A review of the Leptostraca of the British Isles with discussion of the genus

A survey of the Leptostracan fauna of the British Isles collected from routine environmental monitoring samples revealed six species of Nebaliidae to be present in shallow waters. Th ese species were identifi ed as Nebalia herbstii Leach, 1814, Nebalia strausi Risso, 1826, Nebalia borealis Dahl, 1985, Nebalia kocatasi Moreira, Koçak and Katagan, 2007, Nebalia reboredae Moreira and Urgorri, 2009 in Moreira et al., 2009a and Sarsinebalia urgorrii Moreira, Gestoso and Troncoso, 2003. Th e presence of N. strausi, N. kocatasi, N. reboredae and S. urgorrii is reported from the British Isles for the fi rst time. Three further species have been previously reported from the waters surrounding Britain and Ireland but were not recorded in the current study. Th us, the number of species recorded in the British Isles now stands at nine and the range of several species is extended northwards from previous records in the Mediterranean and the Iberian Peninsula. Th e diagnostic characters of Sarsinebalia Dahl, 1985 are discussed and it is suggested that a formal reappraisal of the genus be conducted. A key to European Leptostraca is provided based on adult females. keY words Nebaliidae, Ireland, United Kingdom, identifi cation key, new records.


Leptostraca of the British Isles
Nauplius, 24: e2016006 shallow-and deep-water locations.However, the group is morphologically conservative and the traditionally used identifying characters were inadequate for species separation.Revisions of the group (Dahl, 1985;Moreira et al., 2003;2007;2009a;2009b), including descriptions of several new species, have made it apparent that the list of species reported from the British Isles is inaccurate and the distribution of the previously reported species must be re-evaluated.Mauchline (1984) lists four species of Leptostraca within the fauna of the British Isles.These include Nebalia bipes (Fabricius, 1780), Sarsinebalia typhlops (G.O.Sars, 1870), Nebaliella caboti Clark, 1932 and Nebaliopsis typica G.O. Sars, 1887.Nebalia bipes has historically been widely recorded from British and Irish coasts.Tattersall (1911) recorded it from the Clare Island Survey "at low water at spring tides, under stones and seems to prefer the neighbourhood of black, evil-smelling mud".Fahy (1985) recorded it from the gut contents of sea trout from Mulroy Bay, Co. Donegal.Dahl (1985) provided a revision of the European Leptostraca and demonstrated that N. bipes is not a cosmopolitan species recorded throughout western European coastal waters but instead is a boreal, circum-Arctic species known only from Greenland, Arctic North America, and Svalbard to western Norway.Considering Dahl's (1985) revised distribution of N. bipes and his statement that Nebalia herbstii Leach, 1814 was the commonest species of Nebalia Leach, 1814 of the British Isles and the western coast of France, O' Reilly et al. (2001) suggested that it seems probable that most of the British records of N. bipes should refer to N. herbstii.Correspondingly, it should be considered that all historical identifications of N. bipes from the British Isles should be seen as suspect and should be re-examined.
In addition to N. bipes, historical records of British and Irish Nebaliidae include N. herbstii, Nebalia borealis Dahl, 1985, Nebalia strausi Risso, 1826 and Sarsinebalia typhlops, Nebaliella caboti and Nebaliopsis typica.Sarsinebalia typhlops has previously been recorded (as Nebalia typhlops) by Tattersall (1905) off the west coast of Ireland: 60 miles west of Achill Head in 364 m of water and 50 miles off Cleggan Head, Co. Galway in 220 m of water.Linder (1943) recorded N. typica from the extreme south-west of the continental slope.Mauchline and Gage (1983) also recorded S. typhlops (as Nebalia typhlops), N. caboti and N. typica at >1500 m from the Rockall Trough.Nebalia herbstii is previously recorded from the south-west and western coast of England (Leach, 1814;Dahl, 1985), the Channel Islands, Dublin Bay in the gut of Gaidropsarus mediterraneus (Linnaeus, 1758) and off SW Ireland, location not specified (Dahl, 1985).British records of N. strausi are confined to the Channel Islands (Dahl, 1985) whilst N. borealis has only been reported from Shetland (Dahl, 1985).
The current study examines recent collections of Nebaliidae from inshore and offshore waters around the Irish and British coasts, providing further records for some of the previously known species and demonstrating the occurrence of additional species in these waters.

Material and Methods
Specimens of Nebaliidae were collected from numerous locations during surveys of inshore and offshore waters around the British Isles.The surveys included benthic surveys of Marine Natura 2000 sites, benthic audits of marine fish farms, ecological surveys related to marine aggregate extraction and the oil and gas industry and the U.K.'s Marine Conservation Zone programme.Macrofaunal samples were collected using 0.1 m 2 Day grabs, 0.1 m 2 mini-Hamon grabs, dredges and diver-operated cores but did not deliberately target leptostracans.During these surveys macrofaunal samples were fixed in 10% buffered formalin, sieved over either a 0.5 or a 1.0 mm mesh and the residues were sorted under either a Nikon SMZ-10 or Nikon SMZ-645 stereomicroscope into various taxa and transferred to 70% ethanol prior to identification.The leptostracan material was subsequently re-analysed for the current study.All specimens were compared against published descriptions of the species with all diagnostic characters carefully considered.Where available, accompanying habitat and collection data have been included in the material examined; however, since some of the macrofaunal samples were provided by third-party contractors these data have not always been made available.Where grain size analysis was carried out the substrate classification is based on that of Folk (1954).Where this analysis was not carried out the substrate descriptions were based on visual examination of the sediment in the field.In total,

Leptostraca of the British Isles
Nauplius, 24: e2016006 1,251 leptostracan specimens were examined.Sex of a proportion of the material was determined using the relative length of the antennae to the body to provide an indication of the proportion of males to females but this was not conducted for all lots.Voucher specimens have been deposited in the zoological collections of the National Museum of Ireland -Natural History (NMINH) and Oxford University Museum of Natural History (OUMNH.ZC).Specimens that had previously been cited by O'Reilly et al. (2001) are housed at SEPA.Irish specimens examined by Juan Moreira, ( JM), Universidad Autónoma de Madrid, are retained in his collection.The remaining leptostracan specimens are retained in the collections of the authors (Irish material with Aquafact International Services Ltd. and U.K. material with APEM Ltd.).Line drawings were made using a camera lucida connected to a Nikon YS2-B microscope.Maps are presented for each species discussed.For Sarsinebalia typhlops, Nebaliella caboti and Nebaliopsis typica these are based on the records presented by Tattersall (1905) and Mauchline and Gage (1983) since no material was available to us.For all other species, maps are based only on material reviewed in the present study, given the likelihood of previous misidentifications.Various records of Leptostraca from online datasets and grey literature have not been referenced as confirmation of identifications could not be established.
Remarks.Nebalia strausi is recorded for the first time from Irish waters.Records of N. strausi to date suggest a southerly distribution with the specimens from Mulroy Bay, Co. Donegal, representing the northernmost extent of its range.Features distinguishing N. strausi from other species of Nebalia within the North-East Atlantic include distally acute denticles on the posterior margin of pleonites 6 and 7, in conjunction with an antennular flagellum with more than 10 articles and the exopod of the 2nd maxilla barely extending past the proximal article of the endopod.However, research (Moreira et al., 2004;Koçak et al., 2010) has shown that specimens of N. strausi from different localities can exhibit variation in a number of characters including the shape of the anal scales and the denticles of the posterior border of pleonites 6-7.Koçak et al. (2011) suggested that the existence of cryptic species cannot be ruled out for N. strausi.
Distribution beyond the study area.Izmir Bay, Turkey (type locality).Cyprus and the Aegean Sea to the western Iberian Peninsula and the Canary Islands

Leptostraca of the British Isles
Nauplius, 24: e2016006 (Moreira et al., 2007;Koçak et al., 2011).Based on the previously known distribution and the present records it seems likely that this species is also present along the west coast of France and southern North Sea coasts but this requires confirmation.
Ecology.Gravel to fine sand with mats of Zostera marina, at depths of 4.5-13.8m, %TOM 1.2-4.9(Moreira et al., 2009a).Cobbles and gravel to mud, at depths of 4-23.2 m, %TOM 0.69-18.93(present study).Intertidal on semi-exposed and sheltered rocky shores.The study of their distribution relative to aquaculture cages indicates that this species is most abundant at the edges of the cages as distinct from directly beneath them or further out.
Remarks.The specimens examined agree closely with the description and figures given by Moreira et al. (2007).A comparison of diagnostic characters of Mediterranean species of Nebalia by Koçak and Moreira (2015) highlights useful differences between N. kocatasi and other species, in particular the lateral armature of the 3rd article of the antenna (N.kocatasi has 3 thin setae and 3 spine-like setae with the proximal spine-like seta smallest (Fig. 5A).Nebalia kocatasi is the only species that has been recorded in this study from the intertidal zone.This likely reflects the sampling methodology rather than true habitat preferences.Numerous specimens of N. kocatasi from Kenmare, Co. Kerry, and Finavarra, Co. Clare as well as two specimens from Shetland, had epibionts on the pleonites and on the carapace (Fig. 6).These had a total length of 306~392 µm, capsule length of 219~274 µm and stalk length of 86~141 µm (n=15).A study of these by Gregorio Fernandez-Leborans (Universidad Complutense, Madrid) could not determine their identity but concluded they were most likely protozoans.Over 40 species of chonotrich ciliates are known from Nebaliaceans (Fernandez-Leborans, 2001).
Distribution beyond the study area.Ría de Ferrol, Galicia, Spain (type locality).Based on the previously known distribution and the present records it seems likely that this species is also present along the west coast of France and continental southern North Sea coasts but this requires confirmation.
Ecology.Medium sand at depth of 6.4 m, 1.4 % TOM (Moreira et al., 2009a).Gravelly sand at depths of 5.5-40 m, 0.69-3.1% TOM (present study).Nebalia reboredae appears to be the European species of Nebalia which shows a greater affinity for sandy sediments.
Remarks.The specimens examined agree closely with the description and figures given by Moreira et al. (2009a).Contrary to Dahl's statement that Nebalia herbstii is the dominant Nebalia species in British waters, N. reboredae was the most commonly recorded in this study.British and Irish Distribution.Shetland, western British Isles (Dahl, 1985), south-west and western coasts of England (Leach, 1814), Loch Fyne, Isles of Scilly, Falmouth (present study) (Fig. 1D).

Nebalia herbstii
Distribution beyond the study area.Western France to Spanish border (Walker-Smith and Poore, 2001); northern coast of the Iberian Peninsula (Martínez et al., 2007).
Remarks.Dahl (1985) believed N. herbstii to be the dominant species of the genus in the British Isles.For this reason, many subsequent records of Nebalia from the British Isles have been attributed to N. herbstii.The material examined in the present study demonstrates that far from being the most common species of the genus it is relatively uncommon, in routine samples at least.Dahl (1985) also believed that the species may well occur around the Iberian Peninsula; however, Moreira et al. (2009a) could not confirm the occurrence of N. herbstii there.Ledoyer (1997) recorded its presence in the Mediterranean (Corsica and Bonnieu, France).However, Koçak et al. (2011) suspect that these specimens may refer to

Leptostraca of the British Isles
Nauplius, 24: e2016006 a different species as the dimensions of the maxilla 2 in Ledoyer's specimens differ from that typical of the species [in Dahl's (1985) description the proximal article of the endopod is longer than the distal article, whereas in Ledoyer's specimen they are subequal in length].Subsequent to Dahl's (1985) revision many of the records that were originally ascribed to Nebalia bipes within the British Isles were ascribed to N. herbstii but these records should be treated with caution and deposited specimens re-examined in the light of the new records submitted for the British Isles.All 16 specimens of N. herbstii from Loch Fyne had epibionts similar to those discussed above for N. kocatasi.British and Irish Distribution.Shetland Islands (Dahl, 1985); North Sea, St. Abbs, Scotland, English Channel (present study) (Fig. 2A).
Ecology.Clay, muddy sands to sandy gravels and rocky bottoms at depths of 14 m to 350 m.
Remarks.The specimens examined here agree well with Dahl's description of the species.One of the specimens cited under the name N. herbstii by O'Reilly et al. (2001), with the caveat that only specimens for 1989 had been confirmed, was re-examined here and found to be N. borealis.This species has a northern distribution and the present records extend the southern range of the species southwards in the British Isles to the central English Channel.

Genus Sarsinebalia Dahl, 1985
Remarks.Dahl (1985) created the genus Sarsinebalia to accommodate Nebalia typhlops, and based it on four characters: 1) a complicated rostrum with subterminal spine; 2) the features of the eye; 3) lack of a combrow on the exopod of the first pleopod; and 4) a short exopod of the 2nd maxilla.Dahl (1985) goes on to state that these features are also present in an undescribed species from the Red Sea [according to Moreira et al. (2003) this is corrected to north-west Africa in an unpublished manuscript by Dahl] and two further undescribed species from Australia.Moreira et al. (2003) postulate that the north-west African species could be could be identical to their Sarsinebalia urgorrii Moreira, Gestoso and Troncoso, 2003 whilst the identity of the Australian species is unknown as descriptions of these taxa had not appeared at the time of Dahl's death.To date, five species have been ascribed to Sarsinebalia: S. typhlops, S. urgorrii, Sarsinebalia cristoboi Moreira, Gestoso and Troncoso, 2003, Sarsinebalia biscayensis Ledoyer, 1998, andSarsinebalia kunyensis Ledoyer, 2000.According to Mauchline and Gage (1983) and Dahl (1985), specimens recorded from New Jersey, U.S.A. by Hessler and Sanders (1965) as Nebalia typhlops occidentalis Hessler and Sanders, 1965 differ in several significant characters and could represent a further species.
Walker-Smith and Poore ( 2001) considered Sarsinebalia to be a synonym of Nebalia but only included the type species S. typhlops in their analysis.However, Moreira et al. (2003) retained Sarsinebalia as valid as some (but not all) of the characters proposed by Dahl (1985) hold between S. typhlops, S. urgorrii and S. cristoboi.However, their study likewise overlooked S. biscayensis and S. kunyensis.A recent key to worldwide Nebalia species (Song and Min, 2016) also did not adopt the synonymy of Sarsinebalia suggested by Walker-Smith and Poore (2001).
Since Dahl's (1985) landmark study there have been significant advances in the taxonomy and systematics

Leptostraca of the British Isles
Nauplius, 24: e2016006 of the Leptostraca with around 67% of the currently accepted species having been described since this date with further new species continuing to be described (e.g.Koçak and Moreira, 2015;Song and Min, 2016).Combined with the phylogenetic studies of Olesen (1999) and Walker-Smith and Poore (2001), there is now a greater, if still not complete, understanding of variation in the traditionally used diagnostic characters across the Leptostraca.Many of Dahl's diagnostic characters for Sarsinebalia no longer stand up to scrutiny and now appear to be more indicative of species-rather than genus-level differences.These are discussed in turn here.
1) Complicated rostrum with subterminal spine.All species of Nebalia have a rostral keel (Walker-Smith and Poore, 2001) so in this respect the rostrum of Sarsinebalia species is no more complex.The subterminal spine is present in Sarsinebalia and the undescribed Nebalia sp.B of Walker-Smith and Poore (2001) as well as the less closely related genera of the Paranebaliidae.Additionally, S. biscayensis has a reduced spine (see Ledoyer, 1998), and some mature males of S. typhlops are also noted by Dahl (1985) as having a reduced spine.
2) Features of the eye.Of the five nominal species, only S. typhlops (and possibly the undescribed species of Dahl, 1985) has the 'squarish' eye shape described by Dahl (1985).The eyes of S. urgorrii and S. cristoboi are of a similar but not identical shape, whilst S. biscayensis has long slender eyes (see Ledoyer, 1998) and S. kunyensis appears to have rounded eyes (see Ledoyer, 2000).Eye shape is exceptionally variable in other Nebalia (even as far as being bi-lobed -see Walker-Smith and Poore, 2001) so a 'squarish' eye seems not to be too unusual and certainly within the range of variation covered by other Nebalia.In addition, Hessler (1984) illustrated significant developmental changes in the shape and armature of the eye of Dahlella caldariensis Hessler, 1984.Pigment is absent in S. biscayensis, S. kunyensis and S. typhlops but present in the S. urgorrii and S. cristoboi (but also seems to fade after long-term storage in alcohol, in S. urgorrii at least).As a general rule within the Crustacea, there is a tendency towards unpigmented eyes in deeper water species which may explain its absence in S. biscayensis, S. kunyensis and S. typhlops which are found deeper than S. urgorrii and S. cristoboi.Ommatidia are absent in S. typhlops and S. biscayensis but present in the other three species (also absent in unrelated Speonebalia Bowman, Yager and Iliffe, 1985, Nebaliella Thiele, 1904, and Dahlella Hessler, 1984).
3) Lack of a comb-row on the exopod of the first pleopod.A comb-row is absent in S. typhlops, S. urgorrii and S. cristoboi but present in both S. biscayensis and S. kunyensis.The length of the comb-row appears to be diagnostic (at least partially so) at a species level in Nebalia (see Walker-Smith and Poore, 2001) and the complete absence of a comb-row could just represent the extreme of this variation.
4) A short exopod of the 2nd maxilla.Only S. typhlops and S. biscayensis have an unusually short exopod on maxilla 2, whereas the other species have an exopod longer than the first segment of the endopod.In S. kunyensis and S. biscayensis the division between the two endopod segments is indistinct but the length of the exopod relative to the overall endopod can be used.
Based on this, the only character that is consistent between all 5 species of Sarsinebalia is the presence of a subterminal rostral spine.As already mentioned this spine occurs elsewhere in the Paranebaliidae and in Walker-Smith and Poore's (2001) Nebalia sp.B and so is not unique to Sarsinebalia.A formal reappraisal of Sarsinebalia is outside of the scope of the current study and should await examination of material of the currently included species as it is conceivable that other, as yet un-investigated, characters may unite all or some of the species currently assigned to the genus.Molecular methods may also help resolve the status of the genus.Until such time we continue to use Sarsinebalia here in accordance with Moreira et al. (2003).
Distribution beyond the study area.Galicia, NW Iberian Peninsula (Moreira et al., 2003;2009a).Based on the previously known distribution and the present records it seems likely that this species is also present along the west coast of France and continental southern North Sea coasts but this requires confirmation.
Remarks.Based on the current records, S. urgorrii is the dominant species of Sarsinebalia in shallow British and Irish waters.The pigmentation of the eyes was variable in the present material ranging from dark to orange and occasionally being almost completely absent; when this was the case externally visible ommatidia were still evident.The examined material generally agreed with the description of Moreira et al. (2003) but in one female from the English Channel six spines were noted on the distolateral border of the sixth pleopod rather than four as stated in the diagnosis provided by Moreira et al. (2003).All other characters agreed well with S. urgorrii.British and Irish Distribution. Rockall Trough, 1990-2900m (Mauchline and Gage, 1983), west of Ireland (Tattersall, 1905) (Fig. 2C).
Distribution beyond the study area.Widespread; Red Sea, Lofoten Is. (Norway), Messina, Bay of Naples (Italy), North America from Davis Strait to New Jersey, Australia (Walker-Smith and Poore, 2001).Records should be examined as it is possible that some may refer to undescribed species.
Remarks.Previous British and Irish records of S. typhlops require confirmation in light of the present study where only S. urgorrii was recorded amongst abundant material from shallow waters (much of which was originally identified as S. typhlops following Mauchline, 1984).It is likely that S. typhlops is restricted to deeper waters in the area (Mauchline and Gage, 1983).
Remarks.Although Mauchline and Gage (1983) referred their specimens to N. caboti they did so with some reservation given the uncertainty surrounding the systematics of Nebaliella at that time and stated that comparative studies should be made of all Nebaliella species to help resolve these issues.Such a study was conducted by Walker-Smith (1998) who reviewed Mauchline and Gage's (1983)  British and Irish Distribution.South-west British Isles (Linder, 1943), Rockall Trough (Mauchline and Gage, 1983) (Fig. 2D).
Distribution beyond the study area.Described from the South Pacific midway between New Zealand and Chile (Sars, 1887).Considered to be near cosmopolitan between the latitudes 50°N and 50°S (Mauchline, 1984).West and south-east coast of South America, near Falkland Is., off coast of Ghana, Ivory Coast, south-west Indian Ocean, South Pacific, Scotia Sea (Walker-Smith and Poore, 2001).

discussion
Prior to the present study, five species of Leptostraca were recorded from the British Isles.These are Nebalia herbstii, Nebalia borealis, Sarsinebalia typhlops, Nebaliella caboti and Nebaliopsis typica.In light of the present research, the number of leptostracan species recorded in the British Isles is nine, as outlined above.These records provide further evidence to support Dahl's (1985) removal of N. bipes from the British species list and demonstrate that, despite the previous works, the leptostracan fauna of the British Isles was poorly known.
Our records indicate that the dominant species in shallow, offshore waters of the British Isles are N. reboredae and S. urgorrii, neither of which has previously been formally reported for the region.Many of the current records are from routine monitoring projects, in coastal and offshore waters and collected using grab samplers, dredges and diver cores.The distributions presented here are therefore influenced to a degree by the sampling methods, sampling intensity, target habitats and the drivers of these surveys.For example, we have few records from intertidal areas (the exception being N. kocatasi as outlined above) and comparatively few from very shallow near-shore locations.It is possible that targeted sampling for leptostracans using other methods such as light traps may yield further records of species new to the area and provide a better understanding of the distribution of each species in the region.
Historically, the distribution and identity of leptostracans in European waters has been problematic, although several recent studies beginning with Dahl (1985) have contributed greatly to our understanding of the group in Europe.In order to help clarify species ranges and consistently identify specimens a key to the currently described Leptostraca of Europe is provided below.Given the highly conservative morphology of leptostracans, it is likely that molecular studies will demonstrate that some of the currently accepted species in fact represent species complexes, particularly for some of the more wide-ranging species.Furthermore, as new species continue to be described and further works are likely to provide insights into variability of character states, this key should be regarded as provisional rather than definitive at this stage and identifications should be carefully compared with original descriptions where possible.Several studies, most notably that of Olesen and Walossek (2000), have discussed ontogenetic changes in some taxonomically important characters such as antennae, maxillae and pleopods.Males for some species are unknown or poorly described.The key is therefore based on adult females and using this key for males or immature specimens could result in incorrect identifications.Staining specimens in Methyl Green improves the visibility of certain characters and may aid in the accurate identification of specimens.In particular it makes it possible to discern the relative lengths of the articles of the second maxilla without dissection.

Family
record accepting their identification in addition to providing other charactersLeptostraca of the British IslesNauplius, 24: e2016006 useful for separating N. caboti from its congeners.