Contributions to Zoology, 85 (1) – 2016Emilia Rota; Svante Martinsson; Marco Bartoli; Anneke Beylich; Ulfert Graefe; Alex Laini; Mark J. Wetzel; Christer Erséus: Mitochondrial evidence supports a Nearctic origin for the spreading limicolous earthworm Sparganophilus tamesis (Clitellata, Sparganophilidae)

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Introduction

The aquatic megadrile Sparganophilus tamesis Benham, 1892 (type species of the monotypic Sparganophilidae) was first discovered in River Thames south of Oxford, England, hence the type species name. However, in his description of this species, Benham suggested that cocoons may have been an introduction from North America, via the roots of water plants or attached to timber that had been shipped from the United States (Benham, 1892: p. 175; see Rota et al., 2014). The taxon was never recorded again in Europe until Černosvitov (1945), based on a rich collection of specimens from Windermere, England, pointed out possible errors in the original description – which might have led to misinterpretations and descriptions of synonyms such as Pelodrilus cuenoti Tétry, 1934 from France (see full list of synonyms in Rota et al., 2014). In particular, the interchaetal dorsal interval dd, illustrated by Benham (1892) as broader than half the body circumference, differed by being one-third the body circumference in the worms studied by Tétry (1934) and Černosvitov (1945).

In the meantime, Sparganophilus Benham, 1892 had been found also in North and Central America. Trusting Benham’s account of S. tamesis (ventral position of the outer chaetae and lack of prostate-like glands), three new congeners had been described: one from Illinois, USA (S. eiseni Smith, 1895), and two more from California, Mexico and Guatemala (S. smithi Eisen, 1896 and S. benhami Eisen, 1896). Additional collections had soon extended the distribution of S. eiseni to Ohio, Michigan, Florida, Indiana and Canada (e.g., Smith, 1896; Moore, 1906; Heimburger, 1915). Michaelsen (1918) refuted Moore’s (1895) identifications of S. tamesis from Pennsylvania and New Jersey – suggesting instead that this species was a European autochthon.

Černosvitov (1945), considering the large range of distribution of S. tamesis in England, its occurrence in France and its absence in America, suggested that the genus’ distribution should be explained by the past geological relationships between the American and European continents rather than by accidental importation. Omodeo (1963) followed the trail and hypothesized that a Sparganophilus ancestor reached Europe from North America via a land-bridge across the Atlantic in a relatively ancient age, evolving to the European endemic S. tamesis.

Jamieson (1971), upon re-examination of the type-specimens of S. tamesis, left no morphological grounds for the distinction of North American S. eiseni – either as a specific or an infraspecific taxon – and considered the occurrence of S. tamesis at Kew Gardens as further support for the hypothesis of human transportation. To Gates (1982) it seemed a ‘plausible view that sparganophilids were taken to England since 1500 A.D. and unwittingly by man’, and Sims and Gerard (1985) concurred: ‘as the British records can all be associated with gardens containing imported aquatic plants, e.g., Goring-on-Thames, the type locality, is only a few miles downstream from the Botanic Gardens, Oxford’. Zicsi and Vaucher (1987) agreed on the proposed synonymy of S. eiseni with S. tamesis and could not see any other explanation for its occurrence in Europe (including their new record from Switzerland) than transplantation of aquatic plants from North America to England, and from there throughout the old continent. However, Reynolds (e.g., 1980, 1995, 2008) has always maintained the synonymy of S. tamesis with S. eiseni to be unacceptable, claiming to have identified both species in specimens deposited in the Natural History Museum (London) that had been collected from artificial water habitats in England.

More recently, Bouché and Qiu (1998) have revived the view of the Sparganophilidae as a family suggestive of earlier connections between the two continents, and have even hypothesized the presence in Europe of two endemic species: the more widespread S. tamesis, postglacially re-expanded north of the Alps, and the Swiss S. langi Bouché and Qiu, 1998, which survived in the Rhone-Ebro basin during the glaciations and afterwards recolonized part of the Mediterranean. On the other hand, molecular phylogenetics (Jamieson et al., 2002; James and Davidson, 2012) indicates that the family Sparganophilidae is sister taxon to Komarekionidae, a monospecific earthworm family living in forest soils in midwestern (Illinois, Indiana, Kentucky) and eastern United States (Georgia through Maryland) in and adjacent to the Appalachian Highlands region (Reynolds and Wetzel, 2008; Rota et al., 2014a; Rota and de Jong, 2015).

Stimulated by the recent discovery of well-established populations of Sparganophilus in Germany (Graefe and Beylich, 2011) and Italy (Rota et al., 2014) – in both cases morphologically conforming to the diagnosis of S. tamesis as revised by Jamieson (1971) – we analysed the mitochondrial genetic diversity (COI and 16S) of samples of Sparganophilus from the extreme corners (UK, Germany and Italy) of its distribution area in Europe (see map in Rota et al., 2014:fig. 3) and from two well separated locations in the United States, one of which not far from the type locality of S. eiseni. The newly obtained COI sequences were compared to barcode sequences of North American Sparganophilus available in BOLD (Barcoding of Life Data Systems; Ratnasingham and Hebert, 2007), with the aims to find: molecular evidence on the identity/separation between S. tamesis and S. eiseni, arguments favouring/disproving recent introduction over old endemic distribution, and information about the possible provenience of the European specimens.