Taxonomic ranks and their relationships in a molecular-based taxonomic study.

<p>In this schematic view the taxonomic ranks can be grouped in four different areas discriminated by their information content: individuals lie in the less informative level; a single taxonomic approach identifies morphotypes, MOTU and UCS; integration of data allows the definition of DCL, IOTU and CCS; the last and more informative level contains species. Individuals represent the first level of observation (1). These organisms are grouped on the basis of morphological similarities (2), in a classical taxonomic approach, which may lead to the identification of a species (2a), but can also be one of the inputs of the IOTU (2b). Molecular variability observed among individuals can lead to the definition of MOTUs (3) that, with the addition of more data, can be elevated to the level of DCL (3a) or IOTU (3b). However, in many published works MOTUs are identified within nominal species without additional taxonomic data (3c), being in this sense synonyms of UCS. As a consequence, the information content of MOTU and UCS is variable as identified by the dotted arrows between them. UCS is identified within a species (4), if further taxonomic data are provided it can be elevated to a DCL (4a) or an IOTU (4b). When two or more nominal species are similar at the molecular level for the chosen marker we call this situation Multi Taxa - Molecular Operational Taxonomic Units (MT-MOTUs) (4c). IOTU is the rank reached by a biological entity defined by molecular data coherently coupled with other source of information. When IOTU has reached a sufficient level of information it can be elevated to the rank of a CCS (5), which following a formal description will become species (6). The “+” in the left up corner of each box indicates that within each taxonomic rank, more than a single entity belonging to that rank can occur. MOTU is defined according to <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0040122#pone.0040122-Floyd1" target="_blank">[7]</a>; UCS, DCL and CCS are defined according to <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0040122#pone.0040122-Vieites1" target="_blank">[17]</a>, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0040122#pone.0040122-Padial1" target="_blank">[18]</a>.