Hemolymph Ecdysone and Electrolytes during the Molting Cycle of Crayfish: A Comparison of Natural Molts with Those Induced by Eyestalk Removal or Multiple Limb Autotomy

Abstract

In studies of crustacean molting, eyestalk removal (ER) or multiple limb autotomy (MLA) has traditionally been used to precipitate ecdysis. This study was undertaken to determine whether the general physiological condition of the crayfish Procambarus clarkii, at 23° C, induced to molt either by ER or MLA, is comparable to that of animals molting naturally. Hemolymph was sampled every 2 d up to and for 3 wk following ecdysis. Samples were assayed for ecdysone (as an endocrine measure) and inorganic electrolytes (Ca, Na, Cl, K, Mg), which were selected for study since regulatory hormones originate in the eyestalk. Following surgery, ER crayfish underwent lethal ecdysis within 13 d, sooner than MLA crayfish (36 d). In the control group, ecdysone peaked approximately 5 d before ecdysis. For induced molts, the ecdysone peak was two times larger and was more protracted. Calcium was elevated 15 d before natural molts as a result of cuticular reabsorption, then returned to baseline level and exhibited a transitory decrease at ecdysis as a result of water loading. The MLA crayfish had a similar Ca profile, except that the peak occurred earlier and in advance of the ecdysone peak. A premolt Ca peak was not observed in ER crayfish; levels of all ions were greatly reduced at ecdysis because of excessive water uptake. The ecdysone and Ca profiles did not appear to be linked in any treatment. Sodium and Cl profiles were very similar in all treatments, essentially decreasing as a result of water uptake around the time of ecdysis. Potassium remained constant in control crayfish. In MLA crayfish it decreased around the time of ecdysis, and in ER crayfish it exhibited both a postsurgical decrease and a more pronounced decrease at ecdysis. Magnesium in control and MLA crayfish decreased in premolt and remained low, possibly contributing to exoskeletal mineralization; however, levels were elevated in ER crayfish. Collectively, the data demonstrate that ER crayfish experience severe hemolymph dilution at ecdysis as a result of increased water influx, which may contribute to death. The eyestalks appear to contain factors that regulate water and electrolyte balance. Although MLA crayfish molted successfully, the profiles for ecdysone and electrolytes, while superficially similar to those of control crayfish, were sufficiently different that the two groups cannot be considered physiologically or metabolically equivalent.