528 THE BIOLOGY OF MARINE ANIMALS 



with a yellow tail. Kinoshita (36) concludes that testicular hormones are 

 responsible for maintaining the sexual coloration of the male fish. 



In the gobiid Chloea sarchynnis of Japan, it is the female which develops 

 sexual coloration during the spring breeding period. In this condition the 

 lower jaw, throat, ventral fin and anal fin become jet black. The alteration 

 in colour is due to a localized increase in the number of melanophores, 

 increased melanin content and melanophore expansion. Assumption of 

 breeding dress is paralleled by an increase in ovarian interstitial tissue, and 

 the changes in melanophores characteristic of the breeding dress can be 

 induced by injection of oestrogenic preparations (37). 



In summary it may be said that the primary receptor for chromatophore 

 responses in teleosts is the eye, and photic stimulation of this organ gives 

 rise to impulses which are registered in the brain and which lead to colour 

 responses through the autonomic nervous system. Efferent impulses from 

 the c.n.s. may follow either one of two pathways, through the hypo- 

 physial stalk to the pituitary body, or through rami communicantes to the 

 sympathetic trunks and peripheral nerves. The intermediate lobe of the 

 pituitary secretes a chromatophorotropic hormone, intermedine, which 

 causes melanophore expansion, but in many fishes this mechanism is over- 

 ridden by nervous control. Concentrating nerve fibres cause melanophore 

 contraction, and when these are cut the pigment cells are paralysed and 

 expand. Dispersing nerve fibres have also been postulated, but the evidence 

 is indirect and unsatisfactory. Xanthophores and erythrophores are also 

 subject to nervous and hormonal control, and respond independently of 

 the melanophores. 



RHYTHMIC COLOUR CHANGES 



Many animals show a persistent daily rhythm in chromatic activity, even 

 under conditions of constant illumination or constant darkness. Such 

 rhythms may last for long periods, at least four to eight weeks in test 

 animals (isopods, crabs) held in the dark. In Uca pugilator (the fiddler 

 crab) there is an endogenous diurnal rhythm in which the animals darken 

 by day and blanch by night : the changes in hue are produced by dispersion 

 of the black and white pigments in the day phase of the cycle, and con- 

 centration of the same pigments in the night phase of the cycle. So strong 

 is the diurnal rhythm in this species that it largely masks the background 

 response. These daily changes in the chromatic behaviour of Crustacea 

 depend upon an endogenous rhythm in the secretion of chromatophoro- 

 tropins. Diurnal rhythms of Crustacea have attracted most attention, but 

 chromatic rhythms also have been discovered in various lower vertebrates. 

 A diurnal rhythm, partially concealed by responses to background and 

 incident illumination, may reveal itself in various ways. Animals in day 

 and night phases may show differences in degree of response to injected 

 chromatophorotropins, or there may be a strengthening of those back- 

 ground responses which correspond with the phase of the rhythm then 

 operating, and a weakening of those which are antagonistic. Thus, crabs 



