CHAPTER XVI 

 LARVAL DEVELOPMENT AND METAMORPHOSIS 



Page 



Anatomy of trochophore and veliger 355 



Morphology of larval shell - 363 



Attachment and metamorphosis 3(i5 



Dispersal of larvae 369 



Reaction of larvae 371 



To the external environment 371 



To angle of surface 372 



To the properties of surface 373 



Gregariousness 373 



Artificial rearing of oyster larvae 374 



Bibliography 376 



The anatomical structure of an oyster larva is 

 known primarily from works on the development 

 of 0. edulis by Horst (1883), Huxley (1883), 

 Dantan (1917), and Erdmann (1935). Fragmen- 

 tary information regarding other species is found 

 in publications of Stafford (1913) on 0. lurida; 

 Prytherch (1934) on C. virgmka and Fujita 

 (1934); on C. gigas. Larval histology is described 

 in a comprehensive paper by Erdmann (1935), 

 and fate of larval organs in the metamorphosis 

 of 0. edulis is discussed by Cole (1938b). 



The voluminous literature on the ecology and 

 biology of oyster larvae of 0. edulis and other 

 species has been reviewed by Korringa (1941) in a 

 lengthy publication which places emphasis on 

 spawning and the setting of oysters. An abun- 

 dance of ecological data found in the reports of 

 Federal, State, and private organizations con- 

 cerned with the conservation and management 

 of oj'ster bottoms, deals mainly with the time of 

 appearance and setting of oj^ster larvae. Rela- 

 tively little is known about the factors which 

 control the life and behavior of the larvae, and 

 only a few studies have been made in recent 

 years on larval physiology, nutritive requu-ements, 

 and metabolism. However, advances in the tech- 

 nique of artificial rearing of oyster larvae from 

 fertilized eggs (Loosanoff and Davis, 1963a, 1963b) 

 now make it possible to obtain a continuous 

 supply of larvae of known age regardless of the 

 season of the j^ear. This advantage may stimulate 

 future studies of larval physiology. 



FISHERY bulletin: VOLUME 6 4, CHAPTER XVI 



ANATOMY OF TROCHOPHORE AND 

 VELIGER 



The sHghtly flattened embryo which forms at 

 the completion of cleavage does not increase in 

 bulk during embryonic development and is about 

 40ju to 50iLi along its dorso-ventral dimension, about 

 the same size as the egg. The two polar bodies may 

 still be attached to some of the embryos and a 

 tuft of robust cilia marks their anterior ends. The 

 larva, which at this moment begins to swim, is 

 called trochophore from the Greek "trochos," a 

 wheel: and "phero," to bear. 



The formation of the trochophore results from 

 the epiboly, i.e., the midtiplication of small ecto- 

 dermic cells, their arrangement around the single 

 and much larger macromere, and invagination of 

 the endoderm. At the early stage of larval de- 

 velopment, described by Horst (1883) for 0. edulis, 

 the invagination of endodermic cells (fig. 333, en.) 

 marks the position of the blastopore, bl., (from 

 the Greek "blastos," bud, and "poros," passage), 

 a channel which leads to the archenteron (the 

 cavity of the gastrula). A small invagination of 

 the ectodermic cells at the animal pole of the 

 larva indicates the location of a saddlelike shell 

 gland (sh.g.), which at the later stages gives rise 

 to the larval shell called prodissoconch (from the 

 Greek "pro," before, "dissos," double, and 

 "kongche," conch or shell). 



The invagination of the blastopore (fig. 334, 

 bl.) becomes deeper and narrower; the mesoderm, 

 me., is formed; and the shell gland, sh., increases 

 in size. At the trochophore stage (fig. 335) the 

 blastopore is closed and the mouth, m., is formed 

 aliove it; the ectodermic cells, ec, develop cilia 

 and are now called the trochoblasts. They form 

 a cihated ring or prototroch, which functions as 

 an organ of swimming. The position of the pro- 

 totroch is indicated in fig. 335 by two ectodermic 

 cells, c, with cilia. 



As the development of the trochophore ad- 

 vances, the prototroch forms a ciliated crown at 



355 



