Freshwater Snail Physella Research Articles

Predator avoidance by the freshwater snailPhysella virgata in response to the crayfishProcambarus simulans.

We examined predator avoidance behavior of the freshwater snailPhysella (=Physa) virgata in response to the crayfishProcambarus simulans. In both laboratory and field enclosure experiments snails crawled above the waterline for 2 h or longer, then returned to the water.Physella virgata react to chemical signals given off by crayfish actively foraging on conspecific snails; they do not react to inactive crayfish. Low dissolved oxygen, crowding, and disturbance of the sediments did not elicit "crawlout" behavior. Crawling above the waterline, by reducing the probability of encounter between vulnerable, thin-shelled snails and crayfish, is an adaptive response to predation.

Factors Determining Population Density and Size Distribution of a Freshwater Snail in Streams: Effects of Spatial Scale

Eleven spring-fed streams located in central Oklahoma were sampled over a two-year period to evaluate which abiotic and biotic variables were most important in determining the abundance and size distributions of the freshwater snail Physella virgata virgata. To determine if spatial scale and sampling resolution affected the conclusions derived from stepwise multiple regression and principal components analyses, data were analyzed in three ways. An among-stream assessment included data collapsed over time and space for each stream. Within-stream analyses were also conducted on each stream separately, and a between-habitat (pools versus riffles) analysis was performed for each stream separately. At the largest scale, crayfish (Orconectes virilis) abundance and algal biomass were most strongly associated with both the size and abundance of snails among streams. Within-stream analyses suggested that flow rate and substrate particle size were most important in predicting within-stream abundance patterns of snails. Significant differences in both snail size and abundance were found between pools and riffles, with higher abundances and larger snails being found in pools. Variables associated with microhabitat selection of snails also differed between pools and riffles, with flow and substrate particle size being most influential in riffles, while algal biomass and macrophyte densities were most important in pools. Our findings suggest that spatial scale and sampling regime are critical in determining benthic invertebrate size and abundance patterns.

Life-history strategies of a freshwater snail in response to stream permanence and predation: balancing conflicting demands.

Life-history parameters for the freshwater snail Physella virgata virgata were estimated in temporary and permanent streams with and without crayfish (known snail predators). Snails from the permanent stream with crayfish exhibited higher age and size at first reproduction, as well as higher size and age at death, compared to snail populations from both temporary streams and the permanent stream without crayfish. Life-history patterns of snails from the permanent and temporary streams without crayfish were indistinguishable, with small sizes at first reproduction and at death, as well as lower age at first reproduction and at death. The snail population from the temporary stream with crayfish showed an intermediate life-history pattern. The results suggest that snail life-history patterns vary with both predation pressure and habitat stability, exhibiting phenotypically plastic responses to the variables.

Effects of Snail Grazing on the Diversity and Structure of a Periphyton Community in a Eutrophic Pond

Periphyton diversity was highest at low to intermediate levels of grazing by the freshwater snail Physella and suppressed at high grazer densities, in partial support of the intermediate disturbance hypothesis. For the first time, the response curve of algal biomass versus a range of snail densities was used to establish low, intermediate, and high levels of community disturbance. Physella densities corresponding to these levels were added to net enclosures in a small eutrophic pond, to examine differences in attached algal cell densities and diversity after 20 d. Algal standing crop was enhanced in low and depressed in intermediate and higher grazer treatments. Five categories of attached algal response to grazing were identified: (1) filamentous algae suppressed at high grazing pressure; (2) rosette or filamentous taxa suppressed at moderate to high levels of grazing; (3) algae resistant to grazing via sediment-associated recruitment; (4) low profile algae with highest densities at moderate grazing, and; (5) prostrately attached taxa enhanced at moderate and high grazing levels, in contrast to marine macroalgal communities, the primary mechanism mediating community response to different levels of grazing was the morphology of algal attachment.