ROSENTHAL: TROPHIC INTERACTION BETWEEN SEA STAR AND GASTROPOD 



posed primarily of sexually mature individuals 

 (Rosenthal, 1970). K. kelletii less than 40 mm 

 in length were rarely preyed on by P. giganteus. 

 These smaller individuals are more secretive in 

 habit than larger mature A', kelletii. and they 

 characteristically burrow in the substratum. 

 Possibly the burrowing and cryptic behavior of 

 smaller A', kelletii decreases sea star predation 

 by reducing the number of contacts with preda- 

 tory P. giganteus. K. kelletii is considered to be 

 what Paine (1969) referred to as "secondarily 

 preferred prey," since invertebrate species other 

 than K. kelletii were found to be numerically 

 more predominant in the diet of P. giganteus at 

 these four locations (Figure 2). 



Seasonal changes in K. kelletii predation by 

 P. giganteus was not considered to be a factor 

 in these subtidal regions. P. giganteus was ob- 

 served feeding on whelks throughout the year 

 with no noticeable change in the incidence of 

 predation. 



RESPONSE OF Kelletia kelletii TO 

 Pisaster giganteus 



From the number of reports of mollusks re- 

 sponding to predatory sea stars (Feder and 

 Christensen, 1966), I initially expected to ob- 

 serve an avoidance reaction by K. kelletii in the 

 presence of P. giganteus. To test this, 150 K. 

 kelletii between 15 and 138 mm in shell length 

 were brought into the laboratory and maintained 

 in either standing or circulating seawater. The 

 seawater temperature ranged between 16.0° and 

 19.9° C throughout the entire experiment. K. 

 kelletii were tested for any reaction which might 

 be exhibited in the presence of, or while touching, 

 P. giganteus. At times, the whelks responded 

 by siphon extension, shell rocking, or twisting, 

 and a slow sliding movement away from or in 

 the direction of P. giganteus. Many times no 

 shell movement was noted within a 10-min pe- 

 riod. Tests were conducted on whelks that were 

 inactive and on others that were moving, feeding, 

 or spawning. Each K. kelletii was used only 

 once or twice so that continual contact with the 

 sea star would not affect the whelk's reaction. 



At no time did I note an escape or avoidance 

 response by A. kelletii in the presence of the 

 sea star. 



Field observations were similar to those in the 

 laboratory; either A', kelletii did not respond 

 or, at times, they actually were attracted to P. 

 giganteus. The two species were usually found 

 close to one another on these subtidal reefs, and 

 contacts between the two probably are frequent. 

 During convergent feeding, the two species 

 touched or even crawled over one another ; how- 

 ever, at other times the A', kelletii and P. gigant- 

 eus occurred within a few centimeters of each 

 other even though neither species was engaged 

 in feeding. Paine (1969) found a perplexing 

 intimacy of association between three intertidal 

 gastropods and their major predator, the sea 

 star P. ochraceus. In contrast to these two sit- 

 uations, Bullock (1953: 137), stated that "In 

 those seashore situations where predatory stai'- 

 fish and gastropods both occur, it is notable that 

 the two are generally not seen close together." 



One of the routine field experiments was to 

 pick up a P. giganteus underwater and place 

 it on or within a few centimeters of an indi- 

 vidual or group of A', kelletii. On only one occa- 

 sion was a reaction exhibited in K. kelletii out 

 of the hundreds of attempts to stimulate an active 

 response. In this one instance, a group of 11 A. 

 kelletii, which appeared to be searching for food, 

 was encountered 20 m underwater off Del Mar on 

 November 6, 1968. A large P. giganteus was 

 placed approximately 30 cm from the group of 

 whelks, and within a few minutes the sea star ap- 

 proached the K. kelletii. All of the whelks 

 moved away from the approaching sea star; 

 however, two of the K. kelletii were captured 

 by the sea star while the others moved off in a 

 similar direction. This reaction appeared to be 

 an avoidance response although it could have 

 been only random or chance movement on the 

 part of the whelks, regardless of the presence 

 of the sea star. 



CONCLUSION 



The existence of a predator-]jrey relationship 

 between a gastropod and a sea star is not un- 

 usual; however, the continual nonresponsive or 



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