A sieving method for rapid determination of size-frequency distribution of small gastropods. Example of the mud snail Hydrobia ventrosa (Gastropoda: Prosobranchia)

摘要

A sieving system using sieves of predefined mesh size has been developed for rapid size–frequency distribution analysis as a basis for the assessment of the secondary production of the hydrobiid species Hydrobia ventrosa Montagu of Lake Ichkeul in northern Tunisia (Mediterranean). The relationship between shell length (SL) and shell width (SW) SL = e0.5005 ċ SW1.4306 calculated by a geometric mean regression is very highly significant allowing the application of sieves in mesh intervals of 0.1 mm for the determination of size–frequency distribution. The mean difference between sieved and measured size distributions, for different distribution patterns, only slightly exceeds 1%. The regression between shell length and ash-free dry mass (AFDM) fits an exponential model AFDM = e(-2.6591 + 0.9194 SL). Mean difference between estimated and observed biomass is in the 10% range. The elementary composition (C, H and N) of the ash-free dry mass is size-dependent. As a result, the proposed sieving method reduces by at least 20 times the data acquisition time (extraction, sorting and size-measuring), avoiding subsampling without significant loss of accuracy for secondary production studies of small gastropods.