Light and transmission electron microscopy of the haptoral sclerites of the monogenean gill parasite Macrogyrodactylus clarii.

Abstract

The present study represents the first detailed description of the haptoral sclerites of Macrogyrodactylus clarii Gussev 1961. Light microscopy reveals outgrowths of the hamuli roots; two lateral spine-like extensions of the hood-like accessory sclerites; a ridged, fan-shaped distal end of an accessory sclerite; and two thread-like accessory sclerites with biforked ends associated with the pointed hooked region of each hamulus. Transmission electron microscopy (TEM) reveals the presence of parallel tubules in the root of the hamuli and structural differences along the length of the hamulus including the root, shaft, and pointed hooked region. The root consists of two layers, the shaft four layers and pointed hooked region only one layer with dense outer serrations. Characteristic features of the hamulus root are the presence of longitudinally orientated parallel tubules in its central core and parallel electron-dense ridges in the outer layer of its middle region; features not observed in either the shaft or the pointed hooked region. Each hamulus blade of M. clarii is associated with haptoral gland cells producing electron-dense secretory bodies. The 16 marginal hooklets each consist of a blade (sickle) articulating with a handle at the guard region and a domus. TEM revealed structural differences between the handle, the blade at the articulation region, and the distal hooked region. The domus, a filamentous thread-like sclerite at the light microscope level, consists of two electron-dense, fibrous thickenings connected to each other by a cytoplasmic process. Each marginal hooklet is associated with a small cavity and a large reservoir of homogeneous particles and secretory bodies. The possible functions of these structures are discussed in relation to equivalent features in other monogeneans.