[Change of the weight/ foot area coefficient in relation to aggregation in Tarebia granifera]

To analyze specimens dispersion in a population of Tarebia granifera and its relationship with the change of morphometric parameters.

Two samplings by parcels including two types of substrata in opposite aggregation conditions were carried out, and the specimen abundance was compared between both samplings and substrata. The samples were stratified into classes according to shell height and for each of these the coefficient weight/foot-area was calculated and correlated with class size. It was experimentally evaluated whether specimens with a lower foot load had a better persistence in emerged substratum as well as the interaction between two class groups of shell height.

The abundance difference between samplings was t=5.874 (p<0.001), and t=10.15 (p<0.001) between emerged rocky and sandy substrata, only under conditions of high aggregation where foot load decreases with the size for higher specimens (r=-0.98, p<0.05). Experimentally, the abundance of specimens of opposed sizes were mutually limited (r=-0.87, p<0.01), waves' height limits the abundance of smaller snails (r=-0.94, p<0.01) but it is tolerated by larger ones (r=0.72, p<0.05), and foot load diminishes with the waves' height (r=-0.93, p<0.001).

Under high aggregation conditions larger specimens emigrate toward substratum of difficult capture by smaller ones, which are limited by the environment's mechanical influence. The decrease of foot load facilitates specimens' dispersion.