Characterisation of eggs and larvae of Lamellodiscus erythrini (Monogenea: Diplectanidae) using light and scanning electron microscopy.

While the identification of adult monogeneans primarily relies on morphological criteria, the morphology of a number of monogenean larvae (oncomiracidia) is to this day scarcely described. Yet, oncomiracidium plays a crucial role in the life cycle of the parasite, being responsible for the detection and localisation of its host, as well as for its attachment to this host. Few studies investigated the external morphological structures related to these functions, especially in Monopisthocotylea. The present study focuses on the early life stages (egg and oncomiracidium) of Lamellodiscus erythrini Euzet et Oliver, 1967, which are accurately described for the first time by light and scanning electron microscopy. Eggs of L. erythrini are smooth, tetrahedral and extended by a long polar filament. Freshly laid, the egg is brown, opaque, impermeable and becomes transparent as it matures, revealing the larva and its eye spots. When the egg matures, the egg casing exhibits functional weak points all around the operculum through which the larva emerges. The larva of L. erythrini is elongated, cylindrical and has a highly developed ciliation covering three areas: an anterior zone, a pleural zone, and a posterior cone. The ciliated cells are contiguous and are organised in a structured mosaic of spherical droplets, each cilium inserted into one. The larval tegument presents microvilli as well as 9 pairs of dorsal sensilla. The haptor is a closed structure consisting of 14 sclerotised hooklets, 12 arranged in a circle, and one pair positioned at the centre of the haptor. The possible link between these morphological structures and larval behaviour is discussed.