-m2 



Table 7. Concentration and biomass of two sea urchin species in 1 

 samples taken at three positions across grazing band (from Leighton 1971). 



Sample number 

 and position 



Relative 

 density 

 in numbers 

 S. S. 



7„,„a 



fran? pu r 



rr, b 



j%r i% 



8 



Total 

 urchins 

 per m 2 



Relative 

 density 

 in weight 

 S. S. 

 fran. purp. 



j%r (if 



Total 

 urchin 

 weight 

 per m 2 



(g) 



l 



At grazing front 94.0 6.0 



9 m behind front 39.0 61.0 



18 m behind front 15.0 85.0 



32 

 62 



46 



99.3 0.7 

 64.0 36.0 

 33.5 66.5 



a S. fran . = Strongyl ocentrotus f ranciscanus , 

 ^S. purp . = S. purpuratus . 



2486 

 1942 

 1253 



The smaller white sea urchin, 

 Lytechinus anamesus , occurs from southern 

 California to Baja California, Mexico 

 (Morris et al. 1980). The species feeds 

 extensively on smaller algae, particularly 

 foliose reds, and its grazing effects on 

 large kelp are generally much less than 

 that of red and purple sea urchins. 

 Clarke and Neushul (1967) and Dean et al . 

 (1984) reported that high densities of 

 Lytechinus anamesus may remove adult 

 _ by " 



Macrocystis plants by grazing 

 holdfasts and lower fronds. 



through 



Another sea urchin, the diadematid 

 Centrostephanus coronatus , may eat kelp 

 (Vance 1979) , producing very localized 

 effects; however, it is not an aggregating 

 species, and its distribution is normally 

 not extensive in kelp communities, except 

 at some of the Channel Islands and islands 

 offshore of Baja California, Mexico. 



The bat star Patiria miniata (Figure 

 20) may affect algal recruitment by 

 digesting spores and small plants when it 

 everts its stomach over the substratum. 

 It is found in abundance on rocky 

 substrata (4-5 individuals/m 2 in many 

 places) throughout the range of giant 

 kelp. Bat stars are omnivores and 

 scavengers, and also eat tunicates and 



other encrusting animals. The polychaete 

 worm Ophiodromus pugettensis commonly 

 occurs as a commensal on the sea star's 

 oral surface. 



4.4.3.2 Molluscs (Mollusca) . Many 

 species of molluscs feed on kelp forest 

 plants. Particularly prominent are the 

 abundant turban and top snails (Table 6). 

 It would be unusual for the grazing 

 activities of these molluscs to result in 

 the removal of adult plants. They can, 

 however, damage fronds and blades, 

 resulting in the severing of these parts 

 from adult plants. Tegula spp. in Carmel 

 Bay, when particularly abundant (^ 20 

 individuals per plant), has been observed 

 to retard or prevent the growth of new 

 sporophylls on Pterygophora cal ifornica 

 during spring months. If storms remove 

 most large fronds from Macrocystis , these 

 snails can remove the remaining fronds and 

 thus kill the entire plant (Schiel and 

 Foster in prep.). Riedman et al . (1981) 

 and Watanabe (1984a) recorded that the 

 abundances of three species of Tegula were 

 stratified with depth in a Macrocystis 

 forest near Pacific Grove, central 

 California. Tegula brunnea (Figure 20) 

 was the most abundant turban snail in 

 shallow water (-4 m depth), while T. 



64 



