FISHERY BULLETIN: VOL. 70. NO. 1 



■purpuratiis tended to move to the top of a tank 

 and stop when pursued by P. helianthoides in 

 the laboratoiy. 



The 162 D. imhricata which were found feed- 

 ing on whole purple urchins ranged in size (ra- 

 dius) from 79 mm to 166 mm, with 118 mm as 

 the mean value. In comparison, a sample {N = 

 344) of the leather star population within the 

 perimeter of the study area contained individuals 

 from 50 mm to 202 mm and had a mean size 

 value of 108 mm. The purple urchins that were 

 preyed upon ranged from 12 mm to 52 mm in 

 test diameter, with a mean value of 34 mm and 

 a standard deviation of 8.5 mm. For compar- 

 ative purposes we removed 494 S. purpuratus 

 from 16 randomly selected m- quadrats. These 

 urchins ranged from 7 mm to 60 mm in diameter 

 and had a mean value of 39 mm and a standard 

 deviation of 8.8 mm. Using these data we com- 

 pared the mean of the 5. piirpurcitus prey and 

 the mean of the urchin population using a t-test. 

 We found that at the 5% level there was a sig- 

 nificant difference between the two means. It 

 appeared as though the D. imhricata were feed- 

 ing on a smaller size class of S. piirpuratns than 

 was available to them from the urchin popula- 

 tion (Figure 5). 



Landenberger (1968) found that in the lab- 

 oratory large Pisaster spp. tended to eat large 

 mussels. To determine if an optimal predator- 

 prey size relationship existed between D. imhri- 

 cata and S. purpu7'atus, the size of each predator 

 and its prey was recorded as a point on a scatter 

 diagram (Figure 6). A non-parametric corner 

 test of association or independence was then ap- 

 plied to these data. We found that the two var- 

 iables (predator size and prey size) were in- 

 dependent at the 95 Sr level of significance. 

 Therefore, no association between the size of a 

 leather star predator and the size of a purple 

 urchin prey is believed to exist. There was, 

 however, an upper limit to prey size in that we 

 observed no predation on purple urchins greater 

 than 52 mm in test diameter. 



FEEDING AND DIGESTION RATES 



Most of the purple urchins we observed were 

 killed and digested extraorally by leather stars 



5 5 9 5 13 5 17 5 21 5 25 5 29 5 33 5 57 5 41 5 45 5 49 5 S3 5 57 5 el 5 



SIZE (diameter mm) 



Figure 5. — A size frequency histogram comparing the 

 162 S. purpuratus prey with a sample {N = 494) from 

 the urchin population within the Pt. Loma study area. 



following capture. However, purple urchins 

 smaller than about 22 mm in test diameter were 

 usually ingested whole. Kjerskog-Agersborg 

 (1918) found that the sea star, Pycnopodia heli- 

 anthoides, digested large prey extraorally, 

 whereas smaller prey were regularly ingested 

 whole. 



Determining the feeding rate or number of 

 purple urchins which can be eaten by a leather 

 star over a given period of time was perhaps 

 the most inconclusive aspect of the study. The 

 experiment where leather stars and purple 

 urchins were confined within a 4 m- area pro- 

 vided little information on feeding rates in na- 

 ture. Although three urchins were eaten during 

 the first seven days of observation, we observed 

 no further predation after this initial period. 

 We concluded that the enclosure interfered with 

 the normal movements of the experimental an- 

 imals, since the four leather stars were fre- 



210 



