Hormonal Arthropods: A Brief Overview of Moulting Essay

Hormonal Arthropods: A Brief Overview of Moulting Essay

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There is no doubt that arthropods are an extremely successful group of animals, with an estimated 5-10 million species worldwide[1], and this can be attributed to having an exoskeleton; it provides many benefits, such as protection from parasitism and other threats. However, one major disadvantage of having an exoskeleton is the limitations that an inelastic cuticle can place on growth. The exoskeleton provides protection, but when freshly moulted the animal is soft and vulnerable, as well as having limited mobility and use of appendages; many seek shelter before moulting[2]. There are similarities and differences between the moult cycles of all the arthropods, however only crustaceans and insects will be discussed here.

The crustacean moult cycle is initiated when the Y-organ in the head secretes the hormone Ecdysone (E) into the circulatory system[3,4,5]; the Y-organ is the primary source of E[5]. Upon contact with the haemolymph (a combination of blood and tissue fluid) within the circulatory system E is converted to its active form, 20-hydroxyecdysone (20E), by a P450 enzyme[4]. This causes the epidermal cells to secrete moulting fluid, which contains a mixture of enzymes such as proteases and chitinases. The moulting fluid degrades the exo- and endocuticle, and the digested cuticle is reabsorbed to create new cuticle; moult fluid also has a role in initiating the moult. The degraded cuticle is replaced by water or air so that the animal can swell in order to lose the shell. During the passive phase of the moult cycle, the shell splits along the break point caused by the decalcification of the shell, in the crab this is along the epimeral lines. Once the shell has split, the animal wiggles side-to-side to free itself from i...

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...stem: roles of eclosion hormone and ecdysis triggering hormone. Journal of Experimental Biology, Vol. 200, pp. 869-881
19. Kuballa A. & Elizur, A. 2007. Novel molecular approach to study moulting in crustaceans. Fisheries Research Agency Bulletin, Vol. 20, pp. 53-57
20. Vafopoulou, X. & Steel, C. G. 1996. Circadian regulation of a daily rhythm of release of prothoracicotropic hormone from the brain retrocerebral complex of Rhodnius prolixus (hemiptera) during larval-adult development. General and Comparative Endocrinology, Vol. 102, pp. 123-129
21. Steel, C. G. & Vafopoulou, X. 2006. Circadian orchestration of developmental hormones in the insect, Rhodnius prolixus. Comparative Biochemistry and Physiology – Part A: Molecular & Integrative Physiology, Vol. 144, pp. 351-364
22. Feyereisen, R. 1999. Insect P450 enzymes. Annual Review of Entomology, Vol. 44, pp. 507-533

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