Hatching of an estuarine crab : characterization, activity, and putative role of a caseinolytic protease

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The zoeas of an estuarine crab Sesarma haematocheir (akate-gani) hatch synchronously within 0.5 - 1 h from a female. Three aspects of hatching were studied : caseinolytic activity, morphological changes of the embryonic envelope structure, and environmental factors. Hatch water (i.e., the filtered medium in whcih zoeas have hatched) contains a caseinolytic protease, and the molecular weight of this protease was estimated on gel filtration. Transmission electron microscopy shows that the envelope (1.8 mum thick) is composed of three distinct layrs : an outer (E1), thick middle (E2), and thin inner (E3) layr. The outer two layrs, that are structurally the main components of the envelope, were not morphologically altered during hatching, but the inner layr (E3) was clearly altered between 11 h and 3 h before hatching, suggesting its digestion. The assay with isolates of the embryonic envelopes and morphological observations also suggested that the envelope is digested by the hatch water. The time of hatching was strongly affected by low water temperatures, and their effects were much stronger to the embryos detached from the female than female-attached embryos. Salinity was ineffective. Mechanisms of hatching based on the digestion of this (E3) layr has been discussed.