470 THE BIOLOGY OF MARINE ANIMALS 



the external skeleton (shell, chitin, etc.). The blue coloration of many 

 decapod Crustacea, such as the adult lobster Homarus vulgaris, is due to 

 carotiproteins deposited in the exoskeleton, whereas the epidermis is 

 completely concealed beneath the thick calcareous cuticle and is without 

 importance in coloration. In smaller species of Crustacea — for example, 

 the shrimps and prawns, Crangon, Hippolyte and Processa — the cuticle 

 is thin and translucent, and pigments in the epidermis are responsible 

 for surface coloration. The dark colours of some bivalve shells result from 

 a deposition of pigment in the external cuticular layer. The colours of 

 fishes are due particularly to dermal chromatophores, although epidermal 

 chromatophores also have a minor effect. The agencies responsible for 

 structural colours are usually located in the integument ; for example, the 

 layers of guanine crystals producing the metallic sheen of many bony 

 fish. Again, pigments may lie diffusely in tissue spaces, or be localized in 

 cells of the skin. Sometimes the colours from deep tissues, such as muscle, 

 blood, gonads and gut, show through the skin and colour the animal. 

 This is particularly so in certain polychaetes, for example, Pomatoceros 

 triqueter, in which the sexes are distinguishable at maturity by differences 

 in the colour of the abdomen due to the genital products within, the male 

 being cream in colour and the female bright pink or orange. In the nemer- 

 tine Amphiporus pulcher, the eggs are bright red and are visible to the 

 exterior. The polyclad Cycloporus papillosus varies in tint according to 

 the colour variety of Botryllus upon which it is feeding, and thus changes 

 in colour after it has passed to a new and differently coloured host. In 

 this flatworm the colour of the gut contents is visible through the body 

 wall. In Arenicola marina and Capitella capitata, to cite two more examples, 

 the rich red colour of haemoglobin shows through the skin of the animals, 

 and gives them a reddish hue (63, 80). 



Extensive research is only now beginning to unravel many of the in- 

 tricacies concerning the pigments of lower animals, but the functions, and 

 indeed the nature, of many invertebrate pigments are still unknown. In 

 marine animals these substances include carotenoids, porphyrins and other 

 pyrrolic compounds, melanins and indole pigments, purines, naphtho- 

 quinones, various protein substances, and still other materials whose iden- 

 tities are yet to be established. Some of these substances are endogenous in 

 origin, being manufactured by the animal; others are exogenous, being 

 derived from its food, although sometimes chemically altered by the animal 

 assimilating them. 



Chromoproteins are important substances in coloration and in animal 

 physiology. They include the haemoglobins and related compounds, with 

 a prosthetic haem group conjoined to a protein molecule; carotiproteins 

 and melanoproteins, in which the chromatic substance is conjugated with 

 a protein molecule; and the complex respiratory pigments haemocyanin 

 and haemerythrin, which function in oxygen transport. Of the pigments 

 now to be considered, some like the carotenoids, are found in many groups 

 of animals, and are responsible for the colours of many marine forms (17, 80). 



