CHAPTER 2 

 FUNCTIONAL OYSTER BIOLOGY AND AUTECOLOGY 



This chapter summarizes the salient 

 information on oyster biology, especially 

 that relating to the functional position 

 of the oyster in the estuarine ecosystem. 

 Each aspect of oyster biology discussed 

 here is presented as an aid to understand- 

 ing this functional role. 



A number of excellent treatises on 

 oyster biology, including the monograph on 

 the American oyster by Galtsoff (1964), 

 preclude the necessity for another exten- 

 sive treatment. Readers interested in 

 more detail on subjects discussed here 

 should refer to Galtsoff (1964) or other 

 references cited in the chapter. 



2.1 EVOLUTION AND TAXONOMY 



The oyster evolved long ago from an 

 ancestral mollusk into a highly reorga- 

 nized and in some ways simplified form. 

 The major evolutionary steps involved are 

 summarized below as they were recon- 

 structed by Yonge (1960): 



(1) lateral compression of the body 



(2) extension of the mantle to the 

 margins of the shell 



(3) division of the shell into 

 halves (valves) separated by 

 a noncalcareous ligament 



(4) fusion of pallial muscles to 

 form paired adductor muscles 



(5) reduction of head and develop- 

 ment of labial palps 



(6) development of cilia on (paired) 

 gills, and development of a gill 

 feeding function in addition to 

 their respiratory role 



(7) probable reduction of metabolic 

 requirements over that of ances- 

 tral forms 



(8) loss of foot and byssus in the 

 adult life stage 



(9) loss of anterior adductor muscle 



(10) rounding of the body 



(11) development of a horizontal 

 orientation with the left valve 

 down in the adult form 



The currently accepted formal classi- 

 fication of the American oyster is pre- 

 sented below, accompanied by the major 

 morphological and ecological characteris- 

 tics that apply to each category. There 

 is currently some controversy about the 

 classification of some oyster genera and 

 species, and descriptors are not standard- 

 ized, so that different workers have used 

 shell morphology, geographical range, 

 reproductive behavior, and larval shell 

 morphology to classify oysters. New tools 

 of biochemical genetics offer hope of 

 resolving some of the controversial ques- 

 tions in oyster taxonomy. 



For this report, the American oyster 

 will be classified according to Abbott 

 (1974) as follows: 



Class Bivalvia (Pelecypoda) 

 Order Pterioida 

 Family Ostreidae 

 Genus Crassostrea 

 Species virginica 



Each taxon will now be described 

 briefly. 



Class Bivalvia 



This class includes clams, mussels, 

 scallops and oysters. Some general char- 

 acteristics are (1) a shell divided into 

 two valves hinged dorsal ly by a ligament 

 of conchiolin and connected by one or two 

 adductor muscles; (2) a shell usually 

 consisting of three layers: an outer 

 organic horny matrix (conchiolin), a mid- 

 dle prismatic layer, and an inner nacreous 

 or pearly layer; (3) a laterally com- 

 pressed body; (4) either a small head or 

 none at all; (5) a wedge- or hatchet- 

 shaped foot (if present), (6) a mantle 

 extending to the margins of the shell and 

 forming a large mantle cavity, containing 

 ciliated gills (ctenidia) that function in 

 feeding, pumping, and respiration; (7) a 

 crystalline style that releases amylase 

 for starch digestion. 



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