Morphometric comparison of three Southern Ocean Fragilariopsis species

While attempting to assemble a reference image set of Southern Ocean diatoms for training automatic classification algorithms, we encountered numerous specimens which we were unable to classify unequivocally into one of three highly similar Fragilariopsis species. Problems about the delimitation of these species were also raised at the last Polar Marine Diatom Workshop in 2015 in Salamanca. The present study originated from these two sources. Using semi-automated microscopy and image analyses, we assembled a set of 501 specimen images and accompanying morphometric data, and 12 members of the polar marine diatomist community contributed their identification of these specimens independently from each other. After comparing the identification results themselves, we used the morphometric features extracted in an attempt to clarify the nature of morphometric distinction of the three taxa in uni-and bivariate analyses, and performed multivariate classification experiments and tested their agreement with expert consensus opinion. Beyond the specific insights into morphometric distinction of the studied taxa, our study also highlights some of the more generic challenges and possibilities of research at the interface between automatic identification and traditional taxonomy.