Figure 386. — Chain of C. fornicata. Female mollusks, 

 the oldest of the group, are at the bottom ; the males 

 occupy the uppermost position with the hermaphrodites 

 (tr.) between the two groups. From Coe, 1936. 



Wadden Zee and the Limfjord in Denmark, where 

 it successfully competed for space and settled on 

 scattered shells, makuag it impossible for the 

 larvae of 0. edulis to set on them. Furthermore, 

 a large amount of silt and soft mud was deposited 

 by Crepidula and rendered the bottom unsuitable 

 for oyster planting. 



The story of C. fornicata is an excellent illustra- 

 tion of the possible danger of introducing a foreign 

 species, which under new conditions, and in the 

 absence of natural enemies and diseases, may re- 

 produce and survive at a rate which upsets the 

 natural balance of nature. 



Other fouling organisms may be of a seasonal 

 nature. Some of the oyster bottoms along the 

 Atlantic coast of the United States are often 

 covered with miUions of tunicates of the species 

 AloJgida manhattends (De Kay). This ascidian 

 can be so abundant in a dredged sample that the 

 oysters are hidden under the gi'ay mass of tunicates. 

 T observed this condition in the mouth of Chester 

 River, Md.; undoubtedly it occurs in other places 

 along the coast. The fouling by I\Iolgula is sea- 

 sonal; the organism dies and the renuiants are 

 sloughed off in the fall. Among the 29 species of 

 invertebrates collected from oysters suspended 



from a raft in the water of Oyster River, Mass., 

 four species constituted the largest portion of the 

 biomass: Molgula manhattensis, Botryllus schlosseri, 

 Amphitrite ornata, and Balanus halanoides. The 

 worm Amphitrite was found in typical tubes of 

 mud about one-quarter-inch or more in diameter. 



At the height of the fouling season in August, 

 the weight of the animals and plants and of sedi- 

 ment accumulated by them comprised 44 percent 

 of the total weight of a string of oysters. The 

 death of Molgvla in October and the sloughing off 

 of its cases reduced the weight to 11 percent. 

 Later on in November the weight increased to 

 about 17 percent because of the gi'owth of the 

 remaining organisms. 



The shells of li\ang oysters are frequently 

 covered with encrusting Bryozoa. In New Eng- 

 land waters and in Chesapeake Bay the appear- 

 ance of Bryozoa usually precedes the time of 

 setting of oyster larvae. When the oysters com- 

 plete their development, the shell surfaces may be 

 covered with Bryozoa colonies and unsuitable to 

 receive the set of spat. There is a possibility, not 

 fully substantiated, that a great many oyster 

 larvae are eaten by Bryozoa, and Osburn (1932) 

 thinks they are detrimental to the oyster beds in 

 Chesapeake Bay. Marie Lambert made a faunistic 

 study of the Bryozoa collected during the summer 

 on live oysters in the Oyster River near Chatham, 

 Mass. She recorded two species of Endoprocta 

 and five species of Ectoprocta. The most common 

 on oyster shells were Bowerhankia imbricata and 

 Schizoporella unicornis. The latter is an encrust- 

 ing bryozoan (fig. 387) commonly found on oyster 

 gi'ounds of Connecticut, part of Long Island 

 Sound (Hutchins, 1945), and Chesapeake Bay. 



Dense setting of barnacles on oyster shells is 

 very common throughout the range of distribution 

 of C. virginica. In many instances, the space 

 that would have other%vise been available to 

 oyster larvae is already occupied by barnacles, or 

 the spat becomes covered with barnacles and fails 

 to grow. Barnacles have no adverse effect on 

 aduJt oysters. 



The assemblage of invertebrate species found 

 livmg in close association with the oyster reflects 

 the fauna of the region and naturally differs from 

 place to place. In some areas oysters may be 

 almost entu-ely free of fouling organisms, while 

 in others their shells are hidden under a heavy 

 mass of siliceous sponges, hydroids, compound 

 ascidians {Botryllus), and Bryozoa. 



FACTORS AFFECTING OYSTER POPULATIONS 



427 



