GROWTH AND REPRODUCTION OF AN ESTUARINE POPULATION OF THE COLONIAL HYDROID CORDYLOPHORA-CASPIA (PALLAS) IN THE NORTHERN BALTIC SEA

Growth and reproduction of the colonial hydroid Cordylophora caspia we re monitored during the breeding season in natural conditions. In 1987 , a life history study was carried out on the upright stems of the mai n stolen. Mean size of uprights varied cyclically. The first peak coin cided with the peak number of sexual hydranths, after which the mean u pright length decreased, possibly indicating somatic costs of sexual r eproduction. Extrinsic factors like flooding may also have contributed to cyclical changes in upright size. In 1988 and 1989, colonies were reared on experimental plates in the estuary. In 1988, colonies grew u ntil mid July, after which they regressed to a dormant condition and t hen started growing again in mid August. Predation and space competiti on are discussed as possible causes for this dormancy in the middle of the growing season. In 1989, colonies grew continually, with the exce ption of a decline in colony biomass and number of feeding hydranths a t tile end of July, just following the peak of sexual reproduction. Se xual reproduction started in the early stages of colonial development for all years. During early summer, C. caspia allocated resources simu ltaneously in colonial growth and sexual reproduction. However, sexual reproduction had a clear peak in mid summer, and thereafter sexual re production ceased while colonial growth continued.