Demonstrating morphometric protocols using polystome marginal hooklet measurements

Abstract

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.

A new polystomatid (Monogenea, Polystomatidae) from the mouth of the North American freshwater turtle Pseudemys nelsoni

“…Measurements were taken using Eclipse network software (Nikon). Marginal hooklet measurements were obtained from the oncomiracidia that hatched from incubated eggs, following the protocol developed by Du Preez and Maritz (2006).…”

Putting in shape – towards a unified approach for the taxonomic description of monogenean haptoral hard parts

“…As already emphasised by several authors, most morphological features of the haptoral hard parts are indeed highly correlated (e.g. Shinn et al, 1996;Du Preez & Maritz, 2006), and traditional discriminant methods cannot properly accommodate such multicollinearity. This is especially true for small sample sizes where several morphological measurements are used to classify a few individuals [for a comprehensive report and ways to handle multicollinearity, see Vignon (2011)].…”

“…I voluntarily discarded the few publications dealing with morphometrics using a wide range of methods and measurement (e.g. Shinn et al, 1996;Du Preez & Maritz, 2006). While several methods of shape retrocalculation based on linear measurements exist, figures were drawn by hand with the help of a custom-made transparent rule previously calibrated on a computer with Adobe Illustrator.…”

“…Due to the lack of discrimination of traditional methods, several authors have included arbitrary additional points to take into account the maximum amount of shape information (Murith & Beverley-Burton, 1985;Rehulková & Gelnar, 2005) or to find an informative combination of variables (Shinn et al, 1996;Du Preez & Maritz, 2006;Robertsen et al, 2007), leading to the construction of analyses on a case-by-case basis (McHugh et al, 2000;Shinn et al, 2004). As more landmarks are included in an analysis, the minimum number of distances needed to fix the relative landmark positions increases to the point of being unrealistic (Slice, 2007).…”

Identification of Monogenea made easier: a new statistical procedure for an automatic selection of diagnostic linear measurements in closely related species

“…Gussev (1976) has indicated a complex system of measurements to correctly identify monogenean species. However, several studies demonstrated that many morphometric variables are highly intercorrelated, which diminishes their value to discriminate species (Shinn et al 1996;Du Preez and Maritz 2006;Vignon 2011a,b). This problem is particularly important in closely related species (Strona et al 2005).…”