Original paper

Resource use and partitioning by two co-occurring freshwater gastropod species

Lombardo, Paola; Cooke, G. Dennis


Many trophic generalist gastropods exert predation-based control on their trophic resources. However, most grazing experiments have been carried out with single snail species, and little is known about snail effects on trophic resources by coexisting species. Substrate colonization and feeding behavior of Physa spp. (Physidae) and Helisoma trivolvis (Planorbidae) were observed in single- and two-species laboratory trials. Spatial overlap between Physa and H. trivolvis was derived from distributions of snail individuals on trophic (living macrophytes, periphyton, and leaf litter) and spatial resources (aquarium space divided into quadrats). Food consumption was determined as biomass changes with respect to snail-devoid controls. Snails reduced the degree of spatial overlap when coexisting, though Physa appeared to affect H. trivolvis distribution more than vice-versa regardless of snail density. Both species preferred leaf litter as a substrate, and both increased nutrient (phosphorus) concentration of water. Periphyton and leaf litter biomass loss was similar in all snail-present aquaria, with only inedible leaf veins left intact at the end of the 10-day experiment. Biomass of living macrophytes increased in all Physa-only aquaria, and in two-species, high density aquaria, suggesting that macrophyte enhancement may be possible in high-density, Physa-dominated snail assemblages.


gastropodsresource utilizationherbivoryoverlapbehaviorphysa spphelisoma trivolvis