Environmental influences on the formation and germination of hibernacula in the brackish-water bryozoan Victorella pavida Saville Kent, 1870 (Ctenostomata: Victorellidae)

Brackish-water and fresh-water bryozoans produce asexually derived dormant propagules that allow survival of unfavourable conditions and provide a potential means of dispersal. The propagules of brackish-water ctenostome bryozoans are called hibernacula. We monitored the life-cycle of the brackish-water ctenostome Victorella pavida Saville Kent, 1870 in its natural habitat and investigated, in laboratory cultures, the influence of temperature and salinity on the production and germination of hibernacula and on subsequent colony growth. V. pavida is a protected species in the UK, where its only locality is at Swanpool lagoon, Falmouth. Colonies were collected from Swanpool monthly from January 2004 to January 2005. Hibernaculum germination appeared to be triggered by increased water temperature (c. 13 °C) in the lagoon in March and April. In culture, germination was triggered by transfer from 5 °C to 19 °C in a range of salinities; subsequent colony growth was affected by salinity, with strongest growth at 13, 18 and 36 psu, and reduced growth at 5 and 9 psu. At 3.5 psu, hibernacula germinated, but there was no further development. At 36 psu there was an initial lag in growth, but after 30 d the colonies were comparable with those kept at 18 psu. Hibernaculum formation by colonies occurred from June to October, with production increasing towards October. Hibernacula appear not to have long-term viability but merely to permit survival from one year to the next. The results suggest that any changes in the hydrographic regime at Swanpool could have significant consequences for the survival of V. pavida.