Patterns of Hemocyte Production and Release Throughout the Molt Cycle in the Penaeid Shrimp Sicyonia ingentis

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.