Individual Sampling date pH O2 mg/l T° °C Cond µs/cm2 Age (week) Size (mm) Gonad development OP OM OG EM P1 19/03/2008 7.97 8.8 16.2 540 6 0.7 - -   - P1 06/05/2008 7.12 8.3 16.4 615 13 2 - -   -  P1  14/05/2008 7.69 7.86 16 620 14 2.4 +  +     - P1 28/05/2008 7.9 7.74 16.2 632 16 3.5 +++ ++   - P1 12/06/2008 8.04 8.12 16.2 608 18 4.3 +++ +++   +++ P5 22/05/2008 8.13 7.92 16.2 613 15 1.1 - -   - P5 19/06/2008 7.92 7.44 16 620 19 2 ++  -   - P5 26/06/2008 7.94 7.97 16.3 620 20 2.5 ++ ++   - V3 19/03/2008 7.98 8.6 16.4 538 6 1.3 - -   - V3 27/03/2008 8.05 8.4 15.9 593 7 1.9 - - - - V3 24/04/2008 8.1 7.64 15.7 663 11 2.6 - -   - V3 29/04/2008 8.03 8.33 15.7 623 14 3.4 +++ - +++ - V3 15/05/2008 8.13 7.92 15.7 617 15 3.7 +++ + +++ - V3 22/05/2008 8.02 7.82 15.9 624 20 4.5 +++ - +++ - V5 19/03/2008 7.98 8.6 16.4 538 6 1.5 - - + - V5 03/04/2008 8.05 8.4 15.9 593 7 1.9 - - + - V5 06/05/2008 7.97 8.4 15.8 597 8 1.3 - - + - V5 22/05/2008 7.86 7.95 15.8 616 13 3.8 + - +++ - V5 28/05/2008 8.13 7.92 15.7 617 15 3.5 +++   + +++    V5 06/06/2008 7.78 8.39 15.8 630 16 4.4 +++ +++ +++ - V5 12/06/2008 7.86 8.3 16.1 612 17 2.9 ++ - +++ - V5 19/06/2008 7.92 8.21 16 605 18 4.4 +++ +++ +++ - V5 12/06/2008 7.81 8.21 15.8 616 19 4 +++ ++ +++ - P: Potamopyrgus antipodarum, V: Valvata piscinalis, Cond: conductivity, OG: Oogonium, OP: primary oocyte, MO: secondary mature oocyte, EM: Embryon

Table 1: The data presented consist of physicochemical results, size and age of organisms sampled for histological studies and for observations of the development of gonad maturity. The length of the shell (from the spire to the operculum) of each specimen was measured using a binocular magnifier with an integrated micrometer ruler.