ON AN ANANDROMOUS PHENOMENON OF MEGALOPAE OF A GRAPSOID CRAB, VARUNA LITTERATA(FABRICIUS)

Abstract

This paper deals with the anandromous phenomena of megalopa of Varuna litterata (FABRICIUS) (Grapsidae, Varuninae), which was observed at the river mouth of the Hikiji-Gawa, situated on the north-east coast of Sagami Bay. The phenomena were observed in the daytime during the period from Oct. 9th to Nov. 7th of 1972. Tidal influence to the river reaches about 2 km from the river mouth. The adult crab is very rarely found on the northern areas of Sagami Bay, where may be the northern limit of distribution of this species. Each of the megalopa reaches the river mouth from off shore before full tide. The number of megalopa increases to the greatest at the time of full tide after the new moon or the full moon day. The megalopa continues to swim swiftly up the stream, keeping the distance between 10 to 30 cm from the shore. The swarm of megalopae thus reaches even 2 km beyond the limit of tidal influence. The length of carapace of the megalopa varies from 3.7 mm to 4.5 mm, the average size being 4.1 mm.