Morphological and attachment changes of Lamellodiscus theroni (Monogenea: Diplectanidae) during its post-larval development on fish.

Species of the genus Lamellodiscus Johnston et Tiegs, 1922 (Monogenea: Diplectanidae) are characterised by a complex haptor bearing many different attachment elements: two pairs of main hooks joined by medial bars, 14 peripheral marginal hooks and one or two lamellodiscs, formed by several overlapping sclerotised plates (lamellae). These haptoral structures appear gradually during parasite development and, therefore, attachment strategies vary with developmental stage. The main aim of this work was to study the developmental changes of Lamellodiscus theroni Amine, Euzet et Kechemir-Issad, 2007 under experimental conditions, with special attention to the gradual variations in attachment strategies and the pathological implications. Throughout the gradual development of the sclerotised structures, six developmental phases were distinguished in L. theroni: phase I, with only 14 peripheral marginal hooks; phase II, with main hooks (ventral and dorsal) formed; phase III, with ventral bar formed; phase IV, with dorsal bars formed; phase V, with dorsal and ventral lamellodiscs formed; and phase VI, adult stage with male copulatory organ formed. During development, parasites attach to different parts of the first and secondary gill lamellae and the mode of attachment changes from unspecific stage, i.e. based on piercing any flat gill tissue in the early stages, through an intermediate stage when ventral and dorsal main hooks are completely functional and parasites become restricted to the interlamellar space, and finally to the definitive adult attachment stage when lamellodiscs are fully developed. The timing of key events in the development of L. theroni was used to establish adequate intervals for anthelmintic drug administration.