Demonstrating morphometric protocols using polystome marginal hooklet measurements.

The flexible body structure of polystomes (Monogenea: Polystomatidae) encumbers taxonomic classification and species identification. Large intraspecific and limited interspecific variation in the morphology of polystomes further complicates the identification of species. Apart from employing the host-specific nature of the polystomes, taxonomic characterisation relies heavily on the sclerotised skeletal structures, such as the hamuli and the marginal hooklets. The currently accepted measurement system for marginal hooklets appears to be non-optimal and an improved protocol is needed. This paper describes in detail how such a protocol is found by evaluating various sets of measurements statistically in order to identify the most informative combination of parameters. Thirteen measurements of marginal hooklets from 11 different Southern African Polystoma species were taken and evaluated. A new protocol for discriminating between species of Polystoma Zeder, 1800, that employs only three measurements, is proposed. The value of the processes to derive morphometric protocols, as described, is that it is not restricted to a specific taxon, but that it can be amended and applied to any taxonomic grouping.