Towards an Integrative Morpho-molecular Classification of the Collodaria (Polycystinea, Radiolaria)

Abstract

Collodaria are ubiquitous and abundant marine radiolarian (Rhizaria) protists. They occur as either large colonies or solitary specimens, and, unlike most radiolarians, some taxa lack silicified structures. Collodarians are known to play an important role in oceanic food webs as both active predators and hosts of symbiotic microalgae, yet very little is known about their diversity and evolution. Taxonomic delineation of collodarians is challenging and only a few species have been genetically characterized. Here we investigated collodarian diversity using phylogenetic analyses of both nuclear small (18S) and large (28S) subunits of the ribosomal DNA, including 124 new sequences from 75 collodarians sampled worldwide. The resulting molecular phylogeny was compared to morphology-based classification. Our analyses distinguished the monophyletic clade of skeleton-less and spicule-bearing Sphaerozoidae from the sister clades Collosphaeridae (skeleton-bearing) and Collophidiidae (skeleton-less), while the Thalassicollidae was not retrieved as a monophyletic clade. Detailed morphological examination with electron microscopy combined with molecular analyses revealed many discrepancies, such as a mix between solitary and colonial species, co-existence of skeleton-less and skeleton-bearing specimens within the Collosphaeridae, as well as complex intraspecific variability in silicified structures. Such observations challenge a morphology-based classification and highlight the pertinence of an integrative taxonomic approach to study collodarian diversity.

Biology and Ecology of Radiolaria

“…It is currently composed of the three families Collosphaeridae, Collophidiidae, and Sphaerozoidae ( Fig. 8.13 ) (Biard et al 2015 ). Collosphaeridae are characterized by a single spherical shell with pit-like pores, and colonies are composed of tens to thousand cells.…”

“…Sphaerozoidae also form colonies, but each cell is either surrounded by numerous isolated silica spicules or naked. Recent molecular phylogenies demonstrate that the former family Thalassicollidae, composed only of solitary specimens (single cell with or without silica spicules), is not valid as its representatives are scattered throughout other collodarian families (Biard et al 2015 ). To date, a total of ~20 genera and ~80 species are considered as taxonomically valid taxa under ICZN.…”

Optics-based surveys of large unicellular zooplankton: a case study on radiolarians and phaeodarians

“…Solitary collodarians could be a polyphyletic group, according to the branching pattern of collodarians in a DNAbased phylogenetic tree, suggesting that both solitary and colonial types are a part of the same life cycle (Biard et al 2015). We therefore suggest simply treating them as solitary and/or colonial collodarians.…”

Time calibrated morpho-molecular classification of Nassellaria (Radiolaria)

“…S1). The first clade, constituted by 31 sequences, was assigned to Collodaria in Biard et al (2015). A second clade formed by 2 sequences (BS: 94) was highly related to clades B and C (BS: 96).…”

“…Among polycystines, Nassellaria actively feed on a large variety of preys, from bacteria to mollusc larvae (Anderson, 1993;Sugiyama et al, 2008), contributing significantly to trophic webs dynamic of 2012; Decelle et al, 2012;Biard et al, 2015). Acquisition of reference DNA barcode based on single cell sequencing of isolated specimens have also established the basis for further molecular ecology surveys inferring the actual diversity and ecology in the nowadays oceans (Decelle et al, 2013;Biard et al, 2016;Nitsche et al, 2016).…”