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

Aurospio tribranchiata sp. nov. Paterson & Soto

(Figure 1, 8 d, 10, 11, 12, Table 3)

Minuspio sp4 Soto 2008: 89.

Material examined: 33 specimens examined in total.

Holotype: Porcupine Abyssal Plain, RRS Discovery D229, July 1997, 13200#20, 48º49.8’ N, 16º29.62’ W, 4844 m, 1–3 cm, 0.3 mm (NHMUK 2015:1199).

Paratypes: Porcupine Abyssal Plain, RRS Challenger 79, May 1991, 52701#9, 48º51.6’ N, 16º27.4’ W, 4842 m, 2 individual; 52701#25, 48º50.4’ N, 16º29.6’ W, 4844 m, 3 individuals; 52701#47, 48º50.6’ N, 16º29.9’ W 4841 m, 1 individual.

Porcupine Abyssal Plain, RRS Discovery D222, August 1996, 12930#14, 48º50.92’N, 16º30.24’W, 4837 m, 1 individual; 12930#44, 4849.95’N, 1630.2’W, 4839 m, 1 individual; 12930#59, 4850.45’N, 1630.58’W, 4837 m, 1 individual; 12930#68, 48º49.92’N, 16º29.76’W, 4840 m, 2 individual; 12930#73, 48º50.08’N, 16º29.69’W 4839 m, 2 individuals.

1) Aurospio tribranchiata sp. nov. a) Chaetiger 2; b) Chaetiger 3; c) Chaetiger 4; d) Chaetiger 5; scale bar = 100 µm. 2) Aurospio abranchiata sp. nov. from PAP a) Chaetiger 2; b) Chaetiger 3; c) Chaetiger 4; d) Chaetiger 5; e) Chaetiger 8; scale bar = 100 µm.

3) Aurospio abranchiata sp. nov. from canyons a) Chaetiger 2; b) Chaetiger 3; c) Chaetiger 4; Chaetiger 5; scale bar = µm.

Porcupine Abyssal Plain, RRS Discovery D226, March 1998, 13077#19, 4850’N, 1630.05’W, 4846 m, 4 individual; 13077#23, 4849.28’N, 1630.56’W, 4844 m, 1 individual; 13077#59, 48 49.98N, 16 29.96W, 4845 m, 1 individual; 13078#13, 48º50.00’N, 16º30.0’W, 4843 m, 1 individual.

Porcupine Abyssal Plain, RRS Discovery D229, July 1997, 13200#17, 4849.67’N, 1628.72’W, 4843 m, 2 individuals; 13200#20, 48º49.8’ N, 16º29.62’ W, 4844 m, 3 individuals; 13200#47, 4839.29’N, 1630.27’W, 4844 m, 1 individual; 13200#52, 48º49.84’N, 16º29.84’W, 4844 m, 2 individuals.

Porcupine Abyssal Plain, RRS Discovery D231, March 1998, 48 50.51’N 16 29.43’W, 4842 m, 1 individual; 13368#25, 48º 50.51’N, 16º 29.43’W, 3 individuals.

Porcupine Abyssal Plain, RRS Discovery D237 September 1998, 13627#17, 4849.9’N, 16 49.9’W, 4837 m 2 individuals.

Diagnostic features: three pairs of branchiae, first pair starting on chaetiger 3 and the last pair being much longer than the other two.

Description. Small slender species with thorax widening posteriorly then staying the same width; holotype fragment with 20 chaetigers, 3.00 mm long; 0.18 mm wide at chaetiger 1 (not including the chaetae). Colour cream to pale yellow.

Prostomium rounded to rectangular; caruncle long extending to anterior edge of chaetiger 2, well developed, then appearing to extend weakly beyond segment boundary. No eyes. Peristomium separated ventrally and laterally from chaetiger 1, fused dorsally forming a distinct rounded collar. Palps simple up to 5 chaetigers in length, no basal sheath.

Branchiae present on chaetigers 3 to 5; all apinnate, cylindrical, tapering gradually to a rounded point; branchiae on chaetigers 3 and 4 short, about the same length as the accompanying dorsal lamellae; branchiae on chaetiger 5 long, up to five chaetigers in length and two to three times the length of the accompanying dorsal lamellae (Fig. 11.1d; 12 a,c, 13a,c.

Chaetiger 1 with small rounded notopodial lamellae. Notopodial lamellae increase in size with the largest on chaetigers 4 and 5; lamellae triangular on chaetigers 2 and 3, becoming wider and more rounded on chaetigers 5 and 6. Notopodial lamellae in mid-body low and rounded. Dorsal crests low difficult to see but occur on chaetigers 9 to 12.

Neuropodial lamellae on chaetiger 1 small rounded, increasing over chaetigers 2 to 9; lamellae largest on chaetiger 3 (Fig 12 b).

Capillaries arranged on two rows on notopodia and neuropodia, capillaries bilimbate with those in the lower row slightly granulated. Sabre chaetae robust, curved, limbate, slightly or non-granulated, starting on chaetiger 10 or 11; one per fascicle. Neuropodia hooded hooks start on chaetiger 12, up to six per fascicle; two to three pairs of small teeth above the main fang (specimens small difficult to see the exact arrangement), no inner hood; Notopodial hooded hooks not observed. Pygidium unknown.

Methyl green pattern. Band of stain across middle of prostomium or across whole prostomium, diffuse over body.

Remarks. A. tribranchiata sp. nov. is assigned to Aurospio because the branchiae start on chaetiger 3. All specimens of A. tribranchiata, were carefully examined stained with Shirlastain A (SDL international. A textile fibre identification stain - very useful in revealing features, such as scars) and were not able to detect the presence of scars on any of our specimens. As further supporting evidence there was no branchial pair on chaetiger 2 or evidence of their scars even on otherwise complete specimens which had very long branchiae of chaetiger 4 (which could be easily lost) and palps (almost always lost in spionids) still attached. Based on current evidence we have assigned this species to Aurospio until proven otherwise. Other characters such as the shape of the prostomium, lack of an internal secondary hood in the hooded hooks also are common to species in this genus (but see Remarks in the previous species for a discussion on the problems of defining this genus).

A. tribranchiata sp. nov. is similar to A. pilkena Wilson, 1990 and A. banyulensis (Laubier, 1963), in particular the form of the prostomium, having three pairs of branchiae (see Table 4). A. banyulensis also has dorsal crests starting on chaetiger 8, hooded hooks starting on chaetiger 12 and sabre chaetae on chaetiger 10. A. tribranchiata sp. nov. differs in the arrangement of the third pair branchiae which are long whereas on A. pilkena and A. banyulensis they are all the same size. A. longibranchiata differs from A. pilkena in the starting positions of the sabre chaetae and hooded hooks and having dorsal crests (see Table 4).

Etymology. tribranchiata refers to the three pairs of branchiae found on this species. Ecology. A. tribranchiata sp. nov. is one of the dominant species of spionids found on the Porcupine Abyssal Plain, found in densities of between 4 and 24 individuals per m2. This species was found within the sediment at depths between 0–5 cm.

Distribution. This species has only been recorded from the Porcupine Abyssal Plain 48°N, 16° W (Northeast Atlantic Ocean) and from a depth of 4800 m.