Ultrastructure and chaetotaxy of cercarial sensory receptors: A comparative analysis of four species of Digenea (Platyhelminthes)

Date of Completion

January 1999


Biology, Zoology




Previous investigations of cercarial sensory receptors have revealed a great heterogeneity of types. Other studies have suggested that these receptors exhibit chaetotaxic patterns that are consistent at family, genus, and species levels. Combined investigations of receptor morphology and chaetotaxy by means of standard procedures provide a new perspective. By comparing these features among taxa that vary in their relatedness, preliminary analyses of functional and phylogenetic components involved in cercarial sensory systems are possible. In this study, microscopical and neurocytochemical techniques were used to investigate ultrastructure of sensory receptors in conjunction with their chaetotaxy in cercariae with different degrees of relatedness: the allocreadiids, Crepidostomum sp. and Bunodera sp., lecithodendrild Allassogonoporus sp., and opecoelid Podocotyle sp. Freshwater mollusks were collected in the field, isolated, and exposed to artificial light and temperature to induce cercarial emergence. The most active cercariae were collected and processed for light and electron microscopy. For neurocytochemical studies, cercariae were treated with silver nitrate and acetylthiocholine iodide. Chaetotaxic patterns were determined for each species and receptors were identified according to their position in relation to nerve networks. Total numbers of receptors as well as percentages of ventral receptors are consistent among taxa. Different types of receptors were identified with scanning and transmission electron microscopy. Receptors differ in presence or absence of cilia, a tegumentary sheath and a tegumentary collar, cilia number, cilium length, and tegumentary collar length and width. Long and short uniciliated receptors were found on dorsal surface of body and widespread throughout the tegument, respectively. Long multiciliated receptors were detected around the oral opening of Podocotyle sp. Chaetotaxic patterns of families Allocreadiidae, Lecithodendriidae, and Opecoelidae are reviewed. Total numbers of receptors as well as percentages of ventral receptors may reflect an optimal design associated with constraints of cercarial existence. Ultrastructure and site-specificity observations suggest that most receptors are mechanoreceptors. The morphological diversity of receptor types probably reflects their adaptive value for host finding. Studies combining ultrastructure and chaetotaxy of cercarial sensory receptors of other families are encouraged. They may not only allow a better understanding of functional roles of these receptors but also reveal novel synapomorphies. ^