Systematics and Biology of the Two Species of Embiotocid Fishes Referred to the Genus Ditrema in Japan

Abstract

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.