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Hormonal Arthropods: A Brief Overview Of Moulting

1726 words - 7 pages

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 its shell; during the crab’s active phase it releases its legs, then body, and finally its claws.

Even though the crustacean may hide until its cuticle has hardened; amongst many crustacean species the female can only be inseminated when they are soft post-moult[2]. The protein Bursicon stimulates recalcification and is necessary for cuticle tanning; it does not travel to the cuticle via a hormone system, instead it acts directly upon the cuticle itself[14]. The rest of the time there are a few hormones that work to prevent moult; for example, Moult Inhibiting Hormone (MIH). MIH is secreted by the X-organ/sinus gland complex in the eyestalk[6,19] travels in the haemolymph to the Y-organ and inhibits E synthesis and release[6,7]. There are MIH receptors on the plasma membrane of the Y-organ[7,10], and the rate of E synthesis depends on how responsive the Y-organ is to MIH[7]. Crustacean Hyperglycaemic Hormone (CHH) regulates the ion and water uptake which for swelling that help the animal lose its shell[8]; it also regulates carbohydrates[9]. A surge of CHH is released from the eyestalk during premoult[8,9] which also binds to receptors on the Y-organ to inhibit the release of 20E[10]. CHH may also have an effect on the...

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