LAUSR

Morphological features are often the only characteristics suitable for identification of taxa in testate amoebae, especially in ecological and palaeoecological studies. However, whereas the morphology of some species is rather stable it may vary considerably in others. Within the order Arcellinida the genus Trigonopyxis with the type species Trigonopyxis arcula is morphologically highly variable. To identify reliable characteristics for morphology-based taxon delineation we investigated variations in shell size, pseudostome diameter and pseudostome form in T. arcula from three different sites of the Ecuadorian Andes, where these characteristics vary even more than previously described. Further, we investigated if morphological characteristics in Trigonopyxis varied with changes in environmental factors. We studied 951 shells of Trigonopyxis collected along an altitudinal gradient with varying abiotic factors. We established a method for characterization of the pseudostome form, which lead to five different morphotypes. Our results suggest that shell size alone is not an appropriate character for taxon delineation but can be used as an indicator for changes in environmental conditions. In contrast, the pseudostome form might be used for taxon delineation, but likely also varies considerably within taxa. Overall, the study provides an overview of the morphological variability of the genus Trigonopyxis.