Experimental induction of intraspecific morphometric variability in a single population of Halicephalobus cf. gingivalis may surpass total interspecific variability

Although molecular techniques are revolutionising nematode taxonomy, morphological data still form the basis of nematode species descriptions. However, morphological characters show a natural variability that should be taken into account before describing new species. The current study presents the results of an elaborate morphometric study of Halicephalobus cf. gingivalis, including 15 measurements and 13 indices of 540 specimens, the progeny of a single parthenogenetic female and cultured under different temperature and food conditions and measured in different adult age groups, i.e., young adults with a developed vulva but before the onset of oviposition, adults laying eggs, and old, post-reproductive adults near the end of their life cycle. The morphometric characters were analysed using both univariate (analysis of variance) and multivariate (principal components and canonical discriminant analysis) techniques. The main results reveal that the morphometric characters most used in Halicephalobus identification keys have a huge variability within a single progeny, e.g., body length 1.9 times longer than the shortest or ratio VA/tail length 3.9 times larger than the smallest. This variability has a magnitude that has not been observed in nematodes before. Further, by changing the environmental factors, the morphometric characters are influenced to an extent that one could assign - with seemingly 'statistical support' - different 'species' of the genus to different subpopulations. With this experimental study we provide convincing elements to advocate an integrative taxonomic approach and to discourage the description of new species based only on morphometric differences