Hydrobia ulvae and H.ventrosa growth

Growth rate () of H.ulvae and H.ventrosa as function of shell diameter (d0) on the moment of winter line formation; A - H.ulvae (1 - sexually unmatured, 2 - matured individuals); B - H.ventrosa

The growth patterns of H.ulvae and H.ventrosa from the two populations are shown in Figure 1. The snails' growth rate correlates negatively with size at the start of the growth season. The smaller the shell diameter at the moment of winter line formation, the higher angle gain at the date of sampling. The growth rate response of infected snails depends on the trematode species infecting them. Notocotylus sp. does not increase the growth rate of infected individuals (Table 1). Likewise, there are no significant differences in the growth rates between snails infected with Bunocotyle progenetica and uninfected H.ulvae and H.ventrosa. In contrast, infestation by Microphallid trematode species (Maritrema subdollum, Microphallus claviformis and M.pirum) caused a significant increase in the growth rate of H.ulvae . Snails infected by M.pirum had the highest growth rate. It was possible to identify individuals infected by this species before dissection because probably due to enhanced growth the coloration of the periostracum on the last whorl was lighter than usual. The growth rate of H.ulvae individuals infected with Himasthla sp. differs significantly from the growth rate of non-infected ones. A similar effect on growth rates was produced by parthenites of Cryptocotyle sp. in both Hydrobia species.

The high rate of infestation by Cryptocotyle sp. in H.ulvae from the Suhaja bay in 1989 allowed to compare the growth rate of infected and healthy individuals separately in the group of sexually immature (Figure 5, A(1)) and matured (Figure 5, A(2)) H.ulvae. As opposed to matured snails, the immature ones did not increase growth rate when infected with this species.