Adaptations of the intertidal midge Clunio to arctic conditions.

1. The arctic population of Clunio marinus in Tromsö (Norway) (69°39'N) shows a tidal periodicity of emergence (period: 12.4 hours). Emergence occurs between the times of high and low tide. 2. A semilunar periodicity of pupation could not be detected. 3. In accordance with the tidal periodicity of emergence and reproduction, the habitat is situated in the midlittoral zone (area between the high and low water levels of neap tides). High densities of pupation are found in a sandy mud flat. 4. The Tromsö population is compared with European populations in temperate latitudes, which are characterized by a semilunar periodicity of emergence, and a habitat around the infralittoral fringe of the midlittoral zone. The different adaptations of the populations in biological rhythms and preferred habitat are discussed with regard to the geographic differences in photoperiodic conditions.