Reproduction of the colonial hydroidObelia geniculata(L., 1758) (Cnidaria, Hydrozoa) in the White Sea.

Populations of the colonial hydroidObelia geniculatain the White Sea reproduce asexually by frustule formation. Young medusae appear in the plankton during July and August. The number of medusae rarely exceeds 36 per m3, and the average number varies every year from 0.4 to 10 per m3. The size of medusae is smaller than reported from other regions. The umbrella of the largest recorded medusa was only 0.57?mm in diameter and the specimen had just 35 tentacles. Only a few mature medusae were found during the study. The colonies in the White Sea are epiphytic and grow only on laminarian thalli. At the beginning of July there are no colonies on thalli from the upper subtidal zone. By the end of August, colonies ofO.?geniculatahad increased in density to 30 per m2. Hydroid recruitment was attributed to active frustule production by colonies living below that zone. The frustules detach from the stems of the hydroids and are found in plankton. Production of frustules on branches occurs continuously during colony growth until water temperatures climb above 0?°C. We found that water temperature in this Arctic environment is generally too low for medusa maturation and planula development in the species. Propagation by frustule formation is the principal means of reproduction inObelia geniculatawithin the White Sea, and this phenomenon accounts for the species being a dominant epiphyte on laminarian thalli there. [ABSTRACT FROM AUTHOR]