without showing an increase in size, for 14 

 days in concentrations of silt as high as Z 

 g./l- Recent experiments also have suggested 

 that the sizes and shapes of turbidity-creating 

 particles are important in the degree of dam- 

 age they cause eggs and larvae. These studies 

 are now in progress. 



Food and Feeding of Larvae 



Oyster larvae, like other living organisms, 

 must feed to survive and grow. Larvae begin 

 to take particulate food soon after they reach 

 straight-hinge stage. Since the diameter of 

 their gullet is then only about 9 ti , particles 

 they can digest are correspondingly small. 



Most food organisms consumed by the larvae 

 are members of a large group of microscopic 

 marine algae. Not all species of algae, how- 

 ever, are of equal value. The thickness of 

 algal cell walls and the degree of toxicity of 

 metabolic products that algae produce are 

 important factors in determining their quality 

 as larval food. Critical evaluation of a number 

 of micro-organisms has shown that so-called 

 naked flagellates, such as Isochrysis galbana 

 and Monochrysis lutheri , produce few, if any, 

 external metabolites which may unfavorably 

 affect larvae. 



The food value of micro-organisms depends 

 upon how completely they meet the require- 

 nnents of larvae. Sometimes a mixture of the 

 two above-mentioned flagellates, togetherwith 

 two other naked flagellates, Platymonas sp. 

 and Dunaliella euchlora , induced more rapid 

 growth of larvae than equal quantities of these 

 food organisms given separately. All these 

 forms induce better growth than forms with 

 thick cell walls, such as several species of 

 Chlorella . 



Experiments are now in progress to find 

 dried foods that may be used successfully to 

 feed moUuscan larvae. So far, these experi- 

 ments have shown that some dried algae can 

 be used for growing to metamorphosis larvae 

 of some bivalves, such as the common hard 

 clam, Mercenaria mercenaria . Unfortunately, 

 even flagellates, such as I. galbana and M. 

 lutheri , are not good enough, when dried, to 

 maintain oyster larvae. Nevertheless, it is 

 hoped that this problem will be solved soon. 



Metamorphosis, or Setting, of Larvae 



At the close of the free-swimming period, 

 when a larva is about 300 ii , or roughly one 

 seventy-fifth of an inch long, it drops to the 

 bottom to metamorphose. After descending, it 

 crawls for some time and nnay even swim 

 away to a new area until it finds a firm sur- 

 face free of dirt or silt, to which it attaches 

 itself by its left valve. Before, during, and 

 imnnediately after metamorphosis, radical 

 changes occur in the anatomy of the setting 



organism, such as disappearance of the velum 

 and foot and development of a primitive set 

 of gills. Recently metamorphosed oysters are 

 usually called "set" or "spat" (fig. 10). 



In many bodies of water the spat attaches 

 itself at all depths from the surface to the 

 bottom. In Long Island Sound, oysters set 

 even at a depth of 100 feet. In other areas of 

 the Atlantic and Gulf of Mexico coasts setting 

 may be confined to the zone between tidal 

 levels or only near the bottom, regardless of 

 depth. 



Experiments have shown that during setting 

 oyster larvae prefer certain surfaces or 

 materials {fig. 11). If old, clean oyster shells 

 and artificial collectors made of plastic, 

 cement, glass, or resins are placed in the 

 same basin with mature, ready-to-set larvae, 

 the oyster shells will collect considerably more 

 spat, per square foot of surface, than artificial 

 collectors. 



Diseases of Larvae 



It is quite difficult to establish whether, in 

 nature, oyster larvae suffer from various 

 diseases. In the laboratory, however, even 

 under the best conditions, heavy larval mor- 

 talities, which could be ascribed to diseases 

 or parasites, often occur at all ages. In 1954 

 the Bureau's biologists discovered that a 

 fungus, Sirolpidium zoophthorum , maybe re- 

 sponsible for epizootic mortalities in cultures 

 of bivalve larvae, including oysters. Still 

 more recently, it was demonstrated that cer- 

 tain bacteria produce toxins that can retard 

 growth or even kill larvae. Some bacteria 

 that kill molluscan larvae were tentatively 

 identified as belonging to the genus Vibrio or 

 Pseudomonas . 



High temperatures usually favor bacterial 

 growth. It also has been sho'wn that under 

 hatchery conditions bacterial contamination of 

 algal food cultures sometimes causes a sharp 

 decrease in growth of oyster larvae without 

 causing extensive mortality. Soon after the 

 discovery of pathogenicity of fungi and bac- 

 teria, biologists began testing a number of 

 antibiotics and fungicides to prevent and con- 

 trol larval diseases without seriously affect- 

 ing survival and growth of larvae. A number 

 of such substances now are used in proper 

 concentrations with considerable success in 

 pilot oyster and clam hatcheries. 



OYSTER ENEMIES 



Diseases and Parasites 



Oysters in all stages of development are 

 susceptible to attack by numerous enemies. 

 The oyster eggs, early en-ibryos, and larvae 

 are eaten in large numbers by protozoans, 

 ctenophores, jellyfish, hydroids, worms, bi- 

 valves, barnacles, numerous larval and adult 



17 



