Spatio-temporal distribution of Medusae (Cnidaria) in the Bay of Bizerte (South Western Mediterranean Sea)

The abundance, distribution, and species composition of jellyfish were evaluated in relation to trophic and environmental parameters using the data obtained from 24 cruises carried out from January 2004 to December 2005 in the Bay of Bizerte. Twenty four species of medusae were found. 20 Hydromedusae and 4 Scyphomedusae; 6 are new records for the Tunisian fauna: Corymorpha nutans, Paragotoea bathybia, Aequorea aequorea, Clytia mccradyi, Amphinema dinema and Eutima mira. The latter is recorded for the first time in the western Mediterranean. The Hydromedusae were represented by Leptomedusae (8 species), Anthomedusae (7 species), Trachymedusae (3 species) and Narcomedusae (1 species). Analysis of the seasonal occurrence of medusae in the Bay of Bizerte revealed that hydrological and trophic factors are the strongest forcing factors affecting species distribution. The highest average density of hydromedusae was reached in July 2005 (18 ind.m(-3)), and the dominant species during the study period was Aglaura hemistoma, which occurred in peak numbers in July 2005 at B6 (45.2 ind.m(-3)). Mesozooplankton density declined significantly in the winter of 2004 (342.5 ind.m(-3)) due to the dense aggregations of the Scyphomedusae Pelagia noctiluca.