Notes on the use of wood refugia by Physella heterostropha (say) in a thermally disturbed swamp

Physella heterostropha size distributions and densities were measured in Pen Branch delta, a thermally altered swamp of the Savannah River, South Carolina. During cessation of thermal input, measurements were taken in a highly impacted area and more natural area. Of the three substrates sampled, logs, small woody material and benthos, snail density was highest on small pieces of submerged wood and < 20% of the snails on logs were out of the water. Following resumption of thermal input, snail density was four times higher on logs than previously measured. Snails responded to elevated water temperatures by congregating in a narrow band slightly above the water. Snail size did not appear to affect their response. However, the thermal history of the snails influenced their behaviour and survival rates. Logs may be a potential refuge for snails when water temperature are greater greater than 38°C. However, our results indicate that long-term survival in this manner may be impossible.     

Competitive interactions between two successful molluscan invaders of freshwaters: an experimental study

The New Zealand mud snail Potamopyrgus antipodarum (Hydrobiidae) and the pulmonate Physella acuta (Physidae) have invaded freshwaters in many parts of the world and become established. They co-exist in many streams, lakes and ponds in New Zealand, often at high densities. In the present study the effects of intraspecific- and interspecific interactions between the two species on growth and reproductive output were examined in laboratory mesocosms. In 30-day experiments, growth of Potamopyrgus antipodarum was lower in high density treatments than controls providing evidence for competition at higher densities of both snail species. No competitive effect was obtained for Physella acuta when controls were compared with high-density treatments, but growth was reduced at high densities of conspecifics. Numbers of juveniles released by Potamopyrgus antipodarum in 40 day trials declined at high snail densities and were lowest at high densities of conspecifics. Egg production by Physella acuta was also reduced at high snail densities. However, when the two species were kept together at equal densities (total snail density twice that of controls), egg production by Physella acuta was significantly higher than in all other treatments, suggesting facilitation by the congenor. Lastly, in a 10-day experiment, Physella acuta grew faster in water conditioned by Potamopyrgus antipodarum than in Physa-conditioned water, whereas Potamopyrgus antipodarum showed no growth response to Physella-conditioned water. Overall, our results indicate that growth and reproductive output of both snail species are influenced more by the density of conspecifics than the presence and density of the other species. The successful co-existence of the two species in New Zealand freshwaters therefore may be a reflection, at least in part, of few competitive interactions between them.     

Experimental evidence for alternative stable equilibria in a benthic pond food web

Both local and initial conditions have been implicated in causing variation in the strengths of predator effects among natural communities. In this study, I performed a controlled mesocosm experiment using two common snails, Physella gyrina and Helisoma trivolvis, and their insect predator, Belostoma flumineum, to explicitly quantify and understand the causes of previously observed variation in food web interactions in natural ecosystems. The results from a controlled mesocosm experiment showed that: (1) at low productivity, snails were dominated by small‐vulnerable species (P. gyrina), predator effects were strong and predator effects caused a trophic cascade extending to producers; (2) at high productivity, snails were dominated by large‐invulnerable species (H. trivolvis), predator effects were weak and there was no trophic cascade to producers; (3) at intermediate productivity, alternative stable equilibria were implicated, such that depending on initial conditions, snails were dominated by either small‐vulnerable or large‐invulnerable species and predator effects were either strong or weak. Thus, this study represents one of the first to quantify how variation in food web structure can result from the local environment, initial conditions and their interaction.     

Spore ornamentation of Haplosporidium pickfordi Barrow, 1961 (Haplosporidia), a parasite of freshwater snails in Michigan, USA

Spore ornamentation is increasingly recognized as a key character for species differentiation and genus assignment in the phylum Haplosporidia. Unfortunately, spore ornamentation is known for only a small number of described species so it is difficult to assign most species to genera with any confidence. Scanning and transmission electron microscopy were used to determine the presence and morphology of spore ornamentation of Haplosporidium pickfordi collected from the digestive gland of the snail Physella parkeri in Douglas Lake, Michigan. Spores possess filaments that are derived from the spore wall and originate from two separate areas at the posterior end of the spore. When spores are first isolated from host tissue, filaments are fused into a sheet that wraps around the spore, passing under the opercular lid. These filaments gradually unravel when spores are held in water and after about 14 d most filaments project freely from the posterior end of the spore. The number of filaments could not be determined with certainty, but appears to be approximately nine. Filaments are 100 nm in diam. and up to 50 microm in length. The presence of spore wall-derived filaments confirms the placement of the parasite in the genus Haplosporidium.     

Populations of the European freshwater pulmonate Physa acuta are not reproductively isolated from American Physa heterostropha or Physa integra

Abstract. It has long been speculated that Physa acuta , a pulmonate snail widespread and invasive in fresh waters of the old world, may have originated in North America. But the identification of a new-world cognate has been complicated by the confused systematics and taxonomy of the Physidae in America. More than 40 species of physids are currently recognized in the United States, many with variable and overlapping morphology. We have previously established that premating reproductive isolation is negligible among physid snails. Here we report the results from no-choice crosses each involving 2 populations of the widespread American species Physa heterostropha and Physa integra , both with each other and with P. acuta , designed to compare measures of reproductive success between species and between populations within species. Samples of P. acuta were collected from France and Ireland, P. heterostropha from eastern Pennsylvania and South Carolina, and P. integra from southern Indiana and northern Michigan. The 6 intrapopulation controls varied quite significantly in their survival, age at first reproduction, parental fecundity, F₁ viability, and F₁ fertility under our culture conditions. Measures of survival and reproduction in the 6 interpopulation crosses were generally intermediate, but in no case significantly worse than the more poorly performing control. Thus we were unable to detect evidence of reproductive isolation among our 6 populations of snails from 2 continents. All should be referred to the oldest available nomen, P. acuta .     

Winter diet of brown trout Salmo trutta in groundwater‐dominated streams: influence of environmental factors on spatial and temporal variation

Winter diet composition of brown trout Salmo trutta was quantified from November to March in 35 temperate groundwater‐dominated streams in south‐eastern Minnesota, U.S.A., in relation to stream physical characteristics including drainage area, channel slope and influence of groundwater on stream thermal regime. Aquatic invertebrates made up the majority of S. trutta diet in all streams and sampling periods and individual S. trutta typically had consumed 30 or more prey items at each sampling event. Differences in diet composition were greater among streams than between sampling periods within a stream, with Gammarus spp., Brachycentrus spp., Glossosoma spp., Chironomidae and Physella spp. the most common taxa. Landscape‐scale stream characteristics were not significantly associated with S. trutta consumption or diet composition. Winter was period of significant activity in groundwater‐dominated streams, as S. trutta fed on a variety of aquatic prey taxa highlighting the importance of winter base‐flow in moderating S. trutta populations in seasonally cold catchments.     

Predator identity and consumer behavior: differential effects of fish and crayfish on the habitat use of a freshwater snail

Predators can alter the outcome of ecological interactions among other members of the food web through their effects on prey behavior. While it is well known that animals often alter their behavior with the imposition of predation risk, we know less about how other features of predators may affect prey behavior. For example, relatively few studies have addressed the effects of predator identity on prey behavior, but such knowledge is crucial to understanding food web interactions. This study contrasts the behavioral responses of the freshwater snail Physellagyrina to fish and crayfish predators. Snails were placed in experimental mesocosms containing caged fish and crayfish, so the only communication between experimental snails and their predators was via non-visual cues. The caged fish and crayfish were fed an equal number of snails, thereby simulating equal prey mortality rates. In the presence of fish, the experimental snails moved under cover, which confers safety from fish predators. However, in the presence of crayfish, snails avoided benthic cover and moved to the water surface. Thus, two species of predators, exerting the same level of mortality on prey, induced very different behavioral responses. We predict that these contrasting behavioral responses to predation risk have important consequences for the interactions between snails and their periphyton resources. Received: 1 June 1998 / Accepted: 12 October 1998