Eurysilenium, M. Sars, 1870

Boxshall, Geoff A., O’Reilly, Myles, Sikorski, Andrey & Summerfield, Rebecca, 2019, Mesoparasitic copepods (Copepoda: Cyclopoida) associated with polychaete worms in European seas, Zootaxa 4579 (1), pp. 1-69 : 23-24

Eurysilenium sp.

Material examined: 1♀ (non-ovigerous) from Harmothoe fragilis, Vigdis, Stn 9-2 (61.3783°N, 02.104723°E), depth 281 m, 11 June 2005; collected by A. Sikorski; NHMUK Reg. No. 2016.545. 1 ovigerous ♀ and 1 mature ♀ attached dorsally on single Harmothoe impar, Fladden Stn 72 (58.063°N 0.550°E), depth 152 m, collected by J.P. Hartley, 23 November 2015. 2 ovigerous ♀♀ (1 with 2 dwarf males attached) located dorsally on H. impar, Fladden Stn 82 (58.351°N 1.218°E) depth 133 m, collected by J.P. Hartley, 23 November 2015. NHMUK Reg. Nos 2018.135-138 .

Differential diagnosis. Ectosoma of adult female from H. fragilis dorsoventrally flattened, heart-shaped in outline, with anterior margin weakly produced anterolaterally into paired lobate shoulders ( Fig. 9A ). Lateral margins of ectosoma converging posteriorly; posterior margin rounded. Length of ectosoma 1.11 mm, width 1.46 mm. Ectosoma with paired genital apertures located ventrally on posterior surface, either side of mid-line. Genital apertures slit-like, with sclerotized opercular plates anterior to slit and narrow sclerotized bar posterior to slit ( Fig. 9A ). Egg sacs unknown. Stalk originating from flat underside in midline, in anterior quarter of ectosoma. Endosoma elongate, circular in cross-section and tapering distally, terminating in globular swelling.

Male small, about 185 µm in total body length including caudal rami, 165 µm excluding caudal rami; maximum width about 65 µm; body indistinctly 6-segmented ( Fig. 9B ), tapering posteriorly, comprising welldefined, subrectangular cephalothorax and 5-segmented post-cephalothoracic trunk. Fourth trunk somite with paired lobate swellings ventrally. Anal somite bearing paired elongate caudal rami, each distinctly hooked at tip; rounded lobes present on somite lateral to bases of caudal rami. Cephalothorax attached to female by ventral surface; with trace of rostrum mid-ventrally in frontal region. Single pair of stylet-like structures present, either side of midline and anterior to mid-length of cephalothorax.

Remarks. The ectosoma of the female from H. fragilis is heart-shaped whereas that of typical E. truncatum ( Fig. 8B, C ) is more rectangular with its lateral margins tapering only slightly. There are also differences in the position and shape of the genital apertures: they are more posteriorly located in E. truncatum than in the female from H. fragilis , and the relative sizes of the anterior and posterior plates framing the slit-like genital openings are different. In the female from H. fragilis the posterior plates are more bar-like compared to the deeper plates of E. truncatum . However, we have only a single female from H. fragilis . There is an additional difference in the male: the body length of the male attached to the female from H. fragilis is 165 µm, excluding the caudal rami, which is smaller than the 250 µm reported for the male of E. truncatum here and by Lützen (1964a).

The Eurysilenium material from H. impar has more strongly produced anterolateral lobes than the material from H. fragilis and tapers more strongly towards the rear. Such variability in form of the ectosoma has been apparent in E. truncatum right from the original description, in which Sars (1870) figured two ovigerous females; one with a heart-shaped ectosoma bearing spherical egg sacs and the other with large anterolateral lobes, more strongly tapering lateral margins on the ectosoma, and bearing kidney-shaped egg sacs attached eccentrically (cf. Sars 1870, figs 16 and 17). These two syntypes look remarkably different from each other.

It is difficult to interpret such differences. It is clear that in herpyllobiids the shape of the ectosoma of the adult female can vary markedly according to their individual reproductive state, so it is unlikely to be reliably diagnostic of species identity. So, despite these differences noted above in material from particular hosts, it will be necessary to obtain more data, preferably including molecular sequence data, before the taxonomic significance of these differences in ectosomal shape can be ascertained.

The females from H. impar are located dorsally on their hosts, attached between setigers R11 and R15. Their ectosomas are orientated at right angles to the host, with the posterior end directed outwards.

Figures

FIGURE 8. Eurysilenium truncatum M. Sars, 1870. A, detached endosoma of female from Eucranta villosa; B, ectosoma of ovigerous female from Eunone sp., dorsal, with cephalothorax of attached male (arrowed) just visible between genital apertures of female; C, same, ventral view with male removed; D, curved egg sac detached from female from Eunone sp.; E, genital apertures of female with male attached, ventral. Scale bars: A, 1.0 mm, B–D, 0.5 mm, E, 200 µm.

FIGURE 9. Eurysilenium sp. from Harmothoe fragilis. A, ectosoma of female, ventral showing male attached between ventrally-located genital apertures of female; B, adult male, dorsal. Scale bars: A, 0.5 mm, B, 100 µm.