Penaeid molting stages

Growth in crustaceans is not a continuous process. Decapod crustaceans must first loosen the connections between their living tissue and the cuticle. Then they must move out relatively rapidly from the confines of this cuticle, take up water to expand the new, flexible exoskeleton, and quickly harden it so it provides protection and support for locomotion.

Penaeid shrimp molt at intervals of a few days or weeks. Their molt cycle is divided into six stages: early postmolt (stage A), late postmolt (stage B), intermolt (stage C), early premolt (stage D0-D1), late premolt (stage D2-D3), and ecdysis (stage E).

In stage A (Fig. 1), which occurred immediately after ecdysis, a pigmented cellular matrix completely filled the setal bases. During stage B (Fig. 2), the cellular matrix retracted from the setal base and a clear space was easily recognized in the bases. In stage C (Fig. 3), the matrix was absent from the setal bases and the pigment seemed to form an epidermal line at the bases of the setal nodes.

At the D0-D1 stage (Fig. 4), the pigment retracted from the bases of the setal nodes, leaving the old cuticle. During D2-D3 (Fig. 5), the new developing setae were observed. Stage E, the actual shedding of the exoskeleton, occurred at night in less than one minute, which made it very difficult to find animals in this molting stage. Fig. 6 shows the new setae that were extruded from the matrix.