Morphology and functioning of attachment organs of the Polystomatidae (Monogenea)

Abstract

Monogeneans are mainly ectoparasitic on fish, but the family Polystomatidae radiated onto tetrapods and can be found on the skin and gills of the Australian lungfish, in the urinary bladder of frogs, gills and skin of salamanders, cloaca and phalodeum of caecileans, on the eye, nostrils, mouth, cloaca or urinary bladder of freshwater turtles, and on the eye of the hippopotamus. These host organisms are ecologically related through their association with freshwater habitats that favour parasite transmission. Firm attachment is critical to maintain a close relationship with their hosts. Attachment organs usually comprise of several units that are semi related to each other due to the need to form a functional unit. Interactions between subunits are expected to be under stabilising selection, and therefore hinder evolutionary change. Monogeneans are renowned for their effective posterior attachment structures in the form of hooks or hamuli and suckers that secure them, permanently or semi-permanently, to their hosts. The aim of this study was to investigate the morphology and functioning of attachment organs of selected polystomes representing different genera. A number of genera were selected in the study of attachment structures, genera included: Protopolystoma, Polystoma, Eupolystoma, Neopolystoma, Polystomoides and Oculotrema. Light microscopy and scanning electron microscopy was used to study the external morphology. Histology followed by light microscopy, confocal microscopy and enzyme digestion techniques followed by scanning electron microscopy was used to study the internal morphology. It was found that variation in haptoral components do exist, even among congeners, living for example in the bladder and oral cavity of the same host. Environmental factors relating to host ecology need to be taken into account when studying the morphology of monogenean haptors. Such factors play an important role in the adaptation of monogeneans and have possibly led to the change in microhabitats, which in turn explain the variation of haptoral components between parasites. Not all haptoral structures necessarily function in attachment throughout the entire life of the parasite and different haptoral structures are important for attachment to the host at different developmental stages of the parasite.