ROSENTHAL and CHESS: PREDATOR-PREY RELATIONSHIP 



170 



150 



- 130 



i 110 



90 



70 



10 20 30 40 50 60 



Strongylocentrotus purpurafus (diameter mm) 



Figure 6. — Size of the leather star predator vs the size 

 of the purple urchin prey. Open symbols indicate two 

 or more identical data points. 



quently observed crawling on the frame or the 

 wire mesh of the cage. 



Our tagging efforts in the field proved to be 

 somewhat more valuable. The disc tags re- 

 mained attached to the leather stars for at least 

 eight months. During this period we were able 

 to study the feeding behavior of tagged D. im- 

 bricata on an individual basis. The tagging pro- 

 cedure might have inhibited normal feeding be- 

 havior in D. imbricata; however, we observed 

 leather stars actively feeding within three days 

 after being marked. Out of 69 marked individu- 

 als, the maximum rate of urchin predation oc- 

 curred with a 151-mm leather star. This indi- 

 vidual was observed feeding on three purple 

 urchins (32-36 mm) during seven consecutive 

 days of underwater observations. 



Laboratory feeding observations were carried 



out for 20 days. During this period every one 

 of the seven D. imbricata in the water table 

 preyed on at least one S. purpuirdus. One 

 leather star (140 mm) fed on three purple 

 urchins (16-38 mm) during a five-day period. 

 Sixteen of the 76 S. purpuratus present were 

 eaten by the leather stars. The prey ranged 

 from 15 mm to 42 mm in diameter. 



From these observations it was learned that 

 the digestion of a purple urchin's soft tissues 

 usually took between 20 and 48 hours. However, 

 on one occasion a 138-mm D. imbricata complete- 

 ly digested a 22-mm purple urchin in seven 

 hours. Fisher (1928) reported that P. helian- 

 thoides digested the soft parts of Strongylocen- 

 trotus spp. in 24 to 36 hours. The digestion rate 

 varied with the size of the leather star and the 

 size of the prey, as well as the method of di- 

 gestion. Large purple urchins (^22 mm), 

 which were usually digested extraorally, were 

 not digested as rapidly as smaller urchins. As 

 an example, 19 hours were required for a 

 140-mm leather star to digest a 16-mm purple 

 urchin, whereas 28 hours were required for this 

 same leather star to digest a 38-mm purple 

 urchin. 



As pointed out in the section on methodology, 

 it was usually necessary to lift up or turn the 

 leather star over to determine vviiether or not 

 it was feeding on an urchin. Li the laboratory, 

 when we disturbed a leather star that had cap- 

 tured or was in the process of digesting a purple 

 urchin, it retracted its stomach, released hold of 

 the urchin, and moved away from the prey. A 

 similar disruption in feeding behavior was also 

 noticed in the field. By marking the D. imbricata 

 we had a method which we thought would allow 

 us to study the feeding behavior of individual 

 leather stars in nature. However, the tactile 

 sensitivity that most members of this species 

 displayed negated most of the benefit attained 

 from individual recognition. 



One physical factor in nature which appeared 

 to influence predation on S. purpuratus by D. 

 imbricata was water turbulence. The incidence 

 of urchin predation decreased when a long-per- 

 iod swell generated a strong surge along the bot- 

 tom off' Pt. Loma. For example, on 9 January 

 1970, we recorded 48% of the D. imbricata ob- 



211 



