Xenocoeloma Caullery & Mesnil, 1915

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 : 60-61

Genus Xenocoeloma Caullery & Mesnil, 1915

Diagnosis. Adult female body ovoid to cylindrical, highly transformed and lacking traces of external segmentation. Body up to about 6 mm in length. Anterior end attached to host body wall; anterior axopore connecting central axial cavity (axocoel) of copepod with host body cavity. Limbs lacking. Posterior end of body with integumentlined, common genital atrium containing paired oviduct openings and opening to exterior via single atriopore. Egg sacs paired, elongate, multiseriate. Mature adult male reduced to testicular tissue housed within median receptaculum masculinum of female.

Type species: Xenocoeloma brumpti Caullery & Mesnil, 1915 .

Remarks. Xenocoeloma currently comprises two species: Xenocoeloma alleni ( Brumpt, 1897) , a parasite of Polycirrus caliendrum Claparède, 1869 and P. aurantiacus Grube, 1860 , and X. brumpti , a parasite of P. arenivorus ( Caullery, 1915) . Caullery & Mesnil (1919) explored in detail the morphology and internal anatomy of X. brumpti and presented comparisons with X. alleni . They considered that the strongest argument for separating X. brumpti as a distinct species was its utilization of a different host from X. alleni . This argument is largely based on an assumption of strict host specificity which was prevalent at the time. These species utilize congeneric hosts and both were originally described from opposite shores of the English Channel, X. alleni from Plymouth on the coast of the United Kingdom ( Brumpt 1897), and X. brumpti from the vicinity of Luc-sur-Mer on the coast of Normandy, France ( Caullery & Mesnil 1919).

There are morphological differences between the two species: in X. brumpti the length of the adult female is 5 to 6 mm and the width (diameter) is 1.25 mm, giving a length:width (L:W) ratio of about 4.0–4.8:1, whereas X. alleni has a length of 4 mm and a width of 2 mm, giving a L:W ratio of about 2:1 ( Caullery & Mesnil 1919). Bocquet et al. (1965) studied Xenocoeloma parasitic on Polycirrus caliendrum collected at Roscoff: their specimens measured 2.3 mm in length and 1.2 mm in width (after fixation in Bouin’s) and they identified them as X. alleni on the basis of this L:W ratio of 1.94:1. Differences in coloration have also been reported between these two species (see Bocquet et al. 1965), but the summary of coloration patterns given by Gotto (1993) highlights the variability in both body and egg sac coloration. Bocquet et al. (1970) studied the development of the genital system in X. alleni .

Our material includes Xenocoeloma specimens from six different polychaete hosts taken in Norwegian waters, all members of the family Terebellidae . Five of these hosts, Amaeana trilobata (M. Sars, 1863) , Polycirrus norvegicus Wollebaek, 1912 , P. latidens Eliason, 1962 , P. medusa Grube, 1850 , and P. plumosus (Wollebaek, 1912) , are all members of the subfamily Polycirrinae , while the other host is Paramphitrite birulai (Ssolowiew, 1899) , a member of the subfamily Terebellinae . It is possible that cryptic species occur on different hosts. Among hosts of the genus Polycirrus Grube, 1850 the situation is compounded by unresolved taxonomic confusion regarding species found in European waters ( Glasby & Hutchings 2014).