Revision of Sthenelais Kinberg, 1856, Fimbriosthenelais Pettibone, 1971 and Eusthenelais McIntosh, 1876 (Polychaeta, Sigalionidae) in the Northeast Atlantic

Two common sigalionid species from the Northeast Atlantic, Sthenelais boa (Johnston, 1833) and S. limicola (Ehlers, 1864), have never been revised in detail. Although their validity has never been questioned, a number of taxonomic problems related to Sthenelais Kinberg, 1856 and the later established Fimbriosthenelais Pettibone, 1971 remain unresolved. The validity of F. minor (Pruvot & Racovitza, 1895) has been repeatedly discussed, but no agreement reached. Also the validity of Fimbriosthenelais has been at stake, affecting the generic assignment of Fimbriosthenelais zetlandica (McIntosh, 1876), another species present in the area. Among the investigated species of Sthenelais, some where thought to be synonyms of Eusthenelais hibernica McIntosh, 1876, which led us to also include Eusthenelais McIntosh, 1876. We also re-examined Eusthenelais abyssicola McIntosh, 1879, the only other species attributed to the genus, and confirm that it is indeterminable. In total, we investigated 37 nominal taxa reported from the Northeast Atlantic and as a result we consider only five species to be valid: Sthenelais boa, S. limicola, Fimbriosthenelais zetlandica, F. longipinnis (Grube, 1869) and Eusthenelais hibernica. These genera and species are described and discussed herein and an updated identification key to all Northeast Atlantic species of Sigalioninae Gonzalez et al., 2018 is given.


Introduction
Species attributed to the genus Sthenelais Kinberg, 1856 have been among the earliest described sigalionids in the wider Northeast Atlantic (i.e., including the Mediterranean Sea): Sthenelais boa

R e s e a r c h a r t i c l e
urn:lsid:zoobank.org:pub:A1E7F84F-49D0-4DA8-94E6-77E8CA68098F (Johnston, 1833) and S. limicola (Ehlers, 1864) are widely distributed in the area, with both found nearshore and S. limicola also regularly present in offshore habitats. Both species have regularly been reported in the literature (see respective synonymy paragraphs below), but so far they have not been included in any detailed revision of the genus.
Sthenelais Kinberg,1856 was initially established for Sthenelais helenae Kinberg, 1856 from Chile (Southeast Pacifi c). Pettibone (1971) revised several species of the genus and established the new genera Willeysthenelais Pettibone, 1971 and Fimbriosthenelais Pettibone, 1971. With the presence of very long papillae at the basis of the ventral cirri Willeysthenelais is easily distinguished from the other two genera; species attributed to Willeysthenelais are not reported for the area and not treated further in this study.
The distinction between Sthenelais and Fimbriosthenelais is rather subtle. Both genera as defi ned by Pettibone (1971) have very similar parapodia, lacking any long basal papillae on the ventral cirri, and differ by the presence (Fimbriosthenelais) or absence (Sthenelais) of papillae on the parapodial stylodes, which are club-shaped or elongate structures located at the margin of bracts or distally on lobes.
A major problem we encountered during this study is the inconsistency of terminology found throughout the literature, especially with regard to parapodial structures and terms such as papillae, fi mbriae, etc. Below, we establish a standardised terminology, which will enable us to differentiate more clearly between the species described.
Among the species of Sthenelais moved to Fimbriosthenelais by Pettibone (1971) was Sthenelais minor Pruvot & Racovitza, 1895. The type material of S. minor being lost (type locality near Banyuls, Western Mediterranean), Pettibone based her redescription on material from Brest described by Saint-Joseph (1899) as Sthenelais minor?. Chambers & Muir (1997) re-examined Saint-Joseph's specimen and considered it to be a juvenile of Sthenelais boa as they could not confi rm the presence of well defi ned papillae on the stylodes of this specimen. They noted too, that small specimens of S. boa may have a few papillae on some stylodes, that their elytra were covered by sand grains and that simple neuropodial chaetae were present, all of which are distinguishing characters of F. minor as redescribed by Pettibone. Chambers & Muir (1997) also moved Sthenelais zetlandica McIntosh, 1876 back from Fimbriosthenelais to Sthenelais without further comments or analysis of the genera and species involved. Barnich & Fiege (2003) and later Gil (2011) discussed the potential synonymy of F. minor with S. boa and the taxonomic implications regarding the validity of the genus Fimbriosthenelais without reaching a fi nal conclusion. Today, with many more specimens, type material and extensive literature studied, we are able to present a long overdue revision of species attributed to Sthenelais and morphologically similar genera in the Northeast Atlantic and Mediterranean Sea.

BARNICH R. & VAN HAAREN T., Revision of NE Atlantic Sthenelais and similar genera
The suggested synonymy of Sthenelais jeffreysi McIntosh, 1876 andS. heterochaeta McIntosh, 1897 with Eusthenelais hibernica McIntosh, 1876 (see Eliason 1962;Gil 2011), leads us to also include Eusthenelais McIntosh, 1876 in this study. However, the presence of dorsal cirri on segment 3 puts this genus closer to Neoleanira Pettibone, 1970 and clearly differentiates it from Sthenelais and Fimbriosthenelais, which lack any dorsal cirri (see McIntosh 1876bMcIntosh , 1900Amoureux 1972;Núñez et al. 2015). Table 1 gives a summary of all important generic characters discussed herein.
In total, we investigated 37 nominal taxa attributed to Sthenelais, Fimbriosthenelais and Eusthenelais present in the wider Northeast Atlantic. Table 2 summarises the current status of these taxa; further details are found in the Synonymy and Remarks sections below.
The current revision is restricted to material from the Northeast Atlantic and the Mediterranean Sea; records of the species under consideration outside this area should be treated carefully and need further confi rmation. Based on the results of this revision an updated identifi cation key to all species of the subfamily Sigalioninae Gonzalez et al., 2018 found in the area is given below.

Material and methods
The type and additional specimens investigated for this study are deposited in the collections of the following institutions: Eurofi ns TvH collection, Amsterdam; National Museums Scotland (NMS); Natural History Museum of Denmark (NHMD); Senckenberg Natural History Museum, Frankfurt (SMF); The Natural History Museum, London (BMNH); Thomson Environmental Consultants, Guildford (TUM).
Specimens were studied using Leica MZ9.5 and WILD M8 stereo microscopes and Nikon Eclipse E400 and Leica Diaplan compound microscopes, the latter equipped with Nomarski interference contrast. Drawings are based on digital photographs taken with a Canon PowerShot G12 and fi nalised using Procreate and Photoshop.
In fi gures of anterior ends, the anteriormost elytra were either missing or removed.
Evaluation of parapodial bracts and lobes requires careful examination. Localising bracts and stylodes is best achieved on a detached, but unmounted parapodium under the stereo microscope. In order to assess the presence of papillae on the stylodes, the parapodium needs to be mounted on a slide with a coverslip and examined at higher magnifi cation using a compound microscope.
The length of the specimens is measured from the anterior margin of the prostomium to the posterior border of the last segment (pharynx not included, if everted), while the width is taken at the widest segment, including parapodia but excluding chaetae. Sigalionids are long-bodied and often fragmented; thus, comparing the length of individuals is rather diffi cult. But the width (being maximal in the anterior body region) is rather easy to measure and thus most useful for comparing body sizes.

Terminology
A general overview of the terminology used in the identifi cation of sigalionid genera is given in Aungtonya (2003). The most important identifi cation characters used in this revision are illustrated in detail in Figs 1-2 (anterior end and parapodia of Sthenelais limicola) and Fig. 3 (stylodes and digitiform extensions on parapodia of S. boa). Understanding and correctly assessing the different appendages on the prostomium and anterior part of the body can be rather challenging: they tend to be entangled after fi xation or may be missing. We follow Aungtonya & Eibye-Jacobsen (2018) in using "dorsal tentacular crest" instead of "ctenidium" (see Pettibone 1971) for the structure present dorsally on the tentaculophores in all generic diagnoses presented herein.
European Journal of Taxonomy 740: 138-171 (2021) Also, the parapodial characters are not always easy to examine due to the presence of various bracts and lobes showing a variable number of stylodes and / or digitiform extensions (Fig. 3). Based on the revision of Pettibone (1971), we describe the basic parapodial characters of the genera considered herein as follows: The parapodia are biramous, each with up to three cup-shaped ctenidia dorsal to the notopodia ( Fig. 2A-B). The notopodial acicular lobe is nearly completely encircled by a bract covering the basis of the capillary notochaetae ( Fig. 2A, notopodial bract). The neuropodial acicular lobe is posteriorly nearly completely covered by a bilobed or truncate bract and presents anteriorly two smaller crescent-shaped bracts ( Fig. 2A-B: bilobed neuropodial posterior bract, neuropodial anterodorsal and anteroventral bracts). From these three neuropodial bracts, three groups of neurochaetae emanate: the middle (posterior) group with stout compound falcigers and occasionally compound spinigers; the upper (anterior) group with slender compound falcigers and occasionally compound spinigers and / or simple, spinous chaetae; and the lower (anterior) group with slender compound falcigers.
As defi ned by Pettibone (e.g., 1970Pettibone (e.g., , 1971) parapodial stylodes are club-shaped or elongate structures, resembling giant papillae, present marginally on the bracts or distally on the lobes. Because stylodes can be covered by small papillae, there was a need for another descriptive term for these characteristic structures.
Unfortunately, the description by Pettibone (1971) of the parapodial stylodes and other characters found on the margin of the bracts of the different species is not consistent. When establishing her new genus, Pettibone stated: "The genus Fimbriosthenelais is named for the characteristic fi mbriated or papillated parapodial stylodes." However, the terms "papillae/papillated" and "fi mbriae/fi mbriated" are used for different morphological structures in her revision: stylodes of species of Fimbriosthenelais are described as being fi mbriated or papillated (i.e., covered by fi mbriae = papillae). And also, the margin of the neuropodial bracts in species of Fimbriosthenelais and Sthenelais is described as being fi mbriated (but shows digitiform extensions, also referred to as "papillae" by Pettibone). Additionally, "papillae" cover the ventral body surface in Fimbriosthenelais.
In our understanding, the difference between a papilla and an extension in this context is the degree of separation from the supporting structure: a papilla is more or less separated and placed on top of the structure, whereas an extension is part of the considered structure. Consequently, we propose an adjusted terminology for the different characters discussed here: 'Papillae' = globular or elongate (separated) structures on stylodes or body surface; papillae often have minute sensorial hairs distally.
'Extensions' = digitiform projections of variable length being part of the margin of the neuropodial bracts; a single extension can bear minute papillae and/or sensorial hairs.
Another problematic term used by Pettibone (1971) is "fi mbriate with stylodes" referring to the neuropodial bracts of F. zetlandica for example. In our understanding, the margin of the bract in this species is more or less straight and carries stylodes ( And fi nally, while the genus Fimbriosthenelais was established for species with fi mbriated/papillated stylodes, "fi mbriated" neuropodial bracts (sensu Pettibone) are not only found in Fimbriosthenelais, but also in some Sthenelais species such as S. helenae, type species of the genus, and in S. boa (Figs 3, 4D-E).
Therefore, we suggest not to use "fi mbriated" in descriptions of the neuropodial bracts; they have either digitiform extensions or they carry stylodes. This allows for "fi mbriae/fi mbriated" to be used only in the sense of papillae which cover the stylodes and are the diagnostic character of the genus Fimbriosthenelais.
The three genera considered here present several types of chaetae: spinous capillaries (i.e., slender chaetae with transverse rows of spines and a long capillary tip, which can be simple or bidentate; Fig. 4F), simple spinous chaetae (i.e., moderately stout chaetae with transverse rows of spines in their distal part and a short, pointed, simple tip; Figs 2E, 4G), compound falcigers (i.e., chaetae consisting of a stem and a single or multi-articled blade with a typically curved, bidentate tip; Figs 2D, F-G; 4H-I, 6F-H, 8F-G), compound spinigers (i.e., chaetae consisting of a stem and a multi-articled blade with a simple, pointed tip; Figs 2C, 8E). PROSTOMIUM. Rounded, fused to fi rst segment. Median antenna inserted terminally, with stout, cylindrical ceratophore with lateral auricles and tapering style. Lateral antennae fused to inner dorsal sides of tentaculophores, without ceratophore, distinctly shorter than dorsal tentacular cirri. Paired palps encircled by palpal sheath emerging ventrally to tentaculophores.

Taxonomy
TENTACULOPHORES. With single aciculum, a pair of tentacular cirri, two bundles of capillary chaetae, L-shaped inner tentacular lobe with ciliated ridge and fused to palpal sheath, and dorsal tentacular crest.
SEGMENT 2. With fi rst pair of elytra, biramous parapodia and buccal cirri longer than following ventral cirri. Small ctenidia on lateral lips and medial to ventral cirri in anterior segments. PARAPODIA. Biramous, each with up to three cup-shaped ctenidia dorsal to notopodia, noto-and neuropodial acicular lobes with accessory bracts and smooth stylodes on most parapodia (minute papillae may be present on stylodes in juveniles or in anteriormost parapodia of adults). Notopodial acicular lobes nearly completely encircled by a bract covering the basis of the notochaetae. Neuropodial acicular lobes posteriorly with a large bilobed bract and anteriorly with two smaller crescent-shaped bracts.
CHAETAE. Notochaetae slender, spinous, tapering to capillary, uni-or bidentate tip. Neurochaetae usually compound falcigers and, if present, few compound spinigers and/or simple spinous chaetae; stems of compound chaetae usually with few rows of spines distally. Neurochaetae arranged in three groups: upper group of neurochaetae within anterodorsal bract: mainly slender compound falcigers; few simple, spinous chaetae (may be missing); in some species also few compound spinigers. Middle group of neurochaetae within posterior bract: all stout compound falcigers. Lower group of neurochaetae within anteroventral bract: all slender compound falcigers.

Remarks
The generic diagnosis of Sthenelais is emended for the potential presence of minute papillae on the stylodes found in juveniles or in anteriormost parapodia of adult S. boa and for the presence of compound spinigers found in S. limicola (see below). Wehe (2007) was the fi rst to mention that the capillary notochaetae of S. boa taper to a bidentate or unidentate tip and emended the generic diagnosis of Sthenelais accordingly. We confi rm the presence of bidentate and unidentate (i.e., simple) tips for the capillary notochaetae of S. boa, while those of S. limicola are all simple, see below.
European Journal of Taxonomy 740: 138-171 (2021) Based on our study, Sthenelais currently comprises two valid species in the wider Northeast Atlantic: Sthenelais limicola (Ehlers, 1864), which is the most widespread and common nearshore and offshore, and Sthenelais boa (Johnston, 1833), which is found usually nearshore.
For an extensive list of synonyms, or species previously referred to Sthenelais, which belong to other genera, or species that require further investigation, see Table 2 and the respective paragraphs below.

BARNICH R. & VAN HAAREN T., Revision of NE Atlantic Sthenelais and similar genera
Description PROSTOMIUM. Median antenna with long, smooth, tapering style; ceratophore with large auricles. Lateral antennae fused to inner dorsal side of tentaculophores, very short, not reaching half the length of the dorsal tentacular cirri. Two pairs of eyes present (Fig. 1A).
PARAPODIA. Stylodes without papillae, slender, cirriform. Parapodia of anterior body with stylodes present on anterior side of notopodial bract, on neuropodial acicular lobe and distally on upper and lower parts of large bilobed posterior bracts; number and length of distal stylodes decreasing along body. Margins of anterodorsal and anteroventral bracts smooth. Additional long, dorsal papillae on notopodia, a single papilla on anteriormost segments, up to 5 more posteriorly ( Fig. 2A-B).

Remarks
The description above is emended for the terminology used in describing the neuropodial bracts, for the presence of long, dorsal papillae on the notopodia and for the length and number of stylodes along the body: they are very long on segment 2 (some nearly reaching the length of the chaetae), becoming shorter on the following parapodia; this character led Ditlevsen (1917) to name his new species fi lamentosus.
The syntypes of S. fi lamentosus are in good condition and our examination revealed that they also show the other diagnostic characters of S. limicola as described above. Sthenelais limicola (Ehlers, 1864) predates S. fi lamentosus Ditlevsen, 1917, which becomes its junior synonym.
The type material of Sthenelais haddoni is probably lost; it consisted of a posterior fragment with completely smooth elytra presenting the typical lateral notch of S. limicola (see description in McIntosh 1897a). Sthenelais limicola has priority and S. haddoni becomes its junior synonym.
Compared to other species of the genus, Sthenelais limicola presents a number of remarkable characters: The compound neurochaetae include not only falcigers, but also spinigers. The anteriormost segments of Sthenelais articulata Kinberg, 1856 also show some compound spinigers (see Pettibone 1971), but this type of chaetae was not listed as a diagnostic generic character by Pettibone or any subsequent workers. Compound spinigers in addition to falcigers are found in other sigalionid genera, for example in Eusthenelais hibernica, the type species of Eusthenelais (see below). However, this genus clearly differs by the presence of a pair of dorsal cirri on segment 3.
The presence of additional long papillae dorsally on the notopodia is another remarkable character. So far, such papillae have not been described for any other species of Sthenelais. Similar papillae are found ventrally on the neuropodia of some, but not all, species of Willeysthenelais (see Pettibone 1971). All members of this genus are characterised by additional long papillae on the bases of the ventral cirri.
Consequently, these remarkable characters (presence of spinigers in addition to falcigers and long dorsal papillae on notopodia) might justify the erection of a new genus.
In their phylogenetic study of Aphroditiformia combining molecular and morphological data, Gonzalez et al. (2018) found that Sthenelais limicola formed a clade with Fimbriosthenelais longipinnis and Willeysthenelais diplocirrus (Grube, 1875), while Sthenelais boa formed a clade with Pholoides asperus (Johnson, 1897) and Pholoides dorsipapillatus (Marenzeller, 1893). This seems to confi rm our opinion that the current generic assignment of Sigalion limicola should be reconsidered. A more detailed study combining molecular data of a larger number of Sthenelais species with the emended diagnostic characters described herein would be desirable to justify the erection of a new genus for Sigalion limicola. However, in a personal comment B. Gonzalez stated that there are currently not enough suitable specimens available to conduct a more detailed molecular study.

Distribution and habitat
Widely reported throughout the area. In the Northeast Atlantic present around the British Isles (RB data, based on TUM reference collection, and Chambers 1985), Northern and Central North Sea (RB and TvH data), in the Skagerrak, Kattegat and northern Öresund (Hartmann-Schröder 1996), along the French Atlantic coast (Fauvel 1923), and around the Iberian Peninsula (Núñez et al. 2015). In the Mediterranean Sea confi rmed for the Western Mediterranean and the Adriatic Sea and reported from other areas (Barnich & Fiege 2003). Also recorded from the Northwest and Southeast Atlantic; however, these records require confi rmation. Occurring on muddy substrates at depths of 20 to 1550 m.

Description
PROSTOMIUM. Median antenna with long, smooth, tapering style; ceratophore with large auricles. Lateral antennae fused to inner dorsal side of tentaculophores, very short, not reaching half the length of dorsal tentacular cirri. Two pairs of eyes present (Fig. 4A).

Remarks
The description above is emended for the potential presence of minute papillae on the stylodes of anteriormost parapodia in adults (and in general in parapodia of juveniles) and for terminology used in description of the neuropodial bracts.
After examination of numerous specimens (see above), we agree with Chambers & Muir (1997) that adult Sthenelais boa have smooth stylodes on the majority of parapodia, but can have minute papillae on the stylodes of some anterior parapodia. Also juveniles may have minutely papillated stylodes.
Chambers & Muir (1997) checked Saint-Joseph's specimen of Sthenelais minor?, which was used by Pettibone (1971) for the description of Fimbriosthenelais minor, and concluded that this specimen is a juvenile of Sthenelais boa. Consequently, they listed Fimbriosthenelais minor sensu Pettibone as a synonym, but they did not include Sthenelais minor Pruvot & Racovitza, 1895 in the synonymy. Although the type material of Sthenelais minor is probably lost, the original description and fi gures clearly show the diagnostic characters of Sthenelais boa. We, therefore, confi rm the synonymy of both species.
The presence of minute papillae on the stylodes of Sthenelais boa leaves us with a taxonomic dilemma regarding the validity of the genus Fimbriosthenelais. This has been discussed in the past by Barnich & Fiege (2003) and Gil (2011). According to Pettibone (1971), the genus Fimbriosthenelais was established for species with papillated stylodes, while the stylodes of Sthenelais are smooth. Thus, we could agree with Chambers & Muir (1997) and move all species of Fimbriosthenelais back into Sthenelais.
During this study, however, we noted that under moderate magnifi cation (compound microscope 400 ×) the papillae on the stylodes of Fimbriosthenelais longipinnis and F. zetlandica are much larger and more easily observed than on the stylodes of Sthenelais boa, where, if present at all, they are minute and diffi cult to see. To allow for a comparison of the sizes of these papillae, Fig. 5 presents stylodes of the different species drawn to the same scale.
For the time being, we suggest to emend the respective generic diagnoses and preserve the genus Fimbriosthenelais. Further studies, including also new species described after Pettibone's revision from

BARNICH R. & VAN HAAREN T., Revision of NE Atlantic Sthenelais and similar genera
other parts of the world and combining morphological and molecular data, would be helpful to support the validity of Fimbriosthenelais. (1866) (1855) possibly shows a Sthenelais, but the description is insuffi cient to attribute it to any known species. We therefore consider this synonymy questionable for the time being, as a revision of species and type material outside the Northeast Atlantic is beyond the scope of this paper.

Sthenelais leidyi was a new name established by Quatrefages
In Chambers & Muir (1997) Sigalion carringtonii Carrington, 1865 is listed as a synonym of Sthenelais boa without reference to examination of type material. Mackie & Chambers (1990), however, investigated the syntypes of S. carringtonii and placed it in synonymy with Sigalion mathildae Audouin & Milne Edwards, 1832; this view was already adopted earlier by McIntosh (1900) and Hartman (1959) and is followed herein.

Distribution and habitat
Widely reported throughout the area. In the Northeast Atlantic present around the British Isles (RB data, based on TUM reference collection, and Chambers 1985), in the Skagerrak (Hartmann-Schröder 1996), the southern North Sea (south of the Frisian Front) (TvH data), along the French Atlantic coast (Fauvel 1923) and around the Iberian Peninsula (Núñez et al. 2015). In the Mediterranean Sea present in the Western and Eastern Mediterranean, the Adriatic and the Aegean Sea (Barnich & Fiege 2003). Also recorded from other areas in the Atlantic and Indo-Pacifi c; however, these records require confi rmation. Occurring on various substrates from shallow waters to 200 m depth.
TENTACULOPHORES. With single aciculum, a pair of tentacular cirri, two bundles of capillary chaetae, L-shaped inner tentacular lobe with ciliated ridge and fused to palpal sheath, and dorsal tentacular crest.
SEGMENT 2. With fi rst pair of elytra, biramous parapodia and buccal cirri longer than following ventral cirri. Small ctenidia on lateral lips and medial to ventral cirri in anterior segments. PARAPODIA. Biramous, each with up to three cup-shaped ctenidia dorsal to notopodia, noto-and neuropodial acicular lobes with accessory bracts and distinctly papillated stylodes. Notopodial acicular lobes nearly completely encircled by a bract covering the basis of the notochaetae. Neuropodial acicular lobes posteriorly with a large bilobed or truncate bract and anteriorly with two smaller crescent-shaped bracts.
CHAETAE. Notochaetae slender, spinous, tapering to capillary tip. Neurochaetae mostly compound falcigers and, if present, a few simple spinous chaetae; stems of compound chaetae usually with a few rows of spines distally. Neurochaetae arranged in three groups: upper group of neurochaetae within anterodorsal bract: mainly slender compound falcigers and a few simple, spinous chaetae (may be missing). Middle group of neurochaetae within posterior bract: all stout compound falcigers. Lower group of neurochaetae within anteroventral bract: all slender compound falcigers.

Remarks
The generic diagnosis of Fimbriosthenelais is emended for the presence of distinct (large) papillae on the stylodes to allow for differentiation from Sthenelais (see remarks related to S. boa above).
Based on our study, Fimbriosthenelais currently comprises two valid species in the wider NE Atlantic: F. zetlandica (McIntosh, 1876), which is widely distributed in the area, and F. longipinnis (Grube, 1869), a mainly Indo-Pacifi c species, which is also found in the eastern parts of the Mediterranean Sea.
As discussed above, we agree that F. minor (Pruvot & Racovitza, 1895) is a junior synonym of Sthenelais boa (Johnston, 1833). A fi nal decision on the validity of the genus Fimbriosthenelais would require a complete revision of all species, especially also additional species described since Pettibone's revision (see Aungtonya & Eibye-Jacobsen 2018).

Description
PROSTOMIUM. Median antenna with short, smooth, tapering style; ceratophore with small auricles. Lateral antennae fused to inner dorsal side of tentaculophores, very short, not reaching half the length of dorsal tentacular cirri. Two pairs of eyes present (Fig. 6A).
TENTACULOPHORES. Dorsal tentacular cirri subequal or slightly longer than median antenna, of similar shape. Ventral tentacular cirri about half the length of dorsal ones (Fig. 6A).
ELYTRA. With short, clavate papillae on outer lateral and posterior margin; surface covered by rounded to conical microtubercles ( Fig. 6B-C).
CHAETAE. Notochaetae slender, spinous, tapering to simple capillary tip. All neurochaetae compound falcigers with bidentate tip. Those of anteriormost segments slender with multi-articled blades. In all other parapodia falcigers of similar size and shape with bi-or three-articled blades. Middle neurochaetae slightly stouter than those of upper and lower groups (Fig. 6F-H).

Remarks
The description above is emended for the details regarding shape and size of the stylodes and their associated papillae and for the terminology used in the description of the neuropodial bracts.

Distribution and habitat
Reported from the NE and Southeast Atlantic and the Mediterranean Sea. Also reported from the Gulf of Oman, Indian Ocean. Occurring on muddy substrates from 30 to 560 m depth (see Pettibone 1971;Barnich & Fiege 2003;Wehe 2007). (Grube, 1869)

Description
PROSTOMIUM. Median antenna with moderately long, smooth, tapering style; ceratophore with large auricles. Lateral antennae fused to inner dorsal side of tentaculophores, very short, not reaching half the length of dorsal tentacular cirri. Two pairs of eyes present.
TENTACULOPHORES. Dorsal tentacular cirri longer than median antenna, of similar shape. Ventral tentacular cirri about half the length of dorsal ones.
ELYTRA. With fi liform and shorter, clavate papillae on outer lateral margin; surface of anterior elytra completely covered by rounded to conical microtubercles, in more posterior elytra microtubercles confi ned to areas near margins ( Fig. 7A-B).
PARAPODIA. Stylodes more or less club-shaped, with large, obvious papillae (Fig. 5E). Parapodia of anterior and middle body with stylodes present on anterior side of notopodial bract, on neuropodial acicular lobe and on upper and lower parts of bilobed posterior neuropodial bract. Margins of neuropodial anteroventral bract with digitiform extensions, anterodorsal bract reduced, without stylodes or extensions (Fig. 7C).
CHAETAE. Notochaetae slender, spinous, tapering to simple capillary tip. Upper neurochaetae mainly slender compound falcigers with multi-articled blade and minutely bidentate tip and a few simple, spinous chaetae. Middle neurochaetae stout compound falcigers with short single-or up to three-articled blade and bidentate tip. Lower neurochaetae slender compound falcigers with multi-articled blade and minutely bidentate tip.

Remarks
The description above is emended for the details regarding the shape and size of the stylodes and their associated papillae and for the terminology used in the description of the neuropodial bracts. The species has been extensively studied by Pettibone (1971), Wehe (2007) and Aungtonya & Eibye-Jacobsen (2018); please refer to these works for additional details.
Specimens from the Western Mediterranean described by Gil (2011) seem to agree with F. longipinnis as described by Pettibone (1971) regarding their elytral characters and the fi nely papillated ventral body surface. However, they seem to differ due to a reduced number of stylodes (and those present having no papillae, but just sensorial hairs) and blades of falcigers with not more than three articles. Because of the confusing terminology regarding extensions and papillae on bracts, lobes and stylodes (see above), it is likely that the stylodes described by Gil (2011) as lacking papillae and having just hairs correspond to the extensions of the anteroventral bract. As mentioned by Pettibone (1971) the number of articles of the blades is rather variable and would also fi t the current concept of F. longipinnis. Another possibility

BARNICH R. & VAN HAAREN T., Revision of NE Atlantic Sthenelais and similar genera
is that the specimens are juveniles of either F. longipinnis or maybe F. zetlandica. Unfortunately, those animals were not available for study and thus we cannot confi rm the presence of this species in the Western Mediterranean.
The specimens of F. longipinnis from Cyprus (Eastern Mediterranean), described by Barnich & Fiege (2003) and re-investigated herein, agree in all characters with Pettibone's revised description.
Sthenelais minor var. digitata Fauvel, 1919 was considered a possible synonym of F. longipinnis by Pettibone (1971). At the time of her revision no type material was available, but Wehe (2007) was able to examine the holotype and placed Sthenelais minor var. digitata in synonymy with Fimbriosthenelais hirsuta (Potts, 1910).

Distribution and habitat
Not known to occur in the Northeast Atlantic; presence confi rmed for the Eastern Mediterranean Sea. Otherwise widely reported from the Red Sea and Indo-Pacifi c. Type of habitat unknown; in shallow water down to 75 m depth (see Pettibone 1971;Barnich & Fiege 2003).  PROSTOMIUM. Rounded, fused to fi rst segment. Median antenna inserted terminally, with stout, cylindrical ceratophore with lateral auricles and tapering style. Lateral antennae fused to inner dorsal sides of tentaculophores, without ceratophore, length equal to that of dorsal tentacular cirri. Paired palps encircled by palpal sheath emerging ventrally to tentaculophores.
TENTACULOPHORES. With single aciculum, a pair of tentacular cirri, two bundles of capillary chaetae, L-shaped inner tentacular lobe with ciliated ridge and fused to palpal sheath, and dorsal tentacular crest.
SEGMENT 2. With fi rst pair of elytra, biramous parapodia and buccal cirri longer than following ventral cirri. Small ctenidia on lateral lips and medial to ventral cirri in anterior segments. Remarks Núñez et al. (2015) were the fi rst to present an extended generic diagnosis for Eusthenelais. Their diagnosis is emended herein for the terminology used in the description of the parapodial bracts and stylodes, for characters describing the tentaculophores and the location of the different neurochaetae.
McIntosh established the genus Eusthenelais for specimens of E. hibernica differing from Sthenelais by the presence of compound spinigers in addition to bidentate falcigers (McIntosh 1876b(McIntosh , 1879(McIntosh , 1900. In

BARNICH R. & VAN HAAREN T., Revision of NE Atlantic Sthenelais and similar genera
the original description (McIntosh 1876b), he did not mention the presence of dorsal cirri on segment 3, but he refers to this character in the text and fi gures of his re-description of the same material (McIntosh 1900). The presence of dorsal cirri on segment 3 is of generic relevance and puts Eusthenelais closer to Neoleanira Pettibone, 1970 and clearly differentiates it from Sthenelais and Fimbriosthenelais, which lack any dorsal cirri. Neoleanira on the other hand differs from Eusthenelais by the absence of any bidentate falcigers, all neurochaetae being compound spinigers (see Pettibone 1970).
We agree with Wehe (2007), Gil (2011) and Aungtonya & Eibye-Jacobsen (2014) that the generic name Parasthenelais Amoureux, 1972is invalid. Amoureux (1972 redescribed the species Eusthenelais hibernica based on specimens collected in deep waters off the Galician coast and established a new generic name without valid reason. Currently, Eusthenelais hibernica is the only valid representative of Eusthenelais. Another species assigned to the genus, Eusthenelais abyssicola McIntosh, 1879, was described for specimens from deep waters in the Davis Strait. However, we checked the holotype (BMNH 1921.5.1.622) which is unidentifi able. The description also being insuffi cient, we agree with Hartman (1965) and consider this to be an indeterminable sigalionid.  (Fig. 8)

Description
PROSTOMIUM. Median antenna with long, smooth, tapering style; ceratophore with small auricles. Lateral antennae fused to inner side of tentaculophores, size and shape similar to dorsal tentacular cirri. Eyes indistinct (Fig. 8A).
TENTACULOPHORES. Dorsal tentacular cirri long, size and shape similar to median antenna. Ventral tentacular cirri short, not reaching half the length of the dorsal ones (Fig. 8A).

Remarks
The description above is emended for the terminology used in describing the parapodial bracts and stylodes and the details regarding the shape and location of the neurochaetae. The recently collected specimens of BARNICH R. & VAN HAAREN T., Revision of NE Atlantic Sthenelais and similar genera E. hibernica from the Geikie Slide (Northwest Scotland) were in rather good condition and proved very helpful in supplementing the diagnostic characters of the species. Chambers (1985) noted that the type material of Eusthenelais hibernica is unidentifi able and, consequently, Read & Fauchald (2020) list Eusthenelais McIntosh, 1876 as a nomen dubium in WoRMS. However, especially the presence of both spinigers and bidentate falcigers as described by McIntosh (1876b) is suffi cient to distinguish it from other sigalionid species. The other important generic character, i.e., the presence of dorsal cirri on segment 3, was not mentioned in the original description, but later described by McIntosh (1900) based on the same material (see respective remark related to generic diagnosis above). We examined the syntypes, which are in rather bad condition, but we can confi rm the presence of a pair of dorsal cirri on segment 3 in both specimens.
In both publications (1876b, 1900) McIntosh described Eusthenelais hibernica and Sthenelais jeffreysi as different species. It is not clear why he did not realise that these are in fact synonymous, as for both species he described the two types of neurochaetae and, although he did not mention the dorsal cirri in the text for S. jeffreysi, he clearly fi gured them in his monograph of 1900 (pl. 29 fi g. 4). Eliason (1962) noted that not only Sthenelais jeffreysi, but also Sthenelais heterochaeta McIntosh, 1897 could be possible synonyms of Eusthenelais hibernica. For S. heterochaeta again the dorsal cirri are not mentioned, but the original description of the elytral and chaetal characters leaves no doubt that this is E. hibernica.
We checked the holotype of S. jeffreysi and found it to be unidentifi able; moreover, the type material of S. heterochaeta seems to be lost. But, as explained above, the respective original descriptions are suffi cient and we confi rm Eliason's view that both S. jeffreysi and S. heterochaeta are synonyms of E. hibernica.
Compared to the other species described herein, Eusthenelais hibernica presents a number of remarkable characters: The presence of a pair of dorsal cirri on segment 3 is the main differentiating character (absent in all other species). The auricles are smaller and much less obvious. The lateral antennae are of similar length to the dorsal tentacular cirri (versus distinctly shorter). The neuropodial posterior bract is large and obvious, similar to the one found in S. limicola (versus smaller and much less obvious in S. boa, or the Fimbriosthenelais species).

Distribution and habitat
So far only known from the NE Atlantic: recorded from off Norway, along the western coasts of the British Isles and Ireland, down to Southwest Portugal. Occurring on muddy and sandy substrates from 70 to 870 m depth (see above).

Updated key to the species of Sigalioninae in the Northeast Atlantic and Mediterranean Sea
We follow the latest subfamily designation proposed by Gonzalez et al. (2018). For keys to other species currently assigned to the subfamilies Pelogeniinae, Pholoinae and Pisioninae, see Barnich & Fiege (2003), Martinez et al. (2008) and Meißner et al. (2020).
Please note: characters in the key apply to adults; in younger individuals or juveniles there is likely some variation to be observed, for example in the number of stylodes present or the degree of coverage with ventral body papillae. Elytral characters apply to anterior elytra (but not the fi rst pair) unless otherwise stated. The distribution given for species not treated in detail herein is based on Barnich & Fiege (2003), Wehe (2007) and Núñez et al. (2015).  (Johnston, 1833) (NE Atlantic, Mediterranean, Indo-Pacifi c; nearshore)

Discussion
The aim of the present study was to revise the Northeast Atlantic sigalionid species assigned to Sthenelais Kinberg, 1856, Fimbriosthenelais Pettibone, 1971 and Eusthenelais McIntosh, 1876. One of the challenges in sigalionid taxonomy is the inconsistent use of the descriptive terminology, especially regarding important differentiating characters, such as lobes, bracts and stylodes found on parapodia, etc. In the Terminology chapter above, we discuss the different terms found in the previous literature and establish a set of standardised defi nitions, which allows to compare the different species more easily.
The type species of the genus Sthenelais, S. helenae Kinberg, 1856, was revised by Pettibone (1971), but her study did not include two of the earliest described and most common sigalionids in the wider NE Atlantic, Sthenelais boa (Johnston, 1833) and S. limicola (Ehlers, 1864).
Especially S. limicola has been inadequately described in the past and we here provide a detailed morphological description allowing us also to add two more species, i.e., S. fi lamentosus Ditlevsen, 1917 andS. haddoni McIntosh, 1897, to the current list of synonyms. In our study we found that S. limicola presents two remarkable characters, the presence of spinigers in addition to falcigers and of long dorsal papillae on the notopodia, which might justify the erection of a new genus. The phylogenetic study by Gonzalez et al. (2018) seems to confi rm our opinion that the current generic assignment of S. limicola should be reconsidered. A more detailed study, combining molecular data of a larger number of species of Sthenelais with the emended diagnostic characters described herein, would be desirable to justify the erection of a new genus for S. limicola. However, currently there are not enough suitable specimens available to conduct such a study.
Sthenelais boa, on the other hand, was suitably described by previous authors (for details, see synonymy section above). However, the discovery of small papillae on the stylodes of parapodia in juveniles (and of anterior parapodia in adults) by Chambers & Muir (1997) started a discussion on the validity of the genus Fimbriosthenelais established by Pettibone (1971) for species, which differ from Sthenelais by the presence of papillae on their parapodial stylodes. We agree with Chambers & Muir that minute papillae can be present on stylodes of anterior parapodia in S. boa and consequently we include S. minor Pruvot & Racovitza, 1895 in the list of its synonyms. However, we noted that the papillae on the stylodes of F. zetlandica (McIntosh, 1876) and F. longipinnis (Grube, 1869), the two species present in the considered area, are much larger and more easily observed than those found on S. boa (see Fig. 5, presenting stylodes of different species drawn to the same scale). For the time being, we suggest to preserve the status of Fimbriosthenelais and propose that a further study including new species described after Pettibone's revision from other parts of the world and combining morphological and molecular data should be conducted in order to fi nally decide on the status of this genus.
Based on Eliason (1962), Gil (2011) Chambers (1985). Our study revealed that the original description by McIntosh (1876b) and the subsequent description of the same material by McIntosh (1900) are indeed suffi cient to differentiate this species and genus and we confi rm the validity of Eusthenelais and its type species E. hibernica. Based on additional material recently collected from the Geikie Slide (off Northwest Scotland), we provide an emended diagnosis and description of E. hibernica and confi rm the synonymy of S. jeffreysi and S. heterochaeta with this species.
Another representative of Eusthenelais from deep waters in the Davis Strait, E. abyssicola McIntosh, 1879, was also investigated as part of our revision. We agree with Hartman (1965) and regard this species as indeterminable, its holotype being unidentifi able and the original description being insuffi cient. This leaves E. hibernica to be currently the only valid representative of the genus.
A total of 37 nominal taxa reported to occur in the NE Atlantic and Mediterranean Sea were investigated during this revision and we confi rm the validity of fi ve of them: Sthenelais boa and S. limicola, Fimbriosthenelais zetlandica and F. longipinnis, and Eusthenelais hibernica. The above presented results and conclusions provide the necessary basis for a future study combining morphological and molecular data to help resolve the two remaining problems addressed here: fi rstly, the implications of the occasional presence of minute papillae on the parapodial stylodes of S. boa on the validity of the genus Fimbriosthenelais and, secondly, additional proof to determine whether it is justifi ed to erect a new genus for S. limicola.