OPTIMIZATION OF FERTILIZATION PARAMETERS FOR REARING SURF CLAMS (SPISULA-SOLIDISSIMA)

Laboratory experiments were conducted to determine optimal conditions for ''in vitro'' fertilization of surf clam, Spisula solidissima, oocy tes. Critical variables such as concentration of gametes, number of sp ermatozoa per oocyte, temperature, pH and salinity were investigated a nd assessed in relation to the proportion of monospermic and polysperm ic fertilizations. The optimal concentrations of sperm and oocytes for monospermic fertilizations were 0.8-4 x 10(6) spermatozoa/ml and 5-30 X 10(3) oocytes/ml. Good percentages of monospermic fertilizations (8 0%) were thus obtained at sperm:oocyte ratios of 30:1 to 500:1 with op timal success (95% monospermic fertilizations) at a sperm:oocyte ratio of 50:1 to 100:1. Higher ratios induced increased polyspermic fertili zations while, at lower values, the oocytes remained unfertilized. Tem perature, pH and salinity of natural sea water were varied separately from the standard values of 18-degrees-C, pH 8.0 and 24 parts per thou sand and the success of fertilization was assessed. Temperatures rangi ng from 6 to 24-degrees-C had no detrimental effects upon fertilizatio n and temperatures of 8 to 20-degrees-C appeared adequate for optimal fertilization. Fertilization rates, not affected at pH values ranging from 7.5 to 10.5, decreased rapidly at other values. Fertilization was not affected at salinities ranging from 20 to 35 parts per thousand a nd a salinity range of 23 to 30 parts per thousand was ideal for ferti lization.