Order Pterioida 



The order Pterioida is one of four 

 orders of bivalves, each distinguished by 

 the structure of its gills, and includes 

 pearly and winged oysters, scallops, and 

 the true oysters. These orders are char- 

 acterized by paired gills that are greatly 

 lengthened (compared to the ancestral 

 type) and folded back on themselves to 

 form four demibranchs interconnected by 

 tufts of cilia. The mantles in these tnol- 

 lusks have taken over the sensory function 

 of the molluskan head, including some 

 visual or light-sensing capacity. 



Family Ostreidae 



This family includes a large number 

 of edible and nonedible oysters. They are 

 generally restricted to shallow coastal 

 waters between 44° S and 64° N (Galtsoff 

 1964). Oysters have unequal valves with 

 no hinge teeth except in the prodissoconch 

 or larval shell. In all but their larval 

 stages, oysters have completely lost their 

 byssus (attachment filaments) and foot and 

 have retained only the posterior adductor 

 muscle, which is kidney- or crescent- 

 shaped. 



Genus Crassostrea 



The oysters included in this genus 

 are characterized by extremely variably 

 shaped (ecomorphic) shells, depending on 

 the substrate and current regimes of the 

 habitat in which the oysters grow. Mem- 

 bers of the genus Crassostrea are anatom- 

 ically distinct from their counterparts in 

 the genus Ostrea , in that Crassostrea are 

 somewhat larger at maturity, with a deeper 

 cupped left valve on which they ordinarily 

 rest. ^ They also possess a distinctive 

 asymmetrical space between the right 

 mantle and gill plates, known as the 

 promyal chamber. The promyal chamber is 

 important because it probably permits 

 greater pumping rates, an advantage in 

 silt-laden water (Ahmed 1975). This cham- 

 ber also functions in the reproductive 

 success of this genus. Eggs of Crassos- 

 trea species are small ( -x. 40 y) and are 

 released directly into the water, rather 



Note, however, that reef oysters are usu- 

 ally oriented vertically with both left 

 and right valves pointed upward. 



than being incubated within the mantle 

 cavity, as those of the genus Ostrea . The 

 promyal chamber allows for higher release 

 velocity for eggs and is important for egg 

 dispersal. 



The production of free-living plank- 

 tonic larvae is critical to members of the 

 genus Crassostrea because it promotes ge- 

 netic exchange over wide areas. Oyster 

 larvae have been documented to travel at 

 least 50 km (30 mi). Quayle (1969) and 

 Stenzel (1971) estimated that they could 

 disperse up to 1,300 km (800 mi). 



Probably the most important charac- 

 teristic of the genus Crassostrea , which 

 has permitted almost worldwide distribu- 

 tion, is its ability to tolerate wide 

 ranges of salinity, turbidity, tempera- 

 ture, and oxygen tension. 



The morphology has changed little 

 since the oyster arose during the Triassic 

 period about 190 million years ago. The 

 genus Crassostrea arose during the Creta- 

 ceous period (T35 million years ago) 

 (Stenzel 1971). Representatives of this 

 genus characteristically occur in turbid 

 estuaries with soft ■ bottoms in the Indo 

 Pacific area, Eurasia, Africa, and North 

 and South America. 



Crassostrea virqinica 



The Eastern or American oyster (£. 

 virginica ) is the species that builds the 

 intertidal reefs focused on in this re- 

 port. This species is distributed along 

 the entire east coast of North America, 

 from the Gulf of St. Lawrence in Canada to 

 Key Biscayne, Florida, to the Yucatan and 

 the West Indies; and it has been reported 

 even in Brazil (Gunter 1951). Figure 6 

 (from Ahmed 1975) illustrates this 8,050- 

 km (5,000-mi) range. Crassotrea virqinica 

 prevails over this immense range because 

 of its tolerance to low temperature (Sten- 

 zel 1971). 



Physiological, ecological, and bio- 

 chemical data indicate that £. virqinica 

 has several distinct physiological races 

 (Loosanoff and Nomejko 1955; Menzel 1955; 

 Hillman 1964; Li and Fleming 1967; Ahmed 

 1975). On the other hand, Buroker et al. 

 (1979) concluded that significant genetic 

 distinctions occur only between popula- 

 tions of C. virqinica from Nova Scotia and 



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