Overriding of the Molt-Inducing Stimulus of Multiple Limb Autotomy in the Mud Crab Rhithropanopeus Harrisii by Parasitization with a Rhizocephalan

Abstract

Specimens of the mud crab Rhithropanopeus harrisii underwent ecdysis within 16 days following autotomy of six walking limbs from each, compared to a mean of 40 days for untreated controls. The second ecdysis following autotomy also occurred sooner than that of controls of equivalent size. Crabs parasitized by the rhizocephalan parasitic castrator Loxothylacus panopei did not undergo ecdysis during the duration of this investigation (140 days) whether autotomized or not. Parasitized crabs from which walking legs had been autotomized in the laboratory did not form limb regenerates. However, two parasitized crabs that had been collected from the field bore large regenerates that may have developed before the rhizocephalan-induced anecdysis. It appears that sacculinids prevent ecdysis of their hosts even after proecdysis has been initiated. In a pattern typical of sacculinid-host associations, prevalence of the castrator decreased with increasing host size. In 1956, Bliss noted that five specimens of the Bermuda land crab Gecarcinus lateralis (Freminville) missing six to eight limbs molted sooner than controls, three within two to four months, the other two, within six months. Similar observations were systematized and the advantages of multiple limb autotomy (>4 for G. lateralis) over eyestalk ablation in initiating proecdysis in five species of decapod crustaceans were described in several publications from this laboratory (Skinner and Graham, 1970, 1972; Holland and Skinner, 1976). Subsequently, others have found multiple autotomy an effective trigger of precocious molts in more than a dozen additional species (reviewed in Skinner, 1985). The following summary taken from that review is based on studies of G. lateralis. Within a few days following closure of the wound, a regenerating limb bud forms at the autotomy plane. Regenerates do not grow further while the animal is in anecdysis, but grow rapidly during proecdysis (Bliss, 1960). If a regenerate' is autotomized before a critical period, probably DI, growth of other regenerates ceases and the proecdysial period is extended until a second regenerate is produced. If a regenerate is autotomized following the initiation of D1, however, the sequence of proecdysial events continues without interruption and culminates in ecdysis without the autotomized regenerate having been replaced (Skinner and Graham, 1972; Holl nd and Skinner, 1976). Specimens of Uca pugilator and Uca pugnax are among the species of Crustacea in which multiple limb autotomy induces a precocious molt (Skinner and Graham, 1972); an interruption of proecdysis by limb autotomy can occur during stage DI in U. pugilator(Hopkins, 1982). In megalopae and early postlarval instars of Rhithropanopeus harrisii which have a very brief anecdysis (5-8 days), limb autotomy either (1) lengthens the anecdysial period if limb regenerates are formed or (2) shortens the time to ecdysis and regenerates are formed during the subsequent anecdysial period (McConaugha and Costlow, 1980, 1987). Thus1 proecdysis can be induced, arrested, or unaffected depending on the timing of autotomy. Two factors, limb autotomy factor anecdysis (LAFan) and limb autotomy factor proecdysis (LAFpro), have been proposed to explain this dichotomy (Skinner, 1985). LAFan triggers a precocious ecdysis; LAFpro interrupts proecdysis. To summarize, following the loss of >4 limbs during anecdysis, LAFan is produced and, if a threshold titer is reached, the animal enters proecdysis. In contrast, LAFpro is produced following autotomy of a regenerate during early proecdysis; it prolongs the proecdysial period. Once proecdysis has reached a critical stage, probably apolysis which initiates DI, either LAFpro is not released or it is no longer an effective inhibitor.