FISHERY BULLETIN: VOL. 70, NO. I 



tests were conducted to determine if a similar 

 alarm response existed in S. jmrpuratiis. A 

 purple urchin within a group of urchins was 

 crushed underwater, and the reactions of neigh- 

 boring conspecifics noted for 5 min. In both 

 situations we observed no change in movement 

 or alteration in behavior which could be con- 

 sidered alarm oriented following injury to a con- 

 specific. In place of an alarm response we occa- 

 sionally noted an entirely different reaction from 

 S. purpuratus in the laboratory. If a leather star 

 was disturbed while feeding on a purple urchin 

 or moved away from an urchin test following 

 predation, occasionally other S. purpiirahis in 

 the aquaria approached the conspecific and scav- 

 enged the remains. 



DISCUSSION 



The behavioral responses exhibited by S. pur- 

 puratus when it is disturbed by D. imbricata 

 suggest a well-developed predator-prey rela- 

 tionship. In most instances purple urchins 

 erected globiferous pedicellariae when touched 

 by the four sea stars {D. hnbricata, P. helian- 

 thoides, A. sertulifera, and P. miniata) which 

 are known to prey upon them. In contrast, no 

 evasive or defensive responses were noted in 

 the same purple urchins when they were touched 

 by eight additional asteroid species. It appears 

 as though S. 2)urpuratus either responds to sea 

 stars that are biochemically similar, or through 

 selection the urchin has acquired the ability to 

 recognize particular asteroid species as potential 

 predators. 



The predator-prey association which exists be- 

 tween these two species off Pt. Loma, California 

 may be a regional phenomenon, since the rela- 

 tionshi]) has not been reported from other local- 

 ities along the Pacific Coast. However, from the 

 responses in both laboratory and field situations 

 we believe that the occurrence is probably more 

 widespread than indicated in the literature. The 

 large number (41 '/r ) of .S. purpuratus we found 

 included in the overall diets of leather stars off 

 Pt. Loma, as opposed to the total exclusion of 

 this species in the diets of leather stars off Wash- 

 ington state as reported by Mauzey et al. (1968) 

 is extremely puzzling to us. We can only spec- 



ulate at this time on what could account for this 

 variation in feeding behavior. Selection of po- 

 tential prey by D. imbricata may be determined 

 by the following conditions: (1) Prey density 

 and availability, (2) search time or the time re- 

 quired by the sea star to find and capture suit- 

 able prey, (3) taste or gustatory preferences of 

 the sea star, and (4) some form of associative 

 learning by D. imbricata. 



Strongyloceyitrotus purpuratus appeared to be 

 available to D. imbricata on almost a continuous 

 basis within the study area, since the population 

 of purple urchins was estimated to have a mean 

 density value of 30.2/m2. Predator search and 

 capture time also seems to be related to the den- 

 sity and distribution patterns of the prey, as 

 well as to the avoidance tactics employed by these 

 potential prey. Encounters between the two 

 species on uniform substratum usually resulted 

 in the escape of S. purpuratus; however, when 

 the purple urchins occupied depressions, holes, 

 or crevices along the sea floor, they became more 

 vulnerable to asteroid predation. In response 

 to asteroid predation S. purpuratus has appar- 

 ently evolved countermeasures such as evasive 

 movement, and defensive utilization of spines 

 and poisonous globiferous pedicellariae. The 

 large number (90'^r) of feeding leather stars 

 with purple urchin pedicellariae attached to their 

 epidermis might lead one to suspect that these 

 appendages are ineffective as a defensive mech- 

 anism. The pedicellariae, however, appear to 

 act as an irritant that in certain situations halts 

 the pursuit of a leather star and thus allows the 

 urchin to escape. Marler and Hamilton (1966, 

 p. 142) stated that "there is evidently a subtle 

 and dynamic balance between these different 

 evasive characteristics of the prey species on the 

 one hand and the abilities of the predators to 

 overcome them on the other". The taste or 

 gustatory preferences of individual D. imbricata 

 as compared to a leather star population has not 

 been explored. 



From our observations off Pt. Loma we would 

 expect that at least a few purple urchins would 

 show up in the diets of D. imbricata off Wash- 

 ington, even if other species were "preferred" 

 above 5. purpuratus. Possibly before leather 

 stars prey on live urchins there is a learning 



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