the nearshore subtidal 

 reefs off southern California. Bathymetric dis- 

 tribution appeared to be somewhat similar for 

 K. kelletii and P. giganteus off San Diego County, 



Manuscript accepted March 1971. 



FISHERY BULLETIN: VOL. 69, NO. 3, 1971. 



669 



