A PRELIMINARY STUDY ON ARTIFICIAL CULTIVATION OF SPATS OF MACTRA ANTIQUATA SPENGLER

1.The present paper deals with the artificial fertilization,embryonic de- velopment,metamorphosis and the process of settlement,with special refer- ence to the living habitat of larvae and the artificial rearing of spat,of a com- mon edible clam—Mactra antiquata Spengler. 2.Ripe eggs of superior quality can be obtained by means of collecting eggs through dissection and by washing and settling eggs.Immediately after fertilization,it is necessary to renew fresh seawater,to wash off surplus sper- matozoa and unfertilized eggs,and to keep the developing embryos in a sus- pended condition.Under the favorable environment,the hatching rate can rea- ch as high as 95%,or even higher. 3.The range of favorable temperature for the embryonic development lies between 17～24℃,the most favorable temperature being 20～24℃. 4.The range of favorable salinity for early development,i.e.from fert- ilization of eggs to blastula stage,is closely related to that of adults.The ra- nge of salinity increases with larval development;but the range of favorable salinity for spats appears to be more restricted than for larvae. 5.Within the range of light intensity between 500～2500 lux,the trocho- phore larvae are markedly positive towards light,i.e.positive phototropic. The veliger larvae,on the other hand,are negatively phototropic,but not very marked. 6.The trochophore larvae show a definite reaction towards the color of light,being positive towards green,yellow and white light. 7.When the larvae are rearing outside the laboratory,the filtered sea- water(changed daily about half of the original amount)can fulfill the food requirements of larvae which are capable of developing normally through me- tamorphosis to the stage of attachment. 8.The chief enemies of larvae are certain bacteria and carnivorous pro- tozoans which can be kept under control by changing water constantly,so as to keep the culture-medium clean.The rate of survival of larvae will be enha- nced by taking such measures. 9.Within the range of favorable temperature the larvae require 11～13 days to complete the development from the straight-hinged larvae to the sta- ge of settement.It is possible to accelerate the growth-rate of larvae by rais- ing the temperature. 10.The size of newly-hatched larvae lies between 208 X 189～225 X 209μ. They attach to the substratum by secreting byssus.The larvae under 1 mm. in body-length have,however,no strong desire for settlement. 11.The larvae under 1～2 mm.in body-length begin to lead a more sta- ble life of settlement.Those inhabiting the fine sand are growing faster than those inhabiting the soft mud;whilst those without the means of settlement are growing slowest.