Characteristics of protonephridial terminal organs and the eyes of four species of Trigonostominae (Platyhelminthes : Rhabdocoela)

Abstract

Two complex, phylogenetically informative structures were investigated using transmission electron microscopy in four species within a small taxonomic unit of rhabdocoel turbellarians. A synapomorphic modification of the structure of the protonephridial terminal organs, viz. reduced definition of the filtration ribs, was found in three of the species, Ceratopera sp., Proxenetes fasciger and Trigonostomum setigerum. Modifications of the mitochondrial lensing elements found in the eyes of Beklemischeviella contorta implied that it may join with Ptychopera westbladi to form a branch within the subfamily, while the distribution of lensing from giant mitochondria illustrated the plasticity of this feature. The study demonstrates the potential of such characters to assist phylogenetic resolution, but highlights the danger of using the characteristics of single species to represent larger taxa.