HATCHING SIZE AND THE DISTRIBUTION OF NURSE EGGS AMONG PROSOBRANCH EMBRYOS

Abstract

Some embryos of Thais emarginata attain twice the shell length that others do by the time of hatching (0.9-1.8 mm, +44% to -31% about the mean). Larger hatchlings must have acquired more nurse eggs than smaller ones. Embryos tend to reach the same size when nurse eggs are scarce (in crowded capsules); size differences are most apparent when nurse eggs are in excess. Therefore, competition among embryos contributes little to hatching size differences. Embryos are distributed haphazardly among capsules; some capsules contain only 1 embryo which becomes large, while others contain as many as 33 embryos, each of which remains small. Nurse eggs are distributed more regularly. Embryos are also distributed haphazardly among capsules of Acanthina spirata. Each embryo of A. spirata has on average only 1.67 nurse eggs, and hatching size is correspondingly less variable (0.55-0.75 mm, +15% to -16% about the mean). These nurse-egg feeders reach variable hatching sizes because some embryos share their yolk supplies with many more capsulemates than others.