The Biological Bulletin, Vol 184, Issue 2 230-242, Copyright © 1993 by Marine Biological Laboratory

PHYSIOLOGY

Biochemical Correlates of Estivation Tolerance in the Mountainsnail Oreohelix (Pulmonata: Oreohelicidae)

B. B. Rees and S. C. Hand
Department of Environmental, Population and Organismic Biology, University of Colorado, Boulder, Colorado 80309-0334

Biochemical changes occurring over 7 months of estivation were studied in two species of land snail, Oreohelix strigosa (Gould) and O. subrudis (Reeve), to determine whether differential mortality during estivation is related to different energetic strategies. Laboratory-maintained snails, which were fed ad libitum prior to estivation, were compared with snails collected from the field and induced to estivate without augmenting their energy reserves. In all groups, polysaccharide was catabolized early in estivation, and protein was the primary metabolic substrate after polysaccharide reserves were depleted. Lipid was catabolized at a low rate throughout estivation. Rates of catabolism were largely statistically equivalent between species. Urea and purine bases accumulated during estivation as a result of protein catabolism, with the former being quantitatively more important. In both laboratory-maintained and field-collected snails, the rate of urea accumulation was greater in O. subrudis, resulting in higher tissue urea contents in this species at the end of the 7-month experiment. The tissue concentrations of urea at 7 months ranged from about 150 to 300 mM and were positively correlated (r = 0.99, P = 0.006) with mortality in these snails. Methylamine compounds, a class of compounds that can offset disruptive effects of elevated urea, were measured in one group of O. strigosa at 7 months of estivation and found to be low relative to urea levels. We suggest, therefore, that in the absence of elevated levels of counteracting compounds, urea may reach toxic levels and may be one factor limiting the duration of estivation that is survived by these land snails.