



300 



Microns 

 FiQiiRE 349. — Prodissoconch of 0. edulis Liiine. Arrangement as in figure 347. From Ranson, 1960. 



stops swimming, the velum is partially withdrawn, 

 and the larva begins to crawl on its foot. This 

 behavior may be changed suddenly by the re- 

 sumption of swinnning; the foot may be with- 

 drawn, tlie velum expands again, and the larva 

 swims away. When it is ready to set, the larva 

 crawls until it encounters suitable condition for 

 final attachment. 



Phases of setting of C. rinjinica were recorded 

 by a motion picture camera nearly 30 years ago 

 (Prytherch, 1934) and the photogi'aphs were 

 recently reproduced bj^ Medcof (1961, p. 19). 

 To facilitate photography, the larvae were ce- 

 mented with marine glue on their left valves to a 

 glass slide whicli was tilted at a 45° angle. Under 

 such conditions the larvae had no free choice in 

 selecting the place for attachment, and the records 

 obtained in this manner do not represent normal 

 behavior. The attachment of fully developed 

 larvae can be observed, however, by placing them 







Microns 



300 



Figure 350. — Prodissoconch of 0. lurida Carpenter. 

 Arrangement as in figure 347. From Ranson, 1960. 



in sea water in a petri dish and observing their 

 behavior with a binocular microscope. 



The foot of the larva extends forward, its tip 

 attaches temporarily to the substratum, and the 

 whole body is pulled over by the contraction of the 

 foot. The direction of crawling changes and 

 occasionally reverses as the foot extends at dif- 

 ferent angles. The movement continues for some 

 time, gradually becoming shorter and slower. 

 Finally the foot extends far beyond the edges of the 

 shell, the larva turns sideways with its left valve 

 touching the substratum, and comes to a standstill. 

 The attachment is made permanent when the 

 byssus gland discharges a cementing fluid, which 

 sets within a few minutes (Nelson, 1924). A 

 similar process takes place in tlie setting of 0. 

 eduJis and is probably common to other species of 

 oysters. 



The change from larva to juvenile oyster (spat) 

 then begins innnediately. The process of this 

 metamorphosis is better known for O. edulis than 

 for other species of oysters, for it has been studied by 

 Davaine (1853), Huxley (1883), and more recently 

 by Cole (1938b). The work of early European 

 zoologists influenced the study of the American 

 oyster to such an extent that in several instances 

 the description of the metamorphosis of C. 

 virginica has been repeated almost verbatim from 

 studies on 0. edulis with only slight changes 

 (Ryder, 1883; Jackson, 1888, 1890). A somewhat 

 more detailed account of the transformation of 

 larva into spat of C. virginica and 0. lurida was 

 given by Stafford (1913). 



During the metamorphosis the larval organs 



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FISH AND WILDLIFE SERVICE 



