fed pure cultures of flagellates grown in the so- 

 called "Erdschreiber" medium of the following 

 composition (Gross, 1937): 



Sodium nitrate (XaNOs) 0.1 g. 



Sodium orthophosphate (NazHPOj) 0.02 g. 



Soil extract 50 ml. 



Sea water 1.000 ml. 



Soil extract is made b,y boiling 1 kg. of good 

 potting or garden soil with 1 1. of distilled water 

 in an autoclave for 1 hour. The flask is set aside 

 for 2 or 3 days, and the muddy dark fluid is de- 

 canted and sterilized by heating to the boiling 

 point. After standing 3 to 4 weeks the suspended 

 particles settle on the bottom, and the trans- 

 parent brown or red fluid is poured into another 

 container and boiled for a short time. Boiling of 

 the medium sliould be avoided once the required 

 quantities of nitrates and phosphate have been 

 added. 



Since the six flagellates used in these experi- 

 ments (Bruce, Knight, and Parke, 1940) were not 

 identihed and were labeled only by letters, incon- 

 sistencies in the results reported may be attributed 

 to the appearance in the culture of other species, 

 or, as the authors state, "to the supervention of 

 factors outside experimental control." The au- 

 thors suggest that one of these conditions may be 

 the fact that larvae from dift'erent oysters are not 

 equally viable. 



The feeding of oyster larvae (O. edulis) with 

 pure cultures of nannoplankton was repeated by 

 Walne (1956). In this case the larvae were kept 

 in vessels of 1 1. capacity without change of sea 

 water, and the species of flagellates grown in 

 cultui'es were identified. Among the Chloro- 

 phyceae, only Pyramimonaft grossii Parke gave 

 consistently good results. Tests made with Chlo- 

 rella stiijmaiophora Butcher seemed to indicate 

 that those clilorococcales which have a thick cell 

 wall are poor food for oyster larvae. The best 

 results were obtained with Isochrysis galbana 

 Parke, a chrysophycean of about 5ai to 6m in 

 length. Prymnesium parvum Carter was found 

 to be toxic to larvae. So far there is no proof 

 that the species of flagellates used in these experi- 

 ments form a significant component of the natural 

 population of nannoplankton and that their pres- 

 ence in estuaries is necessary for larvae living 

 under natural conditions. 



Imai and Hatanaka (1949, 1950) reported that 

 the larvae of C. gigas can be reared on a culture 

 of colorless flageflate, Monas sp., which abounds 

 in brackish waters of Japan. The authors believe 



LARVAL DEVELOPMENT AND METAMORPHOSIS 



that the flagellate of the Monas type plays an 

 important role in the production of oysters in 

 Japan. The possibility remains, however, that in 

 their experiments other flagellates were present in 

 the culture of Monas enriched with glucose, cane 

 sugar, nitrates, and phosphates. 



The pelagic life of C. virginica and C. gigas, and 

 probably of all oviparous oysters, is longer than 

 that of larviparous 0. edulis and 0. lurida. Con- 

 sequently, the rearing of these oviparous larvae 

 under artificial conditions presents additional dif- 

 ficulties. Considerable advances in the rearing of 

 larvae of various bivalve species were made by 

 Loosanoff, Davis, and their collaborators at the 

 Bmeau of Commercial Fisheries Biological Lab- 

 oratory, Milford, Conn. Phases of the work are 

 summarized by Loosanofl' (1954) and LoosanofT 

 and Davis (1963a, 1963b). Oysters were induced 

 to spawn by increasing the temperature and by 

 adding sperm suspension (seep. 30S, Chapter XIV). 

 The fertilized eggs were freed from debris by 

 passing the water through a series of fine screens 

 and placed in 5-gallon earthenware jars until free- 

 swimming larvae emerged. Then the water was 

 changed every 24 to 48 hours by straining it 

 through fine sieves which retained the larvae. 

 The sea water in which the larvae lived was filtered 

 thi-ough cotton to remove detritus and zooplank- 

 ton. Aeration and mechanical agitation were 

 considered unnecessary if the water was changed 

 every other day. The larvae were given measured 

 amounts of cultm-es of various micro-organisms. 

 In general the results obtained in Milford cor- 

 roborate the findings of British investigators. 

 Davis (1953) established that oyster larvae can 

 utilize as food the following species of flagellates: 

 Dicrateria inornata, Chromulina pleiades, Isochrysis 

 galbana, Hemiselmis rufescens, and Pyramimonas 

 grossii. Chlorella sp. was used only by advanced 

 larval stages and not by young veligers. 



The utilizable flagellates were added to the 

 rearing tanks at the rates of 15,000 and 25,000 

 cells per ml. per day but no toxic effects were 

 noticed in these heavy concentrations, and the 

 larval oyster population of approximately 5,000 

 per 1. showed satisfactory growth. The actual 

 number of flagellates ingested by the larvae was 

 not determined, but the inference was made that 

 "the rate of growth of oyster larvae had an inverse 

 relation to the number of larvae per unit volume" 

 (Davis, 1953). Cole (1939) states that a popula- 

 tion of 20,000 to 30,000 small flagellates per 1 ml. 



375 



