Pholoe inornata, JOHNSTON, 1839

PHOLOE INORNATA JOHNSTON, 1839 View in CoL

( FIGS 4, 7)

Pholoe inornata Johnston, 1839: 437–438 View in CoL , pl. XXIII, figs 1–5; Chambers, 1985: 19–20, figs 13a–b, 18a–d, pl. A: 1–2, pl. B: 1–2.

Pholoe synophthalmica Claparède, 1868: 389 View in CoL , pl. III, fig. 1.

? Pholoe minuta McIntosh, 1900 View in CoL : pl. XXXIV, figs 16, 17 (in part; mix of several species in the publication). Not: Pholoe minuta Hartmann-Schröder, 1971: 78 View in CoL , fig. 24a–d.

Type locality: UK, England , Berwick Bay , amongst c o n f e r v a e b e t w e e n t i d e m a r k s. A c c o r d i n g t o Chambers (1985): Berwick Bay, Berwick on Tweed, Northumberland, England .

Non-type material: North Atlantic Ocean: UK, St. Baldred’s Cradle, East Lothians, Tunninghame, 56°01’25”N, 02°34’57”W (56°1.4167’N, 2°34.950’W), 2 October 1974, intertidal, 2 af, 1 pf (NHMD-298021), Cullercoats, Dove, 19 March 1984 (NHMD-298020). Skagerrak: N Jütland, Skarreklit, 57 ⁰09’33”N, 09 ⁰01’23”E (57°9.550’N, 9°1.383’E), 20 September 1968, depth ~ 2 m, between red algae and Electra , 6 complete, 3 af, 4 mf, 3 pf (NHMD-298018), Koster Fjord, off Råssö, S of Tjärnö, Klinken (= Pomatoceros reef), depth 15 m, dredge, 31 July 1985, 1 complete, 3 af, 2 pf, 2 mf (NHMD-298019); Baltic Sea, Flensburg Fjord, 54°47.12’N, 9°58.57’ E, 3 July 1996, depth 15 m, sand, 2 complete, 3 af, 1 mf SEM ( ZSRO-P 366), 54°47.1’N, 9°58’E, 2 November 2017, depth 10 m, sand, 2 af ( ZSRO-P 2515), Fehmarnsund, 54°22’N, 11°15.5’E, 25 October 2017, depth 17 m, sand, 1 complete ( ZSRO-P 2514); Mediterranean Sea, Adria: Croatia, Šipan, Šipanska Luka Bay, 9 October 2001, depth 18 m, between serpulid tubes, 1 af, 1 mf ( ZSRO-P 1166). Specimens collected for DNA work, fixed in 96% ethanol, morphologically examined: Great Britain: Plymouth, The Sound, 50.358333, -4.48333 (50°21.500’N, 4°28.999’W), 16 March 2011, depth 10–15 m, coarse shell gravel ( ZMBN 127117, 127120). Sweden: Koster Area, Klinken, 58.8616667, 11.1972222 (58°51.700’N, 11°11.833’E), April 2015, depth 2–4 m, mud, soft bottom; Pomatoceros reef ( ZMBN 127124).

Diagnosis: Specimens are pale to strongly pigmented, often with orange pigment on the ceratophore of the median antenna; two pairs of closely set black eyes, often with anterior and posterior pair fused; dorsal and ventral tentacular cirri with three to five distinct spine-like papillae; mid-dorsum in larger specimens not completely covered by elytra; elytral papillae long and slender, distally capitate, elongated on posterior elytra; facial tubercle absent; lateral antenna absent; parapodia with only few long and few short simple papillae, long terminal papillae (stylodes) absent; neurochaetae heterogomph compound chaetae with one row of short teeth along the blade (only seen in

SEM).

Description: Largest complete specimen fixed in formalin with about 44 chaetigers 6 mm in length and 1.0 mm wide. Other examined specimens mostly incomplete, between 4.1 and 5.3 mm long and 0.6– 1.0 mm wide, with 34–44 chaetigers.

Body short, linear, depressed ( Fig. 4D); ventral surface with evenly distributed short papillae, denser on the anterior two thirds of the body than posteriorly. In larger specimens, elytra of the middle body region leaving a narrow mid-dorsal gap along the body uncovered ( Fig. 4D), but in smaller specimens, mid-dorsum completely covered by elytra. First pair of elytra rounded ( Fig. 4G), in succeeding segments reniform and anteriorly notched, in posterior segments transversally elongated ( Fig. 4H); segments without elytra with nodular lobes in the position of elytrophores; first elytron with marginal, submarginal and central papillae ( Fig. 4), only area that overlaps with first elytron on opposite side without papillae, in all succeeding segments elytra with several to many marginal and submarginal papillae along the lateral and the posterior margins ( Fig. 4H); elytral papillae cirriform to slightly knobbed distally, with broad base, without articulations ( Fig. 4G–I), in posterior elytra cirriform papillae longest, implying spiny appearance of the worm; elytral surface with or without pigment, some specimens (often fixed in formalin for several years) with pale elytra ( Fig. 4D), others with strongly or weakly pigmented brownish elytra.

Prostomium with smooth cirriform median antenna without articulations, ceratophore of median antenna occasionally of orange colour ( Fig. 4A–C); lateral antennae absent ( Fig. 4B); with two pairs of closely set, black eyes, often anterior and posterior pair of eyes fused ( Fig. 4C), anterior pair larger. Facial tubercle absent; sometimes short papilla present below the median antenna and above the mouth opening being the position of the facial tubercle; ventral fold of mouth opening with fringe of about 15–20 cirri ( Fig. 4J).

Tentacular segment achaetous, with two pairs of cirriform tentacular cirri rising from a tentaculophore ( Fig. 4B); tentaculophore with few simple papillae; dorsal tentacular cirrus slightly longer than ventral one, both tentacular cirri with three to five distinct spine-like papillae ( Fig. 4A–C). Palps massive, tapering ( Fig. 4A).

Parapodia biramous ( Fig. 7); notopodium shorter than neuropodium, notopodium of conical shape at the end, without terminal papillae; few (one to three) long papillae present on its anterior lower edge and usually three prominent papillae along its posterior lower edge ( Fig. 7); neuropodium tapering, longer than notopodium, with few long and few short simple papillae mainly at the ventral surface, long terminal papillae (stylodes) absent ( Fig. 7); cirriform ventral cirrus present on neuropodia, ventral cirrus at first chaetiger (buccal cirrus) anteriorly oriented and considerably longer than on following chaetigers, ventral cirri otherwise laterally oriented. Both podial lobes bearing single stout acicula; notopodium with long spinous capillaries and short stout geniculate capillaries with serrations; neurochaetae compound falcigerous heterogomph chaetae with one row of short teeth on the blade, serrations near the tip of the shaft present ( Fig. 4E, F).

Pygidium with pair of long cirriform to thread-like anal cirri, terminoventrally attached.

Pigmentation: Several specimens with orange or brownish pigment at the base of the median antenna ( Fig. 4C). The orange pigment was also reported by Petersen (1997), even for ethanol-fixed material, in an unpublished ICES workshop document. We can confirm this observation for our material. Elytra either pale or with brown pigment and light-coloured centre (elytral pigment often lost in formalin and also in ethanol-fixed specimens).

Ecology: The species was found in coastal waters of different European seas (see below), usually in water depths less than 50 m. According to the original description, the species was collected at the type locality in the littoral zone between filamentous green algae (confervae).

Geographical distribution: The species occurs from the North Sea to the western Baltic Sea and also in the Mediterranean Sea (based on a single specimen from Croatia morphologically examined in the course of the present study); according to Barnich & Fiege (2003) present in all parts of the Mediterranean except for the central Mediterranean.

Remarks: Pholoe inornata is easily and unambiguously identified by the presence of up to seven robust spine-like papillae on dorsal and ventral tentacular cirri ( Fig. 4A, B), usually arranged in an irregular row on the inner side. Also, the orange pigment often present on the ceratophore of the median antenna is distinct ( Fig. 4C). Moreover, P. inornata is characterized by the absence of lateral antennae and the absence of the facial tubercle, the latter being present in most other Pholoe spp. from coastal European waters, except P. assimilis . However, Petersen (1997) mentions the presence of an inconspicuous facial tubercle and Chambers (1985) observed a retractile papilla-like facial tubercle in P. inornata . In our specimens we sometimes noted the presence of papillae in about the position where a facial tubercle would be found, but do not interpret them as facial tubercle. The presence of papillae near the mouth or proboscis is common in Pholoe spp. , but the presence of papillae in the position of the facial tubercle was not consistent in the examined specimens of P. inornata . We state that the facial tubercle among Pholoe spp. varies regarding its size. If present, it is always found in the typical position below the median antenna and above the mouth opening. Its presence (or absence) is consistent for all specimens belonging to the same species. The facial tubercle is absent in P. inornata . Pholoe inornata can be distinguished from P. assimilis by the absence of distinct spine-like papillae on tentacular cirri in the latter, and by the different dentation of the blades of heterogomph neurochaetae (with two and more rows of teeth in P. assimilis , whereas a single row can be found in P. inornata ). Also, the patterns of parapodial papillae are different, with long papillae being more numerous on the anterior side of the neuropodium in P. assimilis ( Fig. 6) than in P. inornata ( Fig. 7). Another species without facial tubercle is P. minuta . However, this species is so far only reliably reported from Greenland and the western North Atlantic (see Meissner et al., 2017). Some illustrations in McIntosh (1900, fig. 16, pl. XXXIV) of the head region of P. minuta with conspicuous papillae on the surface of the tentacular cirri might show P. inornata . However, other illustrations of the same species in this publication are different. Thus, we conclude that several species are involved in McIntosh’s descriptions of P. minuta .