Distribution of four isopods (Peracarida: Crustacea) in shallow waters of the Bizerte Lagoon (Tunisia, SW Mediterranean Sea)

The distribution of four isopods - Idotea balthica basteri (Pallas, 1772), Idotea chelipes mediterranea Charfi-Cheikhrouha, 1996, Sphaeroma serratum (Fabricius, 1787), and Paracerceis sculpta (Holmes, 1904) - at Menzel Jemil (Bizerte Lagoon, North Tunisia) was analysed and then related to the variation of temperature, depth and plant biomass. The abundance of isopods showed monthly fluctuations with maxima in August and minima in December 2010. The most dominant species was I. b. basteri followed by S. serratum, with a spatial gradient in the distribution pattern of isopod species in the study area. The high density of juveniles in algae and seagrass highlights the important role played by this habitat as a nursery for species. Both dominant species were tolerant to temperature variations, with some preference values (19.73-27.73°C in I. b. basteri; up to 28°C in S. serratum) and salinity (36.7-38.6 psu in I. b. basteri; 39-39.2 psu in S. serratum). Multiple regression analysis between environmental variables and species abundance showed a positive relationship with biomass of Gracilaria bursa-pastoris and Ulva lactuca for the two species of Idotea and Sphaeroma, respectively, while abundance of P. sculpta was positively related to temperature. Biotic and environmental linking (BIOENV) analysis confirmed the greatest influence of temperature and vegetation biomass on the temporal distribution of these isopod species. A spatial distribution pattern according to successive depths was revealed.