Hormonal Effects on Calcium Metabolism in Crustacea

Cyclic shifts of calcium in the exoskeleton and soft tissues,as they are related to the intermolt cycle in crayfish, arereviewed. Regulatory factors, derived from the eyestalk, influencelevels of exoskeletal calcium; eyestalk extracts prepared fromanimals in premolt decrease shell calcium, while reciprocallyextracts from animals in intermolt increase it when these hormonalsources are injected into animals in the premolt stage (D₀-D₄). In addition, premolt eyestalk extract results in an increasein gastrolith calcium. In the exchange of calcium between theanimal and its environment there is evidence for differentialdepositionof recently available calcium in the exoskeleton. Further, intermoltand early premolt animals maintained in Ca⁴⁵-labelled waterfor 15 days concentrate it 4 and 3—fold in the exoskeletonand stomach, respectively. However, removal of a molt-inhibitingfactor through ablation of eyestalks results in a 20 and 40—foldincrease in incorporation inthese same sites relative to environmentalconcentrations. Treatment with mammalian parathyroid extract mobilizes bothexoskeletal and gastric calciumand leads to a rise in bloodcalcium. However, there is little or no effect on levels ofexoskeletal citric acid. Further, citric acid is higher in thecrayfish carapace during stage C, the period of mineralization,than in stage D, the period of demineralization. There are both similarities and differences between the effectsof crustacean and mammalianregulating factors with respect tothe direction and extent of mineralization. Biochemical studiesshould elucidate the mechanisms regulated by these hormones.     

Crustecdysone Mediated Changes in Crayfish

Molting in crayfish, essential for growth is preceded by a decreasein total organic content during premolt. Mineral, as calcium,is conserved in gastroliths after mobilization from the exoskeleton.Crustecdysone treatment results in gastrolith formation by intactcrayfish in intermolt, state C, but does not influence the rateof gastrolith formation in eyestalkless animals. Moreover, therate of gastrolith formation is not increased at higher hormonedoses; conversely, less dense, abnormal gastrohths result, whileapolysis is evident by 48 to 60 hours after treatment. It appearsthat crustecdysone promotes reabsorption of the organic matrixsince treatment with this hormone does not lead to a measureabledecrease in calcium content of shell based on dry weight. Premolt crayfish normally have a higher amino acid pool in alltissues than do those in intermolt, and eyestalk removal fromintermolt crayfish results in an increase in the free-aminoacid level of all tissues by 24 hours; crustecdysone treatmentresults in a significant increase in muscle amino acids by 12hours after a hormone dose of 5 µg/g body weight. In thecontext of integumentary growth and protein synthesis low dosesof crustecdysone (1 µg/g) increase the in vivo incorporationof amino acid into hypodermis, but not hepatopancreas, by 24hours, and the increase is significant by 36 hours; a higherdose (5 µg) does not change this rate of incorporation.Selective increase in hypodermis protein is not evident in acidphosphatase levels from 12 to 48 hours, but respiration of thistissue is significantly elevated within two hours after hormoneinjection.