Overwinter Tissue Degrowth in Natural Populations of Freshwater Pulmonate Snails (Helisoma Trivolvis and Lymnaea Palustris)

Abstract

When populations of aquatic invertebrates including molluscs) are subjected to seasonal periods of starvation, many individuals survive by major shifts in metabolic budgeting. These shifts can result in degrowth, defined as decrease in individual biomass, in some cases involving loss of structural proteins. Snails after degrowth can be recognized in natural populations because their "oversize" shells provide evidence of former larger tissue biomass. Fall and spring samples were taken from four selected populations of Helisoma trivolvis and four of Lymnaea palustris in upstate New York. Both are freshwater pulmonate snails, with a widespread and relatively euryoecic distribution. Shell : tissue ratios were calculated, and overwinter changes are significant (P <.001) for six of the populations. Overwinter tissue degrowth is demonstrated in three populations of Helisoma and one of Lymnaea, with overwinter growth occurring in one population of Hlisoma and two of Lymnaea. The actual dry—mass values of tissues found in spring individuals can be compared with values predicted from their individual shell masses in spring (using regressions for the same populations in fall). Surviving individuals of Helisoma in the three populations with degrowth can have tissues at 19—116% of the predicted mass (with 56 out of 59 below 100%) and, in the population of Lymnaea showing degrowth individual values ranging from 58 to 169% (with 19 out of 22 below 100%). Mean values for the three Helisoma populations are 75.3, 71.7, and 58.7% and for the degrowth Lymnaea populations, 85.3%. This field demonstration of degrowth is discussed in relation to other laboratory studies of starvation in molluscs and in relation to actuarial bioenergetics of natural populations. Capacity for true degrowth can be claimed to confer adaptive advantages in certain environments and is of considerable ecological and evolutionary significance.