Altitudinal variation in size and composition of eggs in the land snail Arianta arbustorum

Abstract

Egg size and egg provisioning are crucial for the survival of offspring in invertebrates without postlaying egg care. The effects of elevation and size of the mother on egg size and on nitrogen and carbon concentrations in eggs were examined in eight populations of the land snail Arianta arbustorum over an altitudinal gradient from 370 to 2340 m in Switzerland. The dry mass of single eggs ranged from 1.48 to 2.79 mg and decreased with increasing altitude. Adult snail size (shell volume) also decreased with increasing elevation as did clutch size and reproductive investment (clutch size x egg dry mass) of mothers. Hatching success of eggs varied among populations, but showed no altitudinal pattern. When differences in parental shell size were taken into account, correlations between altitude and egg size, clutch size, and reproductive investment disappeared. Thus, the altitudinal decreases in egg size, clutch size, and reproductive investment were mainly due to smaller snail sizes at higher elevations. The nitrogen concentration in eggs ranged from 3.4 to 4.5% and decreased with increasing elevation. In contrast to egg and clutch size, the altitudinal variation in nitrogen concentration of eggs cannot be explained by differences in snail size. The carbon concentration in eggs ranged from 31.1 to 33.1%, but showed no altitudinal variation. Nitrogen and carbon concentrations in the eggshell and egg fluid in eggs from four populations were examined separately. The nitrogen concentration in the eggshell ranged from 0.7 to 1.8% and was lower than that of the egg fluid (5.1-5.8%). Similarly, the carbon concentration in the eggshell was lower (20.2-22.8%) than that in the egg fluid (35.8-0.2%). Interpopulation differences in egg composition may affect growth, size, and survival of hatchlings and the extent of egg cannibalism in A. arbustorum.