ИСТИНА

In Arctic latitudes life cycles of marine organisms are adapted to annual changes of temperature. Sexual reproduction is usually a seasonal event. Each species builds gonads and produces gametes in a definite season. The time of sexual reproduction of invertebrates usually depends on environment. But medusozoan cnidarians prepare themselves for reproduction in advance. Scyphozoan polyps and hydroids living on the bottom substrata produce planktonic medusae which swim and feed to growth and sexual reproduction. In my study I investigated life cycles of hydrozoans and scyphozoans from the White Sea. Medusae and hydroids with medusae buds were collected from the sea in different seasons of the year. The annual experiments allowed to study life cycles of several species in laboratory. As a result I determined the temperature which is needed for medusae production in several species inhabiting the White Sea. Four reproduction strategies were found in the studied species: 1) Species producing medusae as a result of lowering the temperature (Catablema vesicarium, Halitholus cirratus, Rathkea octopunctata, Bougainvillia superciliaris). 2) Species which need a long period of minimal temperature to produce medusae (Cyanea sp., Aurelia aurita, Sarsia lovenii). 3) Species producing medusae after a small increase of temperature in spring (Obelia longissima). 4) Species producing medusae after increase the temperature to maximum in summer (Stauridiosarsia producta). The results are useful to determine how the climate changes affect marine populations.