Ultrastructure of the surface structures of Allodiscocotyla diacanthi (Polyopisthocotylea: Monogenea) from the gills of the marine teleost fish, Scomberoides tol

Abstract

Scanning and transmission electron microscopic studies have been made of the surface architecture of the fish-gill parasite, Allodiscocotyla diacanthi. Observations of the haptor region have revealed the presence of cushion-like supports at the base of each clamp, together with a pair of anchor-hooks. Other topographical features observed included a midventrally positioned genital atrium and a ventrolateral vaginal pore. Tegumental serrations, composed of electron-dense bars, partially cover the hindbody, and their presence may serve in the adhesive attitude of the worm, in response to the flow of displacement water currents. The tegumental syncytium contains numerous dense granules and lucent vesicles, the contents of which are released into the cytosol of the syncytium. The clamp sclerites are invested with tegument which, in some specimens, contained inclusions resembling bacteria. Groups of neurons containing characteristic dense-cored vesicles were observed frequently in the clamp region. Each jaw of the clamp is distinguished by the presence of approximately 175 non-ciliated putative sensory endings, and a pair of cone-like sensillae occurs on the ventral surface of the lappet. Additionally, the entire body surface of the worm is covered with some 3000–4000 unicilated structures that are presumed to be sensory in nature.