ARTIFICIAL PROPAGATION OF OYSTERS. 7 



them release the spawn naturally than to sacrifice the parent oysters 

 by stripping it from them together with considerable debris and 

 imripened eggs, which quickly foul the water. When spawning 

 first begins the nmning water is shut off and in some cases the 

 spawning is temporarily arrested by draining and flushing out the 

 tank and refilling with a fresh supply, after which spawning takes 

 place more vigorously than ever. 



When the oysters have ceased spawning they are immediately re- 

 moved from the tank and the eggs allowed to remain in the warm 

 water until the first swimming stage is reached. The transfer of the 

 swimming embryos to the rearing tanks may be accomplished in 

 either one of two ways — first, by pouring them through No. 20 bolt- 

 ing silk to remove any possible debris or enemies which may have 

 gotten into the water; or second, by having a steady stream of water 

 from the surface of the spawning tank to the rearing tanks, which 

 will carry over only the most vigorous of the swimming embryos 

 and leave any unfertilized eggs, debris, etc., behind. 



In one instance (on July 26) a group of 11 month-old oysters 

 spawned for over five minutes at a temperature of 69^°. A sample 

 of this observed in a watch glass developed as normally as that of 

 the older oystei"s. 



EMBRYONIC DEVELOPMENT. 



The young oyster that develops from the egg is extremely unlike 

 the adult and undergoes a variety of changes until some time after it 

 sets, wlien it acquires the asymmetrical characteristics of the adult. 

 The fertilized egg develops into a swimming form in from 6 to 10 

 hours at moderate temperatures, and when about two days old 

 acquires two tiny shells that continue to grow and surround the body 

 of the larva. This early stage is characterized by a straight hinge 

 where the shells are joined together, and precedes the most critical 

 period in the development of the larva. At the time that the straight 

 hinge disappears and the larva becomes more rounded in shape, it 

 has apparently used up the yolk supply from the egg and is begin- 

 ning to take in microscopic food from the water. If it survives this 

 period, it can be reared through the succeeding stages without diffi- 

 culty. 



As the larva increases in size one valve becomes much deeper than 

 the other and has a more prominent umbone, which enables one to 

 distinguish the oyster larva from that of any other bivalve.^ The 

 deeper valve is the left one and that by which the oyster later be- 

 comes attached. After swimming about for a period of two or three 

 weeks the larva becomes about one-third of a millimeter (one 

 seventy-fifth of an inch) long, and if the proper conditions are 

 present it " sets." 



SETTING. 



Tlie terms " fixation," " spatting," and " setting " are applied to 

 the attachment of the larva to some firm, clean surface with which 

 it comes in contact. The left valve becomes attached by means of a 

 shelly secretion of the left lobe of the mantle, which serves to cement 

 it to the surface the larva has selected. In artificial propagation the 



•The oyster and the oyster industry of the Atlantic and Gulf coasts. By E. P. Chur- 

 chill, jr. Appendix VIII, Report U. S. Commissioner of Fisheries, 1919 (1921). Wash- 

 ington. 



