Xenocoeloma orbicularis Boxshall & O'Reilly & Sikorski & Summerfield 2019, sp. nov.

Xenocoeloma orbicularis sp. nov.

Type Material: Holotype ovigerous ♀ attached to head of Paramphitrite birulai (Ssolowiew, 1899), Statfjord B, Stn 3-5 (59.1735°N, 01.843177°E), depth 144 m, 20 June 1999; collected by A. Sikorski; NHMUK Reg. No. 2015.3035.

Differential diagnosis. Female body orbicular with flattened underside (Fig. 27D, E); maximum length 0.87 mm, maximum width 0.95 mm, maximum height in lateral view 0.80 mm. Body lacking traces of segmentation and without vestiges of paired limbs. Egg sacs paired, both egg sac stalks originating from common genital aperture located posterodorsally on body (Fig. 27E); egg arrangement multiseriate; egg sac length unknown (both sacs incomplete in holotype). Body attached to host via broad, featureless stalk, located on ventral surface just anterior to middle of body. Colour of body white.

Etymology. The name of the new species refers to the shape of the ectosoma of the mature female.

Remarks. This unusual parasite is placed in the Xenocoelomidae because it shares the possession of a posteriorly-located common genital atrium, within which both egg sacs originate. It is placed in Xenocoeloma because the body is carried external to the host. The body of the ovigerous female differs markedly in proportions from the two previously described species of Xenocoeloma: in X. brumpti and X. alleni the L:W ratio of the cylindrical ectosoma of the adult female is 3.42 to 4.8:1 and about 1.75 to 3.39:1, respectively, whereas in the new species the body is wider than long and is orbicular rather than cylindrical. The establishment of a new species to accommodate this single specimen should be treated as a working hypothesis to be tested when more material and molecular data are available.

The host of the new species belongs to the subfamily Terebellinae and, given that all other known hosts of Xenocoeloma species belong to the subfamily Polycirrinae, the utilization of a host from a different subfamily could be interpreted as supporting evidence justifying the establishment of the new species. However, as noted below, another xenocoelomid, Aphanodomus terebellae, has now been reported from six different hosts also representing two subfamilies of Terebellidae.