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Although the crustacean crustacean hyperglycemic hormone/molt‐inhibiting hormone/gonad‐inhibiting hormone neuropeptides have been studied extensively in the last two decades and several neuropeptides from the shrimp Metapenaeus ensis have been cloned, the functions of most of these neuropeptides remained putative. In this article, we describe the use of recombinant protein and an RNA interference approach to study the reproductive function of the previously reported molt‐inhibiting hormone (MeMIH‐B) in M. ensis. When hepatopancreas and ovary explants were cultured in medium containing recombinant MeMIH‐B, the vitellogenin gene (MeVg1) expression level was upregulated in a dose‐dependent manner, reaching a maximum in explants treated with 0.3 nm recombinant MeMIH‐B. Shrimp injected with recombinant MeMIH‐B showed an increase in vitellogenin gene expression in the hepatopancreas. Moreover, a corresponding increase in the vitellogenin‐like immunoreactive protein was detected in the hemolymph and ovary of these females. Injection of MeMIH‐B dsRNA into the female shrimp caused a decrease in MeMIH‐B transcript level in thoracic ganglion and eyestalk. These shrimp also showed reduction of vitellogenin gene expression in the hepatopancreas and ovary. Furthermore, the hemolymph vitellogenin level was also reduced in these animals. In summary, the results from recombinant protein and RNA interference experiments have demonstrated the gonad‐stimulatory function of MeMIH‐B in shrimp.

The production and release of hemocytes was evaluated throughout the molt cycle in the shrimp Sicyonia ingentis. Hematopoiesis occurs in paired epigastric hematopoietic nodules (HPN) which consist of an extensive network of vessels. Hemocytes are produced within the walls of these tubules and released into the vessel lumens. During molt stage C (intermolt), few cells were present in the tubule wall; most of these were hematopoietic stem cells. Elevated mitotic rates during stages C to D1-2 (2-4%) led to the production and rapid release of individual hemocytes, primarily granulocytes. Although the mitotic rate progressively declined from stage D3-4 until after ecdysis (stage A1), the maturing hemocytes accumulated within the tubule walls. Around ecdysis, production of hyaline hemocytes exceeded that of granulocytes. Large groups of these hemocytes were channeled into the vessel lumens immediately after molting. Mitotic rates increased again during stages A2 and B with the number of hemocytes in the tubules reaching seven times that of stage C. Morphological stages in the transition of hematopoietic stem cells into hyaline hemocytes and granulocytes are described, and a model of decapod hemocyte maturation is presented.

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