Gordius aquaticus Linnaeus
Diagnosis. – Dimensions as in table 1, from 75 to 987 mm. Male: body colour uniform pale brown (the specimens studied had no pale spots or dark ring behind the calotte, but see comments in Schmidt-Rhaesa, 1997). Anterior end acuminate, with subterminal mouth opening. Cloacal opening oval. Postcloacal crescent with a total length from 62.2 to 723.3 µm, and a maximum width from 12.5 to 87.0 µm in the specimens studied. Posterior lobes short and broad, between 42.3 × 33 and 400 × 73.3 µm (Fig. 1A); apex round, with sparse spiniform processes 15 µm long in the inner area of the lobe, no bristles or other spiniform structures on the lateral sides of the lobes (Fig. 1B). Cuticle smooth, only marked with fine subcuticular lines (Fig. 1C), with scattered acuminate tubercles no higher than 5.3 µm.
Fig. 1. Gordius aquaticus, male. A, terminal lobes and postcloacal crescent; B, apex of the terminal lobes; C, cuticle. b, bristles; co, cloacal opening; l, lobes; pc, postcloacal crescent; t, tubercle. Scale bar of A= 1.000 µm; B= 100 µm, C= 10 µm.
Female: anterior end acuminate. Posterior end not bilobed, with a central cloacal opening. Cuticle as in males. See tables 1 and 2 for the material studied.
Remarks. – The Iberian specimens of G. aquaticus differ in some characters with respect to the description given in Schmidt-Rhaesa (1997) (e.g. cloacal opening not round, with some tubercles and bristles on the tail lobes, only visible using SEM, Fig. 1B). We consider these differences attributable to incomplete descriptions or to intraspecific variation. The identity of our specimens as G. aquaticus was established based on the smooth cuticle and the type of post-cloacal crescent, characters which present a higher constancy.
There is a strong length variation among the specimens studied, even among specimens from the same locality (Table 2). It is unknown if this variation corresponds to differences in the host or to other environmental or genetic factors (although previous studies suggest that there is no correlation between the size of the host and the length of the parasite, Schmidt-Rhaesa, 1997; de Villalobos et al., 1999). There are no significant length differences between the measured males and females (as measured with a two-tailed t-Student, p > 0.5).
The species is distributed in western and central Europe (excluding UK) (Schmidt-Rhaesa, 1997).
Ecological notes. – Most of the specimens were collected either in ponds in a mountain pasture (with small streams nearby) (locality 4) or in a mountain stream (locality 9), on different types of substratum and vegetation (Table 2). Known hosts include large species of Dytiscidae and Carabidae (Coleoptera) (Schmidt-Rhaesa, 1997).