Assessing the spatial distribution of the gastropod Olivella semistriata by using whole-sediment avoidance and recolonization assays

Abstract

In situ observations seem to indicate that the spatial distribution of the snail Olivella semistriata on the Ecuadorian coast (city of Manta) is influenced by urban discharges. Therefore, contamination could determine habitable areas for snails. Firstly, the present study evaluated the ability of O. semistriata individuals to detect local contamination and avoid inhabiting contaminated sediments. Secondly, the ability of snails to recolonize contaminated sediments under recovery was studied. Sediment samples were taken in five points (El Murciélago beach - reference point, El Puerto, La Poza, Río Burro, Los Esteros, and Río Muerto) and evaluated for avoidance and recolonization trials. The tests were performed in a non-forced exposure system in which a contamination gradient was formed by mixing the test sample and the reference sediment. Higher avoidance percentage was observed in the samples of Rio Burro and Rio Muerto. Regarding recolonization tests, the reference sample was always preferred over all test samples. As there is no physical barrier to avoid the displacement of organisms between the studied areas, it is suggested that the absence (visual field observation) of snails in the Rio Burro and Rio Muerto sediments is due to the organisms' ability to avoid those areas. We can conclude that the spatial distribution of the snail may be directly affected by the presence of contaminants in the sediment, by triggering avoidance or even prevent colonization of contaminated areas. Contamination may, therefore, act as a chemical barrier that could cause habitat disruption and isolate populations.