Crustaceans - Hormones in Growth and Reproduction
Arthropods have a tough and frequently hard exoskeleton made up of cuticle composed of chitin and protein. The cuticle imposes restrictions on the growth of the animal. If not the cuticle is shed, growth is not probable to any extent. The periodic shedding of cuticle is known as moulting. Thus, moulting is closely related to growth in arthropods. In crustaceans, two hormones control the moulting process. The Y-organ generates the moulting hormone, the ecdysone, a steroid. Under the effect of ecdysone the animal moults. So it is a moult-inducing hormone having a positive effect on moulting. Though, moulting in crustaceans is as well under the inhibitory control of another hormone produced by the X-organ and released at the sinus gland. This hormone has a negative effect on moulting. If the inhibitory hormone concentration in the blood dominates, the animal moults. It is the balance among the two hormones that determines moulting in crustaceans. However, it is not certain whether the inhibitory hormone acts on the Y organ decreasing the release of moulting hormone ecdysone or it acts directly at the level of the epidermal cells where ecdysone also acts. Anyway, the effect is similar. You will see that in many animals, two or more hormones of antagonistic effects may control very frequently one function, the balance determining the result. Reproduction in crustaceans is as well under hormonal control. The X-organ-sinus gland complex is involved here also. It is easy to remove the X-organ-sinus gland system of crustaceans, which are situated in the eyestalk. Removal of the eyestalk, termed as eyestalk ablation, would result in the removal of the endocrine structures. Removal of the eyestalk from the adult female crustacean results in the acceleration of yolk deposition in the oocytes; size and width of the ovaries will therefore increase. Occasionally it results in precocious egg-laying too. Injection of an extract of the eyestalks of the adult females into eyestalk-ablated animal will reverse the trend. Such kinds of experiments led to the conclusion that in common animals ovarian growth is inhibited by a neurosecretory hormone from the eye stalk, namely, X-organ-sinus gland complex. When the hormone is absent, the ovaries start growing, depositing yolk in the oocytes. Although a number of other hormones controlling ovarian and testis activity have been postulated, sufficient proof to prove their existence is tacking. An exception is the androgenic gland hormone of some crustaceans.