Morphological and biochemical criteria for the separation ofCardium glaucum(Bruguière) fromCardium edule(L.)

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Abstract
The cockles Cardium edule (L.) and C. glaucum (Bruguiere) often form sympatric populations in inner Danish waters. In these populations morphological characters hitherto used as diagnostic criteria overlap. Two biochemical and two morphological methods were applied on pure C. edule and C. glaucum populations, namely 1. Electrophoresis of malate dehydrogenases in horizontal starch gels. 2. Disc electrophoresis of tetrazolium oxidases in polyacrylamid systems. 3. Identification of an edge between the lateral and posterial sides of the cockle, and 4. Measurements of the ligament length to shell width ratio. All four methods correlated when applied on allopatric populations, dividing these into two well defined groups. Methods 1, 3 and 4 were applied to each of 735 individuals from six Limfjord populations with mixed characters. These populations were clearly separated into two groups with the use of methods 1 and 3, but not with method 4. The results show that the two forms have habitat-independent differences and that they constitute two genetically isolated species without hybridization.