Egg capsule formation and hatching in the marine snail Littorina sitkana

Abstract

The process of egg capsule formation and hatching in Littorina sitkana , which includes the secretion of albumen layer, egg covering, egg capsule and jelly layer, has been studied with light and electron microscopy. Fertilization occurs in an expanded portion of the oviduct, or in the lumen of the albumen gland. There the zygotes are coated with a thick layer of albumen, followed by a thin egg covering in the covering gland. Specialized cilia with dense flexible tips manipulate the secretions into place. The characteristic shape of the egg capsule is moulded inside the capsule gland by opposing lips of the gland, which are under muscular control. The two halves of the capsule gland contain different types of gland cells, the secretions of which differ both in content and quantity. The resulting egg capsule is heterogeneous with a thick half composed solely of loose filamentous secretions and a thin half comprising a stratified mixture of loose and dense filamentous secretions. The dense filaments degrade more slowly. Thus during larval development the thick half of the egg capsule weakens first. When the juvenile snail is close to hatching it uses both its shell and radula to wear down and chew through the egg covering. The pre-hatching juvenile crawls around the inside surface of the capsule and periodically exerts pressure on the capsule wall by rapidly expanding the foot and shell. Eventually the thick side of the capsule fractures and the juvenile uses the thin side as a platform for thrust and escape. Rapid dissolution of parts of the egg capsule during hatching indicates that a hatching enzyme may be released.