Horsehair worms (Phylum Nematomorpha) are worms with life cycles including a parasitic phase in the body cavity of terrestrial insects, mainly beetles, crickets or praying mantids. Few species develop in aquatic insects, e.g. Gordius albopunctatus Müller, 1927 (see Schmidt-Rhaesa & Kristensen, 2006). At maturity, they leave their host and reproduce in freshwater (5 species are marine), where they lay eggs, from which larvae hatch and infect aquatic paratenic hosts (Hanelt et al., 2005; Schmidt-Rhaesa, 2013).

To date, only five studies have examined Nematomorpha from the Iberian Peninsula. Baker (1985) reported a single undetermined specimen; later, De Villalobos et al. (2001) reported four species (Gordius aquaticus Linnaeus, 1758, Gordius plicatulus Heinze, 1937, Gordionus wolterstorffii Camerano, 1888 and Paragordius tricuspidatus Dufour, 1828) and Schmidt-Rhaesa & Cieslak (2008) added two new species: Gordionus barbatus and Paragordionus ibericus. Recently Schmidt-Rhaesa & Martínez (2016) described the species Gordius gonzalezi, from material collected in the region of Extremadura. Gerlach (1978) cited an additional four species from the Iberian fauna but without any further information on the locality of collection, deposition of voucher material or reference. In this note we present a new record of the species Gordius albopunctatus Müller, 1927 to be added to the Iberian fauna.

Material studied: 1 ♂, 1 ♀ of Gordius albopunctatus Müller, 1927. Locality: Rascafría, puente del Perdón, río Lozoya. Madrid. Guadarrama Range, Sistema Central. Date of collection: 22.08.2019. Leg. J.E. Expósito López de Felipe. Det. A. Schmidt-Rhaesa. Deposited in the Zoological Museum Hamburg (ZMH) (accession number ♀: ZMH V13460; accession number ♂ ZMH-V13461). The specimen was collected manually in the riverside and transferred to ethanol (70%). Pieces of the midbody region and the posterior end of the male were dehydrated in an increasing ethanol series, critical point dried and coated with gold in a sputter coater. Observations were made using a LEO scanning electron microscope (SEM) 1524.

The diagnosis of the species is made by a combination of two characters according to Schmidt-Rhaesa & Kristensen (2006): the presence of white dots (hence the specific name, albopunctatus) and polygonal areoles on the cuticle (Fig. 1A-C). The posterior end of the male (Fig. 1D) has the postcloacal crescent characteristic for the genus Gordius. Interestingly, while most species of Gordius develop in terrestrial hosts, this species has been exclusively reported from aquatic hosts, including caddisflies (Trichoptera), especially Potamophylax cingulatus (Stephens, 1837) (Schmidt-Rhaesa & Kristensen, 2006) and Potamophylax latipennis (Curtis, 1834) (in Heinze, 1941 as Stenophylax stellatus Curtis, 1834).

This record notably widens the distribution of this species towards southern Europe (currently is cited in Germany, Poland and Belgium) (Schmidt-Rhaesa, 2010, 2013).

Although the ecological importance of horsehair worms in freshwater ecosystems has been underestimated in many limnological studies, recent studies have shown this group plays a significant role in terrestrial and aquatic ecosystems. Specifically, they are relevant in the energy flow between terrestrial and aquatic trophic nets and in modifying the seasonality of spatial energy subsidies, which can be important for the dynamics of the community (Sato et al. 2008, 2011, 2012; Takimoto et al. 2009).