ROSENTHAL. CLARKE, and DAYTON: ECOLOGY OF A STAND OF GL'SiNT KELP 



The trophic role displayed by S. franciscanus 

 in southern California kelp communities has been 

 adequately described (Limbaugh, 1955; Leighton, 

 1971; North, 1971). Strongylocentrotus francis- 

 canus is important because it often overexploits 

 its algal resources in areas of high sea urchin 

 density. Despite the fact that this species is 

 highly motile, foraging movements on attached 

 kelps were not witnessed in this kelp bed. The 

 S. franciscanus population appeared to be sus- 

 tained by snagging detached pieces of macro- 

 algae that drifted along the sea floor. The only 

 predator we observed feeding upon live S. fran- 

 ciscanus in this area was the labrid Pimelome- 

 topon pulchrum. 



Tethya aurantia, a hemispherical shaped 

 sponge, attains a circumference of at least 25 cm 

 and is usually found attached to rocks and con- 

 solidated sediments. There were two mortalities 

 and three recruitments recorded in the fixed 

 quadrats. One mortality was attributed to sedi- 

 ment burial, and Dermasterias imbricata was 

 observed feeding on Tethya in the vicinity of the 

 transect. Rosenthal and Chess (1972) reported 

 Dermasterias to be a predator of Tethya in the 

 sublittoral zone off Pt. Loma, Calif. 



Astrometis sertulifera was the most abundant 

 (O.lS/m^) sea star encountered off Del Mar (Table 

 5). This estimate of abundance is probably con- 

 servative because Astrometis is relatively small 

 (4 to 6 cm in radius) and individuals are some- 

 what cryptic in habit. The most frequent habitat 

 of Astrometis off Del Mar was the undersides 

 of rocks and the interstices of Macrocystis hold- 

 fasts. It appears to be ideally suited for preying 

 upon the assemblage of organisms found associ- 

 ated with the holdfasts of Macrocystis. Astro- 

 metis was observed eating juvenile Kelletia kel- 

 letii (gastropod), Conus californica (gastropod), 

 Mitrella sp. (gastropod), Styela montereyensis 

 (ascidian) and juvenile Strongylocentrotus pur- 

 puratus (echinoid) in this study. Leighton (1971) 

 mentioned that Astrometis feeds heavily on juve- 

 nile sea urchins off southern California. No pred- 

 ators of Astrometis were observed in the study 

 area; however, Rosenthal and Chess (1972) found 

 that 4% of 437 feeding Dermasterias were eating 

 Astrometis. 



The gorgonian Lophogorgia chilensis is one of 

 the most exquisite macroinvertebrates in this 

 region. It usually grows in an upright manner 

 attached to solid substratum. A few small colonies 



were found growing attached to Muricea cali- 

 fornica. Lophogorgia was not nearly as abundant 

 as M. californica or M. fruticosa; the mean den- 

 sity during August 1972 was 0.04/m2 (Table 5). 

 There was one mortality and one recruitment 

 recorded in the 48-m2 quadrat area during 1968- 

 72. No predators of Lophogorgia were observed 

 in this kelp bed and causes of colony mortality 

 are probably similar to those of Muricea 

 spp. 



The purple sea urchin, Strongylocentrotus 

 purpuratus , ranged in size from 15 to 68 mm, with 

 a median size of 42 mm (n = 82). In contrast 

 to the low density in this location (O.OS/m^), other 

 kelp stands off southern California have been 

 found to contain as many as 77 S. purpuratus /yd^ 

 (Leighton, 1971). The most frequent habitat of 

 S. purpuratus off Del Mar was the undersides 

 of rocks; however, individuals were also found 

 on consolidated mounds and siltstone ledges. A 

 few individuals were noted within the holdfasts 

 of adult Macrocystis; however, we believe the 

 incidence of holdfast infestation is low in com- 

 parison to other San Diego County kelp beds. 

 Strongylocentrotus purpuratus is a herbivore 

 well known to overexploit its algal resources 

 (Leighton, 1971; North, 1971). In the Del Mar 

 area most of the purple sea urchins were observed 

 eating detritus and drift algae; rarely were the 

 attached macroalgae grazed upon. 



Dermasterias imbricata (Rosenthal and Chess, 

 1972), Astrometis sertulifera (Leighton, 1971; 

 pers. observation), and Pi?nelometopon pulchrum 

 (Limbaugh, 1955; pers. observation) are three 

 known S. purpuratus predators which inhabited 

 the study area. 



Pisaster giganteus is one of the most abundant 

 and widely distributed sea stars found off southern 

 California. Although visually conspicuous at Del 

 Mar, Pisaster was not nearly as abundant 

 (O.Ol/m^) as it was in some of the other kelp beds 

 (i.e. Pt. Loma, La Jolla, Catalina Island) we 

 surveyed between 1968 and 1972. The diet of 

 Pisaster has been partially quantified (Rosenthal, 

 1971). Although it was occasionally observed 

 scavenging, it primarily feeds on live animals 

 which, in this area, include Astraea undosa 

 (gastropod), Conus californicus (gastropod), Kel- 

 letia kelletii (gastropod), Botula falcata (pelecy- 

 pod), Hinnites multirugosus (pelecypod), Litho- 

 phaga plumula (pelecypod), Parapholas cali- 

 fornica (pelecypod), Pennitella penita (pelecypod). 



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