Progonus muelleri (Levinsen, 1881) and Derogenes varicus sensu lato

The monotypic genus Progonus was erected by Looss [38] for P. muelleri, first described from waters off Greenland [35]. The two genera Derogenes and Progonus are very similar with the only difference being the presence of a cyclocoel in P. muelleri vs. blindly ending caeca in species of Derogenes [19, 35, 49]. We examined body sections of representatives of both genera, mainly D. varicus s. s. from Limanda limanda (Fig. 5C) and P. muelleri from M. scorpius and the two species can also be readily distinguished by the seminal vesicle, located in the anterior third of the midbody of D. varicus s. s. vs. in the posterior third of the midbody in P. muelleri . Additionally, the pars prostatica is far shorter in P. muelleri (see Fig. 5).

We examined additional specimens of Derogenes spp. from T. Odhner’ s collections at the Invertebrates collection in the SMNH (Figs. 9 and 10), all identified and labelled as D. varicus, which we consider herein as representative of the D. varicus species complex or D. varicus s. l. The hosts were Argentina sphyraena, Brosme brosme, Molva molva, and Platichthys flesus . Corresponding morphometrical data are indicated in Table 7. Overall, specimens from the previously mentioned hosts share the same anatomical features, concerning the organisation of gonads and genital terminalia (Figs. 9B, 9D, 9F, 9H, 10B). Derogenes varicus s. l. from the previously mentioned hosts share the sausage-shaped appearance, except D. varicus s. l. from B. brosme that has a stockier appearance. The testes in D. varicus s. l. from A. sphyraena and the one from H. hippoglossus differ by having more longitudinally elongated testes. The most striking one is D. varicus s. l. from A. sphyraena that also differed slightly by the organisation of the genital terminalia (Fig. 9B), the shape of vitelline masses being larger and elongated, by having a smaller body and smaller organs but the measurements of eggs overlapped. However, the number of specimens measured from T. Odhner’ s collection is low and thus we refrained from comparing morphometrical values. Since we are attempting to delineate species within the D. varicus species complex using integrative taxonomy, and as molecular sequence data are lacking for Derogenes from A. sphyraena, B. brosme, M. molva, and P. flesus, we consider Abbreviations: NEA, Northeast Atlantic.

specimens from the previously mentioned hosts as D. aff. varicus or D. varicus s. l., pending further examinations.