ROSENTHAL and CHESS: PREDATOR-PREY RELATIONSHIP 



Figure 3. — Globiferous pedicellariae from S. purpnratus 

 attached to the arm of a leather star following contact 

 between the two species in the laboratory. 



contacts between the two species off Pt. Loma 

 appears to be quite frequent. 



Both species are conspicuous and abundant 

 members of the benthic community within the 

 study area. Strongylocentrotus ptirpuratus was 

 found in densities between and 100 individuals 

 per m^, with 30.2/m- as the mean value within 

 the 16 randomly placed m- quadrats. Purple 

 urchin density appeared to vary with the type of 

 substratum. The greatest concentrations of pur- 

 ple urchins were found along or within the silt- 

 stone ledges. 



We found D. hnbricata in densities between 

 and 7 individuals per 100 m- in the 2,400 sq m of 

 sea floor that was examined quantitatively. 

 Leather stars were most abundant near siltstone 

 ledges and rocky outcrops, where the highest 

 concentrations of purple urchins were also found. 



In most instances leather stars were observed 

 feeding on purple urchins which were located in 



holes or depressions (Figure 4), under ledges, 

 or in the holdfasts of giant kelp. Rarely was 

 urchin predation observed on uniform substrat- 

 um devoid of irregularities or discontinuities in 

 the bottom. In the field we have observed S. 

 pitrpurahis reacting to the presence of D. hn- 

 bricata. When an approaching leather star dis- 

 turbed an urchin tactually, it usually evoked a 

 running or escape response. The S. purpuratus 

 that inhabited substrata where avoidance ma- 

 neuvers were possible usually escaped from pur- 

 suing leather stars. However, when purple 

 urchins occupied depressions or irregularities 

 along the sea floor, they became more vulner- 

 able to asteroid entrapment and predation. 



Figure 4. — Leather star feeding on a purple urchin. 

 The sea star has pressed its oral surface into a depres- 

 sion to reach the prey. 



In the laboratory most of the purple urchins 

 that were preyed upon were captured on the sides 

 of the water table at the water-air interface. 

 Predation, in this case, appeared to be an arti- 

 fact of the aquarium, since the two species do 

 not experience this water-air barrier in the sub- 

 littoral zone. However, this interface was one 

 of the few physical obstacles besides the corners 

 of the water table that hampered the escape of 

 the purple urchin, and thus allowed a pursuing 

 leather star to capture it. In a similar obser- 

 vation, Mauzey et al. (1968) found that S. 



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