PIGMENTS AND COLOURS 493 



colours are produced by blue, red and yellow substances, together with 

 minute granules of a white material, all of which are present in each animal 

 variety although in different proportions. Some breeding experiments have 

 indicated that blue and brown are due to simple dominant-recessive 

 allelomorphic factors. Crosses between blue and brown types give worms 

 with blue and brown crowns in the ratio 3:1. Blue is thus dominant, brown 

 recessive ; the inheritance of orange colour has not been determined (26, 27). 

 The situation in the littoral isopod Sphaeroma serratum is more complex, 

 for there are five principal types of colour patterns, determined by the 

 interaction of five pairs of multiple alleles and, in addition, an independent 

 gene pair, rubrum. Summarized results for the complex colour phases 

 are — 



albicans : quadruple recessive, ddllooss 

 discretum: triple recessive, Dd or DD, /boss 

 lunulatum: double recessive, LI or LL, ooss 

 ornatum: single recessive, Oo or 00, ss 

 rubrum: dominant, R 

 normal colour: homozygote, rr. 



It was determined that the several colour phases remain stable from one 

 year to another at any one station, but that there is considerable variation 

 between stations. Discretum and albicans were in the majority at all 

 stations on the French coast, while the other mutants were rarer or some- 

 times lacking. Polychromatism in the harpaticid copepod Tisbe reticulata 

 has also been analysed genetically. Five major phenotypic colour phases 

 occur among females of this species, and are due to variations in caroti- 

 proteins. The different phases are determined by several series of allelo- 

 morphic factors (4, 5). 



The polymorphic phases of these various animals, Metridium, Pomato- 

 ceros, Botryllus, Sphaeroma, etc., appear to be non-adaptive and are not 

 themselves subject to environmental selection. By analogy with other 

 animals studied in much greater detail, it is unlikely that random drifting 

 of mutations through such large and widespread populations would suffice 

 to maintain such colour phases. Rather the factors responsible for the 

 production and organization of pigments are linked with other genes 

 having greater or less survival value, and it is selection of the latter that 

 causes the apparent balance of colour phases in different areas (22). 



Sexual Coloration 



Sexual differences in coloration appear to be adventitious in most 

 marine invertebrates in which they occur, and result frequently from differ- 

 ences in the reproductive elements tinting the body, or from structural 

 alterations associated with spawning. A striking difference in coloration 

 between the sexes occurs in certain species of sapphirine copepods 

 (Sapphirina maculosa, S. nigromaculata) in which the males are beautifully 

 iridescent. This is attributed to a deposition of guanine crystals in special 



