Microdistribution of three genera of small terrestrial snails (Stylommatophora: Pulmonata)

Abstract

The microdistribution of three genera of small terrestrial snails was determined by recording the number of individuals visible on or above the litter surface in terraria held out-of-doors. Observations were made at various times throughout the 24-h day from May to August. Members of each genus spent most of their time hidden in the litter; of the individuals in each population, never more than 44% of Vertigo, 70% of Discus, and 42% of Euconulus were visible during an observation. Moisture conditions seemed to influence the number of snails observed on or above the litter surface more than either temperature or light. When humidity was allowed to track relatively dry environmental conditions, proportions visible under given temperature and light conditions were significantly lower in all three genera than when humidity conditions were held near the dew point. The proportion of the population visible at the surface also varied diurnally but response to temperature was greater than to light conditions, and peaked between 6 and 15 °C. In all three genera the proportions visible at the surface diminished at higher temperatures, regardless of humidity. Of the snails visible some were moving and others motionless when observed. Relatively more individuals of Euconulus were moving when seen than of Discus. Vertigo showed the lowest proportion moving. Temperatures at which the greatest proportions of snails were moving were also those at which the greatest proportions were observed on or above the litter surface; consequently more were moving at night than during the day.