488 THE BIOLOGY OF MARINE ANIMALS 



algal carotenoids, with greater light intensity. Actinia equina, however, 

 lacks algae, and light exerts a direct effect on pigmentation in this animal. 



Slow transformations in the colours of animals are known as morpho- 

 logical colour changes, in contradistinction to rapid chromatophore 

 responses, and have been studied especially in fishes. Early experiments by 

 Cunningham (10) showed that young flat fishes (Platichthys flesus), when 

 kept in tanks illuminated from below, slowly became darkly pigmented 

 on their undersurface. Similarly, the American summer flounder Para- 

 lichthys dentatus produces melanophores on its undersurface when it is 

 illuminated from below in black surroundings, but this does not occur in 

 white surroundings. But blinded fish produce melanin on the lower sur- 

 face as the result of inferior illumination irrespective of whether the sur- 

 roundings are black or white, a result due to direct action of light on the 

 skin. On the other hand, the American Gulf fluke Paralichthys albiguttus, 

 when kept on a white background, showed a decrease in melanophores 

 amounting to 30 % in 11 days, through the ejection of pigment cells through 

 the skin. Other fish kept for several weeks on a black background showed 

 an increase in epidermal melanophores (Fig. 11.6(#)). Killifish Fundulus 

 heteroclitus similarly reveal a loss in melanin when kept for some time on 

 white backgrounds, and an increase when a black background is substi- 

 tuted (66, 68, 75). 



Morphological colour changes involve carotenoids as well as melanins. 

 Thus, specimens of Fundulus majalis increased their yellow chromato- 

 phores after several weeks on yellow and black backgrounds, but showed 

 a decrease on blue and white backgrounds. The green fish Girella nigricans 

 showed an accelerated loss of xanthophylls when held for long periods in 

 white containers. Some other fishes, Fundulus parvipinnis and Gillichthys 

 mirabilis, however, maintained their level of xanthophylls on backgrounds 

 of different colours. Among invertebrates the shrimp Palaemonetes vulgaris 

 decreased its carotenoid content (astaxanthin and carotene) when main- 

 tained upon a white background, while other specimens, in dark surround- 

 ings, increased their pigment-content (Fig. 11.6(6)) (16, 17, 75). 



Morphological as well as physiological colour changes depend upon the 

 albedo — that is, the ratio of reflected to incident light — and are relatively 

 independent of total illumination over a wide range (see p. 508). In experi- 

 ments carried out with the long-jawed goby Gillichthys mirabilis on white, 

 grey and black backgrounds, it was found that the amount of melanin in 

 the skin was greatest when the albedo was low. Fish maintained on a 

 white background for 87 days had relatively little integumentary melanin; 

 those on grey backgrounds had more ; and those on black backgrounds had 

 the greatest quantity. Comparable experiments concerned with quanine- 

 deposition in Girella nigricans have shown that the amount of guanine in 

 the skin varies with the albedo, being greatest in fish kept on light back- 

 grounds (76, 77). 



There is, consequently, a close relationship between physiological and 

 morphological colour change. Background conditions which favour a dis- 



