As the larvae grow, their length, measured 

 parallel to the hinge line, continues to be about 

 5 to 10// more than their width, extending per- 

 pendicularly from the hinge to the ventral edge 

 of the shell. This condition persists until 

 larvae reach about 85 to B0^. At 95 to 100/; 

 the length and width are about equal, but from 

 then on the increase in width is more rapid 

 than in length. When the length reaches 125 to 

 130|i, the width already exceeds it by about 

 8/1, and this disparity remains until the larvae 

 reach metamorphosis (when they change to a 

 more oysterlike form). 



A well-pronounced black spot, called an 

 "eye", develops when the larvae are about 

 250-275/i long and remains throughout the rest 

 of the free-swimming period. Most larvae 

 undergo metamorphosis; i.e., begin to set, 

 between 275 and 315 // although occasionally 

 free-swimming larvae may be as long as 

 355 u . The larvae are highly active and remain 

 in suspension throughout most of the free- 

 swimming period. Large larvae, measuring 

 about 200 fi or more, tend to gather on the 

 water surface in hatchery vessels and form 

 small "rafts" that float just below the surface 

 film. If disturbed, the larvae composing the 

 "rafts" swim apart, but congregate again in 

 a short time. 



Growth of larvae is affected by many condi- 

 tions but chiefly by food and temperature. 

 Their effects will be discussed later. How- 

 ever, even though larval cultures sometimes 

 originate from the same spawnings of the same 

 parents and are grown under identical condi- 

 tions in the same vessel, individual larvae nnay 

 show a widely different rate of development 

 and growth and, therefore, metamorphose at 

 different times. For example, in one healthy 

 culture of larvae kept at about 73° F. the first 

 individuals began to set 18 days after fertili- 

 zation. The intensity of setting gradually in- 

 creased and remained heavy for 17 days, but 

 some larvae continued to swim another 10 

 days before metannorphosing. Thus, setting 

 of this presumably homogeneous culture of 

 larvae continued for 27 days. Occasionally, 

 however, the larvae in some uncrowded cul- 

 tures nnay be of relatively uniform sizes 

 (fig. 8). 



Contrary to the old opinion, oyster eggs 

 and larvae, if protected against disease- 

 causing organisms and toxic substances, are 

 rather hardy and are able to withstand sharp 

 changes in their environment. For example, 

 if laboratory-grown larvae kept at a tempera- 

 ture of 72° F. are placed in the refrigerator 

 for 6 hours and then returned to room tem- 

 perature, most will recover and continue to 

 develop normally. It seems unlikely, therefore, 

 that ordinary short-term temperature varia- 

 tions of a few degrees, occurring in natural 

 waters, can be responsible for heavy mortality 

 of larvae. 



It will be shown later in this article that 

 eggs and, especially, larvae of oysters can 

 also endure significant changes in salinity 

 and turbidity. They can withstand strong me- 

 chanical disturbances, such as vigorous water 

 motions created by winds of hurricane propor- 

 tions, without ill effects. On the other hand, 

 eggs and larvae show sensitivity to even traces 

 of certain chemicals present in sea water. 

 Some of these are natural substances released 

 from the bottom soil or produced by plants 

 and animals living inthe sea. Other substances, 

 such as insecticides, weedicides, oils, organic 

 solvents, and detergents, may strongly affect 

 eggs and larvae even if the substances are in 

 minute concentrations. For example, of com- 

 monly used insecticides, DDT is one of the 

 most toxic to oyster larvae because, even at a 

 concentration of 0.05 part per million (p. p.m.), 

 it killed nearly all of these organisms. How- 

 ever, another common insecticide, Lindane, 

 even when present in sea water at a concen- 

 tration of 10.0 p. p.m., caused no appreciable 

 mortality among larvae of the Eastern hard 

 shell clam, Mercenaria mercenaria . Effects 

 of each insecticide, therefore, should be eval- 

 uated separately. 



Metabolites, substances released into sea 

 water by many aquatic micro-organisms, 

 especially the one-celled organisms called 

 dinoflagellates, seriously affect oyster eggs, 

 larvae, and adults. Dinoflagellates are the 

 forms responsible for the so-called "red 

 tide" in Florida and in other States, which 

 sometimes kills not only shellfish but finfish. 

 Experiments at one of the Bureau's biological 

 laboratories have shown that fertilized oyster 

 eggs placed in sea water containing a large 

 number of dinoflagellates became unfavorably 

 affected, and most of the eggs were unable to 

 develcp into normal shelled larvae. 



Effects of Temperature on Eggs and Larvae 



Experiments to determine temperature 

 limits for development of oyster eggs showed 

 that at 60° F. none of the eggs reached normal 

 straight-hinge stage, although a few developed 

 as far as early shelled larvae. At 64° F., 

 however, about 97 percent of the eggs developed 

 to fully formed straight-hinge stage. In ex- 

 periments at 86° F., a temperature found in 

 southern waters, most fertilized eggs de- 

 veloped into normal straight-hinge larvae. 

 At 92° F., however, about half of the eggs 

 reached straight-hinge stage, and many were 

 abnormal. 



At all favorable temperatures, growth of 

 oyster larvae depends, to a large extent, upon 

 the food available. Thus, when kept at the 

 same temperature, larvae given relatively 

 poor food, such as microscopic green algae 

 called Chlorella , grew less rapidly than larvae 

 given better food. Nevertheless, even when 



14 



