shell has sufficient space for only a few spat to 

 grow to maturity, heavy concentrations of spat 

 are of doubtful value for reproduction and may 

 even be liarmful by creating overcrowded 

 conditions. 



ARTIFICIAL REARING OF OYSTER 

 LARVAE 



Early attempts to rear o.yster lar\ae under 

 artificial conditions produced uncertain results. 

 Sometimes a small number of spat were obtained, 

 but the experiments could not be repeated under 

 similar conditions. At that time oyster larvae 

 were placed in 5-gallon carboys, and the water 

 was aerated and circulated. At 2-day intervals 

 the larvae were concentrated by centrifuging and 

 transferred into fresh sea water (Wells, 1920). In 

 another method, tried with only partial success, 

 the larvae were reared in slowly running sea water 

 which was filtered through a 2-inch layer of white 

 sand or porous stone (filtrose) placed on the 

 bottom of a container. The rates of filtration 

 and of addition of new water were regulated by a 

 valve placed below the filtering layer (Prytherch, 

 1924). In both types of experiments no food was 

 added to the containers under the assumption 

 that enougli was present in the water. Then 

 Gaarder and Sparck (1933) and Gaarder (1933) 

 studied the food of the larva of 0. ecbiUs in Nor- 

 wegian oyster ponds and made what may be con- 

 sidered the first significant step toward solving the 

 problem of rearing larvae under artificial cotuH- 

 tions. Sparck observed that the water of the 

 ponds contained considerable numbers of a small 

 green unicellular alga which later on was isolated 

 and cultured in the laboratory. It appeared to 

 l)e a species of Chlorella which was consumed by 

 tlie larvae. Studies by these investigators re- 

 vealed also that nannoplankton of tlie ponds con- 

 sisted principally of small green algae and flagel- 

 lates measuring from 2m to 3m. Fertilization of 

 experimental tanks by the addition of liquid 

 maimre greatly increased the production not only 

 of Chlorella but also of various diatoms, chiefly 

 Nitzschia, flagelhites, various large unicellular 

 green algae, and bacteria. In this enriched water 

 a few larvae grew to a size of 300m but failed to 

 attach (Sparck, 1927). After it was found lliat 

 ( 'hlonlla is present in the Norwegian oyster ponds, 

 in experimental tanks in Conway, Wales, and in 

 certain experimental basuis m Denmark, Kandler 

 (1933) attempted to grow oyster larvae on a diet 



374 



of this alga alone but had little success. This led 

 liim to conclude that oyster larvae are unable to 

 digest Chlorella, which left the intestine apparently 

 unchanged. Feeding experiments with Carteria 

 and Chlamydomonas were also unsuccessful. More 

 critical experiments conducted at Conway, Wales, 

 showed that the larvae are unable to utilize 

 nonmotile green algae such as Chlorella and 

 Collomyra but that yellow-brown chrysomonads 

 (not identified but designated as flagellate C) 

 gave satisfactory results (Cole, 1937). 



The Conway experiments demonstrated that 

 organic enrichment of the water of the large 

 tanks was consistently successful in giving rise to 

 a good crop of flagellates with the resulting good 

 growth and setting of larvae (Cole, 1939). The 

 most satisfactory fertilizer was the meat of the 

 shore crab Carcinus ground with sand and heated 

 to the boiling point. The suspension of meat was 

 added to a 90,000-gallon tank at the average rate 

 corresponding to 12.5 medium-sized crabs per day 

 for a period of 3 to 4 weeks. Production of nan- 

 noplankton was judged by pH readings, and as 

 soon as the readings reached 8.3 to 8.4 and the 

 tank had a distinct slight cloudiness, no more 

 crab meat was added. 



Evidence presented by Cole showed that 

 growth and attachment of 0. edulis larvae in 

 tanks were significantly increased by organic en- 

 riclnnent which stimulated the development of 

 the nannoplankton. Under laboratory conditions 

 the oyster larvae grew and set satisfactorily in 

 the water containing cultures of Platymonas 

 tetrahele. The larvae of oysters and other bivalves 

 apparently are not able to swallow microorganisms 

 which exceed 8m, but according to Thorson (1950) 

 the size of nannoplankton normally devoured by 

 larval forms is smaller (2m to 3m). 



Difficulties in obtaining reproducible results 

 from using organic enrichment for rearing larvae 

 suggested that variations in the composition and 

 quantity of nannoplaidvton may be responsible. 

 To determine the food requirements of 0. edulis 

 larvae, Bruce, Knight, and Parke (1940) isolated 

 from sea water six flagellate organisms ranging in 

 size from 1.5m t" ~m i" diameter. A known num- 

 l)er of ovstor larvae were introduced into glass 

 vessels filled witii 10 1. of uncontaniinated, sterile 

 sea water whicii was stirred and aerated. The 

 water was changed continuously by a drop feed; 

 the loss of larvae was prevented by covering the 

 outflow tubes with bolting silk. Tiie larvae were 



FISH AND WILDLIFE SERVICE 



