Disentangling diatom species complexes: does morphometry suffice?

Abstract

Accurate taxonomic resolution in light microscopy analyses of microalgae is essential to achieve high quality, comparable results in both floristic analyses and biomonitoring studies. A number of closely related diatom taxa have been detected to date co-occurring within benthic diatom assemblages, sharing many morphological, morphometrical and ecological characteristics. In this contribution, we analysed the hypothesis that, where a large sample size (number of individuals) is available, common morphometrical parameters (valve length, width and stria density) are sufficient to achieve a correct identification to the species level. We focused on some common diatom taxa belonging to the genus Gomphonema. More than 400 valves and frustules were photographed in valve view and measured using Fiji software. Several statistical tools (mixture and discriminant analysis, k-means clustering, classification trees, etc.) were explored to test whether mere morphometry, independently of other valve features, leads to correct identifications, when compared to identifications made by experts. In view of the results obtained, morphometry-based determination in diatom taxonomy is discouraged.

Peer Review #3 of "Diatom identification including life cycle stages through morphological and texture descriptors (v0.2)"

“…The reason for including such dataset was to perform a comparison study of the discriminant power of the feature descriptors used by (Mann et al, 2004) and ours. Dataset 3 was recently analyzed by (Blanco et al, 2017) and the reasons for its analysis were the same as the previuos case. Dataset 1 explicitly includes different stages of the life cycle of the diatoms, which increases the degree of intraclass variability.…”

“…Figure 13 shows a perfect cluster separation. In (Blanco et al, 2017) the authors concluded that only morphometric measurements based on taxonomic keys such as the length/width ratio or the striae density are not sufficient for diatom classification when classes are similar. They obtain correct classification rates between 40% and 70% when classifying six different species of the Gomphonema genera (see Table 3) with different classifiers and clustering algorithms.…”

“…Traditionally, diatom identification has been made by expert biologists. They usually use morphometric measures, such as length and width, and other frustule characteristics, like striae density, and they make the identification comparing specimens with previously described diatom in the literature (Blanco et al, 2017). This task is challenging due to a huge number of diatom species, similarities between species and life cycle related changes in shape and texture.…”

Peer Review #1 of "Diatom identification including life cycle stages through morphological and texture descriptors (v0.1)"

“…The camera is connected to the computer through an USB2.0 connection, providing an image size of 2592 x 1944 pixels. Images corresponding to (Mann and Bayer, 2018;Blanco et al, 2017) in Table 1, 2 and 3 have been provided by the authors. Table 1 form the dataset used in this paper for a total of 703 images of eight different classes.…”

Shape outline extraction software (DiaOutline) for elliptic Fourier analysis application in morphometric studies

“…These shells (valves) are held together by extracellular polymeric substances (EPS) and silicate bands (copulae/girdle bands). The primary morphological parameters (valve length, valve width, and stria density) can be used in identifications, but may not be sufficient for species identifications (e.g., Blanco et al., ). The shape of the diatom typically forms a definable face and a rectangular‐like shape in side view.…”