Prionospio branchilucida Paterson, Neal, Altamira, Soto, Smith, Menot, Billett, Cunha, Marchais-Laguionie & Glover, 2016, sp. nov.

Prionospio branchilucida sp. nov. Altamira, Glover, & Paterson

(Figure 1, 2.3, 5, 8a, Table 2)

Material examined: 41 specimens examined in total.

Holotype: Kaplan Clarion-Clipperton Fracture Zone East Site, RV New Horizon, February, March 2003, BC 830 14˚ 55.85N 119° 2.97W 4076 m. specimen KP77 (NHMUK 2015: 1118).

Paratypes. Kaplan Clipperton-Clarion Fracture Zone East Site: RV New Horizon, February–March 2003, BC 847 15° 1.98N 119° 0.02W 4078 m, 1 individual.

Kaplan CCFZ Central Site (IFREMER Nodinaut campaign): RV L’Atalante, May–June 2004, BC 876/KGS17 14°3.4024’N 130°5.5851’W 5012 m, 2 individuals (KP319, KP333; BC 877/KGS19 14°2.9823’N 130°5.6489’W 5027 m, 3 individuals (KP338, KP348), KP; BC 879/KGS26 14°3.3980’N 130°5.5828’W 5012 m, 1 individual (KP368); BC 880/KGS27 14°2.7524’N 130°5.4972’W 5041 m, 1 individuals (KP379); BC 878/KGS20 14°3.4687’N 130°5.5994’W 5012 m, 1 individual (KP443).

ABYSSLINE 0 1 CCFZ: RV Melville October 2013: Stn A, 1352.900 N 11628.0 W 4171 m, 1 individual (CRS 1493, BC_03, in 2-5 cm fraction); Stn C, 1347.601 N 11642.185 W 4081 m, 1 individual (CRS 1504, BC_05, in 0- 2 cm fraction); Stn G, 1345.727 N 11627.824 W 4110 m, 2 individuals (CRS 1529, BC_09, in 5-10 cm fraction); Stn I, 1345.001 N 11630.799 W 4036 m, 2 individuals (CRS 1532, BC_10, in 0-2 cm fraction); Stn J, 1354.113 N 11635.442 W 4163 m, 2 individuals (CRS 1542, BC_13, in 0-2 cm and 2-5 cm fractions); Stn L, 1343.597 11640.200 4160 m, 1 individual (CRS 1545, BC_14, 0-2 cm sub-sample).

Equatorial Pacific (EqPac): RV Thomas Thompson: EqPac November 1992 BC15 5°N 140°W m. 4 individuals

Cap Verde Abyssal Plain: RSS Discovery 12600# 10 September 1993, 0–1 cm, 21º 3.2’ N 31º 11.0’ W 4543 m, 4 individuals;

12600#28, October 1993, 0–1 cm, 21º 4.8’ N 31º 11.1’ W 4613 m, 1 individual;

12600#32, October 1993, 0–1 cm, 21º 3.6’ N 31º 10.0’ W 4545 m, 1 individual.

Madeira Abyssal Plain: RSS Discovery 12174 #11 August 1990, 1– 3 cm, 31° 4.4’ N 21° 10.3’ W 4936 m, 1 individual; 12174#11 August 1990, 3– 5 cm, 31° 4.4’ N 21° 10.3’ W 4936 m, 1 individual; 12174# 16 August 1990, 0–1 cm, 31° 4.9’ N 21° 9.4 W 4947 m, 1 individual; 12174#16 August 1990, 1– 3 cm, 31° 4.9’ N 21° 9.4 W 4947 m, 1 individual; 12174#43 August 1990, 3– 5 cm, 31° 5.3’ N 21° 10.9’ W 4947 m, 1 individual; 12174#53 August 1990, 3– 5 cm 31° 5.2’ N 21° 1.2’ W 4942 m, 1 individual; 12174#60, August 1990, 0–1 cm 31° 6.0’ N 21° 10.0’ W 4947 m, 2 individuals; 12174#60, August 1990, 1– 3 cm 31° 6.0’ N 21° 10.0’ W 4947 m, 1 individual; 12174#60, August 1990, 3– 5 cm 31° 6.0’ N 21° 10.0’ W 4947 m, 2 individuals.

Other material studied. Kaplan Clipperton-Clarion Fracture Zone East Site: RV New Horizon, February– March 2003, BC 822 14° 57.34N 119° 1.18W 4038 m, 1 individual; BC 834 14° 59.95N 118° 58.07W 3982 m, 1 individual;

Diagnostic features. Two pairs of branchiae on chaetigers 2 and 3, sabre chaetae lacking, dorsal crests from chaetiger 7, neuropodial hooded hooks from chaetiger 11.

Description. Body cylindrical, narrow throughout; holotype 1.86 mm long for 20 chaetigers, (0.62 mm up to chaetiger 10); 0.08 mm wide at chaetiger 1. Prostomium rectangular with flat, entire anterior margin; one pair prostomial peaks present; caruncle short with rounded apex, extending to anterior margin of chaetiger 2. Eyes absent. Peristomium incomplete dorsally, not forming lateral wings, separate from chaetiger 1 (Fig.5 a).

Branchiae from chaetiger 2, two pairs, very small. Branchiae on chaetiger 2 digitiform, blunt-tipped, inserted lateral and slightly posterior to dorsal lamellae (Fig. 5 c,d); branchiae approximately one-half chaetiger long, subequal to notopodial lamellae; branchiae on chaetiger 3 conical, inserted posterior to dorsal lamellae and obscured by them, one quarter the length of lamellae (Fig. 2.3b)

Notopodial lamellae start on chaetiger 2, sub-quadranular, slightly angled toward dorsal midline; largest on chaetiger 3, sub-quadrangular with slightly produced tips overlapping dorsal midline, three times as long as lamellae of chaetiger 2; thereafter, notopodial lamellae decrease in size, becoming low and subtriangular by chaetiger 7 (Fig. 2.3f). Dorsal crests present from chaetigers 7 to chaetiger 17.

Neuropodial lamellae start on chaetiger 1, papilliform, length equal to one-fifth chaetiger length; neuropodial lamellae on chaetigers 2–5 leaf-like, narrow with short tips; neuropodial lamellae largest on chaetiger 3, length equal to chaetiger length and nearly twice that of chaetiger 2, neuropodial lamellae thereafter decreasing in length and increasing in width, becoming broadly ovoid by chaetiger 9. Interparapodial pouches absent.

Notochaetae limbate capillaries, long, inserted in two densely packed rows from chaetigers 2–8 with up to 12 chaetae per row; thereafter, notochaetae reduced in number and density; all notochaetae angle forward on chaetigers 1–8 and anterior row always with shorter. Neurochaetae limbate capillaries, inserted in two distinct rows to chaetiger 8, up to eight chaetae per row, anterior row always shorter; thereafter, number of chaetae decrease, rows become indistinct. Neuropodial sabre chaetae not observed through 20 chaetigers (all specimens incomplete, the longest fragment comprises 30 chaetigers). Neuropodial hooded hooks present from chaetiger 11; shaft narrow with slender main fang, surmounted by two to three fine small teeth in one row, up to six chaetae per fascicle, inner hood absent; notopodial hooded hooks not observed. Pygidium not observed.

Methyl green pattern. Banding on subdistal portion of prostomium and distal portion of peristomium; partial banding on anterior chaetigers, beginning at bases of notopodial lamellae, and encircling venter

Remarks. This species is characterised by the small translucent digitiform branchiae on chaetiger 2, having only three pairs of branchiae and by the lack of sabre chaetae.

Few species of Prionospio have only three pairs of branchiae, a character more commonly found in the genus Paraprionospio. Only P. aucklandica Augener, 1923 has been recorded with three pairs but in that species the branchiae are all pinnate, dorsal crests occur only on chaetiger 7, and sabre chaetae are present from chaetiger 10. P branchilucida sp. nov. resembles P. h e r m e s i a sp. nov. in apinnate branchiae and double rows of anterior chaetae; the two species differ in the number of branchiae, the form of the first pair of branchiae, extent of caruncle, and relative size of branchiae and notolamellae (see Table 2).

Etymology. branchia —a gill and lucida —meaning clear, referring to the transparency of the first pair of branchiae.

Distribution. This species has been recorded from the Central Pacific, the Equatorial Pacific (EqPac), and the Madeira and Cap Verde Abyssal Plains in the Northeast Atlantic. Bathymetric distribution ranges from 4800 m to 5041 m.