Detachment velocity, dispersion rates and microhabitat preferences of Pomacea canaliculata (Caenogastropoda: Ampullariidae) from Southern Pampas

SEUFFERT, MARÍA EMILIA; MARTÍN, PABLO RAFAEL

Tucumán

Workshop; 4th International Workshop on the Biology of Ampulariidae; 2010

CONICET-AGENCIA-MINCyT

Pomacea canaliculata inhabits lentic and lotic water bodies but in the latter is usually found in places with little current. Our objectives were to determine the detachment velocity of this species, to investigate the effect of water current on dispersal and the rate of net displacement and to characterize its hydrodynamic microhabitat in streams located in southwestern Buenos Aires Province (Encadenadas del Oeste Basin).Trials were conducted in a laboratory flume in which one snail was placed at a time; after the snail got hold on the bottom, current velocity was gradually increased until the snail´s detachment. The time spent in various activities, the orientation of the snails relative to the current and the detachment velocity were recorded; as control, the same experiment was conducted without water current. In the same channel, snails´ net displacement was obtained at three fixed current velocities: 0, 0.15 and 0.30 m.s 1.In February 2010, 100 marked snails were released in a 50 m-long reach of the Guaminí stream and were sought 24 h later. For each recovered snail the net displacement vector was calculated and then it was correlated with the current velocity encountered along this vector. In February 2008 we sampled at two sites of the Guaminí and Pigüé streams selected for their diversity of microhabitats. The mean current velocity was recorded in each place a snail was found and also in a set of systematic points located in a regular grid.The mean detachment velocity was 0.98 m.s-1 and ranged from 0.07 to 1.64 m.s-1, indicating that the snails can withstand relative strong currents during short periods of time. The frequency distribution of detachment velocities was bimodal, with a group of snails below and one above the mean velocity. Sexually undifferentiated snails showed a clear association with the group of high detachment velocities, suggesting that small snails would be more resistant to current. Most of the snails that resisted high current velocities were positioned with the cephalopodium facing upstream. Individuals in running water spent more time attached to the substrate and less time crawling and ventilating the lung relative to snails without current. Water current interfered with the normal extension of the siphon and forced the snails to crawl to the surface in order to renew the air of the lung. The different current velocities did not influence the mean net displacement, since in the three treatments it was not different from zero.In Guaminí stream, most individuals dispersed a short distance downstream from the release point and crawled through sites with very low current velocities. In both the canal and the stream, a small proportion of snails detached and drifted downstream, suggesting the existence of different dispersal mechanisms. Considering the two sites sampled, we found significant differences between the places with snails and the systematic points; all snails were located at current velocities below 0.3 m.s-1 (range: 0 - 1.61 m.s-1) and 75% of them were found at places with no current al all. These places were mostly located close to the shore and presented abundant macrophytes and fine substrates rich in organic matter (sapropel). The association of P. canaliculata to sites with little current could be explained by the decrease in the energy costs resulting from keeping attached to the substrate and to the interference caused by the current in the performance of routine activities like feeding, lung ventilation and copula.