Experiencing the salt marsh environment through the foot of Littoraria irrorata: Behavioral responses to thermal and desiccation stresses

摘要

Behavioral responses to environmental conditions can determine both the microclimate surrounding an organism, as well as how an organism experiences that microclimate. The salt marsh snail Littoraria irrorata (Say) employs two types of behaviors that potentially affect its likelihood of experiencing thermal and/or desiccation stress: 1) retracting its foot into its shell and 2) vertically migrating on the marsh grass Spartina altemiflora. One aim of our study was to determine how the use of retracting behaviors by L irrorata modifies its ability to tolerate thermal and desiccation stresses, and the interactions between these stresses. A related goal was to elucidate whether the snails move vertically on the stalks of S. altemiflora to avoid thermal and/or desiccation stresses. In the laboratory, snails were kept in 10 biomimic (-potential body) temperature (25-45 ℃ in 5 ℃ increments) and vapor density (VD) deficit (~3 g/m~3 and -17 g/m~3) treatments to determine how they use retracting behaviors to avoid thermal and desiccation stresses. Performance measurements of water loss, body temperature, and mortality were made in relation to behavioral responses, with independent measures from 1.5 to 9 h. In the South Carolina salt marsh, snails' movements and retracting behaviors were monitored and compared to their body temperatures and microclimate conditions. Measurements were made in the high and low marsh (characterized by the height of S. altemiflora) as a function of height in the canopy. We found that the snails used retracting behaviors to shift their ability to tolerate thermal and desiccation stresses by changing their capacity for evaporative cooling through mantle exposure. L. irrorata consistently responded to emersion, or potential desiccation stress, by retracting into its shell, and continued to avoid water loss even under high thermal stress. Both field and laboratory experiments indicated that the snails' behavioral avoidance of thermal stress was severely limited by simultaneous avoidance of desiccation stress. Furthermore, snail movement was largely restricted to periods of tidal inundation in the low marsh and did not reflect behavioral avoidance of abiotic stresses. The use of retracting behaviors by L irrorata to manipulate its tolerance to temperature and humidity levels exemplifies the importance of behavior as a functional trait that determines its climate space. Viewing behavior at a mechanistic level provides a more accurate picture of how organisms experience their environment and how these impacts translate to interactions at the community level.