Laonice (Sarsiana) alberti Sikorski, Langeneck & Pavlova 2021, sp. nov.

Laonice (Sarsiana) alberti Sikorski, Langeneck & Pavlova sp. nov.

(Figures 1, 2 A–H, 4)

LSID: urn:lsid:zoobank.org:act: 4D214346-F738-43B2-AEE0-A6D82B53EDA8

Laonice sp.— Langeneck et al. 2017: 143.

Laonice (Sarsiana) sp. A—Langeneck et al. 2019: 87.

Material examined. Holotype (MOM INV-0022694) and paratype (MOM INV-0022695): Malta Escarpment, Central Mediterranean Sea, R / V Urania, Project BIOFUN, St. 1, 36.5310°N 15.4340°E, 1200 m, 8– 19.05.2009. Paratype (MNCN 16.01 /18547): Balearic slope, NW Mediterranean Sea, R / V Urania, Project BIOFUN, St. 28 B 2400, 38.9433°N 3.6469°E, 2346 m, clayish silt, 07.05.2010. Paratype (DBUA0002347.01): open slope of Blanes submarine canyon, NW Mediterranean Sea, R / V Garca del Cid, Project DOS MARES, Cruise DM3, trawl code DM3 P2, 41.5055N 02.8500E, 1200 m, 10.10.2012.

Additional non type material, possibly belonging to the same species (DBUA0002235.03): Mercator mud volcano, Gulf of Cadiz, off Morocco, Northeast Atlantic Ocean, R / V Maria S. Merian, MSM 01-03, St. 242, 35.2978ºN 6.6468ºW, 350 m, 06.05.2006.

Description. All specimens incomplete (50 chaetigers in holotype), consisting of anterior fragments 0.8–0.9 mm wide.

Prostomium longer than wide, triangular (holotype) to bell-shaped (paratype MOM INV-0022695), with very small apical incision (Figs 1 and 2A); antero-lateral corners connecting to peristomium by very thin and nearly inconspicuous ventral folds hidden in the groove between prostomium and peristomium (Figs 1 and 2B). Prostomium narrowing backwards at first gradually until the fore level of the first pair of parapodia, and then abruptly, merging posteriorly with the caruncle. One pair of faint eyespots (paratype MOM INV-0022695). Occipital antenna very short, erect and finger-like, inserted at the end of the prostomium, just at the rear level of the first pair of parapodia. Palps lost from all specimens. Nuchal organs as double U-shaped ciliary bands extending posteriorly to chaetigers 5–7 (Fig. 2A). Body width uniform along its length.

Branchiae from chaetiger 2, continuing posteriorly to the end of fragments (chaetiger 38 in holotype; chaetigers 31–39 in paratypes,). Branchiae free, initially slightly longer than the notopodial postchaetal lamellae, increasing its length significantly just after the end of the nuchal organs and becoming twice as long as the notopodial postchaetal lamellae after chaetiger 10–12. Length of the longest branchiae up to half of body width.

Notopodial prechaetal lamellae not particularly conspicuous. Longest notopodial postchaetal lamellae at chaetigers 5–12, gradually diminishing posteriorly. Notopodial postchaetal lamellae of the first 3 segments with moderately pronounced upper tips (Fig. 2D), upper margins of all subsequent notopodial postchaetal lamellae broadly oval (Fig. 2 E–F).

Neuropodial prechaetal lamellae not visible. Neuropodial postchaetal lamellae triangular throughout the body (Fig. 2 D–E).

Continuous transverse dorsal crests absent in midbody segments, where the notopodial postchaetal lamellae are fused with quite long crests on the dorsal surface, however without connecting mid-dorsally (Fig. 2C).

Inter-parapodial pouches present from chaetigers 21–34, until the posterior end of the fragments.

Anterior parapodia with capillary chaetae arranged in two vertical rows both in notopodia and neuropodia. Neuropodial hooded hooks appearing from chaetiger 35–38, numbering 8–10 per bundle, appearing bidentate in the lateral view (Fig. 2G), but with paired apical teeth above main fang (Fig. 2H). Single sabre chaeta appearing in the lower part of the neuropodial bundle from chaetiger 20–22 (Fig. 2F).

Pygidium unknown.

No specific pigmentation observed.

Staining in Methyl Green. No specific staining pattern observed.

Etymology. The species is dedicated to Professor Alberto Castelli (University of Pisa, Italy), who provided the material that enabled the establishment of the new species, in recognition of his numerous and important contributions to the knowledge of the Mediterranean fauna of polychaetes.

Distribution. Northeast Atlantic Ocean, Mediterranean Sea: Blanes submarine canyon, Balearic slope and Malta escarpment, (Fig. 4). 1200–2346 m. Probably: Gulf of Cadiz (Mercator mud volcano), 350 m.

Remarks. Laonice (Sarsiana) alberti sp. nov. belongs to the subgenus Sarsiana due to the presence of short nuchal organs with a narrow range of length variation, and due to a prostomium and peristomium fused only by a membrane, inconspicuous when observed in dorsal view (although the membrane may also be hidden in the groove between prostomium and peristomium). While the material examined only showed neuropodial hooks, notopodial hooks might be present in the posterior-most parapodia of complete specimens, as they are known to occur in other species within the subgenus Sarsiana, and especially in those having prostomium and peristomium fused by a membrane. The new species has relatively short nuchal organs. Two species of the subgenus Sarsiana: L. (S.) papillibranchiae Ward, 1981 and L. (S.) junoyi Aguirrezabalaga & Ceberio, 2005 are characterized by having also short nuchal organs extending up to chaetigers 4–6 and 6–7 respectively (5–7 in L. (S.) alberti sp. nov.), though the prostomium is not fused with the peristomium in any way. In addition, inter-parapodial pouches in those species appear on chaetiger 12 and 8–9, respectively, against 21–34 in the new species. Two other deep-sea species, namely L. (S.) asaccata Sigvaldadóttir & Desbruyres, 2003, and L. (S.) shamrockensis Sikorski, 2003, also have short nuchal organs (up to chaetigers 4 and 5, respectively), but their prostomium is clearly fused with the peristomium in the anterior margin.