FISHERY BULLETIN: VOL. 69. NO. 3 



at times even attractant behavior the whelk dis- 

 plays in the presence of this potential predator 

 appears to be unique. P. g if/an tens and K. kelletii 

 are trophically interrelated. Trophic interaction 

 is based on a predator-prey relationship and simi- 

 larities in the diet of both species. The prey 

 species consumed by P. giganteiis and K. kelletii 

 are similar; however, the method of feeding-, as 

 well as the physical condition of the prey, is 

 usually dissimilar. 



These observations leave many unanswered 

 questions on the behavioral relationship between 

 a potential pre.v organism and a predatory sea 

 star. K. kelletii is preyed upon by P. giganteiis; 

 however, the whelk does not respond to sea star 

 contact as has been reported in other gastropod- 

 asteroid interactions. The sea star is a major 

 predator of A', kelletii. although the whelk makes 

 up less than 10 S^ of the sea star's diet. The 

 factors which limit predation of K. kelletii by 

 P. giganteus are unknown, since both species 

 are such conspicuous and extremely abundant 

 inhabitants of the sublittoral zone off San Diego 

 County. Feder (1963) studied intertidal sea star 

 predation on gastropods and noted that species 

 which exhibited avoidance responses in the 

 presence of asteroids were not preyed upon in 

 proportion to their abundance or availability in 

 the intertidal. K. kelletii does not exhibit avoid- 

 ance or escape responses in the presence of P. 

 giganteus, and yet it does not appear to be eaten 

 in proportion to its accessibilit.v or abundance 

 in subtidal areas off San Diego. A general feed- 

 ing preference by the sea star for attached or 

 nonmotile prey is thought to limit predation 

 on K. kelletii in these locations. The cryptic and 

 burrowing habits of juvenile K. kelletii may 

 further reduce ]3redation on the smaller whelks 

 by limiting the numl)er of contacts with j^reda- 

 tory sea stars. Paine (1969) suggested that co- 

 existence between a major predator and prey 

 is possible as long as the prey species does not 

 reach a more preferred status in the diet of the 

 I)redator. 



It can only be speculated that the escape re- 

 sponse of K. kelletii in the presence of the sea 

 star has either not evolved or possibl.v was lost 

 through continual contact and convergence on 

 a similar trophic level. Perhaps other behavioral 



activities such as feeding may be of greater se- 

 lective value to K. kelletii than is a species-spe- 

 cific avoidance response. Quite possibly the 

 whelk benefits more by not actively avoiding 

 P. giganteus than it would by continually run- 

 ning from this potential predator. The chances 

 for K. kelletii to feed on moribund or dead or- 

 ganisms would be increased, since both species 

 are attracted to these food items. 



SUMMARY 



1. Trophic interaction between the sea star 

 P. gigantens and the gastropod K. kelletii is 

 based on a predator-prey relationship and simi- 

 larities in the diet of both species. 



2. P. giganteus, a highly opportunistic in-eda- 

 tor, was observed feeding on 32 identifiable spe- 

 cies of invertebrates. The sea star exhibited 

 a preference for prey which was either immo- 

 bilized or sedentary in habit, since these forms 

 were preyed upon most heavily. 



3. K. kelletii, a carnivorous scavenger, usually 

 feeds on moribund or dead organisms that it 

 finds resting on the sea floor. 



4. K. kelletii and P. giganteus feed together 

 on common food items. Convergent feeding be- 

 tween these two species was a recurring behav- 

 ioral pattern which was observed repeatedly in 

 the four subtidal areas. 



5. P. giganteus is the major identifiable pred- 

 ator of A', kelletii off San Diego County; how- 

 ever, the whelk makes up less than 10 9f of the 

 prey observed to be captured by the sea star. 

 The whelk is believed to be of secondary im- 

 portance as food of the sea star, since inverte- 

 brates other than K. kelletii were utilized more 

 often. 



6. A', kelletii did not display a species-si)ecific 

 avoidance or escape response in the presence of, 

 or while in contact with, P. giganteus. 



7. Both species appear to be highly successful 

 and abundant organisms of the sublittoral zone, 

 and coexistence is believed possible as long as 

 predation by the sea star on A', kelletii is not 

 excessive. 



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