HISTOLOGICAL ANALYSIS OF THE EGG CAPSULE OF THE OVOVIVIPAROUS POLYSTOMATID MONOGENEAN, PSEUDODIPLORCHIS-AMERICANUS

Transmission of Pseudodiplorchis americanus is restricted to the brief period when its host, a desert toad, enters water to spa,vn. The para site accumulates its entire annual reproductive output within an elong ated uterus during the 10-11-month period of host hibernation, Embryos of P. americanus, at all stages of development, are retained within t he uterus which eventually becomes packed with around 150 encapsulated infective larvae. Recently formed eggs, which comprise a fertilized o vum and 2-3 vitelline cells, are closely surrounded by a primary eggsh ell which stains positively for acidic proteins and keratin. Initially , during passage along the proximal uterus, the egg capsule is only 60 mu m in diameter, but as it passes to the distal uterus it expands to 800 mu m in diameter to accommodate the growing larva. Due to chemica l alterations or complete replacement of the shell, the final (seconda ry) egg capsule is a large sac-like structure composed of elastin, The flexible nature of this shell maximizes the numbers of infective larv ae which can be stored in utero. TEM studies have revealed this capsul e to be composed of multi-laminate membranes with a specialized cytopl asmic lining involved in a unique mechanism for embryo nutrition, This is the first report of an elastin-type eggshell within the Monogenea. (C) 1997 Australian Society for Parasitology. Published by Elsevier S cience Ltd.