Observations on Egg Hatching in the Estuarine Crab Sesarma haematocheir

A female of the terrestrial crab Sesarma haematoeheir incubates 30,000-50,000 eggs on her abdomen. After 1 month of embryonic development, zoeae larvae are released into estuarine waters within 3-5 sec by means ofvigorous fanning motions of the abdomen. Hatching (breakage of the outer egg membrane) occurs on land just before larval release. The release behavior itself does not cause rupture of the egg case, nor has the presence of a "hatching enzyme" been obviously demonstrated. Hatching seems to be induced by mechanical rupture of the egg case. The pressure responsible for hatching may be produced either by the larva itself, or by osmotic swelling of thin inner membranes encasing the larva, although neither of these hypotheses is sufficient at present to explain the complete hatching mechanism. If hatching is explained by such mechanisms, then there remains the question ofhow hatching is synchronized among the large number of embryos attached to the female. Hatching of detached embryos is synchronized to some extent, but the degree of synchronization is less than that occurring in the larvae carried by the female. This observation suggests that stimuli from the female are important in establishing highly synchronized hatching. The ecological significance of the hatching system is also discussed. time is also synchronized within a batch of eggs attached to a female. Little is known about the hatching mechanism in Crustacea. It may be mechanical, nonmechanical, or a combination of both. Osmotic hatching has been suggested for a number ofcrustaceans. In this hypothesis, the rigid outer egg membrane may be burst by osmotic pressure inside the thin inner membrane (Yonge 1937, Marshall and Orr 1954, Davis 1965), or by pressure caused by the swelling larva itself (Davis 1964). The crab Sesarma haematocheir burrows on land surrounding small estuaries in Japan and along the coastlines of Southeast Asia. In early summer, a female incubates 30,00050,000 eggs in her folded abdomen for ca. I month. She emerges at riverside or seashore where the zoeae are released. About the time of high tide at night, the females immerse themselves in water and vigorously flex their abdomens as clouds of newly hatched larvae are swept away. This larval release behavior is completed in 3-5 sec, and all the zoeae are released from the pleopods. After larval release, the females return to land and incubate