A weighted taxonomic matrix key for species of the rotifer genus Synchaeta (Rotifera, Monogononta, Synchaetidae).

Tanja Wilke,Wilko H Ahlrichs,Olaf R P Bininda-Emonds

doi:10.3897/zookeys.871.36435

Abstract

A new, weighted matrix identification key for 34 largely undisputed species of Synchaeta was created with the aim of providing comparable, detailed and diagnostic character sets for each species that can be applied to live and/or preserved specimens. As part of this process, 14 species of Synchaeta were intensively re-investigated with respect to their habitus and trophi morphology using binocular, light, and scanning electron microscopy, which, together with behavioural observations, revealed several new discriminating characters. Whenever possible, missing information for any character was added for the remaining species from the literature, with the two recently described species Synchaeta arcifera and Synchaeta squamadigitata being considered for the first time in an identification key. Beyond its completeness, our key has two distinct advantages. First, the characters are supported by detailed illustrations of their respective character states whenever possible to both simplify identification and minimize any uncertainty in the descriptions themselves. Second, the new approach of weighting the characters according to their reliability, robustness and/or ease of determination was employed. This latter approach is especially advantageous for soft-bodied rotifers such as species of Synchaeta, where, for example, several external characters can be influenced by preservation and are therefore less diagnostic or reliable. Although the key is as comprehensive as possible, information for many species remains missing for many characters, thereby highlighting the need for additional comprehensive and detailed species (re-)investigations within Synchaeta.

Highlights

The rotifer genus Synchaeta (Monogononta, Synchaetidae) comprises approximately 37 to 39 valid and truly planktonic species, of which approximately half occur in brackish and/or marine habitats (Hollowday 2002)
Adherence to diverse objects a. absent or only transiently and only when disturbed b. long-lasting adherence without any twisting movement about the longitudinal axis c. long-lasting adherence combined with a twisting movement about the longitudinal axis
We hope to facilitate the reliable identification of both live as well as of preserved specimens, in part by highlighting those features that are more susceptible to the effects of preservation and, more generally, by indicating the reliability of different characters or individual character states for species identification

Summary

Introduction

The rotifer genus Synchaeta (Monogononta, Synchaetidae) comprises approximately 37 (see Segers 2007) to 39 (see Jersabek et al 2018) valid and truly planktonic species, of which approximately half occur in brackish and/or marine habitats (Hollowday 2002). The several comprehensive revisions and keys of Synchaeta that exist (e.g., Voigt 1956–1957; Ruttner-Kolisko 1972; Koste 1978; and most recently Hollowday 2002) tend to be restricted in that they limit themselves to describing the most concise set of characters that delimit each species. This represents a useful simplifying strategy, the inherently incomplete data set it entails presents two distinct disadvantages. More importantly, direct comparisons among species are usually not possible because the species are often described using different sets of characters

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Conclusion