FISHERY BULLETIN: VOL. 70. NO. 1 



served (.V r^ 60) to be feeding on S. piirpuratus; 

 in contrast in this same area on 3 June 1970, 

 only 13''; of the leather stars observed {N — 58) 

 were eating urchins. On the former day we 

 noted that it was extremely calm underwater, 

 while on the latter a very strong" surge prevailed 

 along the bottom at a depth of 15 m. We felt 

 tiiat there was a correlation between water move- 

 ment and urchin predation by D. imbricata, al- 

 though we had no quantitative measurement of 

 this parameter. Feder (1956 and 1970) found 

 that poi)ulations of P. ochracens ate much less 

 in relatively unprotected intertidal areas than 

 in areas which provided the sea stars better pro- 

 tection from wave action. 



SPECIES-SPECIFIC REACTIONS 



Strongylocentrotus purpuratus reacted to the 

 presence of D. imbricata by exhibiting defensive, 

 as well as escape or avoidance, responses. No 

 visible responses were exhibited by S. purpurat- 

 us which could be considered predator-induced 

 until it was actually touched by a leather star. 

 The podia or tube feet of the purple urchin 

 appeared to be active sites for the reception of 

 chemical and tactile stimuli (Figure 7) . In reg- 

 ular urchins, such as S. purpuratus, the podia 

 are symmetrically spaced and arranged in five 

 double rows along the sea urchin's test. These 



podia are capable of extending nearly twice the 

 length of the primary spines and function some- 

 what like the tentacles or antennae of other ma- 

 rine invertebrates. If viewed from the aboral 

 surface, the area of sensory reception surround- 

 ing S. purpuratus can best be illustrated by draw- 

 ing a circle around the urchin, with the locus 

 of the points touching the tips of the fully ex- 

 tended podia. 



In the laboratory sea urchin podia were 

 touched with various biotic and abiotic objects 

 to determine which ones elicited avoidance or 

 defensive responses. When touched with human 

 skin, a glass rod, or a "non-predatory" inverte- 

 brate, the podia in contact usually retracted and 

 then were extended again. However, if the podia 

 of a inirple urchin touched a leather star, the 

 urchin displayed definite signs of predator 

 awareness: in the region of the tactile stimulus, 

 the podia retracted rapidly, spines were de- 

 pressed, and the poisonous globiferous pedicel- 

 lariae gaped and became erect (Figure 8). In 

 most instances the urchins moved in the opposite 

 direction of the leather star stimulus. 



In another experiment we tested the response 

 of S. purpuratus to additional stimuli, using the 

 gaping of the globiferous pedicellariae as an 

 overt sign of predator recognition. The test an- 

 imals were taken from two populations of S. 

 purpuratus. One group was from the jetty in- 



M. > '^ 



y-d 



FiGURR 7. — .S. purpiiratim with e.xtendod podia ju.st 

 prior to initial contact witli tho arm of a leather star. 



Figure 8. — The defensive behavior of S. purpuratus im- 

 mediately after being touched by D. imbricata. 



212 



