Aurospio abranchiata, Paterson, Gordon L. J., Neal, Lenka, Altamira, Iris, Soto, Eulogio H., Smith, Craig R., Menot, Lenaick, Billett, David S. M., Cunha, Marina R., Marchais-Laguionie, Claire & Glover, Adrian G., 2016

Paterson, Gordon L. J., Neal, Lenka, Altamira, Iris, Soto, Eulogio H., Smith, Craig R., Menot, Lenaick, Billett, David S. M., Cunha, Marina R., Marchais-Laguionie, Claire & Glover, Adrian G., 2016, New Prionospio and Aurospio Species from the Deep Sea (Annelida: Polychaeta), Zootaxa 4092 (1) : 24-28

Aurospio abranchiata sp. nov. Neal, Paterson & Soto

( Figures 8 c, 10, 13, 14, Table 3 )

Prionospio sp B: Paterson et al., 2011

Material examined: 97 specimens examined in total.

Holotype: Cascais canyon: RRS Charles Darwin, cruise 179, April–May 2006, St. 56823#2 2006.04.27, 38°18.01’ N, 09°47.02’ W, 3218 m, megacore (NHMUK 2015:1043).

Paratypes: Portuguese margin: Nazaré canyon RSS Discovery 297 August 2005, St. 15755#1 39º30.62 ’N, 09º56.19’ W, 3461 m, 3 individuals; St. 15760#1, 39º30.02’ N, 09º56.17 ’W 3465 m, 5 individuals; St. 15762#1, 39º30.02’ N, 09º56.22’ W, 3464 m, 2 individuals; St. 15758#6, 39º34.99’ N, 10º19.00’ W, 4335 m, 5 individuals; St. 15765#2, 39º35.00’ N, 10º19.04’ W, 4336 m 4 individuals.

RSS Charles Darwin cruise 179 April–May 2006, St. 56851#1, 39°29.99’ N, 09°55.97’ W, 3517 m, 2 individuals; St. 56851#2, 39°29.99’ N, 09°56.01’ W, 3517 m, 2 individuals.

Setúbal canyon: RSS Charles Darwin cruise 179 April–May 2006, St. 56804#5 2006.04.21 38° 09.27’N 09° 36.93’W 3275m 8 individuals; St. 56804#6 2006.04.21 38° 09.26’N 09° 36.94’W 3275m, 10 individuals; St. 56806#1 2006.04.21 38° 09.29’N 09° 36.96’W 3275m, 10 individuals; St. 56810#1 2006.04.23 38° 09.22’N 09° 37.02’W 3224m, 1 individual; St. 56816#1 2006.04.25 38° 09.27’N 09° 36.94’W 3275m, 14 individuals; St. 56842#1 2006.05.0 5 38° 06.45’N 09° 59.94’W 4482m, 2 individuals.

Cascais canyon: RSS Charles Darwin cruise 179 April–May 2006, St. 56823#2 2006.04.27 38° 18.01’N 09° 47.02’W 3218m, 9 individuals; St. 56828# 1 18.02’N 09° 46.98’W 3199m, 1 individual.

PA P: RRS Challenger II 79 May 1991: 52701 # 5 48º51.0’N 16º30.0’W, 4840 m, 1 individual.

RRS Discovery D222 September 1996: 12930#39 48º49.95’ N, 16º29.4’ W, 4840 m, 1 individual.

RRS Discovery D226 March 1998: 13077#23, 48º49.28’ N, 16º30.56’ W, 4844 m, 2 individuals.

RRS Discovery D229 July 1997: 13200#20, 48º49.8’ N, 16º29.62’ W, 4844 m, 2 individuals; 13200#47 48º39.29’ N, 16º30.27’ W, 4844 m, 1 individual.

RRS Discovery D231 March 1998: 13368#36, 48º49.78’ N, 16º30.17’ W, 4845 m, 2 individuals; 13368#42, 48º50.08’ N, 16º29.88’ W, 4844 m, 1 individual; 13368#44, 48º 49.7’ N, 16º30.12’ W, 4844 m, 1 individual.

Diagnostic features. Lack of branchiae, enlarged square-shaped notopodial lamellae on third segment; neuropodial lamellae largest on chaetiger 3 forming long, rectangular with rounded edges, wing-like structure which sometimes extends horizontally away from body.

Description. Small and slender species, holotype incomplete with 45 segments, measuring 5.65 mm long for 45 segments and 0.2 mm wide (width at chaetiger 1). Colour in alcohol pale yellow. Body narrow and cylindrical, of uniform width throughout. Prostomium rounded, anterior margin entire, broadly rounded, prostomial peaks absent; short blunt caruncle, extending to anterior of chaetiger 1; eyes not observed. Peristomium well developed, ventrally and laterally distinct, dorsally partially fused to chaetiger 1, forming a collar around prostomium, without forming distinct lateral wings.

Branchiae absent. Notopodial and neuropodial postchaetal lamellae reduced in chaetiger 1; notopodial lamellae on chaetiger 2 nearly square-shaped, meeting at midline and covering the dorsum in larger specimens but more widely separated in smaller ones (Fig. 11.2a,3a); largest on chaetiger 3; lamellae large, nearly square, almost meeting at the midline, covering the entire dorsal surface of the segment in large specimens but not in smaller ones (Fig.11.2b,3b). Notopodial postchaetal lamellae on chaetigers 4–5 smaller, about half the size of those on chaetiger 3, changing from square to rounded shape from chaetiger 6 onwards, all well separated, not covering the dorsum (Fig11.2c,2d,3c,3d). Distinct dorsal crests from chaetiger 8–12. Interparapodial pouches absent.

Neuropodial lamellae on chaetiger 2 square-shaped with rounded corners; on chaetiger 3 very large, rectangular with rounded edges (somewhat indented on outer rim), wing-like structure which sometimes extends horizontally away from body. The neuropodial postchaetal lamellae in other segments becoming more rounded and of similar size.

Capillaries in two rows in both notopodia and neuropodia. Sabre chaetae present singly from chaetiger 10 in neuropodia; stout, strongly curved, no granulation. Neuropodial hooded hooks present from chaetiger 11–12, up to 4–5 par fascicle; at least five pairs of small teeth above the main fang; square-shaped primary hood, secondary hood present, rudimentary. Notopodial hooks present singly from chaetiger 38. Pygidium unknown.

Methyl green pattern. Strong stain remains on sides of segments 5–17, even six months after staining.

Remarks. The absence of branchiae together with the shape and size of both notopodial and neuropodial lamellae on third segment are diagnostic characteristics of this species. The absence of branchiae poses a difficulty in assigning this species to a genus with certainty. Within the Spionidae the genera Spiophanes Grube, 1860 and Spiogalea Aguirrezabalaga and Ceberio, 2005 , are characterised by a lack of branchiae. However, the specimens described here do not belong to these genera because they lack modified, hook-like setae in the neuropodia of chaetiger 1, which are found in Spiophanes and the two chitinous plates surrounding the peristomium, which are characteristic of Spiogalea . The specimens also lack the characteristic arrangement of neuropodial chaetae found in the mid-chaetigers of Spiophanes described by Meißner & Hutchings (2003) and Meißner (2005).

In overall appearance this species bears close resemblance to species belonging to the Prionospio complex. Our species does not seem to belong to either Paraprionospi o or Orthoprionospio based on the first segment having chaetae and the first chaetiger not being completely separated from peristomium usually found in Paraprionospi o and Orthoprionospio . The genus Streblospio can be eliminated because our specimens lack a ridge across chaetiger 1 and a prominent hood or collar across the dorsum of chaetiger 2; the form of the neuropodial multidentate hooks is also different.

The problem of generic assignment arises when trying to determine whether the species belongs to the genus Aurospio or Prionospio . Aurospio was separated from Prionospio primarily on branchiae staring from third chaetiger, their shape, and that they are partially fused at their base to the notopodia. An additional character cited by Maciolek (1981) is the absence of a secondary hood in the hooks of Aurospio . Sigvaldadottir (1998) emphasised a lack of prostomial peaks, a short caruncle reaching to the anterior margin of chaetiger 1; pointed dorsal lamellae in mid-body segments, the absence of dorsal crests, and sabre chaetae starting on chaetiger 10 or earlier in her analyses of Prionospio .

However, A. banyulensis (Laubier, 1966) and A. pilkensis (Wilson, 1990) do not have the branchiae fused to the base of the notopodia. Also, many species of Prionospio do not have secondary hoods in the neuropodial hooded hooks (Wilson 1990). Many of the characters highlighted by the cladistic analyses of Sigvaldadottir (1998) are actually also found in species of Prionospio . For example, short caruncles can be found in P. coorilla Wilson, 1990 ; P. pulchra Imajima, 1990 ; P. somaliensis Cognetti-Varriale, 1988 ; and P. vari gat a Imijima, 1990. Such overlap blurs the distinction between the genera and so we are left with the characters of the branchiae. Our species lacks these key characters. Thus it becomes difficult to place this new species into a genus. We do not believe that the absence of branchiae justifies erection of a new genus or subgenus within Prionospio –complex, but provides further support for view of Wilson (1990) and Sigvaldadottir (1998) that the differentiation of genera/subgenera based primarily on branchial arrangement is not justified. Therefore, based on the overall body shape, shape of the prostomium and shape of the dorsal lamellae, particularly the pair associated with chaetiger 3, this new species most closely resembles the characters displayed by species of Aurospio .

Aurospio dibranchiata , A. foodbancsia Mincks et al., 2008, and A. banyulensis appear to have large rectangular-shaped dorsal lamellae on chaetiger three, often orientated towards the mid-line. By contrast the dorsal lamellae of chaetiger 3 in Prionospio are usually described as foliose or auricular and pointed with a rounded tip. A. pilkena is an exception to this rule, having been placed in Aurospio presumably because the branchiae start on chaetiger 3.

However, such characters have not been completely assessed by comparison with all other species of Prionospio and the assignment to Aurospio is principally by the general impression of the specimens, in particular the rounded shape of the prostomium. This is not a satisfactory conclusion. A major review of the generic boundaries, using molecular as well as morphological characters, will be needed to resolve this problem.

Etymology. abranchiata —refers to the absence of branchiae.

Ecology. A. abranchiata sp. nov. was the second most abundant spionid species found in the canyons of the Iberian margin by the HERMES programme. However, it achieved higher densities in mid-depth canyons (around 3400 m) than in deep-water canyons with highest densities found in Cascais canyon, where it replaced P. vallensis sp. nov. as the most dominant spionid species. Cascais canyon is considered to be the most quiescent of the three canyons studied along the Portuguese margin. Cascais canyon also had the lowest abundance, but highest diversity and evenness, of polychaete assemblages. Aurospio abranchiata sp. nov. was also found in Setúbal canyon at 3400 m during RRS Discovery cruise 186 in 1989, where it was highly dominant, contributing nearly 30% of the total polychaete abundance. The increased density during D186 compared with densities recorded during the HERMES program is difficult to interpret as different mesh-sized sieves were used. A. abranchiata sp. nov. is a very slender species and larger numbers could have been caught in the 0.3-mm mesh used during D186 than in 0.5-mm and 1- mm mesh used during the HERMES sampling program.

The species is also quite numerous in the sediment samples from the Porcupine Abyssal Plain where it reaches abundances of 40 individuals per metre2.

The absence of branchiae provides an interesting insight into evolutionary adaptations in these deep-sea spionids. The deep-sea species of Aurospio — A.dibranchiata , A.foodbancsia , and A. abranchiata sp. nov. show a reduction both in the size of branchiae and also in the number. A. dibranchiata has two pairs of small branchiae, A. foodbancsia has one pair while A. abranchiata sp. nov. has no branchiae. The absence of branchiae in A. abranchiata sp. nov. could be compensated by extremely large notopodial lamellae, which are also a diagnostic feature of this species.

Distribution. Porcupine Abyssal Plain at 4800 m, Nazaré, Setúbal and Cascais canyons along Portuguese margin at 3199–4485 m.

Figures

FIGURE 8. Drawings of hooded hooks of a) Prionospio branchilucida sp. nov.; b) Prionospio kaplani sp. nov.; c) Aurospio abranchiata sp. nov.; d) Aurospio tribranchiata sp. nov. (left hook shown without hood). Bar scale = 0.001 mm (a – d). e) Prionospio amarsupiata sp. nov.; f) Prionospio hermesia sp. nov. g) Prionospio vallensis sp. nov. Bar scale = 0.001 mm (e – g).