CORRESPONDENCE OF THE SEASONAL PATTERNS OF THE BRINE SHRIMP, ARTEMIA SALINA (LEACH, 1819) (ANOSTRACA) WITH SEVERAL ENVIRONMENTAL FACTORS IN AN ARID SOLAR SALTERN (SFAX, SOUTHERN TUNISIA)
Résumé
We investigated the correspondence of the seasonal patterns of Artemia salina with several environmental factors along a natural salinity gradient in four ponds (M1, M2, M3, and B1) located in the arid solar saltern of Sfax (southern Tunisia). The shape of the frontal knobs of males showed the dominance of the autochthonous species, A. salina. Artemia abundance and fecundity were assessed. Biometrics of A. salina stages were studied by comparing cyst size and body length. A. salina abundance displayed a clear seasonal and spatial variation. Abundance of live Artemia ranged between 0.25 × 103 and 1414 × 103 ind m−3. The highest numbers were recorded in pond M3, as nauplii (771.8×103 ind m−3 corresponding to 31.8 g m−3). This resulted from low male and female numbers (34 and 17×103 ind m−3, respectively). Artemia cysts were predominant (92.96% of total Artemia), and grouped in small (216 μm) and large (243 μm) sized cysts, which correlated negatively with temperature (r = −0.4, N = 294, p < 0.01) and salinity (r = −0.39, N = 294, p < 0.01), and positively with female fecundity (r = 0.6, p < 0.05). The large (270 μm) and small (216 μm) cysts suggested the presence of also the species A. parthenogenetica and A. franciscana in the saltern, respectively. Fecundity showed a summer-autumn distribution that correlated significantly (−0.6 and −0.9, N = 35, p < 0.01) with both temperature and salinity. A. salina performed best in M2 (more than 80 cysts per brood), at about 200 g l−1 and 26.8◦C, with gravid females representing 20%. A. salina from the Sfax solar saltern was unable to withstand extreme temperatures (35◦C), but tolerated extreme salinity (330 g l−1). Abundance, biometrics, and reproductive descriptors of A. salina appeared to be governed chiefly by temperature and salinity, the physical structure of the saltern, and food availability, such as the unicellular green alga, Dunaliella salina. Nauplii (mean length: 401 μm) from small-sized cysts encountered in the saltern might be a valuable food source for hatcheries of some small-mouthed marine fish.