Population genetic analysis of Ensis directus unveils high genetic variation in the introduced range and reveals a new species from the NW Atlantic

Abstract

The final publication is available at Springer via https://doi.org/10.1007/s00227-012-2006-6. How to cite:Vierna J, Jensen KT, González-Tizón A, Martínez-Lage A (2012) Population genetic analysis of Ensis directus unveils high genetic variation in the introduced range and reveals a new species from the NW Atlantic. Mar Biol 159: 2209-2227. AbstractWe report current genetic variation of populations of the razor shell Ensis directus (Conrad 1843) (Mollusca: Bivalvia: Pharidae) in native (North American) and introduced (European) ranges using nuclear and mitochondrial sequence-based markers. We expected less variation within the introduced range, especially considering the frequent mass mortality events observed in Europe since the species was recorded for the first time in 1978. However, we found higher variation in Europe. The possible significance of temporal fluctuations of genetic variation, limited effect of random genetic drift, and multiple introductions are discussed. Interestingly, the multiple-introduction hypothesis contrasts with the gradual colonisation of European coastal waters but is supported by trained clustering analysis and by the intensity of transatlantic shipping. Genetic and morphometric evidence strongly supports that examined individuals from a supposed E. directus population from Newfoundland (Canada) belong to a separate species. This new Ensis is formally described here and named E. terranovensis n.sp.In this work, we have studied nucleotide variation at four sequence-based molecular markers, including mitochondrial, nuclear multi-copy, and nuclear single-copy regions in order to: (1) compare current genetic variation in native and introduced ranges to assess to what extent potential bottlenecks and mass mortality events in Europe have impacted diversity, (2) obtain information about the possible origin of European individuals, and (3) study population structure in the native range of the species. Besides, we serendipitously detected a very divergent population from Conception Bay, Newfoundland (Canada) that is proposed to be a new Ensis species based on genetic and morphometric evidence. This new Ensis species from the NW Atlantic is formally described here. Materials and methods Specimens and lab proceduresWe studied a set of 148 razor shells from native and introduced sites (see Table 1; Fig. 1 for details). Razor shells were preserved in 100 % ethanol, and they were identified in the lab as E. directus according to shell morphology (Cosel 2009). Identifications of some of the specimens (Table 1) were confirmed by Rudo von Cosel (Muséum national d'Histoire naturelle, Paris, France) and included in his article on the taxonomy of Atlantic Ensis (Cosel 2009). One individual from the related species E. minor Dall, 1899 (collected off Christmas Bay, Texas, USA) was included as outgroup in some of the analyses.

Climate-induced range shifts of the American jackknife clam Ensis directus in Europe

“…However, differences were observed on modelled niches and spatial projections. Limits of the ecological niches modelled with both ENMs (minimum and maximum thermal tolerances and bathymetric preference) do not differ greatly and are consistent with the ecological knowledge of the clam (Cardoso et al 2006(Cardoso et al , 2009Dannheim and Rumohr 2012;Dauvin et al 2007;Dekker and Beukema 2012;Freudendahl et al 2010;Ghertsos et al 2000;Palmer 2004;Vierna et al 2012;Beukema and Dekker 1995). The main differences between NPPEN and MAXENT results are generated by the shape of the ecological niche (fringe of the niche steeper with MAXENT than with NPPEN).…”

“…the tendency of a species to retain ancestral ecological characteristics (Wiens and Graham 2005). Genetic variations of E. directus in native and introduced sites were recently studied by Vierna and colleagues (Vierna et al 2012). Against all expectations, they found higher genetic variation at mitochondrial and nuclear markers in the introduced area (Europe) than in its native range (North America).…”

“…The mollusk subsequently colonized rapidly the surrounding German coasts (Von Cosel et al 1982;Mühlenhardt-Siegel et al 1983;Swennen et al 1985), reached Denmark andNetherlands in 1982, Belgium in 1986 and France in 1991 (Luczak et al 1993). The American jackknife clam was also observed along several coastlines in England (Von Cosel 2009), in south Wales (Vierna et al 2012) and in the Cantabrian Sea (Arias and Anadón 2012). The dispersal of E. directus is facilitated by an actaeplanic free-swimming larval stage (between 2 and 4 weeks), which allows the species to reach a distance up to 125 km from the source population (Armonies 2001).…”

“…In the regions where E. directus was introduced, interspecific competition with their native counterparts occurs for food and space. The species presents frequent mass mortality events (Vierna et al 2012;Cadée et al 1994) during which dead razor clams accumulate on beaches and become an important source of food for seabirds (Freudendahl et al 2010;Tulp et al 2010). The decomposition of organisms has a negative effect on tourism (odors and sharp shells on the sand) and can damage fishermen's trawls (Dewarumez et al 2011).…”

Species delimitation and DNA barcoding of Atlantic Ensis (Bivalvia, Pharidae)

“…), where it now occurs from the North Sea to the Cantabrian Sea (Arias & Anadón ; Vierna et al . ). Another representative from Atlantic North America is the recently discovered E. terranovensis Vierna & Martínez‐Lage, , which was found off Newfoundland (Canada) and whose possible co‐occurrence with E. directus is still unknown (Vierna et al .…”