Original paper

Abundance, Population structure and Production of Hydrobia ventrosa (Gastropoda: Prosobranchia) in a Mediterranean brackish lagoon, Lake Ichkeul, Tunisia

Casagranda, Caterina; Boudouresque, Charles; François Francour, Patrice

Abstract

Abundance, growth and production of the mud snail Hydrobia ventrosa (Montagu) were studied in the Ichkeul wetland, northern Tunisia. H. ventrosa occurred at annual mean densities (biomass) from 370 ± 78 individuals/m2 (0.17 ± 0.03 g ash-free dry mass/m2) in the macrophyte-free area to 23 475 ± 9 876 individuals/m2 (9.86 ± 3.48 gAFDM/m2) in the macrophyte covered areas. Mean aboveground macrophyte biomass was maximum in September followed by a complete breakdown of the Potamogeton pectinatus L. meadow from October onward due to high salinity. Only the meadow of Ruppia cirrhosa (Petagna) Grande at Tinja remained in place. Abundance and biomass of H. ventrosa were positively related to the macrophyte biomass (p <0.001). age class i was recruited twice: in may-june and in september. the first recruited cohort reached a mean length of 2.03 ± 0.04 mm in november. in the second recruited cohort, no growth was found after november, and their mean length remained around 1.29 ± 0.02 mm. maximum length of age class ii rarely exceeded 2.85 mm (mean = 2.78 ± 0.07 mm) and the average life span is approximately 13–15 months. the mean annual production of Hydrobia ventrosa for the whole lagoon (90 km2) was 11.86 g AFDM/m2 (3.51 g C/m2, 0.34 g N/m2) i.e. 85% at Sejnene area (30.5 km2), 1% in the Centre (49.6 km2), 4% at Tinja area (3.0 km2) and 11% at Joumine area (6.9 km2). The annual P/B ratio was about 3. On the basis of this value, H. ventrosa consumed 23% of the energy income from the macrophytes during the study period and a third (36%) of the secondary production was available to the next trophic level, which is suggestive of the importance of Hydrobiids in lagoonal ecosystems.

Keywords

mud snailhydrobiidsdeposit-feeding invertebratessalinity preferencelagoonal ecosystems