Abstract
The variability, taxonomy, and euryhalinity of C. edule and C. glaucum is discussed. The name, C. lamarcki, commonly used for C. glaucum during the last decade has to be abandoned. Descriptions of the spermatozoa are given; their difference strongly support the view that C. edule and C. glaucum are two distinct species. On the basis of occurrence and salinity preference the author suggests that C. glaucum is a brackish-water species, which may have evolved from a population of C. edule in the Mediterranean during its isolation from the Atlantic in the late Miocene and early Pliocene.
Investigations on a sand flat in Trondheimsfjorden indicated that in 1967 and 1968 the main spawning of C. glaucum took place earlier than that of C. edule. Laboratory experiments on the reactions of the two species to extreme salinities showed C. glaucum to be the more euryhaline, which is in agreement with the recorded occurrences in nature.
