among our sequenced specimens; in addition we also find the undescribed species 8, 10, 12, 13, 14, 15, 21, 25, 26, and 27, and together these undescribed species represent c. 50% of the sequenced specimens. Greenland Sea and the area South of Iceland are dominated by specimens of *T. atlantis* (species 16), *T. gracilis* (species 3), and *T. williamsae* (species 2), but there are also quite a few undescribed species present here as well, but as the sample size is not as large as in Skagerrak, Norwegian coast and shelf, and Barents Sea the results are not really comparable.

Looking at the depth distribution for the different species in a given geographic area, we can see that most species overlap in depth, and there is, in most cases, no clear sorting of species at different depths (Fig 12). In the depth range 150–250 meters in the Norwegian coast and shelf region, we have nine species present, and they all more or less overlap and are present at most of the localities that we have been able to sample, e.g. in the area from Trondheim in the north to Bergen in the south, 11 species are found (Figs 6–8), indicating that they do not inhabit specific habitats like fjords or the open ocean. For most of our samples we have used a sledge, a dredge or a beam trawl, all these gears sample material from an unspecified area of the sea floor. But often, at least when a sledge is used, the area sampled is an apparently flat uniform habitat of mud. In 49 out of the 89 sites from where we have sequenced more than one specimen, we found more than one species (Table 1), and in the most species-rich samples, five different species of *Terebellides* were found, e.g. site NCS24, a sample from a flat mud bottom from 534 meters depth in the Trondheimsfjord. There are few samples taken with a grab, but in one of them (BS5) we found two species co-occurring. Anyway, as we did not sequence all specimens from all samples, it is difficult to assess how many species of *Terebellides* that do co-occur at the same site. For many of the sites only one or a few specimens were sequenced, thus it is likely that diversity for each separate site is underestimated, but when looking at a slightly larger scale this should not be the case.

Apart from the fact that so many species of *Terebellides* still go under the radar, and that these unknown and undescribed species are so common, and even constitute a major part of the diversity both in number of species and specimens, one other thing struck us: the extreme diversity of haplotypes found in *COI* among some species. The most note-worthy are *T. gracilis* (species 3), *T. williamsae* (species 2), *T. atlantis* (species 16), and species 8, where almost all specimens sampled and sequenced have their own unique haplotype (Figs 7 and 8, Table 3). This variation rarely has led to an amino-acid substitution within the species, and in *T. atlantis* (species 16) all 48 haplotypes found among the 55 specimens produce the same exact amino-acid sequence. As the sample size varies a lot between different species, it is difficult to make a direct comparison in haplotype diversity, but one thing to note is that all those four species mentioned above are found at greater depths than a couple of 100 meters (Figs 7 and 8), in contrast with two other species that also are well represented in the material, i.e. *T. shetlandica* (species 1), and *T. stroemii* (species 6), that are found at more shallow depths (Fig 7). The relatively low genetic diversity among these shallow water species may be explained by that they have been more affected by the recurrent ice ages that have occurred during the last 1.8 million years [57], than the species living at greater depths have been. Even so it is hard to understand and explain the extremely high diversity of haplotypes, and how it is maintained, in these deeper-living species, but see [58] that also reported high haplotype diversity in *Aonidella cf dayi*, for possible explanations to this phenomenon.

Our angle on this study has been a molecular one, in order to find out how many species that occur in North East Atlantic waters, and the full morphological investigation has to await forthcoming studies. The main purpose of the morphological examination conducted in this paper has been to connect the described and known morphological species to the correct, or at least the best, molecularly recognized species. It is our hope that we in the future will be able to find
morphological characters that will help in standard morphological identification down to at least a
group of possible species, and in the best of worlds also down to species level. Molecular data from
this study will be vital to help us to sort out when this latter task is obtainable and when it is not.

Much water has passed under the bridge since Holthe [59] published his book on Terebello-
morpha in the North East Atlantic, when he discussed the supposed cosmopolitan distribution
of *T. stroemii*. He acknowledged that the worldwide reports were due to a confusion of closely
related species, but nevertheless stated that 'I do not suspect that there are more than one spe-
cies in the Norwegian material'. Still in these days, most *Terebellides* in the North East Atlantic
are routinely identified as *T. stroemii*, and our comprehensive study make it clear that this is a
severe underestimation of the true diversity among *Terebellides*. We do not think that *Terebel-
lides* is an unusual example of cryptic species, on the contrary, when morphospecies are prop-
erly assessed molecularly, in terms of sampling strategy and number of specimens analyzed
(e.g. [60]), it is commonplace to find more than one species, sometimes several, in the material.
Already Grassle [61] asked the question 'How common are cryptic polychaetes' when she and
her husband had discovered six cryptic species of *Capitella capitata* after an oil spill in West
Falmouth, Massachusetts in September 1969 [62]; we think we now have taken a small step
further towards the answer to this long-held question.

**Supporting information**

**S1 Appendix. COI-unique.nex.** Alignment, in nexus-format, with the unique 271 COI-sequences.
(NEX)

**S2 Appendix. COI-all.nex.** Alignment, in nexus-format, with all 462 COI-sequences.
(NEX)

**S3 Appendix. ITS2x-unique.nex.** Alignment, in nexus-format, with the unique 136 ITS2-sequences, aligned with X-INS-i in MAFFT.
(NEX)

**S4 Appendix. ITS2x-all.nex.** Alignment, in nexus-format, with all 402 ITS2-sequences, aligned with X-INS-i in MAFFT.
(NEX)

**S5 Appendix. ITS2s-unique.nex.** Alignment, in nexus-format, with the unique 136 ITS2-sequences, aligned with RNAsalsa.
(NEX)

**S6 Appendix. ITS2s-all.nex.** Alignment, in nexus-format, with all 402 ITS2-sequences, aligned with RNAsalsa.
(NEX)

**S7 Appendix. COI_and_ITS2s.nex.** Alignment, in nexus-format, including specimens with both COI and ITS2-data, used in the STACEY analysis.
(NEX)

**S8 Appendix. CONCATx.nex.** Concatenated alignment, in nexus-format, of COI, 16S rDNA, ITS2s, and 28S rDNA, including specimens with data from three of the four genetic markers.
(NEX)

**S9 Appendix. CONCATs.nex.** Concatenated alignment, in nexus-format of COI, 16S rDNA, ITS2x, 28S rDNA, including specimens with data from three of the four genetic markers.
(NEX)
morphological characters that will help in standard morphological identification down to at least a
group of possible species, and in the best of worlds also down to species level. Molecular data from
this study will be vital to help us to sort out when this latter task is obtainable and when it is not.

Much water has passed under the bridge since Holthe [59] published his book on Terebello-
morpha in the North East Atlantic, when he discussed the supposed cosmopolitan distribution
of *T. stroemii*. He acknowledged that the worldwide reports were due to a confusion of closely
related species, but nevertheless stated that 'I do not suspect that there are more than one spe-
cies in the Norwegian material'. Still in these days, most *Terebellides* in the North East Atlantic
are routinely identified as *T. stroemii*, and our comprehensive study make it clear that this is a
severe underestimation of the true diversity among *Terebellides*. We do not think that *Terebel-
lides* is an unusual example of cryptic species, on the contrary, when morphospecies are prop-
erly assessed molecularly, in terms of sampling strategy and number of specimens analyzed
(e.g. [60]), it is commonplace to find more than one species, sometimes several, in the material.
Already Grassle [61] asked the question 'How common are cryptic polychaetes' when she and
her husband had discovered six cryptic species of *Capitella capitata* after an oil spill in West
Falmouth, Massachusetts in September 1969 [62]; we think we now have taken a small step
further towards the answer to this long-held question.

**Supporting information**

**S1 Appendix. COI-unique.nex.** Alignment, in nexus-format, with the unique 271 COI-sequences.
(NEX)

**S2 Appendix. COI-all.nex.** Alignment, in nexus-format, with all 462 COI-sequences.
(NEX)

**S3 Appendix. ITS2x-unique.nex.** Alignment, in nexus-format, with the unique 136 ITS2-sequences, aligned with X-INS-i in MAFFT.
(NEX)

**S4 Appendix. ITS2x-all.nex.** Alignment, in nexus-format, with all 402 ITS2-sequences, aligned with X-INS-i in MAFFT.
(NEX)

**S5 Appendix. ITS2s-unique.nex.** Alignment, in nexus-format, with the unique 136 ITS2-sequences, aligned with RNAsalsa.
(NEX)

**S6 Appendix. ITS2s-all.nex.** Alignment, in nexus-format, with all 402 ITS2-sequences, aligned with RNAsalsa.
(NEX)

**S7 Appendix. COI_and_ITS2s.nex.** Alignment, in nexus-format, including specimens with both COI and ITS2-data, used in the STACEY analysis.
(NEX)

**S8 Appendix. CONCATx.nex.** Concatenated alignment, in nexus-format, of COI, 16S rDNA, ITS2s, and 28S rDNA, including specimens with data from three of the four genetic markers.
(NEX)

**S9 Appendix. CONCATs.nex.** Concatenated alignment, in nexus-format of COI, 16S rDNA, ITS2x, 28S rDNA, including specimens with data from three of the four genetic markers.
(NEX)
**S10 Appendix. CONCATmito_ML.tre.** Resulting tree with support values, using Maximum Likelihood on mitochondrial data only.
(TRE)

**S11 Appendix. CONCATmito BI.tre.** Resulting tree with support values, using Bayesian inference on mitochondrial data only.
(TRE)

**S12 Appendix. CONCATnucls_ML.tre.** Resulting tree with support values, using Maximum Likelihood on nuclear data only, with salsa-aligned ITS2-sequences.
(TRE)

**S13 Appendix. CONCATnucls BI.tre.** Resulting tree with support values, using Bayesian inference on nuclear data only, with salsa-aligned ITS2-sequences.
(TRE)

**S14 Appendix. CONCATnuclx_ML.tre.** Resulting tree with support values, using Maximum Likelihood on nuclear data only, with xinsi-aligned ITS2-sequences.
(TRE)

**S15 Appendix. CONCATnuclx BI.tre.** Resulting tree with support values, using Bayesian inference on nuclear data only, with xinsi-aligned ITS2-sequences.
(TRE)

**S16 Appendix. CONCATs_ML.tre.** Resulting tree with support values, using Maximum Likelihood on the combined mitochondrial and nuclear data set, with salsa-aligned ITS2-sequences.
(TRE)

**S17 Appendix. CONCATs BI.tre.** Resulting tree with support values, using Bayesian inference on the combined mitochondrial and nuclear data set, with salsa-aligned ITS2-sequences.
(TRE)

**S18 Appendix. CONCATx_ML.tre.** Resulting tree with support values, using Maximum Likelihood on the combined mitochondrial and nuclear data set, with xinsi-aligned ITS2-sequences.
(TRE)

**S19 Appendix. CONCATx BI.tre.** Resulting tree with support values, using Bayesian inference on the combined mitochondrial and nuclear data set, with xinsi-aligned ITS2-sequences.
(TRE)

**S20 Appendix. COI_TCS_log.rtf.** Log-file from the TCS-analysis on COI-all.
(RTF)

**S21 Appendix. ITS2x_TCS_log.rtf.** Log-file from the TCS-analysis on ITS2x-all.
(RTF)

**S22 Appendix. ITS2s_TCS_log.rtf.** Log-file from the TCS-analysis on ITS2s-all.
(RTF)

**S23 Appendix. COI_GMYC_code_nodes.pdf.** Topology from the GMYC-analysis on COI-unique with node numbers.
(PDF)

**S24 Appendix. COI_GMYC_log.rtf.** Log-file from the GMYC-analysis on COI-unique.
(RTF)
**S10 Appendix. CONCATmito_ML.tre.** Resulting tree with support values, using Maximum Likelihood on mitochondrial data only.
(TRE)

**S11 Appendix. CONCATmito BI.tre.** Resulting tree with support values, using Bayesian inference on mitochondrial data only.
(TRE)

**S12 Appendix. CONCATnucls_ML.tre.** Resulting tree with support values, using Maximum Likelihood on nuclear data only, with salsa-aligned ITS2-sequences.
(TRE)

**S13 Appendix. CONCATnucls BI.tre.** Resulting tree with support values, using Bayesian inference on nuclear data only, with salsa-aligned ITS2-sequences.
(TRE)

**S14 Appendix. CONCATnuclx_ML.tre.** Resulting tree with support values, using Maximum Likelihood on nuclear data only, with xinsi-aligned ITS2-sequences.
(TRE)

**S15 Appendix. CONCATnuclx BI.tre.** Resulting tree with support values, using Bayesian inference on nuclear data only, with xinsi-aligned ITS2-sequences.
(TRE)

**S16 Appendix. CONCATs_ML.tre.** Resulting tree with support values, using Maximum Likelihood on the combined mitochondrial and nuclear data set, with salsa-aligned ITS2-sequences.
(TRE)

**S17 Appendix. CONCATs BI.tre.** Resulting tree with support values, using Bayesian inference on the combined mitochondrial and nuclear data set, with salsa-aligned ITS2-sequences.
(TRE)

**S18 Appendix. CONCATx_ML.tre.** Resulting tree with support values, using Maximum Likelihood on the combined mitochondrial and nuclear data set, with xinsi-aligned ITS2-sequences.
(TRE)

**S19 Appendix. CONCATx BI.tre.** Resulting tree with support values, using Bayesian inference on the combined mitochondrial and nuclear data set, with xinsi-aligned ITS2-sequences.
(TRE)

**S20 Appendix. COI_TCS_log.rtf.** Log-file from the TCS-analysis on COI-all.
(RTF)

**S21 Appendix. ITS2x_TCS_log.rtf.** Log-file from the TCS-analysis on ITS2x-all.
(RTF)

**S22 Appendix. ITS2s_TCS_log.rtf.** Log-file from the TCS-analysis on ITS2s-all.
(RTF)

**S23 Appendix. COI_GMYC_code_nodes.pdf.** Topology from the GMYC-analysis on COI-unique with node numbers.
(PDF)

**S24 Appendix. COI_GMYC_log.rtf.** Log-file from the GMYC-analysis on COI-unique.
(RTF)
**S25 Appendix. COI_GMYC_support.xls.** Support-values for nodes from the GMYC-analysis on COI-unique.
(XLS)

**S26 Appendix. ITS2x_GMYC_code_nodes.pdf.** Topology from the GMYC-analysis on ITS2s with node numbers.
(PDF)

**S27 Appendix. ITS2x_GMYC_log.rtf.** Log-file from the GMYC-analysis on ITS2s.
(RTF)

**S28 Appendix. ITS2x_GMYC_support.xls.** Support-values for nodes from the GMYC-analysis on COI.
(XLS)

**S29 Appendix. ITS2s_GMYC_code_nodes.pdf.** Topology from the GMYC-analysis on ITS2s with node numbers.
(PDF)

**S30 Appendix. ITS2s_GMYC_log.rtf.** Log-file from the GMYC-analysis on ITS2s.
(RTF)

**S31 Appendix. ITS2s_GMYC_support.xls.** Support-values for nodes from the GMYC-analysis on COI.
(XLS)

**S32 Appendix. STACEY_log.txt.** Log-file from STACEY analysis on the COI_and_ITS2s data set.
(TXT)

**S33 Appendix. Distances_COI.xlsx.** Uncorrected distances from COI-all data set.
(XLSX)

**S34 Appendix. Distances_ITS2s.xlsx.** Uncorrected distances from ITS2s data set.
(XLSX)

**S35 Appendix. Uniqhaplo.pl.** Pearl script originally downloaded from the web page of Dr. Naoki Takebayashi at University of Alaska Fairbanks (Department of Biology and Wildlife).
(PL)

**S36 Appendix. Specimen list.** List of sequenced specimens with voucher specification, site ID (see Table 1), sequence ID, and GenBank accession numbers.
(DOCX)

## Acknowledgments

We would like to give our greatest thanks to the staff and crew on all scientific expeditions mentioned in the material and method section. Special thanks to Stefan Agrenius for donating specimen 2045_4 from Byfjorden. We also would like to thank Juan Moreira (Universidad Autónoma de Madrid, Spain) for the line drawings in Fig 2.

## Author Contributions

**Conceptualization:** Arne Nygren, Maria Capa.

**Data curation:** Arne Nygren, Maria Capa.
**S25 Appendix. COI_GMYC_support.xls.** Support-values for nodes from the GMYC-analysis on COI-unique.
(XLS)

**S26 Appendix. ITS2x_GMYC_code_nodes.pdf.** Topology from the GMYC-analysis on ITS2s with node numbers.
(PDF)

**S27 Appendix. ITS2x_GMYC_log.rtf.** Log-file from the GMYC-analysis on ITS2s.
(RTF)

**S28 Appendix. ITS2x_GMYC_support.xls.** Support-values for nodes from the GMYC-analysis on COI.
(XLS)

**S29 Appendix. ITS2s_GMYC_code_nodes.pdf.** Topology from the GMYC-analysis on ITS2s with node numbers.
(PDF)

**S30 Appendix. ITS2s_GMYC_log.rtf.** Log-file from the GMYC-analysis on ITS2s.
(RTF)

**S31 Appendix. ITS2s_GMYC_support.xls.** Support-values for nodes from the GMYC-analysis on COI.
(XLS)

**S32 Appendix. STACEY_log.txt.** Log-file from STACEY analysis on the COI_and_ITS2s data set.
(TXT)

**S33 Appendix. Distances_COI.xlsx.** Uncorrected distances from COI-all data set.
(XLSX)

**S34 Appendix. Distances_ITS2s.xlsx.** Uncorrected distances from ITS2s data set.
(XLSX)

**S35 Appendix. Uniqhaplo.pl.** Pearl script originally downloaded from the web page of Dr. Naoki Takebayashi at University of Alaska Fairbanks (Department of Biology and Wildlife).
(PL)

**S36 Appendix. Specimen list.** List of sequenced specimens with voucher specification, site ID (see Table 1), sequence ID, and GenBank accession numbers.
(DOCX)

## Acknowledgments

We would like to give our greatest thanks to the staff and crew on all scientific expeditions mentioned in the material and method section. Special thanks to Stefan Agrenius for donating specimen 2045_4 from Byfjorden. We also would like to thank Juan Moreira (Universidad Autónoma de Madrid, Spain) for the line drawings in Fig 2.

## Author Contributions

**Conceptualization:** Arne Nygren, Maria Capa.

**Data curation:** Arne Nygren, Maria Capa.
**Formal analysis:** Arne Nygren, Joan Pons, Maria Capa.

**Funding acquisition:** Arne Nygren, Maria Capa.

**Investigation:** Arne Nygren, Julio Parapar, Torkild Bakken, Maria Capa.

**Methodology:** Arne Nygren, Maria Capa.

**Project administration:** Arne Nygren.

**Resources:** Arne Nygren, Julio Parapar, Karin Meißner, Torkild Bakken, Jon Anders Kongsrud, Eivind Oug, Daria Gaeva, Andrey Sikorski, Robert André Johansen, Pat Ann Hutchings, Nicolas Lavesque, Maria Capa.

**Software:** Joan Pons.

**Supervision:** Arne Nygren.

**Validation:** Arne Nygren, Maria Capa.

**Visualization:** Arne Nygren, Julio Parapar, Karin Meißner, Maria Capa.

**Writing – original draft:** Arne Nygren.

**Writing – review & editing:** Arne Nygren, Julio Parapar, Joan Pons, Karin Meißner, Torkild Bakken, Jon Anders Kongsrud, Eivind Oug, Daria Gaeva, Pat Ann Hutchings, Nicolas Lavesque, Maria Capa.

## References

1. Bickford D, Lohman DJ, Sodhi NS, Ng PKL, Meier R, Winker K, et al. Cryptic species as a window on diversity and conservation. TREE. 2007; 22: 148–155. https://doi.org/10.1016/j.tree.2006.11.004 PMID: 17129636

2. Nygren A. Cryptic polychaete diversity: a review. Zool Scr. 2014; 43: 172–183. https://doi.org/10.1111/zsc.12044

3. Brasier MJ, Wiklund H, Neal L, Jeffreys R, Linse K, Ruhl H, et al. DNA barcoding uncovers cryptic diversity in 50% of deep-sea Antarctic polychaetes. R Soc open sci. 2016; 3: 160432. https://doi.org/10.1098/rsos.160432 PMID: 28018624

4. Oug E, Bakken T, Kongsrud JA. Original specimens and type localities of early described polychaete species (Annelida) from Norway, with particular attention to species described by O.F. Müller and M. Sars. Mem Mus Vic. 2014; 71: 217–236.

5. Hutchings PA, Kupriyanova E. Cosmopolitan polychaetes—fact or fiction? Personal and historical perspectives. Invert. Syst. 2017. Forthcoming.

6. Williams SJ. The status of Terebellides stroemi (Polychaeta; Trichobranchidae) as a cosmopolitan species, based in a worldwide morphological survey, including description of new species. In: Hutchings P. A, editor. Proceedings of the First International Polychaete Conference, Sydney: Linnean Society of New South Wales; 1984. pp. 118–142.

7. Imajima M, Williams SJ. Trichobranchidae (Polychaeta) chiefly from the Sagami and Suruga Bays, collected by R/V Tansei-Maru (Cruises KT-65~76). Bull Natl Mus Nat Sci Ser A Zool. 1985; 11: 7–18.

8. Solís-Weiss V, Fauchald K, Blankesteyn A. Trichobranchidae (Polychaeta) from shallow warm water areas in the Western Atlantic Ocean. Proc Biol Soc Wash. 1991; 104: 147–158.

9. Bremec CS, Elías R. Species of *Terebellides* from South Atlantic Waters off Argentina and Brazil (Polychaeta: Trichobranchidae). Ophelia. 1999; 5: 177–186. https://doi.org/10.1080/00785326.1999.10409407

10. Hutchings PA, Peart R. A revision of the Australian Trichobranchidae (Polychaeta). Invertebrate Taxonomy. 2000; 14: 225–272. https://doi.org/10.1071/IT98005

11. Parapar J, Moreira J, Helgason GV Taxonomy and distribution of *Terebellides* (Polychaeta, Trichobranchidae) in Icelandic waters, with the description of a new species. Zootaxa 2011; 2983: 1–20.

12. Schüller M, Hutchings PA. New species of *Terebellides* (Polychaeta: Trichobranchidae) indicate long-distance dispersal between western South Atlantic deep-sea basins. Zootaxa. 2012; 3254: 1–31.
**Formal analysis:** Arne Nygren, Joan Pons, Maria Capa.

**Funding acquisition:** Arne Nygren, Maria Capa.

**Investigation:** Arne Nygren, Julio Parapar, Torkild Bakken, Maria Capa.

**Methodology:** Arne Nygren, Maria Capa.

**Project administration:** Arne Nygren.

**Resources:** Arne Nygren, Julio Parapar, Karin Meißner, Torkild Bakken, Jon Anders Kongsrud, Eivind Oug, Daria Gaeva, Andrey Sikorski, Robert André Johansen, Pat Ann Hutchings, Nicolas Lavesque, Maria Capa.

**Software:** Joan Pons.

**Supervision:** Arne Nygren.

**Validation:** Arne Nygren, Maria Capa.

**Visualization:** Arne Nygren, Julio Parapar, Karin Meißner, Maria Capa.

**Writing – original draft:** Arne Nygren.

**Writing – review & editing:** Arne Nygren, Julio Parapar, Joan Pons, Karin Meißner, Torkild Bakken, Jon Anders Kongsrud, Eivind Oug, Daria Gaeva, Pat Ann Hutchings, Nicolas Lavesque, Maria Capa.

## References

1. Bickford D, Lohman DJ, Sodhi NS, Ng PKL, Meier R, Winker K, et al. Cryptic species as a window on diversity and conservation. TREE. 2007; 22: 148–155. https://doi.org/10.1016/j.tree.2006.11.004 PMID: 17129636

2. Nygren A. Cryptic polychaete diversity: a review. Zool Scr. 2014; 43: 172–183. https://doi.org/10.1111/zsc.12044

3. Brasier MJ, Wiklund H, Neal L, Jeffreys R, Linse K, Ruhl H, et al. DNA barcoding uncovers cryptic diversity in 50% of deep-sea Antarctic polychaetes. R Soc open sci. 2016; 3: 160432. https://doi.org/10.1098/rsos.160432 PMID: 28018624

4. Oug E, Bakken T, Kongsrud JA. Original specimens and type localities of early described polychaete species (Annelida) from Norway, with particular attention to species described by O.F. Müller and M. Sars. Mem Mus Vic. 2014; 71: 217–236.

5. Hutchings PA, Kupriyanova E. Cosmopolitan polychaetes—fact or fiction? Personal and historical perspectives. Invert. Syst. 2017. Forthcoming.

6. Williams SJ. The status of Terebellides stroemi (Polychaeta; Trichobranchidae) as a cosmopolitan species, based in a worldwide morphological survey, including description of new species. In: Hutchings P. A, editor. Proceedings of the First International Polychaete Conference, Sydney: Linnean Society of New South Wales; 1984. pp. 118–142.

7. Imajima M, Williams SJ. Trichobranchidae (Polychaeta) chiefly from the Sagami and Suruga Bays, collected by R/V Tansei-Maru (Cruises KT-65~76). Bull Natl Mus Nat Sci Ser A Zool. 1985; 11: 7–18.

8. Solís-Weiss V, Fauchald K, Blankesteyn A. Trichobranchidae (Polychaeta) from shallow warm water areas in the Western Atlantic Ocean. Proc Biol Soc Wash. 1991; 104: 147–158.

9. Bremec CS, Elías R. Species of *Terebellides* from South Atlantic Waters off Argentina and Brazil (Polychaeta: Trichobranchidae). Ophelia. 1999; 5: 177–186. https://doi.org/10.1080/00785326.1999.10409407

10. Hutchings PA, Peart R. A revision of the Australian Trichobranchidae (Polychaeta). Invertebrate Taxonomy. 2000; 14: 225–272. https://doi.org/10.1071/IT98005

11. Parapar J, Moreira J, Helgason GV Taxonomy and distribution of *Terebellides* (Polychaeta, Trichobranchidae) in Icelandic waters, with the description of a new species. Zootaxa 2011; 2983: 1–20.

12. Schüller M, Hutchings PA. New species of *Terebellides* (Polychaeta: Trichobranchidae) indicate long-distance dispersal between western South Atlantic deep-sea basins. Zootaxa. 2012; 3254: 1–31.
13. Schüller M, Hutchings PA. New species of *Terebellides* (Polychaeta: Trichobranchidae) from the deep Southern Ocean, with a key to all described species. *Zootaxa*. 2013; 3619: 1–45. PMID: 26131462

14. Parapar J, Hutchings PA. Redescription of *Terebellides stroemii* (Polychaeta, Trichobranchidae) and designation of a neotype. *J Mar Biol Assoc UK*. 2015; 95: 323–337. https://doi.org/10.1017/S0025315414000903

15. Jirkov IA, Leontovich MK. Identification keys for Terebellomorpha (Polychaeta) of the eastern Atlantic and the North Polar Basin. Invertebrate Zoology. 2013; 10: 217–243.

16. Hutchings PA, Nogueira JMM, Carrerette O. Terebelliformia. In: Purschke G, Westheide W, editors. *The Handbook of Zoology Online*. 2017. Forthcoming.

17. Rouse GW, Pleijel F. Polychaetes. Oxford: Oxford University Press; 2001.

18. Parapar J, Moreira J, O'Reilly M. A new species of *Terebellides* (Polychaeta: Trichobranchidae) from Scottish waters with an insight into branchial morphology. *Mar Biodivers. 2016;* 46: 211–225. https://doi.org/10.1007/s12526-015-0353-5

19. Parapar J, Moreira J, Martin D. On the diversity of the SE Indo-Pacific species of *Terebellides* (Annelida; Trichobranchidae), with the description of a new species. *PeerJ* 2016; 2313. https://doi.org/10.7717/peerj.2313

20. Curtis MA. Life cycles and population dynamics of marine benthic polychaetes from the Disko Bay area of West Greenland. *Ophelia*. 1977; 16: 9–58. https://doi.org/10.1080/00785326.1977.10425460

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22. Duchêne JC. Données sur le cycle biologique de la Polychète sédentaire *Terebellides stroemi* (Terebellidae) dans la région de Banyuls-sur-Mer. *C R Acad Sc Paris*. 1977; 284: 2543–2546.

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33. Nygren A, Eklöf J, Pleijel F. Arctic-boreal sibling species of *Paranaitis* (Polychaeta, Phyllodocidae). *Mar Biol Res*. 2009; 5: 315–327. https://doi.org/10.1080/17451000802441301

34. Kearse M, Moir R, Wilson A, Stones-Havas S, Cheung M, Sturrock S, et al. Geneious Basic: an integrated and extendable desktop software platform for the organization and analysis of sequence data. *Bioinformatics*. 2012; 28: 1647–1649. https://doi.org/10.1093/bioinformatics/bts199 PMID: 22543367

35. Clements M, Posada D, Crandall KA. TCS: a computer program to estimate gene genealogies. *Mol Ecol*. 2000; 9: 1657–1659. https://doi.org/10.1046/j.1365-294x.2000.01020.x PMID: 11050560

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21. Willemoes-Suhm R. Biologische Beobachtungen über niedere Meeresthiere. Z Wiss Zool. 1871; 21: 380–396.

22. Duchêne JC. Données sur le cycle biologique de la Polychète sédentaire *Terebellides stroemi* (Terebellidae) dans la région de Banyuls-sur-Mer. C R Acad Sc Paris. 1977; 284: 2543–2546.

23. De Queiroz K. Species concepts and species delimitation. Syst Biol. 2007; 56: 879–886. https://doi.org/10.1080/10635150701701083 PMID: 18027281

24. Blindheim J. Oceanography and climate. In: Skjoldal HR, editor. The norwegian sea ecosystem. Trondheim: Tapir Academic Press; 2004. pp. 65–96.

25. OSPAR. Quality status report 2010. London: OSPAR Commission; 2010.

26. Yashayaev I, Seidov D, Demirov E. A new collective view of oceanography of the arctic and north atlantic basins. Prog Oceanogr. 2015; 132: 1–21. https://doi.org/10.1016/j.pocean.2014.12.012

27. Ingvaldsen R, Loeng H. Physical oceanography. Ecosystem Barents Sea. Trondheim, Norway; Tapir Academic Press; 2009.

28. Sjölin E, Erseus C, Källersjö M. Phylogeny of Tubificidae (Annelida, Clitellata) based on mitochondrial and nuclear sequence data. Mol Phylogenet Evol. 2005; 35: 431–441. https://doi.org/10.1016/j.ympev.2004.12.018 PMID: 15804413

29. Palumbi SR. Nucleic acids II: the polymerase chain reaction. In: Hillis DM, Moritz C, Mable BK, editors. Molecular Systematic. second edition. Sunderland, Massachusetts, USA: Sinauer; 1996. pp. 205–247.

30. Folmer O, Black MB, Hoeh WR, Lutz RA, Vrijenhoek RC. DNA primers for amplification of mitochondrial cytochrome c oxidase subunit I from diverse metazoan invertebrates. Mol Mar Biol Biotechnol. 1994; 3: 294–299. PMID: 7881515

31. Bely AE, Wray GA. Molecular phylogeny of nauidid worms (Annelida: Clitellata) based on cytochrome oxidase I. Mol Phylogenet Evol. 2004; 30: 50–63. https://doi.org/10.1016/S1055-7903(03)00180-5 PMID: 15022757

32. Le HL, Lecointre G, Perasso R. A 28S rRNA-based phylogeny of the gnathostomes: first steps in the analysis of conflict and congruence with morphologically based cladograms. Mol Phylogenet Evol. 1993; 2: 31–51. https://doi.org/10.1006/mpev.1993.1005 PMID: 8081546

33. Nygren A, Eklöf J, Pleijel F. Arctic-boreal sibling species of *Paranaitis* (Polychaeta, Phyllodocidae). Mar Biol Res. 2009; 5: 315–327. https://doi.org/10.1080/17451000802441301

34. Kearse M, Moir R, Wilson A, Stones-Havas S, Cheung M, Sturrock S, et al. Geneious Basic: an integrated and extendable desktop software platform for the organization and analysis of sequence data. Bioinformatics. 2012; 28: 1647–1649. https://doi.org/10.1093/bioinformatics/bts199 PMID: 22543367

35. Clements M, Posada D, Crandall KA. TCS: a computer program to estimate gene genealogies. Mol Ecol. 2000; 9: 1657–1659. https://doi.org/10.1046/j.1365-294x.2000.01020.x PMID: 11050560

36. Katoh K, Misawa K, Kuma K, Miyata T. MAFFT: a novel method for rapid multiple sequence alignment based on fast Fourier transform. Nucleic Acids Res. 2002; 30: 3059–3066. PMID: 12136088

37. Stocsits RR, Letsch H, Hertel J, Misof B, Stadler PF. Accurate and efficient reconstruction of deep phylogenies from structured RNAs. Nucleic Acids Res. 2009; 37: 6184–6193. https://doi.org/10.1093/nar/gkp600 PMID: 19723687
38. Koetschan C, Förster F, Keller A, Schleicher T, Ruderisch B et al. The ITS2 Database III—sequences and structures for phylogeny. Nucleic Acids Research 38:D275–9. https://doi.org/10.1093/nar/gkp966 PMID: 19920122

39. Maddison WP, Maddison DR. Mesquite: a modular system for evolutionary analysis. Version 2.5. 2008.

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41. Fujisawa T, Barraclough TG. Delimiting species using single-locus data and the Generalized Mixed Yule Coalescent Approach: A revised method and evaluation on simulated data sets. Syst Biol. 2013; 62: 707–724. https://doi.org/10.1093/sysbio/syt033 PMID: 23681854

42. Jones G. Algorithmic improvements to species delimitation and phylogeny estimation under the multi-species coalescent. J Math Biol. 2017; 74: 447–467. https://doi.org/10.1007/s00285-016-1034-0 PMID: 27287395

43. Darriba D, Taboada GL, Doallo R, Posada D. jModelTest 2: more models, new heuristics and parallel computing. Nat Methods. 2012; 9: 772. https://doi.org/10.1038/nmeth.2109 PMID: 22847109

44. Ronquist F, Teslenko M, van der Mark P, Ayres DL, Darling A, Höhna S, et al. MrBayes 3.2: efficient Bayesian phylogenetic inference and model choice across a large model space. Syst Biol. 2012; 61: 539–542. https://doi.org/10.1093/sysbio/sys029 PMID: 22357727

45. Silvestro D, Michalak I. raxmlGUI: a graphical front-end for RAxML. Org Divers Evol. 2012; 12: 335–337.

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47. Drummond AJ, Suchard MA, Xie D, Rambaut A (2012) Bayesian phylogenetics with BEAUti and the BEAST 1.7. Mol Biol Evol. 2012; 29: 1969–1973. https://doi.org/10.1093/molbev/mss075 PMID: 22367748

48. Brower AVZ. Rapid morphological radiation and convergence among races of the butterfly *Heliconius erato* inferred from patterns of mitochondrial-DNA evolution. Proc Natl Acad Sci USA. 1994; 91: 6491–6495. PMID: 8022810

49. Bouckaert R, Heled J, Kühnert D, Vaughan T, Wu CH, Xie D, et al. BEAST 2: A Software Platform for Bayesian Evolutionary Analysis. PLoS Comput Biol. 2014; 10: e1003537. https://doi.org/10.1371/journal.pcbi.1003537 PMID: 24722319

50. Leigh JW, Bryant D. PopART: Full-feature software for haplotype network construction. Methods Ecol Evol. 2015; 6: 1110–1116. https://doi.org/10.1111/2041-210X.12410

51. Swofford DL. PAUP*: Phylogenetic Analysis Using Parsimony. Version 4.0b. Sunderland, Massachusetts: Sinauer Associates. 2002.

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54. Knowlton N. Molecular genetic analyses of species boundaries in the sea. Hydrobiologia. 2000; 420: 73–90.

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57. Andersen BG, Borns HW. The ice age world: An introduction to quaternary history and research with emphasis on North America, and Northern Europe during the last 2.5 million years. Oslo: Scandinavian University Press; 1997.

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59. Holthe T. Polychaeta Terebellomorpha. Marine Invertebrates of Scandinavia, 7. Oslo: Norwegian Universities Press; 1986.

60. Nygren A, Pleijel F. From one to ten in a single stroke—resolving the European *Eumida sanguinea* (Phylodocidae, Annelida) species complex. Mol Phylogenet Evol. 2011; 58: 132–141. https://doi.org/10.1016/j.ympev.2010.10.010 PMID: 21034835
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39. Maddison WP, Maddison DR. Mesquite: a modular system for evolutionary analysis. Version 2.5. 2008.

40. Pons J, Barraclough TG, Gomez-Zurita J, Cardoso A, Duran DP, Hazell S, et al. Sequence-based species delimitation for the DNA taxonomy of undescribed insects. Syst Biol. 2006; 55: 595–609. https://doi.org/10.1080/10635150600852011 PMID: 16967577

41. Fujisawa T, Barraclough TG. Delimiting species using single-locus data and the Generalized Mixed Yule Coalescent Approach: A revised method and evaluation on simulated data sets. Syst Biol. 2013; 62: 707–724. https://doi.org/10.1093/sysbio/syt033 PMID: 23681854

42. Jones G. Algorithmic improvements to species delimitation and phylogeny estimation under the multi-species coalescent. J Math Biol. 2017; 74: 447–467. https://doi.org/10.1007/s00285-016-1034-0 PMID: 27287395

43. Darriba D, Taboada GL, Doallo R, Posada D. jModelTest 2: more models, new heuristics and parallel computing. Nat Methods. 2012; 9: 772. https://doi.org/10.1038/nmeth.2109 PMID: 22847109

44. Ronquist F, Teslenko M, van der Mark P, Ayres DL, Darling A, Höhna S, et al. MrBayes 3.2: efficient Bayesian phylogenetic inference and model choice across a large model space. Syst Biol. 2012; 61: 539–542. https://doi.org/10.1093/sysbio/sys029 PMID: 22357727

45. Silvestro D, Michalak I. raxmlGUI: a graphical front-end for RAxML. Org Divers Evol. 2012; 12: 335–337.

46. Ezard T, Fujisawa T, Barraclough TG. SPLITS: SPecies' Limits by Threshold Statistics. R package version 1.0; 2009 [cited 2017 Nov 14]. Available from: http://R-Forge.R-project.org/projects/splits/

47. Drummond AJ, Suchard MA, Xie D, Rambaut A (2012) Bayesian phylogenetics with BEAUti and the BEAST 1.7. Mol Biol Evol. 2012; 29: 1969–1973. https://doi.org/10.1093/molbev/mss075 PMID: 22367748

48. Brower AVZ. Rapid morphological radiation and convergence among races of the butterfly Heliconius erato inferred from patterns of mitochondrial-DNA evolution. Proc Natl Acad Sci USA. 1994; 91: 6491–6495. PMID: 8022810

49. Bouckaert R, Heled J, Kühnert D, Vaughan T, Wu CH, Xie D, et al. BEAST 2: A Software Platform for Bayesian Evolutionary Analysis. PLoS Comput Biol. 2014; 10: e1003537. https://doi.org/10.1371/journal.pcbi.1003537 PMID: 24722319

50. Leigh JW, Bryant D. PopART: Full-feature software for haplotype network construction. Methods Ecol Evol. 2015; 6: 1110–1116. https://doi.org/10.1111/2041-210X.12410

51. Swofford DL. PAUP*: Phylogenetic Analysis Using Parsimony. Version 4.0b. Sunderland, Massachusetts: Sinauer Associates. 2002.

52. esri.com [Internet]. Redlands, California; Environmental systems research institute. ArcGis Desctop: Release 10 [cited 2017 Nov 14]. Available from: http://support.esri.com/en/Products/Desktop/arcgis-desktop/arcmap/10-4-1

53. ngdc.noaa.gov [Internet]. 2-minute gridded global relief data (ETOPO2v2) June, 2006. World data service for geophysics, boulder [cited 2017 Nov 14]. Available from: https://ngdc.noaa.gov/mgg/global/etopo2.html.

54. Knowlton N. Molecular genetic analyses of species boundaries in the sea. Hydrobiologia. 2000; 420: 73–90.

55. Knowlton N. Sibling species in the sea. Annu Rev Ecol Syst. 1993; 24: 189–216.

56. GBIF Secretariat. GBIF Backbone Taxonomy. Checklist Dataset [cited 2017 Nov 14]. https://doi.org/10.15468/39omei Available from https://www.gbif.org/species/2326250

57. Andersen BG, Borns HW. The ice age world: An introduction to quaternary history and research with emphasis on North America, and Northern Europe during the last 2.5 million years. Oslo: Scandinavian University Press; 1997.

58. Meißner K, Bick A, Guggolz T, Götting M. Spionidae (Polychaeta: Canalipalpata: Spionida) from seamounts in the NE Atlantic. Zootaxa 2014; 3786(3): 201–245. http://dx.doi.org/10.11646/zootaxa.3786.3.1

59. Holthe T. Polychaeta Terebellomorpha. Marine Invertebrates of Scandinavia, 7. Oslo: Norwegian Universities Press; 1986.

60. Nygren A, Pleijel F. From one to ten in a single stroke—resolving the European Eumida sanguinea (Phylodocidae, Annelida) species complex. Mol Phylogenet Evol. 2011; 58: 132–141. https://doi.org/10.1016/j.ympev.2010.10.010 PMID: 21034835
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62. Grassle J, Grassle JF. Sibling species in the marine pollution indicator *Capitella* (Polychaeta). Science. 1976; 192: 567–569. PMID: 1257794