Plates of Acantharia drawn by Ernst Haeckel at the end of the 19th century
In 1858, the german biologist Johannes Müller from the University of Berlin first described Acantharia that he originally named “Acanthometren”. He was studying and teaching human physiology but became fascinated by the forms in marine plankton at the end of his carrier. He collected cells of Acantharia in the Mediterranean sea (Italy and South of France) with his own plankton net, the “Müller’s net”. During field trips to the island of Helgoland (North Sea), the professor initiated and encouraged his students like Ernst Haeckel to study marine plankton. In the wake of his professor, Ernst Haeckel worked on Acantharia and produced the first taxonomic framework in 1887 in his impressive publication “Die Radiolarien”, where he named and described 372 species. This first classification was then slightly modified by Mielck (1907) and Popofsky (1904a, b, 1906). The main diagnostic characters used for species identification were the different elements of the skeleton, such as the length, form and junction of the spicules.
In 1926, a russian Wladimir Schewiakoff from the University of St Petersburg spent his last three years of his carrier to study Acantharia at the Stazione Zoologica in Naples. Contrary to his predecessors, Schewiakoff observed and worked mainly on living cells, allowing him to define more morphological characters (myonemes, central capsule, membranes etc.), but also to investigate the ecology and physiology of the Acantharia. In his monograph published in 1926, he presented the revised taxonomy of Acantharia containing 130 species. Although few modifications have been done since this time (Trégouboff, 1953; Reshetnyak, 1981; Bernstein et al., 1999; Febvre et al., 2000), this morphology-based classification has not been revised by molecular tools. Currently, the class Acantharia comprises about 50 genera and 150 species, distributed in 18 families and 4 orders: Holacanthida, Chaunacanthida, Symphiacanthida et Arthracanthida (Figure 1). Schewiakoff considered Holacanthida and Chaunacanthida as the “primitive” forms of Acantharia.
Morphological identification of delicate and uncultured protists like Acantharia is a very difficult task. Diagnostic characters are very few and their observation often requires specific preparation under the microscope. In addition, some characters can be destroyed during the sampling collection and/or change drastically through the cell ontogenesis. All this suggests that taxonomy of Acantharia may contain errors and need to be compared with a molecular phylogeny to identify the conflicts.
Contrary to other Radiolaria (Polycystinea) and Foraminifera that have a complete fossil record, Acantharia do not fossilize since their skeleton in celestite dissolves very quickly when the organism dies (Beers et Stewart, 1970). The evolution of Acantharia therefore remains enigmatic. Inferring a molecular phylogeny can be a promising approach to reveal the evolutionary pathway of the Acantharia.
Molecular phylogeny of Acantharia
From single morphologically-identified acantharian cells, isolated from various oceans, we recently established the first comprehensive molecular phylogeny of Acantharia (Decelle et al 2011). Our phylogenetic analyses based on 18S rDNA and partial 28S rDNA revealed the existence of 6 main clades, sub-divided into 13 sub-clades. The polyphyletic nature of acantharian families and genera demonstrates the need for revision of the current taxonomy. This molecular phylogeny, which highlights the taxonomic relevance of specific morphological criteria, such as the presence of a shell and the organisation of the central junction, provides a robust phylogenetic framework for future taxonomic emendation.