WITHIN-CLUTCH AND BETWEEN-CLUTCH VARIATION IN EGG SIZE AND NUTRIENT CONTENT IN THE LAND SNAIL ARIANTA-ARBUSTORUM

1. Life-history theory often assumes that energy and nutrient contents are correlated with egg size. I assessed the within- and among-clutch variation in egg size, nitrogen (N) and carbon (C) content of eggs of the land snail Arianta arbustorum and examined their intercorrelations. I also examined whether the diet of the mother snail (Petasites albus or lettuce) and/or the time of the reproductive season when the eggs were laid affected the variation in egg size and nutrient content. 2. The volume of single eggs ranged from 5.5 to 26.3 mm3 (n = 956, grand mean 13.4 mm3). The overall range of the nitrogen concentration of the eggs was 3.1-5.0% (grand mean 4.1%), and that of the carbon concentration 28.6-34.9% (32.2%). The nitrogen concentration indicates that eggs of A. arbustorum have a protein concentration of 25.5%. 3. The within-clutch variation in egg size expressed by the coefficient of variation averaged 11.1% for egg volume, 8.4% for wet weight and 8.3% for dry weight. Corresponding values for the N and C concentrations were 3.7 and 1.6%. Thus, egg size was in general more variable than the nutrient concentration of the eggs. In all triats the within-clutch variation was significantly smaller than the among-clutch variation, indicating that individual snails produced eggs of relatively constant size and nutrient content. 4. Considering mean clutch values, the nutrient contents (in mg) scaled isometrically with egg size. On the within-clutch level the N content correlated with egg size in only 22 out of 36 clutches (61.1%), and the C content in only 23 of 36 clutches (63.9%). 5. Volume and wet weight but not dry weight were significantly larger in eggs produced by snails maintained on a lettuce diet than in eggs from snails fed on P. albus. This suggests a difference in water content of eggs laid by snails eating different diets. Furthermore, eggs produced by snails maintained on P. albus were more variable in dry weight than eggs produced by snails fed on lettuce. 6. Neither egg size nor nutrient composition in snails fed on both diets changed over the course of the reproductive season. 7. The dry weight of eggs from snails eating P. albus decreased with increasing clutch size, indicating a possible trade-off between egg size and number. This was not the case in snails maintained on lettuce.