Record of the ‘Arctic’ marine gastrotrich Diuronotus aspetos (Paucitubulatina) from the southern North Sea

Abstract

A decade ago, Todaro et al. (2005) described a conspicuous genus of marine mesopsammic Gastrotricha. Both known species of Diuronotus are characterized by the possession of one pair of primary plus one pair of secondary adhesive tubes on their furcal appendages. Diuronotus and a second taxon, Musellifer, are both in the family Muselliferidae based on the shared presence of an elongate, ciliated snout-like region called the muzzle and the peculiar ultrastructure of their spermatozoa (Leasi & Todaro 2008, Balsamo et al. 2010). One specimen of Diuronotus aspetos Todaro, Balsamo & Kristensen, 2005 was encountered in sublittoral sand sampled on May 3, 2012 with a Van Veen grab deployed aboard RV Senckenberg at the shoal Langes Riff, northward island Wangerooge, North Sea (N53°49.640′ E007°50.207′), at a depth of 15.5 m. Important morphometric measures are as follows: 436.7 μm total body length, 117 μm pharynx length, pharyngeo-intestinal junction at U26.8 (Fig. 1a, c). Compared to the original description, these lower values are related to the adolescence of the specimen (reproductive organs were absent). However, species-specific traits were still evident such as the keeled, hemi-elliptic scales with a concave posterior edge (Fig. 1b), a mouth with protrusible processes (Fig. 1c, see also Balsamo et al. 2010), and pairs of long (37 μm) secondary and short (14.5 μm) primary adhesive tubes (Fig. 1a, d–f). The two described species of Diuronotus have so far only been recorded from a very limited geographic distribution. D. rupperti Todaro, Balsamo & Kristensen, 2005 is reported from a beach on the island Laeso, Denmark, while D. aspetos was recorded twice (in 1979 and 2006) from shallow sublittoral sand near the forsaken settlement Skansen on Disko Island, Greenland (Fig. 1g, Todaro et al. 2005, Balsamo et al. 2010). A third putative species of Diuronotus possibly inhabits sediments of the east coast of North America (Fig. 1g, see discussion in Todaro et al. 2005). Currently, D. aspetos is hypothesized to represent a ‘High Arctic’ species (Todaro et al. 2005). The new record from the North Sea (Temperate Northern Atlantic realm, see Spalding et al. 2007) indicates i) a wider geographic distribution and ii) a wider temperature tolerance of D. aspetos. However, future surveys have to demonstrate if stable populations of this species exist beyond the Arctic marine realm.