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

Biochemical changes occurring over 7 months of estivation were studied in two species of land snail, Oreohelix strigosa (Gould) and 0. 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 es- tivation, were compared with snails collected from the field and induced to estivate without augmenting their energy reserves. In all groups, polysaccharide was catab- olized early in estivation, and protein was the primary metabolic substrate after polysaccharide reserves were de- pleted. Lipid was catabolized at a low rate throughout estivation. Rates of catabolism were largely statistically equivalent between species. Urea and purine bases ac- cumulated during estivation as a result of protein catab- olism, with the former being quantitatively more impor- tant. 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 disrup- tive effects of elevated urea, were measured in one group of 0. 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 com- pounds, urea may reach toxic levels and may be one factor limiting the duration of estivation that is survived by these land snails