Herpyllobius arcticus Steenstrup & Lütken, 1861

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 : 10-14

Herpyllobius arcticus Steenstrup & Lütken, 1861

Syn: Sarsilenium crassirostris Leigh-Sharpe, 1926

Norwegian material examined: 1♀ with ♂ attached, from Harmothoe fragilis Moore, 1910 , Huldra, Stn 1-5 (60.85328°N, 02.65085°E), depth 121 m, 23 May 2004; collected by A. Sikorski; NHMUK Reg. No. 2015.2954 . 2♀♀ on 2 specimens of H. fragilis, Kakshauger, Stn 16-2,4 (59.7336°N, 02.556497°E) depth = 100m, 12 June 2000; collected by A. Sikorski; NHMUK Reg. No. 2015.2955-2956. 1 immature ♀ from H. fragilis, Statfjord C, Stn 6-3 (61.23832°N, 02.043427°E), depth 140 m, 15 June 2005; collected by A. Sikorski; NHMUK Reg. No. 2015.2957. 1 immature ♀ with 3 ♂♂ attached, from H. fragilis, Gullfaks Satelitter, Stn F /G2-4 (61.0931°N, 02.27979°E), depth 133 m, 0 2 June 2005; collected by A. Sikorski; NHMUK Reg. No. 2015.2958 . 1♀ from Antinoe sp., Snorre, Stn Ref 10-8 (61.5115°N, 02.007002°E), depth 311 m, 0 3 June 1999; collected by A. Sikorski; NHMUK Reg. No. 2015.2959 . 1♀ from Harmothoe antilopes McIntosh, 1876 , Statfjord N, Stn Ref 8-3 (61.4815°N, 01.852°E), depth 266 m, 0 8 June 1999; collected by A. Sikorski; NHMUK Reg. No. 2015.2960 .

British material examined: 2♀♀ from Harmothoe impar , Belfast Lough, North Channel Sludge Disposal Site, Stn BL 1.2, (54 o 44.98’N, 0 5 o 33.06’W), depth 64 m, 27 March 1985; collected by Matt Service. 2 ovigerous ♀♀ located laterally on tail of Gattyana cirrhosa fragment, Firth of Clyde, Irvine Bay, SEPA Stn Z (55° 34.75’N, 04° 45.20’W), depth 40 m, September 1987 (see O’Reilly et al. 2011), NHMUK Reg. Nos 2017.481-482. 2 ovigerous ♀♀ from H. impar, Deepwater Sludge site, Belfast Lough (54° 45’18”N, 05° 29’36”W), depth 85.5 m, 30 March 2004; collected by Unicomarine. 1♀ from H. impar, Unico 38842, AFBI NI, NMMP, Stn NMP2e (54° 59.977’N, 5° 33.357’W), depth 120 m, 12 February 2006. 1 immature ♀ from H. extenuata, Unico – 39938, EHS - SSDM06 Survey - Belfast Lough / North Channel, Stn DGBc (54° 45.050N, 05° 29.582W, depth 86m.) 0 8 September 2006. 1 immature ♀ from H. impar, Unico 39956, EHS - SSDM06 Survey - Belfast Lough / North Channel, Stn DGR1c (54° 47.683N, 05° 33.025W), depth 83.5 m, 0 8 September 2006. 1 ovigerous ♀ from Harmothoe sp. (Unico 39960), Belfast Lough / North Channel, EHS-SSDM06 Survey, Stn DGR2-b (54° 43.537N, 05° 28.395W), depth 87.4 m, 0 8 September 2006. 1♀ from Harmothoe sp., Unico 42471, Survey EHS-SSDM07, Belfast Lough / North Channel, Stn DGM-a (54° 45.00’N, 05° 29.59’W), depth 86 m, 11 April 2007. 1 ovigerous ♀, from Harmothoe sp., Unico. 44856, EHS-FEPA08, Stn SDCS-a, (54° 50.51’N, 05° 42.85’W), depth 17 m, 0 8 April 2008. 1 ovigerous ♀ from Harmothoe ? impar, Unico 46998, Survey EHS-FEPA09, Belfast Lough/North Channel, Stn DEM-b, (54° 45.00’N, 05° 29.59’W), depth 86 m, 24 February 2009. 1♀ from Harmothoe sp., Unico 47568, Mourne Coast, EHS Stn ANA-C (54° 44.12’N, 05° 55.04’W), depth 19 m, 0 5 May 2010. NHMUK Reg. Nos 2018.112-121.

Differential diagnosis based on females from Harmothoe fragilis . Adult female ectosoma almost spherical; mean diameter 1.08 mm, ranging from 0.95 to 1.20 mm in ovigerous females. Genital swellings small, subrectangular (220 to 240 µm in height, by 165 to 170 µm wide) separated by gap of about 75 µm. Four minute sclerotized dots arranged in arc on postero-dorsal surface, above genital swellings. Egg sacs short and thick (1.00 mm in length by 0.56 mm in diameter), typically as long as or just longer than mean diameter of ectosoma. Stalk originating from underside, just anterior to genital swellings. Endosoma forming irregular 3-dimensional mass produced into processes of varying size.

Final copepodid stage of male (from immature female on H. fragilis ): body cyclopiform ( Fig. 2A ), total length 246 µm, measured from tip of frontal process to posterior margin of caudal rami; maximum body width 80 µm. Prosome comprising cephalosome plus 3 pedigerous somites: cephalosome with tapering frontal process (attachment structure of adult male) protruding through frontal margin just dorsal to base of rostrum. Rostrum ( Fig. 2B ) weakly developed, wider than long, with weakly concave tip. First urosomite bearing rigid seta at each posterolateral corner; second urosomite less than half length of anal somite. Anal somite elongate, about twice as long as wide and bearing paired caudal rami; each ramus about twice as long as wide and armed with 3 plumose setae.

Antennule 3-segmented ( Fig. 2C ), setal formula: 4 + ae, 1 + ae, 5 + ae. Antenna absent. Oral region located immediately behind rostrum on ventral surface of cephalosome; defined by complex arrangement of paired chitinous elements of obscure homology. Maxilliped ( Fig. 2D ) comprising short syncoxa, elongate basis and distal subchela consisting of unarmed endopodal segment plus long curved apical claw. Legs 1 to 3 biramous, coxae of each joined by intercoxal sclerite; basis unarmed, with protruding, rounded inner margin: legs 1 ( Fig. 2E ) and 2 each with 2-segmented exopod and 1-segmented endopod: first exopodal segment unarmed, second with 4 setae; endopod with 3 setae in both legs. Leg 3 ( Fig. 2F ) with 1-segmented rami: exopod with 4 setae, endopod with 2 setae. Leg 4 represented by single rigid seta at corner of first urosomite ( Fig. 2A ).

Remarks. The ovigerous females from H. fragilis are smaller than the material examined by Lützen (1964a) which typically had an ectosomal diameter in the range of 1.2 to 1.5 mm, although one specimen was 1.9 mm. Lützen did not report the host of his material although he noted that the most common host in northern Atlantic waters was Harmothoe imbricata . Lützen (1964a) also reported that the other known hosts included H. extenuata , H. impar , Austrolaenilla mollis and Gattyana cirrhosa . Two of the hosts reported here, H. fragilis and Antinoe sp., are new. This species has often been confused with H. polynoes but Lützen (1964a) unravelled the convoluted nomenclatural history and provided full synonymies for both species.

This species typically attaches to the side of the host in the anterior part of the body (commonly setigers 2 to 10 according to Lützen (1964a)), although O’Reilly et al. (2011) reported two females on the posterior setigers of Gattyana cirrhosa . The location on the host of the new material examined here varied from setiger 4 to setiger 20. A virtual horizontal section along a polychaete host infected with an H. articus reveals the extent of the endosoma located within the coelom of the host and visible on both sides of the gut ( Fig. 4 ).

The final copepodid stage of the male enclosed a fully formed adult male (stippled in Fig. 2A ) which was attached to the female by the tip of the frontal process extending out through an opening in the copepodid stage exuvium. This process is the “neck” of the bottle-shaped adult male. The paired spermatophores of the adult male were visible through the exuvium of the preceding copepodid and occupied most of the space within the prosome. The copepodid of H. arcticus is very similar to that of H. polynoes as figured by Lützen (1968). The setation of the apical segment of the antennule carries 4 setae plus an aesthetasc in the latter species ( Lützen 1968, fig. 3b) compared to 5 setae plus an aesthetasc in H. arcticus ( Fig. 2C ). The segmentation and setation of legs 1 to 3 is the same in both species except Lützen (1968, fig. 3e) shows only 1 seta on the endopod of leg 2 whereas 3 are present in H. arcticus . These two minor setation differences may be the result of damage caused to the fragile setal elements during dissection.

Description of female from Harmothoe antilopes : ectosoma of ovigerous female laterally compressed with deeply furrowed surface ( Fig. 3A ): anterior part of ectosoma narrowing and directed ventrally; posterior margin bearing large paired genital swellings ( Fig. 3B ). Ventral surface flattened; dorsal surface expanded and rounded. Cement glands paired, located laterally about in middle of body; gland strongly curved ( Fig. 3A ). Genital apertures about 320 µm high and 194 µm wide ( Fig. 3B ). Egg sacs multiseriate, cylindrical, about 948 µm long and 500 µm in diameter. Stalk about 112 µm in diameter, located just posterior to mid-level of ectosoma. Endosoma ( Fig. 3A ) with basal mass extending into digitiform processes of different sizes; largest processes about 750 µm in length.

Description of male from Harmothoe antilopes : Adult male attached by means of frontal process to posterior surface of female, dorsal to genital apertures (arrowed in Fig. 3A, B ). Body bottle-shaped ( Fig. 3C ), about 206 µm in total length, comprising slender frontal process (“neck” of bottle) and rounded posterior part containing 2 spermatophores. Each spermatophore about 100 µm long and 45 µm in maximum width, with anteriorly directed tube extending towards frontal process.

Remarks. The single female from H. antilopes was attached laterally, close to the rear end of its host. It is ovigerous and has a bilaterally compressed ectosoma with a deeply-furrowed surface, suggesting that it had only recently extruded the paired egg sacs. The “spent” state of the female renders shape comparisons problematic. However, it differs from H. polynoes in its location on the host, since H. polynoes always attaches dorsally to the prostomium of its host ( Lützen 1964a). This female has a broad stalk and a massive endosoma bearing digitiform processes whereas H. polynoes has a slender stalk and its endosoma is always flattened ( Lützen 1964a). The other common species in Scandinavian waters is H. arcticus although this species rarely attaches to the posterior part of the host. This female is similar to H. arcticus in the form of the endosoma: its body length of about 1 mm is smaller than typical for H. arcticus but the difference might be due in part to its reproductive state. There are some differences such as the slightly larger diameter and slightly more anterior location of the stalk compared to typical H. arcticus (as redescribed by Lützen 1964a), However, all of these differences, as well as the relatively large size of the genital apertures of this specimen, could reflect the “spent” reproductive status of the female. Therefore, this female, despite its unusual location near the rear end of its host, is tentatively identified as H. arcticus .

Figures

FIGURE 2. Last male copepodid of Herpyllobius arcticus Steenstrup & Lütken, 1861 from female on Harmothoe fragilis. A, exuvium of last copepodid stage containing adult male (stippled), showing outline of spermatophores, dorsal; B, rostrum, ventral; C, antennule; D, maxilliped, E, leg 1 and intercoxal sclerite; F, leg 3 and intercoxal sclerite. Scale bars: A, 100 µm, B, D, 10 µm, C, E–F, 25 µm.

FIGURE 3. Herpyllobius arcticus Steenstrup & Lütken, 1861 from Harmothoe antilopes. A, habitus of ovigerous female with male attached (arrowed), lateral view showing position of cement glands (dotted); B, ectosoma of female, posterior view showing attachment site of male (arrowed) dorsal to genital apertures; C, adult male containing paired spermatophores. Scale bars: A, 0.5 mm, B, 250 µm, C, 100 µm.

FIGURE 4. A virtual horizontal section through polychaete host showing endosoma (arrowed) of adult female of Herpyllobius arcticus Steenstrup & Lütken, 1861 either side of host gut within coelomic cavity. Scale bar: 1 mm.