Shell Color Polymorphism in Macoma balthica L. (Bivalvia, Tellinidae) from the White and Barents Seas

Abstract

Representatives of the family Tellinidae stand out among Bivalvia because of the brightness and variety of their shell color. M. balthica , a common representative of the shallow-water infauna of boreal and subarctic seas exhibits intraspecific variation of shell color. This variation was formally described and interpreted as genetic polymorphism by Cain [6]. Shell color was used as a genetic character in some population studies of macoma [3, 5, 10]. Until now, no hypotheses on the mechanisms of shell color inheritance have been advanced. In the present work, a hypothetical scheme of the shell color inheritance is suggested and the spatial pattern of the color morph frequencies in populations of the mollusks from the White and Barents seas is analyzed. We analyzed 12 quantitative samples of M. balthica from different parts of the White and Barents seas (Fig. 1). The sample size varied from 58 to 275 specimens, all of them older than three years. The color of the inner side of the shell was determined. Four qualitative variants of the shell color were found in macomas from the White and Barents seas: the white, red, orange and yellow morphs. We consider this variation to be combinatorial, regarding the presence or absence of a pigment in the shell (white or not white) and the color of the pigmented shells as two independent characters: “presence of a pigment” and “shell color,” respectively. It seems unnecessary to introduce additional characters to this scheme, such as the “dichromancy of the shell” (when the top of the shell is pigmented brighter) and the intensity of pigmentation. These characters varied only in some of the samples, and this variation was not qualitative. The following genetic interpretation of the polymorphism is suggested. The color of pigmented shells is determined by two pigment forms: yellow and red. Synthesis of the pigments and their deposition in the shells are determined by a unique diallelic “shell color” locus. Homozygotes for one allele have red shells, homozygotes for the other allele have yellow shells. Heterozygotes deposit both pigments in their shells, and their color is orange. The possibility of expression of the “shell color” locus is determined independently by one or several loci. Nonpigmented (i.e., white) specimens could not be genotyped with respect to the “shell color” locus.