ROSENTHAL. CLARKE, and DAYTON: ECOLOGY OF A STAND OF GIANT KELP 



of Diopatra tubes represent living worms. 

 Emerson considers Diopatra a selective but 

 omnivorous deposit feeder. We have seen frag- 

 ments of Macrocystis attached to its tube ap- 

 parently serving a dual function of camouflage 

 and food reserve. The gastropod Kelletia kelletii 

 and the sea star Pisaster giganteus were the only 

 invertebrates seen eating Diopatra in this area. 

 Kelletia was frequently observed penetrating 

 either the sides or opening of the Diopatra 

 tube with an extensible proboscis, and Pisaster 

 was found with its stomach everted into the 

 opening of the tube. 



The solitary ascidian Styela montereyensis 

 usually grows attached to rocks, shells, and 

 Macrocystis holdfasts. Styela was found to live 

 approximately 12 to 20 mo; however, one indi- 

 vidual lived for more than 30 mo. Usually there 

 is a heavy recruitment in the late summer and 

 heavy mortality late the next spring (Figure 7). 

 The fall Styela population remained reasonably 

 constant over 4 yr (1968-72), despite the fluctua- 

 tions in the population during a calendar year. 

 A major cause of mortality in the fixed quadrats 

 was sediment movement and strong water 

 motion that either buried or detached the ascid- 

 ians from the substratum. Three predators of 

 Styela were observed: Pisaster giganteus and 

 Kelletia kelletii frequently were seen feeding on 

 Styela. On five occasions these two species were 

 encountered feeding simultaneously on the same 

 Styela (Rosenthal, 1971). In addition, we found 

 the sea star Astrometis sertulifera eating Styela 

 in this location. 



The gorgonian Muricea californica was the most 



visually conspicuous and the third most abundant 

 (2.53/m2) macroinvertebrate in the assemblage 

 (Table 5). It is a colonial animal usually found 

 growing erect from solid substratum. Muricea 

 californica recruits become visible to the unaided 

 eye when the colony is approximately 1 cm high; 

 at this stage they appear to be at least several 

 months old (Grigg, 1970). A decline in the M. 

 californica population was recorded during the 

 time period of this study. A total of 192 indi- 

 vidual colonies were living within the 48-m^ 

 area in September 1968, but by December 1972 

 the population had declined to 119 colonies. 

 During this time interval there were 147 mor- 

 talities and 74 recruitments recorded in the 

 fixed quadrats (Figure 8). Grigg (1970) found a 

 similar decrease in the M. californica population 

 off Del Mar in which he recorded a relatively 

 heavy mortality with no replacement or recruit- 

 ment during a 1-yr period of observation in 1968. 

 There are several physical factors contributing 

 to the mortality of M. californica in this area. 

 Scouring, holdfast detachment, and burial by 

 sand are important examples. Grigg (1970) felt 

 that two-thirds of the mortality recorded at Del 

 Mar during 1968 resulted from physical abrasion 

 by suspended particles, and one-third from colony 

 detachment. Occasionally, M. californica growing 

 either between the holdfasts of adult Macrocj's^is 

 or in close proximity to an established plant 

 died when the plant became detached and drifted 

 away. In such cases the M. californica were 

 entangled and pulled from the sea floor or were 



5 80 



Figure 7. — Styela montereyensis in the 12 fixed quadrats 



(48 m^). 



Figure 8. — Muricea californica in the 12 fixed quadrats 



(48 m2). 



679 



