Taxonomy of Polyclads

The taxonomic order Polycladida (class Turbellaria) comprises several hundreds of described species. The majority of these (about 200 species in seven genera) are gathered in the family Pseudocerotidae, which can be considered as the best characterized tropical polyclad family of today. Pseudocerotids are the most flamboyantly coloured flatworms and are characterized and distinguished by a ruffled pharynx from the next most speciose family of tropical polyclads, the Euryleptidae (with about 130 species), which have a tubular pharynx. The other genera of pseudocerotids are less speciose and poorly known. Some are even monospecific. For a current and comprehensive taxonomic listing of polyclad flatworms and other turbellarian flatworms go to the web pages of Tyler S. & Bush L.F. 1998, Turbellarian Platyhelminths Taxonomy.

Taxonomic studies on polyclad flatworms are quite difficult. One has to take in account a suite of characters as size, shape, colour and marking, eye arrangement, pseudotentacles, pharynx, topography of gonopores and sucker. In cases, where these characters are not sufficient for identification, comparative morphology of the reproductory system is an essential tool for species identification. Since the serial reconstruction of the male and female reproductive structures is difficult and makes special lab equipment necessary, it is reserved for specialists. Recently, molecular data (DNA sequencing) have been used to tell apart similar polyclad species (see section: Phylogeny).

The following table may be an useful tool for polyclad flatworm identification for the diver and UW photographer. It shows some striking features, which may help in identification of the most commonly observed polyclad genera.