Field and culture observations on Coelocladia arctica (Fucophyceae): growth and reproduction in relation to temperature

Coelocladia arctica (Fucophyceae) is most often reported from arctic and su barctic environments with few records from temperate areas. Young vegetativ e macrothalli of C. arctica are indistinguishable from similar stages in th e type species of Stictyosiphon. S. adriaticus. Later stages of C. arctica. with divisions in the cortical cells formed prior to formation of sporangi al filaments, are indistinguishable from initial plurilocular sporangia dev elopment in Stictyosiphon, for example in S. soriferus. These morphological similarities may explain the very rare reports of this species outside The arctic and subarctic environments. Temperature gradient experiments with o ne strain from Baffin Island (arctic) and one from Greenland (subarctic) sh ow that their upper survival temperature. their ability to form macrothalli , and the characteristic sporangial filaments are almost identical. The Baf fin Island strain, however, seems to adjust its reproduction to the hostile arctic environment in being able to liberate and germinate swarmers only a t temperatures from c. 4 to 13 degreesC. In both cases the optima for growt h and reproduction, as determined by laboratory experiments. lie far above the temperature conditions experienced by this: species in nature.