Pholoe minuta, BICK & GOSSELCK, 1985

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 : 184-186

PHOLOE MINUTA BICK & GOSSELCK, 1985 [IN PART]

Type locality: Norway, Oslo Fjord, Drøbak; no types are known to exist ( Petersen, 1998) .

Non-type material: North Atlantic Ocean : Spitsbergen: Kongsfjorden, 78° 55.7’N, 11° 56.2’E, 1 September 2004, depth 26 m, very fine sand, 32.2 psu, 2.0 °C, 5 af, 5 mf, 1 pf ( ZSRO-P2494 ) , 78° 54.3’N, 12° 15.26’E, 1 September 2004, depth 10–15 m, very fine sand, 1 complete, 7 af, 11 mf ( ZSRO-P2495 ) ; Norway, Oslofjorden , Tollerhausen, 10 November 1992, depth 75 m, det. M. Petersen,>100 af, several complete (NHMD-297276), 1af, 4 mf, 2 pf SEM (NHMD-297276) ; UK, Scotland, Loch Etive , 11 May 1968, soft mud, det. M. Petersen, ~30 af, several mf and pf (NHMD-298015); Kattegat, 56° 17.057’N, 12° 12.153’E, 5 September 1998, depth 30.4 m, 2 af, 3 pf ( ZSRO-P598 ) ; Øresund , 56°06’3”N, 12°29’7”E (56° 6.050’N, 12° 29.117’E), 12 September 1981, depth 17.5–18.0 m, muddy sand and shells, det. M. Petersen, 13 af, several mf ( NHM UK 1998.370) , 1 af SEM ( NHM UK 1998.370a), c. 100 specm. (NHMD-298014); Mecklenburg Bay : 54°12’N, 11°50.3’E, 12 July 1988, depth 18 m, 10 af ( ZSRO-P229 ) , 28 specm., few complete, 2 af SEM ( ZSRO-P228 ) .

Specimens collected for DNA work, fixed in 96% ethanol, morphologically examined: Norway: Barents Sea , Svalbard: 80.010 22.2006 (80° 0.600’ N 22° 12.036’ E), 1 September 2009, depth 171 m ( ZMBN 127059 , 127061 , 127062 , 127063 , 127071 ) ; Finnmark : Sør-Varanger, 69.898333, 30.918167 (69° 53.900’N, 30° 55.0900’E), 15 April 2014, depth 315 m ( ZMBN 127040 ) . Sweden: Koster Area , between Hällsöarna, 58.95, 11.0833 (58° 57’N, 11° 4.998’E), April 2005, depth 70–80 m, mud, soft bottom ( ZMBN 127056 , 127058 ) , Aust-Agder , Arendal, AErøydypet: 58.410233, 8.746017 (58° 24.614’N, 8° 44.761’E), 26 May 2011, depth 90–100 m, soft mud ( ZMBN 127052 ) .

Diagnosis: Specimens often pale, sometimes with orange or brownish cover on the surface; two pairs of closely set black eyes present; mid-dorsum completely covered by elytra; elytral papillae mostly marginal, long, slender, distally capitate, elongated on posterior elytra; facial tubercle absent; lateral antenna absent; notopodium at the base with few large simple papillae along its anterior and posterior edge, neuropodium with several short papillae more or less evenly distributed over the neuropodial surface, short papillae slightly more numerous at the posterior side; neurochaetae heterogomph compound chaetae with two or more rows of short teeth along the blade (only seen in SEM) or blade smooth.

Description: Smallest of Pholoe species from European coastal waters. Largest complete specimen with 37 chaetigers, about 6.1 mm in length and 1.1 mm wide (from Spitsbergen). Other examined specimens between 2.1 and

5.1 mm long and 0.6–1.0 mm wide, with 22–35 chaetigers. Body short, linear, depressed ( Fig. 2C, D); ventral surface with evenly distributed small papillae, more abundant on the anterior half of the body. Mid-dorsum completely covered by elytra ( Fig. 2C, D). First pair of elytra rounded, in succeeding segments reniform and clearly wider than long, in posterior segments becoming rounded again ( Fig. 2B); segments without elytra with nodular lobes in the position of elytrophores; first elytron with mostly marginal and several submarginal papillae, only few papillae next to the centre, in all succeeding segments elytra with marginal and few submarginal papillae; elytral papillae cirriform to distally knobbed, without articulations ( Fig. 2B, D′, E), in posterior segments longest, the latter implying a spiny appearance of the worm; elytral surface usually without pigment ( Fig. 2C), some specimens with orange or brownish cover.

Prostomium with smooth cirriform, slightly knobbed median antenna without articulations ( Fig. 2A); lateral antennae absent; with two pairs of closely set, black eyes, often anterior and posterior pair of eyes fused, anterior pair larger ( Fig. 2C, D). Facial tubercle absent; sometimes short papillae below the median antenna and above the mouth ( Fig. 2A). Tentacular segment achaetous, with two pairs of cirriform tentacular cirri rising from a tentaculophore, dorsal pair rather smooth, ventral pair with few papillae; tentaculophore with simple papillae; dorsal tentacular cirrus only slightly longer and thicker than ventral one ( Fig. 2A). Palps massive, tapering.

Parapodia biramous ( Fig. 6); notopodium shorter of conical shape at the end, without terminal papillae; three to five long papillae arranged in an irregular row along its anterior edge and usually three prominent papillae along its posterior edge, number of papillae along the anterior edge more variable whereas the number of three prominent papillae along the posterior edge was found to be quite invariable ( Fig. 6); neuropodium tapering, longer than notopodium, with several short and few long simple papillae distributed over the surface, few longer terminal papillae (stylodes) only rarely present, usually absent ( Fig. 6); cirriform ventral cirrus present on neuropodia, ventral cirrus at first chaetiger (buccal cirrus) anteriorly oriented and more robust and almost twice as long than on other chaetigers, ventral cirri otherwise laterally oriented. Both podial lobes bearing single stout aciculae; notopodium with long spinous capillaries and short stout geniculate capillaries with serrations; neurochaetae compound falcigerous heterogomph chaetae with two, sometimes three to four, rows of short teeth on the blade ( Fig. 2F–H), serrations near the tip of the shaft present.

Pygidium with pair of long cirriform anal cirri, terminoventrally attached.

Pigmentation: All examined specimens white (without colour) in ethanol with no pigment discernible ( Fig. 2C). According to Petersen (1998), elytra evenly pigmented brownish or blackish and pale in worms from deep water.

Geographical distribution: All along the coasts of northern European waters, also occurring in the White and Baltic Seas, in coastal waters of Spitsbergen; Canada (see Meissner et al., 2017, fig. 9, clade I): Hudson Bay, Resolute Bay; Bering Sea (Alaska).

Remarks: Pholoe assimilis Örsted, 1845 was described from Drøbak, Oslo Fjord ( Norway) in a brief description and at that time distinguished from P. baltica because the eyes are closer together. Also, the anal cirri were mentioned to be long (as long as eight posterior segments). Petersen (1998) resurrected P. assimilis based on comprehensive material from European waters, also including the type locality of the species, and provided a short, illustrated description that is in good agreement with our observations (except the pigmentation that was not observed during the present study). Petersen (1998) also listed P. minuta Bick & Gosselck, 1985 in her synonymy list of P. assimilis . According to the description and illustration provided by these authors, we cannot agree with Petersen’s view in full but would assume that P. assimilis was among the studied material of Bick & Gosselck (1985), since the occurrence of the species can be confirmed herein for central parts of the Baltic Sea. Pholoe assimilis would appear amongst the smallest of European congeners. The species is characterized by two pairs of closely set black eyes, the absence of a facial tubercle, elytra completely covering the dorsum, the presence of long marginal elytral papillae on posterior elytra, the presence of usually three prominent papillae along the posterior edge of the notopodium (seen after staining) and double (sometimes more) rows of teeth on the blade of compound neurochaetae (only observable in SEM). There are two other species from the North Atlantic, which lack a facial tubercle but exhibit a complete elytral cover of the dorsum, P. minuta and P. inornata . The identity of P. minuta has only recently been fixed based on the study of material from the type locality (south-western Greenland) in a combined morphological and molecular approach by Meissner et al. (2017). Accordingly, P. minuta has until now been recorded only in the western North Atlantic. Morphological differences are the rather strong pigmentation of P. minuta opposed to a lack of pigment in P. assimilis . Moreover, short parapodial papillae are numerous along the lower edge of the neuropodium and in particular the notopodium, in both anterior and posterior view, in P. minuta ( Fig. 7), whereas short papillae are far less numerous in P. assimilis , in particular along the notopodium ( Fig. 6). Pholoe inornata can be distinguished from P. assimilis by the single row of teeth on the blades of compound neurochaetae ( Fig. 4F) opposed to two and more rows of teeth in P. assimilis ( Fig. 2G, H). The parapodial papillae are differently distributed in the two species ( Fig. 7). Also, up to five distinct papillae are present on the dorsal and ventral tentacular cirri ( Fig. 4B), the latter being a character unambiguously identifying P. inornata among all Pholoe in the eastern North Atlantic.