0.1 g/mVday, or 40 g/mVyr (200 kcal/m"/ 

 yr) could be estimated as sustained by 

 intertidal oyster reefs from predation by 

 birds. This estimate is obviously clear- 

 ly approximate. 



3.5 COLONIAL ASPECTS OF THE REEF COMMUNITY 



The gregarious tendency of oyster lar- 

 vae has obvious adaptive value in terms of 

 the reproductive success of subtidal oys- 

 ter populations. It is also of great 

 adaptive value for intertidal reef oys- 

 ters. Survival in the upper intertidal 

 zone in the study area may depend on a 

 crowded colonial life style. 



The only single oysters (greater than 

 30 mm in height) or small clusters of oys- 

 ters normally observed in the intertidal 

 zone were either at the lower level of the 

 zone (not much higher than 60 cm above 

 MLW), or they were scattered among stalks 

 of cordgrass ( Spartina alterniflora ), 

 where they were shaded. The only way oys- 

 ter reefs attain their maximum steady- 

 state elevation, or mature stage, is via 

 the slow process of reef accretion based 

 on mutual support and self-shading. 



On the other hand, oysters in the 

 study area in the low intertidal zone or 



subtidal zone are characteristically heav- 

 ily fouled and colonized with boring 

 sponges, i.e., Cliona spp. These oys- 

 ters dre usually thick-shelled, with the 

 stunted shape characteristic of slow grow- 

 ing oysters, particularly in high-salinity 

 areas. It is obvious that relatively few 

 oysters survive in the subtidal zone in 

 these marsh-estuaries and that dead shells 

 are rapidly eroded away by Cliona spp. 



Oyster spatfall may be so dense in 

 some low latitude areas that it consti- 

 tutes a "fouling" situation. This condi- 

 tion (dense spatfall) has not been ob- 

 served on an intertidal reef, however. 

 Neither the density of barnacles nor oys- 

 ter spat appears to be limited by space on 

 a reef. This is perhaps not attributable 

 to a lack of prospective spat but rather 

 to the predatory effects of adult members 

 of the reef community, especially filter 

 feeders like mussels, barnacles, and oys- 

 ters themselves. The vortices set up by 

 the feeding currents of reef community 

 filter feeders could make the reef surface 

 a somewhat dangerous place to settle. This 

 type of density-dependent feedback could 

 explain the relatively even distribution 

 of oysters in the mature reefs and the 

 symmetrical form of the reefs. 



American oyster catchers "loafing" on an oyster reef in South Carolina. These birds, 

 rare over most of their range, are concentrated in coastal South Carolina and Georgia, 

 feeding primarily on reef oysters. They are year-round residents and represent one of 

 the major predators to the oysters. Photo by Wiley M. Kitchens, U.S. Fish and Wild- 

 life Service. 



55 



