each category of pesticide are large enough that it should be possible to 

 select compounds to control pest species without serious damage to commercial 

 shellfish. - modified authors' abstract - D.L. 



450 



Davis, Harry C ., and Herbert Hidu. 1969. 



Effects of turbidity-producing substances in sea water on eggs and larvae 

 of three genera of bivalve mollusks. Veliger 11(4): 316-323. 



Species studied were American and European oyster and hard clam. Oyster 

 embryos are iess tolerant of silt, but more tolerant of kaolin (clay) and 

 Fuller's earth than clam embryos. No embryos of either species developed 

 normally at higher concentrations of silt, but significant numbers of both 

 species developed normally in highest concentrations (4 g/1) of kaolin and 

 Fuller's earth. The larger particles present in silt and, to a lesser 

 extent, in kaolin and Fuller's earth, apparently were mainly responsible 

 for adverse effects on oyster eggs, but smaller particles, most numerous 

 in kaolin, were probably responsible for adverse effects on clam eggs. 

 Silicon dioxide had little effect on clam eggs except when the smallest 

 particles were at high concentrations. Many clam larvae, in the presence 

 of smaller particles of kaolin and Fuller's earth, eventually lost their 

 ability to reject these particles. They then began to ingest particles, 

 the stomach became packed, and larvae died. In kaolin and Fuller's earth 

 clam larvae were better able to reject small particles, hence had lower 

 mortality. Hard clam larvae suffered severe mortality at comparatively low 

 concentrations of the smallest particles of silicon dioxide. Effects of 

 kaolin and Fuller's earth, particularly at high concentrations, on survival 

 and growth of clam larvae were more drastic than would be expected from the 

 effect of silicon dioxide particles of similar size. Silt had no deleterious 

 effect on growth of clam larvae until concentrations reached 1 g/1, and 

 growth was not reduced drastically until silt concentrations were in excess 

 of 2 g/1. Clam larvae showed evidence of feeding even in 4 g/1 of silt, 

 although growth was negligible. Growth of clam larvae was drastically 

 reduced by 0.5 g/1 of kaolin or Fuller's earth, and all larvae were killed 

 by 1 g/1 of either substance. Growth of clam larvae was not seriously 

 affected by silicon dioxide particles ranging from 5 to 50 u at concentrations 

 up to 2 g/1. Even in particles less than 5 p growth of clam larvae which 

 survived was fairly good. More rapid growth of clam larvae in lower con- 

 centrations of silt, kaolin, and Fuller's earth may be due in part to 

 chelation or adsorption of toxins present in seawater or produced by algae 

 or bacterial contaminants introduced with food. The experiments showed that 

 bivalve larvae can tolerate turbidities higher than those normally encountered 

 in natural waters, and that under certain circumstances low concentrations of 

 turbidity-producing materials may be beneficial. But high silt concentrations, 

 such as may be generated by dredging or filling, can be detrimental directly, 

 or through lowered pH. It is possible also that in natural waters disturbing 

 the bottom may release toxic bacteria, and organic enrichment sufficient to 

 stimulate their reproduction. Thus, effects of dredging may be more serious 

 than these experiments might suggest. - J.L.M. 



451 



Davis, Harry C ., and Victor L. Loosanoff. 1953. 



Utilization of different food organisms by clam larvae. Anat. Rec. 117(3): 

 646. 



Different microorganisms were evaluated as larval foods using growth of 

 Venus mercenaria larvae as the criterion. Growth of experimental lots was 

 compared with a control which received no supplemental food. The species 

 used, the numbers of cells fed per cc per day, and the lengths of larvae at 

 8 days were: control 124 p; Chlamydomonas 20,000, 174 p; Chlorella sp. 

 50,000, 165 p; Chromulina pleiades 30,000, 155 p; Isoahrysis galbana 30,000, 

 150 p; and Porphyridium sp. 50,000 122 p. After the 10th day feeding of 

 Porphyridium was discontinued and that culture was given 50,000 cells per 

 cc per day of mixed phytoplankton, chiefly Chlorella. Within 5 days the 

 larvae grew to 185-210 p, the size at which metamorphosis normally begins. 



126 



