Trichobranchicola antennatus Boxshall & O'Reilly & Sikorski & Summerfield 2019, gen. et sp. nov.

Trichobranchicola antennatus gen. et sp. nov.

Type material: Holotype ♀ from Trichobranchus sikorskii Leontovich & Jirkov, in Jirkov, 2001, Askeladd Beta, Stn 9-1 (71.47776°N, 20.44262°E), depth 274 m, 27 June 2007; collected by A. Sikorski; NHMUK Reg. No. 2016.573. Paratype ♀ from T. sikorskii, Askeladd Beta, Stn 9-1 (71.47776°N, 20.44262°E), depth 274 m, 27 June 2007; collected by A. Sikorski. Paratype ♀ from T. sikorskii, Snorre B, Stn 6-3 (61.52512°N, 02.228659°E), depth 349 m, 0 7 June 1999; collected by A. Sikorski. Paratype spent ♀ from T. sikorskii, Snøhvit N, Stn SN9-3 (71.49229°N, 21.07569°E), depth 323 m, 19 June 2007; collected by A. Sikorski. Paratype non-ovigerous ♀ from T. sikorskii, Snøhvit SN, Stn 11-2 (71.48632°N, 21.08778°E), depth 326 m, 22 May 2010; collected by A. Sikorski. 2 paratype non-ovigerous ♀♀ from T. sikorskii, Regionale IX, Stn 3 (71.74983°N, 20.00045°E), depth 267 m, 31 May 2010; collected by A. Sikorski. Paratype ♀ from T. sikorskii, Regionale IX, Stn 6 (71.02718°N, 19.6568°E), depth 194 m, 22 May 2010; collected by A. Sikorski. Paratype ♀ from T. sikorskii, Mareano 2010110, Stn 556 (70° 38.38’N, 19° 23.8’E), depth 281 m, 0 4 August 2010; collected by A. Sikorski. Paratype ovigerous ♀ from T. sikorskii, Marulk, Stn MT1-3 - 4 (65.93923°N, 07.560459°E), depth 370 m, 25 May 2012; collected by A. Sikorski. Paratype ♀ from T. sikorskii, Regionale, Stn Reg 18-5 (65.88345°N, 07.850723°E), depth 384 m, 26 May 2012; collected by A. Sikorski. Paratype ♀ from T. sikorskii, Linnorm, Stn LN13 (64.48857°N, 06.794608°E), depth 215 m, 0 7 June 2012; collected by A. Sikorski. Paratype non-ovigerous ♀ from T. sikorskii, Draugen, Stn DG 3-03-4 (64.33839°N, 07.750162°E), depth 275 m, 11 June 2012; collected by A. Sikorski. Paratype spent ♀ from T. sikorskii, Garn Vest, Stn GAW02-5 (64.34654°N, 07.728725°E), depth 263 m, 12 June 2012; collected by A. Sikorski. Paratype ♀♀ NHMUK Reg. Nos 2016.574-587.

Additional material: 1♀ from Trichobranchus glacialis Malmgren, 1866, Vigdis, Stn 4-3 (61.39162°N, 2.079201°E), depth 279 m, 31 May 1999; collected by A. Sikorski. 1 ovigerous ♀ from Trichobranchus sp., Fish, Stn 9-3 (71.77159°N, 21.21137°E), depth 317 m, 23 May 2010; collected by A. Sikorski; NHMUK Reg. No. 2017.166.

Differential diagnosis. Adult female ectosoma highly transformed, comprising anterior cephalothorax, broad trunk and well-defined posterior genitoabdomen (Fig. 16 A–C), connected via narrow stalk to bulla-like endosoma. Cephalothorax comprising projecting frontal region bearing paired antennules distally and antennae near midline (Fig. 16E), and broader posterior region joining to trunk and bearing paired maxillae at base of frontal process (Fig. 16C). Frontal region directed ventrally, specimen figured in Fig. 16C with frontal process displaced into anteriorlydirected position. Mean body length 240 µm (range 203–255 µm, measured in dorsal view, i.e. excluding ventrallydirected frontal region); mean greatest width 189 µm, with a range of 151 to 210 µm (based on 9 specimens). Trunk dorsoventrally flattened, unsegmented, with 2 pairs of lateral setae posteriorly (arrowed in Fig. 16C), possibly representing vestiges of legs 4 and 5. Cement glands paired, extending through posterior third of trunk (Fig. 16B, C). Genitoabdomen much shorter than wide; bearing large, paired genital apertures plus, on posterior margin, small anal prominence with median anal slit located dorsally (Fig. 16A). Antennule cylindrical, located at distal corner of projecting frontal process and armed with single apical seta (Fig. 16D). Antennae located near midline of frontal process between antennules (Fig. 16E); 2-segmented, proximal segment unarmed, distal segment with paired adhesion pads. Stalk narrow, positioned in mid-line close to antennae (arrowed in Fig. 16E). Form of bulla-like endosoma unclear. Maxillae carried on swellings (Fig. 16C) at base of frontal process, unsegmented but distally subdivided into lobate apical pads (Fig. 16F). Egg sacs uniseriate, typically with 2 or 3 eggs per sac; mean egg diameter 85 µm.

Adult male ovoid, indistinctly divided into anterior cephalothorax and posterior trunk. Cephalothorax bearing paired antennules frontally plus antennae; maxillae located laterally. Oral region indistinct. Trunk lacking defined limbs. Caudal rami apparently absent. Antennules lobate, unsegmented, unarmed. Antennae 2-segmented with paired apical adhesion pads. Maxillae with robust proximal segment and tapering distal segment.

Etymology. The specific name refers to the prominent frontal process bearing the paired antennules and antennae.

Remarks. Only a single male was found but it was a collapsed exuvium and in poor condition. It carried three paired appendages and the antennae were of characteristic saccopsine form with paired distal adhesion pads. The maxillae appeared more lateraly located than in Melinnacheres but the oral region was collapsed. No detail could be ascertained of the posterior extremity of the male body. The male was attached adjacent to a genital aperture of the female but did not contain spermatophores. The two pairs of setae located laterally in the posterior region of the trunk of the female are interpreted here as probable vestiges of legs 4 and 5. The new genus is the only member of the family Saccopsidae to retain vestiges of trunk limbs.

Although its type host is Trichobranchus sikorskii, this parasite also occurs on T. glacialis and an unidentified Trichobranchus sp. The length of the female from Trichobranchus sp. was 242 µm, which is the same as that of the material from the type host T. sikorskii, but its greatest width was 235 µm, which falls outside the range given for the type material. This female carried paired egg sacs, one was damaged and incomplete, but the other contained 6 eggs (mean diameter 114 µm), which were larger than those from females on T. sikorskii. The single female collected from T. glacialis was not ovigerous and was subsequently damaged during dissection from the host. The significance of this variation in body proportions and egg size is not yet apparent, and more material is needed.

Currently the known distribution of T. antennatus gen. et sp. nov. is limited to waters of western and northern Norway. The known depth range of this parasite is 194 to 374 m.