The species referred to the genus Ditrema has received a confusing treatment on their forms and nomenclature here and abroad. Recently Yamane (1964) recognized D. temmincki Bleeker and D. viridis Oshima, and further, divided the former into two forms by the differences of coloration. The present study, based on the specimens of Ditrema collected at Hashirimizu, Miura Peninsula, Kanagawa Pref., has been started in 1963 and continued to the present in a hope to clarify the status of its taxonomy and features of life history, and recognized two species, D. temmincki and D. viridis. However, the former was found not to be divided into different races, forms, or any other systematic groups within the species D. temmincki. The most significant differences found on the life history between the two species were those of number of embryos and its size at birth. The average number of embryos carried by a female was 13.02 for D. temmincki and 29.65 for D. viridis, the differences being nearly twice in every ages of females between the two species. The averaged size of embryos at birth was 57.4 mm long in total length for D. temmincki and 49.6 mm long for D. viridis, and the size gap, regardless size or age of mother fishes considered to be 7-8 mm at least. Such size differences at birth was also evidenced by the number of ridges formed on the scales during prenatal growth. The distribution of both species, investigated by means of nation wide census, is almost overlapped throughout Japan from southern Hokkaido to Kyushu. The preferred habitat taken by the two species was found to be different between them at Hashirimizu; D. viridis lives in shore waters characterized by Zostera vegetation in the bay or inlet, while, usual habitat of D. ternmincki was found in rocky region of outer sea. Similar difference of habitat by species was also well confirmed by the result of the census in the country.
Abstract Laboratory bioassays were made at 66 to 75°F to determine the effects of the organophosphorus insecticide DDVP on the viability of the sanake-head spawn and hatchlings. Sixteen different concentrations ranging from 0.10 to 20 ppm were tested against duplicate samples of eggs placed in cylindrical glass jars holding 8 litres of unchlorinated water. The eggs hatched at all test concentrations. Hatching time increased with the increase of concentration. All eggs hatched at concentrations up to 1.0 ppm within a period of 12 hours. Within the same period, about 60% eggs hatched at 1.6 to 16 ppm, and 55% eggs hatched at 20 ppm; the remainders hatched within 24 hours. Yolk-sac absorption was normal at 0.10 ppm. The yolk absorption gradually decreased at 1.6 to 4 ppm, and totally stopped at 5 ppm or more. The hatchlings were many times more sensitive to DDVP than the eggs. The median survival time of hatchlings gradually decreased with the increase of concentration. The eggs and hatchlings survived well at 0.10 ppm DDVP.