Integrative morphological and molecular investigation of Turbanella hyalina Schultze, 1853 (Gastrotricha: Macrodasyida), including a redescription of the species

Abstract

Early descriptions of Gastrotricha mostly lack information that is nowadays essential for a reliable diagnosis and determination of species. This deficiency covers data on taxonomically important external characters (e.g., scale patterns, collocation of cilia, number and arrangement of adhesive tubes) and/or internal features (e.g., anatomy of reproductive organs). Furthermore, the morphological variability is often not or insufficiently documented. Holding a reliable knowledge about the latter is more relevant than ever before, since the discovery of morphologically cryptic genetic meiofauna species is increasing. It is especially a robust morphometry, amongst other relevant data such as ecology and behaviour, that can yield further evidence supporting or disproving the respective species hypothesis. The marine gastrotrich Turbanella hyalina Schultze, 1853 was the first described species of the whole subclade Macrodasyida but data that have been specified above were mostly missing. This repeatedly caused taxonomic confusion with other congeneric species in the past. Furthermore, a considerable amount of intraspecific genetic diversity was recently discovered among different European populations of T. hyalina as well as evidence for the presence of two cryptic species. In order to reduce deficiency and to warrant a reliable identification of T. hyalina in the future, we have conducted an integrative investigation of specimens of the type locality covering a survey of the morphological variability, an analysis of mtDNA sequence data (COI), and a reconstruction of the muscular system based on confocal microscopy. The latter was included because in the recent past, a hidden species diversity of another marine gastrotrich taxon was discovered due to differences in the muscular system between two geographically separated ‘populations’.While morphometric and meristic variation of T. hyalina broadly overlaps with other congeneric species, our light and electron microscope data bore the presence of up to twelve pairs of reduced ventral adhesive tubes on the posterior trunk section, possibly an autapomorphic and hence diagnostic character of T. hyalina. Furthermore, we were able to prove the absence of a pair of ventral ‘cirrata tubes’ that is present in similar species of the genus. The myoanatomy exhibits several similarities but also distinct differences compared to other con-generic species. Our mtDNA survey uncovered more intraspecific genetic diversity than previously known. However, the data also demonstrate that populations of different northwest European sites belong to the same species. The observed phylogeographic patterns are possibly related to post-glacial population expansions in the studied area and may already demonstrate a certain restriction of the gene flow.