Chromatic behaviour of elasmobranchs

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Abstract
The first recorded observations on the pigmentary effector system of elasmobranchs were negative. Schaefer (1921) recorded thatRaia clavataandR. batismaintained a dark coloration on both white and black “backgrounds”, i. e. immediate visual field. Parker (1933) repeated these tests onR. erinaceaand described visible paling on a white background and darkening on a black background. In collaboration with Porter (1934) the same author demonstrated a similar reaction in the dogfish,Mustelus canis. The observations on colour changes in response to background were extended to elasmobranch fishes of English waters in 1936 when Waring described similar responses inScyllium caniculaand Hogben inRaia brachyura, R. maculata, Rhina squatinaandScylliumsp. In all these species it has been explicitly recorded that the natural response is a slow one. With regard to the receptive mechanism involved in these changes, it is generally agreed that discrimination between white and black backgrounds is visual. The work of Lundstrom and Bard (1932), Hogben (1936a) and Waring (1936) on excision of the separate lobes of the pituitary and the injection of extracts, clearly established that the dark phase in all elasmobranchs is due to the activity of the neuro-intermediate lobe of the pituitary. Transfusion of blood from animals kept on different backgrounds has provided independent evidence for the influence of a blood circulated hormone (Parker and Porter 1934; Waring 1936).

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