Mesodon

Abstract: Population characteristics of two species of land snails (Mesodon roemeri, Bulimulus dealbatus) were investigated in field and laboratory experiments. Field experiments consisted of transplantations of populations and artificial manipulations of temperature, moisture, and light levels in the two environments in which these species occurred. Results showed that the species which was part of an ecosystem with a more variable physical environment (and lower species diversity) exhibited a wider tolerance—niche, higher "r" value, shorter lifespan, smaller litter size, shorter developmental time, more general behavior relative to its habitat, more general food requirements, and smaller maintained biomass than did the species which occurred in a less fluctuating environment. Laboratory experiments confirmed field results on size of the tolerance niches and provided detailed description of the size—specific influences of the physical environment on the growth of Mesodon roemeri. The results are in agreement with a developing theory that relates environmental variability to ecosystem diversity and stability through these population properties.

Abstract: Eighteen species and subspecies in three genera of the Polygyridae were found to have 58 as the typical chromosome number. The chromosomes of Mesodon (Polygyrinae) resemble more closely those of Triodopsis (Triodopsinae) than those of Stenotrema (Polygyrinae). Two species were extensively sampled. In Triodopsis fraudulenta a frequently encountered variation in chromosome number occurs. Individuals have 58, 59, 60, 61 (?) or 62 (diploid) chromosomes. It is suggested that snails can tolerate recurrent duplication of chromosome elements, and that in the Pulmonata as a whole, the nearly unbroken seriation in chromosome number may be attributed to a gradual increase from 34 to 62. In T. fraudulenta snails with extra chromosomes show no detectable increase in the volume of their nuclei nor can they be distinguished morphologically from those with the number of chromosomes at present characteristic of the family in Virginia. In two species (T. fraudulenta and T. tridentata) a single heteromorphic bivalent occurs...

Abstract: Seventy-four specimens from four genera of native gastropods were cooled from 2.0°C to — 3.3°C over a 3 hour period under laboratory conditions. The taxa examined included two terrestrial pulmonates {Anguispira alternata and Mesodon inflecta), a pulmonale known to occupy both permanent and ephemeral aquatic habitats {Physella mtegra), and an aquatic prosobranch {Pleurocera canaliculatum) . There was no mortality in the pulmonates, but 39% mortality occurred in the prosobranch Pleurocera. Additionally, a sample of 43 zebra mussels {Dreissena polymorpha), a non-native but recently introduced species, were aerially exposed to the same temperature cycle. Of the 43 mussels, 20 had been acclimated to 2°C, and 23 acclimated to 15°C. Mortality occurred in both treatments (35% mortality in the 15°C acclimated and 25% in the 2°C acclimated) and did not differ significantly between the two groups. No mortality occurred among controls. INTRODUCTION invertebrates, terrestrial arthropods, and both terrestrial and aquatic vertebrates (Storey & Storey, 1988). Freeze The most common ways in which organisms survive in a tolerance, although less common, has also been reported subfreezing environment are avoidance (including superfor such animals as some marine invertebrates and some cooling) and/or freeze tolerance. Supercooling, the extenvertebrates (Storey & Storey, 1988). sion of the liquid phase below the equilibrium freezing Illinois gastropods occur in a wide variety of habitats point of tissue fluids, has been documented in a diversity and include terrestrial, aquatic, and semi-aquatic species, of animal groups including various mollusks and intertidal These forms may all be periodically exposed to subfreezing Page 68 The Veliger, Vol. 40, No. 1 temperatures, particularly during the spring and fall months when ambient temperatures can vary widely within a 24 hour period. However, there is little information available on how these species tolerate periodic and short-term exposure to subfreezing temperatures. The purposes of this study were twofold: (1) to examine the effects of a temperature cycle that would expose four species of native moUusks to freezing temperatures, and (2) to determine whether an introduced moUusk is more or less able to withstand exposure to subfreezing temperatures compared to the native mollusks we studied. Our study was not intended to determine mortality curves or to investigate the eflficacy of freezing as a control method for the zebra mussel Dreissena polymorpha. We selected a prosobranch, Pleurocera canaliculatum (Say, 1821), which is strictly aquatic and occupies large rivers (Burch, 1982). We also included three pulmonates. One, Physella integra (Haldeman, 1841), Family Physidae, is a basommatophoran pulmonate that occupies both permanent and ephemeral aquatic habitats (Te, 1978). Additionally, two terrestrial stylomatophoran species, Anguispira alternata (Say, 1816), Family Endodontidae, and Mesodon inflecta (Say, 1821), Family Polygyridae, were included. The two terrestrial species are active during spring and fall months and are readily obtained during these seasons. As such, individuals are likely to encounter subfreezing temperatures under natural conditions. We also included specimens of an introduced bivalve, the zebra mussel Dreissena polymorpha (Pallas, 1771), in this study. Zebra mussels attach themselves with a byssus to firm substrates. Included among these substrates are the shells of various species of native unionid bivalves (Nalepa & Schloesser, 1993; Tucker et al., 1993) and species of native gastropods including P. canaliculatum (Tucker,