Enveloping walls, encapsulated embryos and intracapsular fluid: changes during the early development stages in the gastropod Acanthina monodon (Muricidae)

Abstract

Encapsulation of embryos in marine gastropods affords protection for the developing young, whether or not parental care takes place. The capsule wall is laminated and its dimensions change during development. Dissolution of the capsule wall releases dissolved organic matter (DOM) into the intracapsular fluid, providing a nutritional source for the embryo. The capsule wall of Acanthina monodon is composed of three layers, a thin outer layer with projections to the exterior, a thicker intermediate layer containing vacuoles and a thin inner layer that gives rise to the hatching plug. During embryonic development the capsule wall lost 27.2% of its initial mass owing to a pronounced thinning of the outer layer, especially of the projections. The organic fraction in the capsule wall (>85% of total mass) decreased by 23.3%, mostly due to the loss of 52.8% of the protein originally present. The internal layer of the capsule wall became 35.2% thinner during embryonic development, and in capsules containing embryos in the prehatching stage the surface area of a section through the plug was reduced to 2% of its original value. Total protein concentration in the intracapsular fluid decreased by 54% during embryonic development. The dry weight of the encapsulated juveniles immediately before hatching was 96% greater than that of the eggs from which they developed. Total protein concentration increased by 148% between the egg and the advanced veliger stages, then decreased in the prehatching juvenile, presumably as a result of the energy cost of metamorphosis, which occurs within the capsule before the juveniles are released. During development, material was lost from the exterior surface of the outer layer of the capsule wall. In contrast, the inner layer partially dissolved and disintegrated into the intracapsular fluid, providing the developing embryos with a secondary source of nutrition, mainly protein, during the later stages of encapsulation, when the material from the nurse eggs was exhausted.