Molting in stomatopod crustaceans. I. Stages of the molt cycle, setagenesis, and morphology.

The description of stages of the molt cycle in mantis shrimp (emphasizing Gonodactylus but compared in a number of Gonodactylidae, Squillidae, and Lysiosquillidae) includes data on texture, hardness, and color of the exoskeleton; behavior; and the micromorphology of the integument and developing setae. After stage A1, When the exoskeleton feels soft and soapy, and A2, when the abdomen attains parchment consistency, the exoskeleton continues to harden until stage C1, when the telson carinae can no longer be depressed with a fingernail. The propodus (C2) and the merus (C3) of the raptorial appendage are the last elements to become rigid. The meral spot achieves indistinct coloration at B1 and distinct but pale coloration at B2; body color continues to intensify until C4. Changes in behavior are particularly marked before and after the molt. After the molt defensive and avoidance behaviors characterize A1 and A2. Threatening increases from low to intense levels in A2 and B1. Striking and offensive behavior increase from weak to normal intensities from B2 to C4. One or two days before the molt, animals lower activity and dominance and feeding levels, exhibit reclusive behavior, and sometimes seal the cavity entrance. The morphology of setagenesis is particularly valuable for determining the stage of the molt after the exoskeleton hardens. The ratio of setae containing cellular contents throughout the lumen to those condensed into strands separates stages A2 through C3. “Cones,” although variable within and between individuals in some species, form initially in B2 and continue to condense through D. Initiation of C4, D0 and D1 overlaps variously in some individuals. Apolysis (D0) and morphogenesis of new setae (D1-D3) provide cues for identifying approach of the molt. Setagenesis and setal morphology differ among families, genera and some species of stomatopods. The structure and functional morphology of setae can be explained by the form of setagenesis. Passive swelling and active muscular peristalsis effect the shedding of the exoskeleton and setal eversion at the molt. The carapace and rostrum are shed as one unit. Patterns of meral sutures vary among taxa with enlargement of the merus and with subterminal or terminal ischiomeral articulation. Middorsal sutures open only on the sixth and seventh thoracic segments, suggesting loss of the first rather than the last abdominal segment and therefore a distant relationship to other eumalacostracans. Approximate durations of the molt stages in stomatopods are compared to those of 11 crustacean taxa. Extensive overlap seems to occur among midinterval stages, particularly in stomatopods. The developmental plateau occurs in different stages in different taxa. Late postmolt (C), associated with calcification, requires longer in more heavily calcified than in thinly calcified species, except for stomatopods. The physiological events immediately surrounding the the molt (A1–B1 and D3–D4) are compressed in time in crayfish, brachyurans, and especially in stomatopods. Behavioral adaptations maximize this compression of physiological events at the molt in stomatopods. These three groups also possess the most asymmetrical molt cycles of the crustacean taxa examined. It is hypothesized that these modifications of the molt cycle allow opportunistic molting and represent adaptations to agonistic behavior. Temporal variation in behavior and in color over the molt cycle may increase the potential for speciation by processes resembling neoteny.