Gastropods in a changing and stressful environment: Impacts on life history and interspecies interactions

Date

2012-12

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Abstract

Freshwater salinization is a global concern partly attributable to anthropogenic salt contamination. Salts may enter freshwater systems via road run-off, irrigation practices, wastewater effluent and oil and gas production. In addition to anthropogenic causes of increased salinity, climate change projections of temperature increases are likely to increase evaporation rates leaving salts behind in freshwater systems. Particularly impacted areas may be in the Southwest region of the United States, where projections indicate reductions in precipitation and increases in temperature. Shallow ephemeral aquatic systems which depend on rainfall for filling and maintaining sufficient hydroperiods are vulnerable to such climatic shifts and will likely result in increased water temperature, reduced hydroperiods and higher salinity. Understanding how projected climate changes will impact aquatic organisms may be critical for managing systems. Many freshwater systems are inhabited by gastropods, which can comprise a large portion of the biomass and are important prey for many aquatic and terrestrial species. Gastropods are a functional link between primary producers and secondary consumers and, if present in large enough densities, can have an impact on the structure of the community. Snails exhibit adaptive phenotypic plasticity in response to predator threat through predator-induced defenses. In the event of perceived predator threat, snails may alter their behavior and reduce consumption rates which can result in alterations of periphyton abundance and distribution. Hence, understanding how predator pressure, hydroperiod, temperature and salinity impact gastropods may provide insight into higher-level wetland ecological functions.

The first objective of this research was to identify salinity tolerance levels and sensitive life stages of field collected gastropods.  We examined the effects of increased salinity (as NaCl, 250 – 4000 µS/cm, specific conductance) on two sympatric freshwater gastropods (Helisoma trivolvis and Physa pomillia).  Life stage sensitivities were determined by exposing naïve eggs or naïve juveniles (through adulthood and reproduction).  Additionally, progeny eggs from the juvenile-adult exposures were maintained at their respective parental salinities to examine trans-generational effects.  Naïve H. trivolvis eggs experienced delayed development at specific conductance > 250 µS/cm; reduced survivorship was also seen in juvenile H. trivolvis at 4000 µS/cm.  Survival and growth of P. pomilia were not affected by increased salinity following egg or juvenile exposures.  Interestingly, the progeny of H. trivolvis exposed to higher salinity may have gained tolerance to increased salinity whereas P. pomilia progeny may have experienced negative trans-generational effects. This study demonstrated that freshwater snail species vary in their tolerance to salinization and also highlights the importance of multigenerational studies, as stressor impacts may not be readily apparent from shorter-term exposures.  
The second objective of this research was to examine the combined effects of temperature and salinity stress on life history and morphological traits of H. trivolovis. Additionally, we sought to better understand how these climate change-related stressors impact the ability of H. trivolvis to express predator-induced defenses.  Hatchlings of field-collected snails (Helisoma trivolvis) were exposed to a range of environmentally relevant salinities (300 – 4000 µS cm-1) and temperatures (20 – 30 °C) with the high-temperature and high-salinity treatments representing a potential environmental condition under projected climate change scenarios.  Each treatment combination had an alarm/predator cue counterpart. 

Highest mortality was observed in the combined high-temperature/high-salinity treatment regardless of predator cues. Furthermore, morphological changes in shell shape and overall snail size were most affected by the presence of predator cue; however, increased salinity and temperature also influenced snail mass and shell thickness, respectively. Reproduction was severely delayed in snails exposed to predator cue; additionally, overall fecundity was decreased in high-salinity/temperature treatments. Collectively, our data suggest high temperatures and increased salinity alter inherent fitness and predator-induced defenses in snails through behavioral and morphological responses.
The third objective of this research was to evaluate predator avoidance strategies of the snails Physa acuta, H. trivolvis and Biomphalaria glabrata, to better understand how gastropod community dynamics change under predator threat and abiotic stress and to evaluate whether the non-native freshwater snail Biomphalaria glabrata, could successfully establish under local conditions including crayfish predators, reduced hydroperiod and increasing salinity. A microcosm design was employed using a 2 x 2 x 2 factorial design with two treatments of specific conductance (increasing or constant), two hydroperiod treatments (decreasing or constant) and two predator treatments (presence or absence of crayfish). The presence of predator reduced the density and biomass of H. trivolvis and B. glabrata; however, predator reduced only the biomass, not density of P. acuta. P. acuta employed anti-predator behavior that reduced snail size but effectively maintained its biomass within the microcosms. B. glabrata did not appear to respond anti-predator or conspecific cues. Simulated evaporation may have forced interspecies interactions between H. trivolvis and B. glabrata as both species may retreat to deeper water; optimally the two species appear to stratify their locations. Increased salinity did not impact H. trivolvis or B. glabrata density, biomass or size; however, it did increase biomass of P. acuta. Given predicted climate change events in the Southern High Plains and successful predator avoidance behaviors noted in P. acuta, this species may become dominant in many freshwater systems. Combining abiotic and biotic stressors provides insight into potential impacts to gastropod communities. Projected climate related changes may increase salinity in ephemeral ponds; however, by limiting anthropogenic contributions to increased salts may lessen gastropod stress responses in ponds.

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Keywords

Gastropods, Salinity, Predator-prey, Phenotypic response

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