Ultrastructure of spermatozoa and female copulatory organs in preferably asexually-reproducing acoel Convolutriloba retrogemma (Acoelomorpha)

Abstract

Acoela (Acoelomorpha) represents one of the most spectacular animal taxa with debatable phylogenetic position. Particularly, acoel genus Convolutriloba is of special interest due to the inhabitance mostly in marine aquaria and the development of unique ways of asexual reproduction, while the sexual one is rarely described. The aim of the present work is the ultrastructural investigation of reproductive system in preferably asexually-reproducing acoel Convolutriloba retrogemma, type species of the genus. In all specimens studied, the oocytes were absent, despite the spermatozoa and female copulatory organs (seminal bursa with mouthpiece) were being fully developed. The spermatozoon of C. retrogemma represents an elongated filiform cell with a pair of incorporated axonemes with a formula 9 + 0, axial microtubules and three types of cytoplasmic inclusions. The bursa mouthpiece is composed of matrix cells with the electron-dense sclerotized core surrounding the lumen, and electron-transparent periphery containing most cell organelles, as well as the intercalary (glandular) cells between them. In all individuals studied by electron microscopy, the lumen of the mouthpiece is always locked with a “plug” composed of various cellular elements, including the sclerotized actin fibers, preventing the insemination. The comparative morphology of the reproductive system of Convolutriloba and evolutionary role of asexual reproduction within this genus are discussed.