Factors affecting Oithona nana and Oithona similis along a salinity gradient

Abstract

Spatial and temporal variations of reproduction variables of the two cyclopoid copepods Oithona nana Giesbrecht, 1893 andOithona similis Claus, 1866 were evaluated monthly from September 2007 to August 2008 in four ponds at different salinities inthe solar saltern of Sfax, Tunisia, under hypersaline conditions. Along the salinity gradient the abundance of O. nana and O. similisdeclined from 14,900 ± 15,200 ind. m-3(A1) to 6,800 ± 7,200 (C31) and from 13,800 ± 17,100 ind. m-3 (A1) to 3,700 ± 5,800(C31), respectively. The findings showed that the abundance of these Oithona species, particularly the ovigerous females, waspositively correlated with the abundance of Diatomeae, and negatively correlated with salinity. The egg production rate (EPR), theegg diameter, the clutch size, as well as the cephalothorax length of mature females, were all negatively correlated with salinityand positively correlated with the abundance of Diatomeae. Along the existing salinity gradient, the EPR decreased from 4.7 ±0.39 (A1) to 3.2 ± 0.13 (C31) eggs female-1 day-1and from 5.19 ± 0.46 (A1) to 3.66 ± 0.05 (C31) eggs female-1 day-1 for O. nanaand O. similis, respectively, and this could mainly be attributed to the hypersaline conditions. The positive relationship betweenthe abundance of Diatomeae with all traits shows, that these may be a vital component of the diet for the investigated species. Thereduction of body size of Oithona along the salinity gradient demonstrates that such a character could mainly be due to a networkof causes, related with the confinement and not only to salinity