FORMATION OF A FERRUGINOUS ENCRUSTATION A ND PARTIAL DISSOLUTION IN-VIVO OF THE CARAPACE OF CYPRIDEIS-MANDVIENSIS (OSTRACODA) IN THE HYPERHALINE ESTUARY OF THE CASAMANCE RIVER (SENEGAL)

Cyprideis mandviensis can be very abundant in the Casamance estuary, w ith populations up to 500,000 individuals per m(2). These ostracods li ve at the boundary between a muddy organic-rich sediment and an hyperh aline, well oxygenated water. Their environment can become alternative ly more reducing or more oxidizing. Some individuals experience a part ial dissolution of their calcitic test. The remaining organic layer ca n then undergo spectacular deformations. In the same time, a silico-fe rrugineous encrustation progressively covers the test. The protection provided by this encrustation allows the survival of the individuals a s it was established by observations on the field with a stereomicrosc ope. The transformation of the test follows successive stages : 1) ins ide the sediments, sulphoreducing bacteria fix inside and on the test. Using Fe2+ that is abundant in the sediment, they produce iron monosu lphide and iron disulphide; 2) when the oxidizing influence dominates, the iron monosulphide partly oxidate. The resulting iron hydroxide (F eOOH) encrust the test and adsorb other elements such as SiO2, Al2O3 o r P. In the same time, the ion HS- also oxidate, producing sulfuric ac id that provokes a partial dissolution of the calcitic test; 3) moving back to reducing conditions, iron monosulphide can transform into iro n disulphide and more Fe2+ can be adsorbed on the test. These successi ve reactions end in the formation of an encrustation covering the part ially dissolved test. This mechanism allows the survival of the ostrac ods, even if they are very deformed. After their death, the remains of the calcitic test are rapidly dissolved.