Pholoicola, 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 : 24-26

Genus Pholoicola gen. nov.

Diagnosis. Adult female highly transformed and deeply embedded in host with only tip of genitoabdomen protruding through host’s body wall. Body bipartite ( Fig. 10A, B ) comprising large anterior part separated from narrower, tapering posterior genitoabdomen by deep furrow. Anterior part trilobate, with median frontal lobe and paired dorsolateral lobes, containing ovaries, and varying in size with reproductive state of individual female. Genitoabdomen widest at origin on ventral surface of anterior part, narrowing gradually toward rounded posterior extremity bearing paired genital apertures ventrally. Asymmetrical dorsolateral lobes present on genitoabdomen. Conspicuous paired oviducts located within tapering genitoabdomen. No vestiges of any limbs observed; caudal rami lacking; without anus. Egg sacs paired, subspherical ( Fig. 10C ); attached as orientated in Figs 10 B–C.

Adult male body about 122 µm in total length; comprising cephalothorax and ovoid post-cephalothoracic trunk ( Fig. 10D, E ) set at angle to each other. Cephalothorax produced anteriorly into rectangular frontal zone. Single pair of reduced cephalothoracic limbs present, located ventrolaterally on frontal zone; each limb armed with short apical claw. Spermatophore sausage-shaped with long slender sperm tube, attached to body surface of female, adjacent to genital apertures.

Last male copepodid: antennule 5-segmented with aesthetascs on segments 2, 4, and 5; antenna comprising 2 slender segments plus slender apical claw; maxilliped subchelate, comprising small syncoxa, robust basis and slender subchela consisting of proximal endopodal segment plus terminal claw.

Type species: Pholoicola chambersae gen. et sp. nov., by original designation.

Etymology. The name of the new genus is based on Pholoe , the name of the host genus, plus –icola, meaning inhabitant: gender masculine.

Remarks. This genus differs in its fundamental organization from the Herpyllobiidae , Bradophilidae , Phyllodicolidae , and Saccopsidae . Almost the entire body of the transformed adult female is embedded in the host, forming the large lobate endosoma, so that only the tapering genitoabdomen, bearing the paired genital apertures, extrudes through the host’s body wall. The ovaries lie within the main body, with only the oviducts inside the tapering genitoabdomen and the genital apertures located externally at its tip. This body plan differs from the four families listed above, in all of which the endosoma and ectosoma are connected by a well-defined stalk that passes through the host body wall, and in which the ovaries are entirely contained within the ectosoma. Almost the entire body of female Pholoicola gen. nov. is located within the host and there is no defined stalk. It is possible that the anterior trilobate part represents an ancestral endosoma and the posterior genitoabdomen represents an ancestral ectosoma, in which case the constriction between these two parts would represent the original stalk, but more detailed anatomical studies are necessary before any such system of homologies can be robustly established. There is an important difference between the last male copepodid stage of Pholicola gen. nov. and the herpyllobiids, namely that it retains a clawed antenna located adjacent to the base of the antennule. There is no trace of an antenna in the male copepodids of herpyllobiids ( Lützen 1968).

The new genus exhibits some similarities to the recently established Jasmineiricolidae in which the embedded endosoma is much larger than the ectosoma and contains the ovaries, and there is no defined stalk ( Boxshall et al. 2015). However, the Pholoicolidae differs from the Jasmineiricolidae in the lack of a functional anus, in the lack of subchelate maxillipeds in the adult female and in the lack of the rosette-like array of eight slender lobes on the ventral surface of the cephalothorax, which are probably derived from the mouthparts. The ovaries of the Pholoicola gen. nov. are located anteriorly, close to the constriction where the tapering genitoabdomen merges with the endosoma, whereas in the Jasmineiricolidae the ovaries are located anterolaterally within the postcephalic trunk and lateral trunk lobes.

The male of Pholoicola gen. nov. is highly reduced. The paired limbs retained by the male are probably the antennae, given their frontal position on the cephalothorax and their subchelate structure. They appear to be used to secure the male to the female. We did not find any males containing spermatophores ready for deposition. However, a pair of early developing spermatophores containing packed filiform sperm, was located in the cephalothorax and anterior part of trunk, and we consider it likely that the spermatophores are deposited anteriorly (as in Herpyllobius ).

Figures

FIGURE 10. Pholoicola chambersae gen. et sp. nov. A, adult female with 3 males attached, ventral; B, female, dorsal; C, detached egg sac; D, adult male, lateral view showing position of developing paired spermatophores; E, adult male, ventral; F, cephalothorax of male, lateral view showing thickened frontal region and clawed limb; G, spermatophore; H, last male copepodid stage, antennule; I, antenna; J, maxilliped. Scale bars: A, 0.5 mm, B–C, 250 µm, D–E, 50 µm, F–H, 25 µm, I–J 10 µm.