ROSENTHAL; TROPHIC INTER.ACTION BETWEEN SEA STAR AND GASTROPOD 



with the greatest concentrations of each between 

 2 and 40 m in depth. 



The laboratory portion of the study was con- 

 ducted in the experimental seawater aquarium 

 of the National Marine Fisheries Service, Fish- 

 ery-Oceanography Center, La Jolla, Calif. I 

 made direct subtidal observations during day- 

 light hours while scuba diving off San Diego 

 County during the period January 1968-July 

 1970. 



DESCRIPTION OF SUBTIDAL 

 STUDY AREAS 



Four widely separated nearshore locations 

 within San Diego County were selected as field 

 study sites (Figure 1). These sites were se- 

 lected because they varied in depth, substratum, 

 and species composition. 



DEL MAR 



The study area was located within a stand of 

 giant kelp, Macrocystis pyrifera, which lies in 

 15 to 20 m of water about 1 km offshore from 

 Del Mar, Calif, (lat 32° 57' N, long 117n6' W). 

 The kelp bed is characterized by having a rel- 

 atively flat sea bottom with intermittent sand 

 patches and low profile siltstone ledges. The sea 

 floor is relatively homogeneous in appearance 

 except for the occurrence of these ledges, which 

 are less than 1.5 m in height. Pterygophora cal- 

 ifornica, a low standing brown algae, occurs 

 abundantly on the seaward edge of the M. pyrif- 

 era bed. 



POINT LA JOLLA 



The observation site off Point La Jolla (lat 

 32°51' N, long 117°16'30" W) was between 150 

 and 300 m due west of Point La Jolla. The area is 

 characterized by large boulders and undercut 

 sandstone ledges. It is a topographically heter- 

 ogenous substrate containing many microhabi- 

 tats. Portions of the area contain large sand- 

 stone formations which rise vertically to within 

 a few meters of the sea surface. The tops of 



these formations are often covered by surf grass, 

 Phyllospadix torreyi. The observation area 

 ranged between 5 and 16 m deep because of 

 such pronounced vertical changes in relief. 

 Scattered throughout the area are two species 

 of perennial brown algae, Egregia laevigata and 

 Eisenia arbor ea. 



QUAST ROCK 



Quast Rock is situated on an offshore reef 

 about 630 m northeast of Point La Jolla (lat 

 .32°51'30"N; long 117°17' W). The observation 

 area encompassed approximately 225 m^ of this 

 reef. The rock is a sandstone formation with a 

 deep undercut and a cave on the northern end. 

 The substratum supports an extremely diverse 

 benthic invertebrate fauna, largely because of 

 the complexity of the habitat. The area is de- 

 void of giant kelp and only two species of brown 

 algae, Cystoseira osnmndacea and Agarum fim- 

 briatum, were common along the reef. The ob- 

 servation area ranged from 17 m on the top of 

 the rock to approximately 27 m on a lower 

 terrace. 



POINT LOMA 



The study site was located approximately 1.5 

 km offshore from Point Loma, Calif, (lat 32°42' 

 N; long 117° 16' W). The area is within a M. 

 pyrifera stand and the bottom is between 13 

 and 18 m deep. The substratum is predominant- 

 ly rock, siltstone, and sand. Portions of the 

 bottom are interrupted by channels and low re- 

 lief ledges. The shade area under the giant kelp 

 canopy supports an algal undergrowth composed 

 primarily of P. calif ornica, C. osmundacea, and 

 Laminaria farlowii. 



FEEDING BEHAVIOR OF 



Pisaster giganteus 



P. giganteus appears to be an opportunistic 

 predator on the nearshore subtidal reefs off San 

 Diego County. It feeds primarily upon live 

 animals, although it has been observed scaveng- 

 ing on dead fishes and invertebrates. Thirty- 

 two identifiable species of invertebrates, with 

 pelecypods and gastropods making up about 80% 



671 



