ENDOCRINE CONTROL IN THE CRUSTACEA

Abstract

SUMMARY The hormones of crustaceans may be grouped into those which control effector organs (chromatophores, muscles) and those which control the more gradual sequences of growth, development and reproduction; they have been termed energetic and metabolic hormones respectively. Most of the known crustacean hormones originate in neurosecretory centres in the central nervous system. The sinus gland is an important part of an elaborate neurosecretory system in the eyestalk of stalk‐eyed crustaceans and in the head of sessile‐eyed forms. It is largely composed of the swollen terminations of neurosecretory fibres. Some cellular elements have also been detected, but it is not yet known whether they have a secretory function. It is generally accepted that the greater part of the secretory material in the sinus gland has been transported thither along axon fibres from cell bodies in the central nervous system, though it has been suggested that the staining reactions of the sinus gland indicate also the probability of chemical transformation, if not autochthonous secretion. After‐sinus gland ablation the cut stump of the nerve leading to it continues to accumulate secretory material. An indiscriminate use of the term X organ in the eyestalk has led to confusion. The X organ, originally described in detail by Hanstrom, is characterized by its association with a sensory papilla and by its contained concentric‐layered structures, here called ‘onion bodies’. The term X organ has been used by some authors to denote a group of neurosecretory cell bodies, the fibres of which terminate in the sinus gland. It is suggested that the sensory papilla X organ (Hanstrom's X organ) should be distinguished from the other X organs as it differs from them morphologically and physiologically. The post‐commissure organs comprise enlargements of the epineurium of two post‐commissure nerves, containing the terminations of neurosecretory fibres. They have been shown to contain chromactivating hormones. The pericardial organs comprise epineurial enlargements containing fine‐fibre terminations. They lie in the pericardial spaces and have been shown to contain substances active on the heart. The Y organ, a glandular structure which contains hormones affecting the rate of development, is located in the antennary segment of those forms which have a maxillary excretory organ and in the second maxillary segment of those which have an antennary excretory organ. Various groups of neurosecretory cells have been detected in the brain and in thoracic ganglia. The term neurohaemal organ has been proposed to denote those tissues, through which substances produced in neurosecretory cells gain ready access to the blood. The pigment pattern of any one crustacean species is fairly constant, but patterns differ in the different groups. The pigments are contained in monochromatic, dichromatic, trichromatic and tetrachromatic chromatophores. The chromactivating substances which have so far been separated from tissue extracts seem to act differentially on these chromatophore types. The sinus glands and the post‐commissure organs contain a substance, found also in the corpora cardiaca of insects, which concentrates the red pigments in the large red and small red chromatophores of Leander serratus; it has been called the A‐substance. When tissue extracts containing the A‐substance have been allowed to stand for some time the A‐substance disappears and is replaced by one or more substances which affect the red pigment in the small chromatophores but not that in the large chromatophores. The post‐commissure organs contain, in addition to the A‐substance, a substance B which disperses red pigments, and also an A'‐substance which concentrates white chromatophore pigments and at the same time the red pigments of the large red chromatophores. A substance which disperses black pigments in the chromatophores of crabs has been found in a great number of different crustacean species. It is present in the Natantia, but its function in these forms is not yet known. There is some evidence that light may affect chromatophore and eye pigments directly, possibly by sensitizing them to chromactivating hormones. Chromatophore and eye pigments continue to migrate rhythmically even if animals are kept in constant darkness or under constant illumination. One such rhythm in Uca has been shown to be in synchrony with the solar cycle, another with the lunar cycle. The rhythms seemed to be largely independent of temperature, and of the eyes and sinus glands. The migration of distal retinal pigment appears to be controlled by one or two substances originating in the sinus glands; there is some evidence that the proximal and reflecting pigments may be influenced by hormones originating outside the sinus glands. A heart‐accelerating substance found in the pericardial organs is without effect on the chromatophores. The sinus glands release a diabetogenic principle which maintains the blood‐sugar level above a certain minimum. This substance appears to be released in response to stressing agents. In crabs, centres outside the eyestalk may also produce the hormone. The eyestalks seem to exert some measure of endocrine influence over glycogen metabolism, though this may be indirect. There is some evidence that the eyestalks are implicated in an endocrine control of chitin formation and the decomposition of lipoids and carotenoids. The evidence for the hormonal control of protein metabolism is suggestive but incomplete. Removal of the eyestalks has been shown to affect respiratory metabolism, and tissue homogenates in vitro can be affected by eyestalk extracts and by extracts of the sinus glands or of the central nervous system. Respiratory data in crabs deprived of their sinus glands suggest that a hormone controlling oxygen consumption is released at the sinus gland, though formed elsewhere. There is evidence that a sinus‐gland principle influences calcium metabolism, but that this may only be effective in the winter non‐moulting season in those animals which moult annually. The phosphate content of the tissues and the blood changes cyclically with the calcium content and there is some evidence of endocrine control, though the substance appears to be different from that which affects calcium metabolism. The eyestalk appears to contain an antidiuretic and water‐balance regulating hormone. It seems likely that this differs from the ‘moulting hormone’, although their actions seem to be very closely co‐ordinated. The moulting cycle is under the control of at least three hormones and possibly as many as six. The moult‐inhibiting hormone of the eyestalk and brain inhibits the onset of the premoult, the period of preparation for moulting; the moult‐accelerating hormone accelerates and correlates the processes of the premoult once this has begun. Both these hormones may act upon the Y organ, which itself produces a hormone necessary for the moult process. The amount of water taken in at moulting is regulated by the water‐balance hormone of the eyestalk. Ovarian growth is controlled, in both Natantia and Reptantia, by an ovarian‐inhibiting hormone emanating from the eyestalk. Testicular growth is controlled in Brachyura, but not in those Natantia so far investigated, by a testis‐inhibiting hormone emanating from the eyestalk. The ovary of some Malacostraca may secrete a progestational hormone responsible for promoting the development of the brooding characteristics. The vas deferens gland in Amphipoda secretes a hormone controlling the development of the male secondary and accessory sexual characteristics. Some colour‐change hormones and substances which affect the heart beat have recently been separated by chromatography and electrophoresis. Preliminary studies indicate that at least one of the colour‐change hormones contains peptide bonds which are necessary for its activity; one of the active substances in pericardial organ extracts is closely related to 5‐hydroxytryptamine. It is suggested that the ovarian‐inhibiting hormone is steroid in nature.