Figure 387. — Oyster from Oyster River, Chatham, Mass., 

 covered in part by tlie colonies of the bryozoan Schizo- 

 porella unicornis and the compound ascidian (white 

 spots) Amaroucium constellatum. Fouling is beginning; 

 witUn a few days tlie surface of the shell may be com- 

 pletely covered with these two animals. 



The fouling is always seasonal, and with the 

 onset of cold weather many animals and plants die 

 and slough off. Possibly because of the periodicity 

 of fouling, the oysters sur\'ive and with few excep- 

 tions are not affected by the organisms gi-owing on 

 their shells. An exception is the invasion of 

 oyster beds by mussels {Mytilus edulis L.), which 

 in several situations may completely cover an 

 oyster bed with a thick layer of mud mixed with 

 excreta. 



A number of annelids are commonly associated 

 with oyster communities, living between clusters 

 of oysters or in the shells. Sometimes a sur- 

 prisingly large numl)er of worms crawl out of the 

 shell crevices when Epsom salt is added to the 

 water in which oysters were kept. Hartman 

 (1945) lists seven species of annelids inhabiting 

 the spaces between clusters of living oysters. 

 Korringa (1951b) describes more than 30 species 

 of annelids which in Dutch waters live on or in 



the shell of 0. edulis. Except for the boring 

 Spionidae, the worms apparently cause no direct 

 harm to oysters, but some of the mud-gathering 

 species of Nereidae materially increase the deposi- 

 tion of sediment over an oyster bed. No evidence 

 has been found of any other adverse effects of 

 annelids on oyster communities. 



Various siliceous sponges are very common 

 members of the epifauna of oyster bottoms. With 

 the exception of the boring sponges, they do not 

 affect oyster populations. The red sponge, 

 Microciona proliUca, is often found on highly 

 productive oyster bottoms. 



Of the protozoa that live on oyster shells, the 

 stentorlike infusorian, Folliculina sp., commonly 

 inhabits brackish water beds. This relatively 

 large protozoan, measuring from 200 to 800 n, 

 lives in bottle-shaped cases attached to the leaves 

 of Elodea, and Potamogeton found in the mouths 

 of rivers and on shells of other molhisks. During 

 the warm season it rapidly multiplies and appears 

 swimming with other plankton. Mass occurrences 

 of folliculuiids in the Chesapeake Bay were re- 

 corded by Andrews (1915) and in Oosterschelde, 

 Netherlands, by Korringa (1951a). Different 

 species are wdely distributed in the coastal waters 

 of the United States (Andrews, 1944). The num- 

 ber of this infusoria found attached to a single 

 oyster shell has varied from one to several hundred. 

 In many localities along the eastern shore of 

 the United States, oyster beds are frequently 

 overgrown by various algae. Oracillaria confer- 

 voides (Linnaeus) Greville is one of the species which 

 sometimes completely covers an oyster bottom 

 with its thick growth. Huge masses of the plant 

 wash away from the home grounds and pile on 

 beaches. Of the many other algae found growing 

 on oyster shells, several are in some regions as 

 abundant as Oracillaria: Enteromorpha, Ulva, 

 Griffitsia, Ceramium, Chondria, Champia, and 

 Scytosiphon. During experiments on raft culture 

 in Oyster River near Chatham, Mass. in 1956 to 

 1959 (Shaw, 1962), the shells of young oysters 

 suspended in water became covered with a very 

 dense growth of G. confervoides. Since there was 

 no noticeable ill effect on the oysters, an exami- 

 nation was made of the periphyton, the organisms 

 living loosely attached to the plant's branches. 

 The prevailmg form was found to be a diatom 

 Lycosoma sp., which was not present in the river 

 plankton outside the immediate area occupied by 

 Gracillaria. The stomach content of the oysters 



428 



FISH AND WILDLIFE SERVICE 



