Sympatric species, Clunio marinus Hal. and Cl. balticus n. sp. (Dipt., Chironomidae), isolated by differences in diel emergence time.

Two populations of Clunio, an intertidal one and a sublittoral one, were found sympatric in the Kviturdvikpollen near Bergen, western Norway. The times of eclosion and the immediately following reproduction were strictly separated in each species by only a few hours' difference. The emergence time of the intertidal population was strictly correlated with the afternoon low water of spring tides; the population resembles the Helgoland (North Sea) population in nearly all its morphological and physiological characteristics. The sublittoral population emerged nearly every day (during the summer) just after sunset, independent of the tides, with only a slight increase in number during the time of the first and last quarters of the moon. The sublittoral population is similar to the populations from the Baltic Sea, especially in its characteristics of emergence control and sinking of the suffused egg masses. Cross-breeding experiments with stocks from both populations resulted in an intermediary emergence time of the first filial generation which was never observed in the field. Therefore, one can conclude that the sympatric populations are perfectly isolated in nature and that they have the well-defined rank of two species. The intertidal population belongs to the well-known Clunio marinus; the sublittoral population together with well-known open-sea populations of the Baltic Sea are classified as a new species, Clunio balticus n. sp.