Other inhabitants of shells of sub- 

 tidal oysters were virtually nonexistent 

 within reef oysters examined in the Geor- 

 gia study, e.g., worms (Polydora spp. ) 

 were found free in the samples but not 

 inside oysters. Boring sponges ( Cliona 

 spp.) were absent on intertidal oysters 

 but abundant on subtidal oysters and dead 

 shells. Infestation (with Cliona ) results 

 in shell deterioration in subtidal oysters 

 due to shell erosion by Cliona . Infested 

 (with Cliona ) oysters are particularly 

 vulnerable to predation, and the shells 

 are fragmented into pieces which tend to 

 be washed away rather than remaining in 

 situ as substrate for further coloniza- 

 tion. This is one of the principal reasons 

 that subtidal reefs are absent in the 

 study area. Guida (1976) discussed the 

 abundance of Cliona spp. in subtidal oys- 

 ters and oyster shells. No oyster drills 

 or starfish were ever seen on the reefs 

 examined. Parasitic gastropod, Odostomia 

 impressa , was abundant, (up to 5,460/rr,2). 



Insects 



An interesting organism occurring in 

 abundance on oyster reefs in the study 

 area is a collembolan insect, Anurida mar- 

 itima , a true marine insect (Miner 1951 ). 

 The trophic role of a similar intertidal 

 collembolan ( Oudemansia esakii ) in Hong 

 Kong has been described as saprophagic on 

 recently dead macrofauna, including oys- 

 ters (Chan and Trott 1972). Anurida 

 appears to be a true oyster associate 

 since it is only observed on mud flats 

 near oysters. The greatest concentrations 

 are inside dead pairs of oyster shells, 

 which often house masses of live insects 

 along with large numbers of exuviae (shed 

 exoskeletons). Small and covered with a 

 nonwettable cuticle, Anurida is extremely 

 buoyant and would be washed away during 

 flood tides were it not for crevices in 

 oyster shells which allow masses of them 

 to cling together. As in the case of 

 Oudemansia , Anurida probably emerges to 

 the reef surface during ebb tide and 

 retreats before flood tide. Dame (1979) 

 reported a few Anurida ('^6/m2) present in 

 South Carolina reefs and Lehman (1974) 

 reported Anurida from Florida reefs. 



Barnacles 



has been noted in previous sections (see 

 Section 3.1). Dame (1979) did not report 

 C^. fragilis on South Carolina reefs, which 

 may indicate that these reefs were lower 

 in the intertidal zone. Since total bar- 

 nacle density on oyster reefs does not 

 approach the density observed on pilings 

 (Bahr 1974), it appears that unknown fac- 

 tors limit barnacle survival on intertidal 

 reefs. It has been reported that Balanus 

 eburneus reaches maximum density at a 

 elevation of 9 to 14 m below sea level 

 (Relini and Giordano 1969). 



Mud Crabs 



Two of the most characteristic mem- 

 bers of the reef community are the common 

 mud crabs Eurypanopeus depressus and Pano- 

 peus herbstii . observed by Bahr (1974) at 

 mean densities of l,037/m2 and lOS/m^, 

 respectively. They seem to remain quies- 

 cent in the brown horizon during exposure 

 of the reefs but begin active feeding with 

 tidal inundation. Feeding consists of us- 

 ing one or both chelae to scrape the film 

 of algae and detritus from shells in the 

 brown and green horizons. The "grazed" 

 appearance of shells and the fact that 

 neither algae nor detritus accumulates on 

 shells indicate the proficiency of graz- 

 ing. These two crabs are undoubtedly 

 omnivorous, and Bahr (1974) noted Panopeus 

 predation on small oysters on reefs and 

 Eurypanopeus predation on amphipods in the 

 laboratory. Dame (1979) reported much low- 

 er densities of mud crabs on South Caro- 

 lina reefs; he found the two species in 

 approximately equal densities. 



Soft Shelled Clams 



Common occurrence of small soft shell 

 clams in the reef samples was noted by 

 Bahr (1974) at densities ranging up to 

 6,460/m2. No adult clams have been ob- 

 served in reef samples. It appears that 

 clam spat (juveniles) settle on the reefs 

 and survive only temporarily. Mya arenaria 

 has not been reported to range success- 

 fully as far south as Georgia, although 

 adult specimens have been found at Sapelo 

 Island. Dame (1979) did not report find- 

 ing Mya arenaria in South Carolina reefs. 



Mussels 



A marked vertical zonation of Chtha- Kuenzler's (1961) study of the ribbed 

 malus fragilis , one of three barnacle spe- mussel Guekensia demissus (formerly called 

 cies identified from the reef community, M odiolus ) demonstrates that this animal's 



47 



