Studies on the reproductive biology of gastropods: Part III. Calcium provision and the evolution of terrestrial eggs among gastropods

Perhaps the most obvious specialization of molluscs is their ability to form a calcium carbonate shell which encloses and protects the body. In gastropods, shell formation and calcification begin while the embryo is still inside the egg, so that by the time of hatching the neonate has a rather well-formed body shell, the protoconch. Gastropod eggs which are laid in an aquatic environment have direct access to calcium from the surrounding waters. Eggs laid in a terrestrial environment, however, do not have direct access to calcium if the eggs are merely dropped into a hole in the ground or left in leaf litter, the two most common sites for land snails. Instead, the parent must make some modifications of egg composition or laying habit to make available the large amounts of calcium needed during embryonic development.
Calcium provision for the egg has been approached by a number of different ways; what they all have in common is that calcium salts are always deposited on the outside of the egg but do not occur in the egg albumen. It is prcsumed that a high concentration of calcium in the egg albumen is toxic to the zygote. The whole range of the methods of supplying calcium for eggs is surveyed. Some land snails merely coat each egg with calcium rich soil, either directly with the foot or by means of (soil) faecal pellets appressed to the egg shell surface. Other land snails have specialized pouches in the reproductive tract where calcium is kept for coating each egg or capsule. The most successful land snails, the Stylommatophora, take calcium from stores in the digestive gland and/or body shell and transport this as ionic calcium through the blood, across a specialized uterine epithelium to the formative eggs where calcite precipitates.
The present work involves the examination of the eggs of a soleoliferan slug, Veronicella ameghini Gambetta. The egg surface is lightly sprinkled with spherules of calciom carbonate, and the embryo is found to resorb egg shell calcium during development. Faecal strands incorporated into the egg surface are another source of calcium.
Finally, a list of gastropod families known to be terrestrial is presented with a suggestion that the eggs be examined to see if they also support the hypothesis presented here: that successful terrestrial colonization by gastropods involves an ability by the parent so supply the eggs with a large amount of calcium.