The ultimate labyrinth: inner architecture of the seminal receptacle of the apple snail Pomacea canaliculata (Caenogastropoda, Architaenioglossa, Ampullariidae)

GIRAUD BILLOUD, M; CASTRO-VAZQUEZ, A

Congreso; XXXIX Reunión científica anual de la Sociedad de Biología de Cuyo; 2021

Sociedad de Biología de Cuyo

Female Ampullariidae have a large and often pigmented organ that is ontogenetically derived from the pallial gonoduct. This organ receives proximally the visceral oviduct and communicates distally with the vagina. Early authors designated this organ as the uterus, although brooding does not occur in it, but it is where fertilization takes place and where the fertilized ova receive their coverings. In the genus Pomacea, in which monoembryonic eggs are deposited above the water level, perivitellin components are secreted by the uterine parenchyma and surround the fertilized eggs. Each egg is finally covered with a calcareous capsule that hardens on contact with air after oviposition. We have envisaged a thorough study involving 3D reconstruction and TEM of the inner uterine components. We observed that the uterus includes at least three main components (1) the seminal receptacle (SR), (2) the albumen gland and (3) the capsule gland. The seminal receptacle has a whitish colour and fibromuscular walls that contrasts with the bright red background of the albumen gland. It is formed by a spheroidal bulb that receives the visceral oviduct and is continued by a thick hook-shaped duct that leads to the junction with the uterine components. This SR is divided longitudinally by a wall that separates two parallel ducts, one of which is intended for the ascent of sperm and the other for the descent of the ova. The sperm duct reaches the center of the bulb and divides into numerous branches approaching the tortuous oocyte duct, which is distributed near the outer surface of the bulb (see below). In addition, many of the branches of the sperm duct terminate in saccules where the sperm are stored forming palisades. In turn, the thin visceral oviduct widens into the egg duct after entering the bulb. The egg duct has three distinct portions: the first is lined by a thick glandular epithelium with intermingled ciliated cells and has no connections to the sperm duct. In this initial portion a translucent perivitellus is secreted. The second region is lined by a cuboidal epithelium and has numerous connections with the neighboring branches of the sperm duct, from where the spermatozoa pass into this portion of the egg duct, which then becomes the site of fertilization. The third portion is a long coiled duct probably intended to fractionate the mass of fertilized oocytes and the translucent perivitellus into "units", each of which subsequently receives an albumen and the calcareous layer secreted by the capsule gland. The highly complex structure of the seminal receptacle can be easily correlated with the functions it serves, but it is difficult to understand how such a complex structure, with ascending and descending ducts, can originate from the single thin duct we have found in the early stages of its development.