Formation of the Midgut Gland and Localization of Esterase in Early Larvae of Japanese Horseshoe Crab, Tachypleus tridentatus

Abstract

In early stage of the first instar larvae of Japanese horseshoe crab, Tachypleus tridentatus, the midgut gland was not yet completed in the prosoma. A number of yolk granules and some connective tissue networks were partitioned into several compartments in the prosoma. In this stage, esterase activity was detected only in muscular tissues and it was suppressed by eserine, showing its choline esterase nature. Then the compartments became narrower into tubules which developed into the midgut diverticula. The yolk granules in the tubules decreased in number and in size, and the connective tissue networks grew and surrounded the tubules. Just before the first ecdysis, the esterase activity was detected in eosinophil granules appeared in the tubules. The activity was strengthened by the taurocholic acid, as a lipase-like feature of the esterase. In the second instar larvae 3 weeks after the first ecdysis, the midgut diverticula with eosinophil granules concomitant with lipase-like esterase were completed. Slender epithelial cells that were observed in adult female diverticula and loaded with these granules were rarely observed in these young diverticula. From the present results, the midgut gland involved in digestion and absorption of nourishment in larvae was shown to be accomplished around ecdysis between the first and the second instars.