492 



THE BIOLOGY OF MARINE ANIMALS 



TABLE 11.3 



Numbers of Chromatophores per Square Millimetre 

 in Stripes of Two Species of Labrids 



Melanophores 



Erythrophores and xanthophores 



Iridocytes 



Melanophores 

 Xanthophores 

 Iridocytes 



Stripes 



Black 



Blue 



Green 



Thalassoma bifasciatum (from Tortugas) 



718 

 0-550 

 3,000 



346 



566 



3,000 



Yellow 



462 

 0-190 

 3,000 



Green 



Thalassoma duperrey (from Hawaii) 



271 I 596 



1,057 527 



3,000 3,300 



(From Goodrich (28).) 



ing interspecific cross between the mackerel Scomber scombrus and the 

 killifish Fundulus heteroclitus, some embryos showed, unchanged, the 

 distinctive green chromatophores of the mackerel; others, the red chroma- 

 tophores of the killifish; and still others bore chromatophore types of 

 both parents. Here it appears that the chromatophores are determined by 

 the variable genetic constitution inherited from the two specifically 

 different parents. Marine species are also rich in colour varieties, and by 

 the study of such cross-fertile animals information about the inheritance 

 of colour patterns can be obtained. 



Colour varieties are common in coelenterates, occurring in the Scypho- 

 medusae (Cyanea capillata), Alcyonaria {Eunicella verrucosa), Actiniaria 

 (Epiactis prolifera) and Madreporaria {Madrepora prostata). As previously 

 indicated, some colour variation is of environmental origin, depending on 

 food and light. In other species the colour varieties are remarkably stable 

 and are genetically determined, as in Metridium senile in which four 

 principal phases are found in nature living side by side under identical 

 conditions (Fig. 11.1). Some species showing pronounced colour phases in 

 other phyla are the bread-crumb sponge Halichondria panicea, the nemer- 

 tine Lineus ruber, many molluscs such as the flat periwinkle Littorina 

 littoralis, and the colonial ascidian Botryllus schlosseri. In Metridium, as 

 in so many other coelenterates, asexual buds are produced and a given 

 variety is propagated asexually for many generations, thus forming an 

 asexually produced clone. 



Genetic studies of the polychaete Pomatoceros triqueter and the isopod 

 Sphaeroma serratum reveal something about the inheritance and signifi- 

 cance of polymorphism in marine invertebrates. The first species occurs in 

 nature in three distinct forms having blue, brown and orange tentacles. In 

 Oslo Fjord, where the animals were collected, the brown varieties were 

 found most abundantly, blue next and orange rather infrequently. The 



