Nelson 1979, 1980a, 

 b; Brawl ey and Adey 

 They appear to have 



their effects on subtidal seaweeds in the 

 South Atlantic Bight. Studies of some of 

 these smaller herbivores show that they 

 are rarely resource limited (Zimmerman et 

 al . 1979; Stoner 1980c) but are often 

 strongly affected by their predators 

 (Young et al . 1976; Young and Young 1978; 

 b, 1981; Stoner 1980a, 

 1981a, b; Edgar 1983). 

 only 1 imited impact on 

 most seaweeds (Carpenter 1986) because 

 their predators usually keep them well 

 below carrying capacity. However, their 

 potential impact is great. Brawl ey and 

 Adey (1981a) demonstrated that amphipods 

 could have a large effect on recently 

 established algal communities, and P. 

 Dayton and M. Tegner (Scripps Institution 

 of Oceanography; pers. comm.) have 

 recently observed giant kelp plants 

 ( Macrocyst i s ) on the west coast being 

 completely consumed by amphipods when 

 amphipod-consuming fishes were missing 

 from nearshore communities because of 

 events related to the El Nino phenomenon. 



Amphipods and polychaetes can also 

 significantly damage seaweeds that are 

 very resistant to fish grazing. Hay and 

 coworkers (Hay et al . 1987, 1988; Paul et 

 al . 1987) have recently shown that 

 seaweeds avoided by omnivorous fishes are 

 often selectively consumed by amphipods 

 and polychaetes (Figure 27) and that the 

 seaweed secondary metabolites that 

 effectively deter feeding by fishes often 

 do not affect, or may even stimulate, 

 feeding by amphipods and polychaetes. 

 They suggest the following reasons for the 

 evolution of this pattern. Because small, 

 relatively sedentary herbivores like tube- 

 building amphipods and polychaetes live on 

 the plants they consume, they should view 

 plants as both foods and living sites. 

 Since large, mobile herbivores like fishes 

 commonly move among, and feed on, many 

 plants, they should view plants primarily 

 as foods and rarely as potential living 

 sites. In the South Atlantic Bight, where 

 fishes that consume plants are also 

 important predators on amphipods and 

 polychaetes (Table 2), seaweeds avoided by 

 fishes should represent safer living sites 

 for small grazers. Thus, small, 

 relatively sedentary herbivores should 

 evolve a preference for seaweeds that are 

 well defended against fishes because if 

 they are living on unpalatable seaweeds 



DIPLODUS HOLBROOKI 



N--8 

 Duration = I9h 



Figure 27. Feeding preferences of two omnivorous 

 fishes and two invertebrate grazers common in the 

 South Atlantic Bight. Lines through the top of each 

 histogram represent ±1 standard error (data from Hay 

 et al. 1987, 1988; M. Hay unpubl.). 



they should experience less predation than 

 if they are living on seaweeds preferred 

 by fishes. There are now several 



documented cases of South Atlantic Bight 

 or Caribbean amphipods and polychaetes 



41 



