Reference: Biol. Bull. 167: 378-389. (October, 1984) FORMATION, ORGANIZATION, AND COMPOSITION OF THE EGG CAPSULE OF THE MARINE GASTROPOD, ILYANASSA OBSOLETA CHARLES H. SULLIVAN' AND TIMOTHY K. MAUGEL Laboratory for Ultrastructural Research, Department of Zoology, University of Maryland, College Park, Maryland 20742 Abstract Embryos of the marine mud snail Ilyanassa obsoleta undergo early development within an egg capsule. After about a week of encapsulation, embryos hatch by releasing a chemical substance that removes the plug found at the apex of a capsule. However, the mechanism of action of this hatching substance remains poorly understood. To study how the hatching substance functions, we examined the composition of the egg capsule, particularly the plug region, to determine what the "substrate" of the hatching substance might be. We have also examined the formation and organization of the egg capsule to determine the origin and identity of the regions of a capsule that the hatching substance must remove. The results show that the Ilyanassa egg capsule is organized into four layers, the outer three of which are composed of protein and carbohydrate. Portions of the two inner layers of the capsule wall extend into the capsule apex and form the plug, which is dissolved by the hatching substance. The isolated capsule plug region contains three major glycoproteins resolved on sodium dodecyl sulfate-polyacrylamide gels. Therefore, the hatching substance may be a pro-tease similar in action to the enzymes released by many other embryos at hatching. Introduction The egg capsules of prosobranch gastropod molluscs are examples of the various types of extracellular envelopes that surround the embryos of animals. Egg capsules are produced in the maternal oviduct by a process described in detail for four species of gastropods by Fretter ( 1 94 1 ). The process may be similar for other species as well. Eggs are released from the ovary and are fertilized in the upper regions of the oviduct. The fertilized eggs, in groups of up to several hundred, move down the oviduct to a specialized region, the albumen gland, and are embedded in the secretions of this gland. A sphincter between the albumen gland and the next region of the oviduct, the capsule gland, relaxes and the embryos are forced into the protein and mucus secretions of the capsule gland. This movement of the embryos leaves an opening in the secretions that is subsequently sealed by the operculum. Ultimately, the soft capsule passes out of the oviduct, through the mantle cavity, and along a temporary groove on the right side of the foot to the ventral pedal gland. Within this latter gland, the capsule is sculptured into a species-specific shape, hardened, and attached to a substrate. The time an embryo spends within the egg capsule is variable (Fretter and Graham, 1962). Some gastropod embryos emerge within a week as swimming premetamorphic veliger larvae (Scheltema, 1 967), so that hatching can be a relatively early developmental Received 4 January 1984; accepted 17 July 1984. ' Present Address: Department of Biology, Gilmer Hall, University of Virginia, Charlottesville, Vir-ginia 22901. Send all correspondence to Dr. Sullivan at the present address. 378
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