Euchonicola parvus, Boxshall & O’Reilly & Sikorski & Summerfield, 2019

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 : 55-56

Euchonicola parvus gen. et sp. nov.

Type material: Holotype ovigerous ♀ from Euchone sp., northern North Sea , NW Hutton Field, BP x 0 54, 10,000N (61.18358°N, 01.30915°E), depth 148 m, summer 2002; collected by P. R. Garwood; NHMUK Reg. No. 2016.589.

Differential diagnosis. Ectosoma of holotype contracted showing apparent segmentation of trunk ( Fig. 24A ). Ectosoma comprising large cephalothorax, 4-segmented trunk and distinct abdomen. Total length of ectosoma 270 µm: cephalothorax about 50 µm long (in dorsal midline) with maximum width of 118 µm, comprising about 19 % of total body length, viewed dorsally. Cephalothorax attached to host via short stalk ( Fig. 24B ) connecting to illdefined discoid endosoma embedded in host ( Fig. 24A ). Post-cephalic trunk subdivided into 3 trunk somites, genital region bearing paired genital apertures, and tapering abdomen. First post-cephalothoracic trunk somite about 45 µm long by 117 µm wide, second 36 µm long by 119 µm wide, third about 66 µm long by 120 µm wide and produced into rounded lobes at posterolateral angles. Genital region about 21 µm long by 84 µm wide (measured at anterior margin). Abdomen tapering posteriorly; about 46 µm long, bearing posteriorly-directed, cylindrical caudal rami. Antennules lobate. Antenna subchelate, with robust basal segment and curved apical claw. Egg sacs uniseriate, each containing 3 large eggs ( Fig. 24A ).

Etymology. The name of the new species alludes to the small size of this species compared with its congeners.

Remarks. The new species can be distinguished from its two congeners (described above) by its relatively small body size. The ectosoma of the ovigerous holotype female is only 270 µm in length compared to 406 to 443 µm and 465 to 530 µm in E. caudatus gen. et sp. nov. and E. linearis gen. et sp. nov., respectively. The body segmentation appears to be better defined than in both other species but this appearance may be an artefact of contraction of the trunk after extrusion of the egg sacs, and might not therefore be a reliable diagnostic character. The abdomen tapers more markedly in E. parvus gen. et sp. nov. than in either E. caudatus gen. et sp. nov. or E. linearis gen. et sp. nov., and the caudal rami have no proximal swelling.

This species is known only from the holotype which was attached to the fifth setiger of its host, an unidentified species of the sabellid genus Euchone . The type locality of E. parvus gen. et sp. nov. in the northern North Sea is only 5 km north of one of the sites from where E. caudatus gen. et sp. nov. was collected. It is possible both species occur on the same host. Small Euchone specimens can be difficult to identify and it was initially thought that the host of E. parvus gen. et sp. nov. and perhaps E. caudatus gen. et sp. nov. might be the undescribed “ Euchone x ” referred to by Cochrane (2003). However, other new Euchone have also been described from the area ( Giangrande et al. 2017) so designating the hosts of these copepods would require careful re-assessment.

Figures

FIGURE 24. Euchonicola parvus gen. et sp. nov. Holotype. A, ovigerous holotype female attached to setiger 5 of host; dorsal view with extent of endosoma within host indicated; B, frontal margin of head showing location of lobate antennule and subchelate antenna relative to stalk. Scale bars: A, 100 µm, B, 25 µm.