Pholoe baltica, ORSTED, 1843

MEIssNER, Karin, Götting, Miriam & Nygren, Arne, 2020, Do we know who they are? On the identity of Pholoe (Annelida: Sigalionidae: Pholoinae) species from northern Europe, Zoological Journal of the Linnean Society 189, pp. 178-206 : 187-190

PHOLOE BALTICA ÖRSTED, 1843

( FIGS 3, 6)

Pholoe baltica Örsted, 1843: 14–15 , figs 21, 34–36, 40; Petersen, 1998: 1374–1375, fig. 1E–H.

Pholoe tuberculata Southern, 1914: 57–59, pl. VI 14A–L.

Type locality, according to Petersen (1998): Øresund near Hellebaek and Kattegat near Skagen, Denmark, from muddy bottoms; no types are known to exist.

Non-type material: Specimens with lateral antennae: North Atlantic Ocean : Norway: 60°29.51’N, 2°49.18’E, 4 May 1998, depth 112 m, 1 af ( ZSRO-P689 ) , 60°23.03’N, 2°47.05’E, 4 May 1998, depth 102 m, 1 af ( ZSRO-P699 ) , Oslofjord, Lagøy, N of Drøbak, 22 March 1984, depth 27 m, sandy mud, Ockelmann dredge, 1 af (NHMD-298017), 59°44’04”N, 10 ⁰33’49”E (59°38.358’N, 10°37.331’E), station 10-10, October 2014, depth 200 m, sand/mud, 96% EtOH ( ZSRO-P2550 ) , 59°38.694’N, 10°36.700’E, station 10–13, October 2014, depth 130 m, gravel/mud/stones, 96% EtOH ( ZSRO-P2551 ) ; Kattegat : 57°46.788’N, 10°52.294’E (supposedly not far from suggested type locality Skagen, pers. comment), 8 September 1998, depth 47.5 m, 4 af, several mf, 1 af SEM ( ZSRO-P1816 a), off Frederikshavn, Denmark, 4 August 1980, depth 27 m, soft bottom (mud), Ockelmann dredge, 4 af, several mf, 1 af SEM (NHMD-298016) ; Baltic Sea: Great Belt , 55°22.60’N, 11°00.0’E, 17 May 1995, depth 38 m, muddy sand, 4 af ( ZSRO-P 365 ) , Mecklenburg Bay , 54°18.902’N, 11°32.970’E, 30 October 2004, depth 25.7 m, 2 af, 2 mf ( ZSRO-P 1668 ) . Specimens without lateral antennae: North Atlantic Ocean: Scotland, Loch Creran , st. 12 ABD II, Box 4, 6 November 1972, soft mud, Van Veen grab, ~ 90 specimens (several af, many mf, few pf or complete) (NHMD-297277), 3 af, 2 mf, 1 pf SEM (NHMD-297277) ; North Sea : 55°47.665’N, 6°47.913’E, 16 May 2002, depth 28 m, 1 af ( ZSRO-P1547 ) ; Skagerak, Gullmarnfjord , 23 July 1993, depth 20 m, mud, 4 af, 1 mf ( ZSRO-P 318 ) , 12 August 2009, depth 20–40 m, 1 af ( ZSRO-P2015 ) ; Kattegat : 57°46.788’N, 10°52.294’E, 8 September 1998, depth 47.5 m, 17 af, several mf, 1 pf ( ZSRO-P 1816 b), 56°17.057’N, 12°12.153’E, 5 September 1998, depth 30, 4 m, 1 af ( ZSRO-P 597 ) ; Baltic Sea: Mecklenburg Bay , 54°15.396’N, 11°52.380’E, 2 May 1999, depth 21, 8 m, 1 complete, 2 af, 1 mf ( ZSRO-P 1805 ) . Specimens collected for DNA work, fixed in 96% ethanol, morphologically examined: Norway: Møre og Romsdal, Sande, 62.27842, 5.45413 (62°16.705’N, 5°27.248’E), 21 July 2012, depth 169–188 m ( ZMBN 127092 ) , Hordaland, Radøy, Mangersfjorden , 60.6363, 4.955833 (60°38.178’N, 4°57.350’E), 7 February 2006, depth 37 m ( ZMBN 127112 , 127113 ) , Bergen, Vatlestraumen , 60.33885, 5.88083 (60°20.331’N, 5°52.850’E), 17 March 2008, depth 23 m ( ZMBN 127107 , 127110 ) , Sletten , 60.28158, 5.20288 (60°16.895’N, 5°12.173’E), 12 May 2007, depth 40 m, gravel ( ZMBN 127102 ) , Rogaland, Karmøy, Karmøysundet , 59.28789, 5.32506 (59°17.273’N, 5°19.504E, 8 June 2014, depth 74–79 m, mud ( ZMBN 127086 ) . UK: S of Shetland Islands, 58.592264, -2.349642 (58°35.536’N, 2°20.9792’W), 18 July 2008, depth 70 m, sand with shell fragments ( ZMBN 127093 ) .

Additional material examined: Pholoe tuberculata ( Southern, 1914), co-type: Ireland , Blacksod Bay , W 180, shore, 15 March 1911, coll. Irish Fisheries, 1 af, dissected parapodia ( NHM UK: 1914.12.12.63). Pholoe petersenae Ravara & Cunha, 2016 , paratypes: NE Atlantic, Gulf of Cadiz, Captain Arutuynov mud volcano, 35°39.740’N, 7°19.960’W, station 180, depth 1323 m, 2 af ( NHM UK 2016.349–351) .

Diagnosis: Elytral papillae usually moniliform or appearing articulated; facial tubercle present; pair of lateral antenna present or absent; elytral cover of anterior dorsum incomplete; notopodia usually with 4–5 long papillae arranged in an irregular row along the lower anterior edge and 3–5 prominent papillae along the posterior edge; neuropodia with several to numerous simple papillae distributed mainly posteriorly and ventrally over the surface, several long terminal papillae (stylodes) present; neurochaetae heterogomph compound chaetae with one row of teeth on the blades (teeth can be lost).

Description: Most examined specimens incomplete, between 1.8 and 11.4 mm long and 0.3–2.0 mm wide, with 29 to 61 chaetigers. Largest examined complete specimen with 60 chaetigers 11.4 mm long and 2.0 mm wide.

Body short, linear, depressed ( Fig. 3F); ventral surface with longitudinal groove and papillate laterally. Mid-dorsum usually not completely covered by elytra, uncovered gap at dorsal midline most prominent in large specimens but also present in smaller specimens ( Fig. 3E, F). First pair of elytra rounded, in succeeding segments subrectangular to reniform, sometimes with anterior notch, further posterior and in posteriormost segments more transversely elongate and of oval shape; segments without elytra with nodular lobes in the position of elytrophores; each elytron with papillae at lateral and posterior margins and in submarginal position ( Fig. 3J), on anterior elytra papillae also at more central parts of the elytral surface; elytral papillae often moniliform or articulated, sometimes pseudoarticulated ( Fig. 3I, J).

Prostomium with smooth, distally tapering median antennae without articulations ( Fig. 3A–D); pair of lateral antenna present or absent (see Remarks) ( Fig. 3A–C, D); with two pairs of closely set, black eyes, usually anterior and posterior pair of eyes fused ( Fig. 3C, H). Facial tubercle present, conspicuous and usually easy to detect ( Fig. 3A, C, D). Tentacular segment achaetous, with two pairs of cirriform tentacular cirri rising from a tentaculophore, dorsal tentacular cirrus slightly longer than ventral one ( Fig. 3A, C, D), the latter with few simple papillae whereas the dorsal one is rather smooth, tentaculophore with few papillae ( Fig. 3A, D). Palps massive, tapering ( Fig. 3D).

Parapodia biramous ( Fig. 6); notopodium short of conical shape at the end, without terminal papillae, usually 4–5 long papillae arranged in an irregular row along its lower anterior edge and 3–5 longer papillae along its posterior edge ( Fig. 6: second and third rows); neuropodium tapering, longer than notopodium, with several to numerous simple papillae distributed mainly posteriorly and ventrally over the surface, several long terminal papillae (stylodes) present ( Fig. 6: second and third rows); cirriform ventral cirrus present on neuropodial ( Fig. 6), sometimes pseudoarticulated, ventral cirrus at first chaetiger (so-called buccal cirrus) anteriorly oriented and more robust and longer than on other chaetigers, otherwise laterally oriented. Both parapodial lobes bearing single stout aciculae which can penetrate the epidermis; notopodium with long spinous capillaries and short stout geniculate capillaries with serrations; neurochaetae compound falcigerous heterogomph chaetae with one row of teeth on the blade, teeth can also be lost, few serrations distally on the shaft ( Fig. 3G, K)).

Pygidium with terminoventrally attached pair of long, cirriform anal cirri ( Fig. 3F).

Pigmentation: Pigmentation either present or absent: some specimens lacking pigmentation entirely, others with yellow, brownish or black plaques on elytra and dorsal surface. Petersen (1998) described some diffuse dark pigment between the eyes, which could also be recognized in some specimens examined in the course of the present study ( Fig. 3H).

Ecology: Mostly in muddy sediments between 27 and about 50 m water depth. At Clew Bay ( Scotland), also between Laminaria roots, dredged in 9–11 fms (16.5– 20.0 m) ( Southern, 1914).

Geographical distribution: Off Ireland: Blacksod Bay, Clew Bay, off southern Norway, Oslo Fjord, German Bight (North Sea), Kattegat, Skagerrak, Mecklenburg Bay (western Baltic Sea).

Remarks: Pholoe baltica was originally described by Örsted (1843) from the Kattegat and Öresund. In his description he mentions and clearly illustrates acuminate anterior projections from the head (‘… prominentias duas acuminatas product ’ could be translated to English as ‘prostomial peaks’) and three pairs of tentacular processes. The tentacular processes were later interpreted as median antenna and facial tubercle above each other in the middle, and the dorsal and ventral pairs of tentacular cirri to the sides ( Petersen, 1998). The also mentioned acuminate anterior projections from the head are most interesting. Petersen (1998) referred to these projections as ‘lateral antennae’ and emphasized that Örsted (1843) was the first to recognize lateral antennae in Pholoe . Morphologically, most similar to P. baltica is the meanwhile unaccepted species P. tuberculata Southern, 1914. In the course of the present study, type material of P. tuberculata was available from the BMNH ( NHM UK 1914.12.12.63). The specimen was labelled ‘co-type’ and thus might be part of the syntype series or a paratype. The original description of P. tuberculata by Southern (1914) is detailed and our examination of the type revealed good agreement with it. Lateral antennae could not be found in the type specimen and we concluded their absence in P. tuberculata (as their presence was also not mentioned in the original species description). Other important morphological characters are shared between the two species (e.g. presence of both a conspicuous facial tubercle and moniliform elytral papillae, presence of neuropodial stylodes). Petersen (1998) included P. tuberculata as potential junior synonym of P. baltica . Unfortunately, this listing was not discussed by the author. As we know, Petersen (1998) was aware of Örsted’s observation of lateral antennae in P. baltica , but did not comment on the situation in P. tuberculata . However, based on the presence of lateral antennae in P. baltica and their absence in P. tuberculata , the two species can be distinguished and, accordingly, could be considered distinct. In the course of our study we found anterior processes (lateral antennae) in some specimens from Norway, the Kattegat and the Baltic Sea. The lateral antennae are easily missed and are only seen after staining (e.g. with methyl green) in LM ( Fig. 3C) or also in SEM in well-preserved specimens ( Fig. 3A, B). If lateral antennae were present (in addition to other diagnostic characters, see Diagnosis) these specimens were initially identified by us as P. baltica , whereas their absence identified them as P. tuberculata . The occurrence of parapodial papillae seemed more pronounced in specimens without lateral antennae (‘tuberculata’), but also varied among specimens with lateral antennae (‘baltica’) ( Fig. 6). Additional consistent differences were not found. However, the results of our molecular studies did not support the hypotheses that specimens with and without lateral antennae belong to two different species, occurring sympatrically in the study area. The single COI haplotype and the single haplotype of 28S rDNA found in the two specimens available for DNA-analysis of ‘tuberculata’ are also found among specimens of ‘baltica’. We have thus to conclude that the presence of lateral antennae, in combination with other characters (see diagnosis), is not a reliable character for the identification of P. baltica , nor of P. tuberculata in case of their absence. Based only on molecular data, we have to regard P. tuberculata as junior synonym of P. baltica , as suggested by Petersen (1998). We are unable to explain the background of this variation in P. baltica . There are no reports about sexual dimorphism in Pholoe , and we did not find ovigerous specimens in our samples to test this hypothesis.

A species reported to also exhibit prostomial terminal peaks is Pholoe petersenae Ravara & Cunha, 2016 from deep waters of the north-east Atlantic. We examined the paratypes of this taxon and support the view that this is a distinct species. Most conspicuous are the long cirriform terminal extensions of the notopodia.

Pholoe baltica is morphologically similar to P. longa . The two species exhibit moniliform elytral papillae, the elytral cover of the dorsum is incomplete, a facial tubercle is present, and the compound neurochaetae have blades with one row of teeth. Lateral antennae are absent in P. longa . The number and size of neuropodial stylodes is reduced in P. longa , whereas they are conspicuous in P. baltica ( Fig. 6). Pholoe longa is known to occur in the western North Atlantic along the coasts of Greenland, Canada and Alaska ( USA), whereas P. baltica is widespread in coastal waters from the North to Baltic Seas. The two species are also genetically distinct.