Exitomelita lignicola Tandberg & Rapp & Schander & Vader 2013, sp. nov.

Exitomelita lignicola sp. nov. Tandberg and Vader

(Figures 2–6)

Type material

Holotype ZMBN 87905. Paratypes ZMBN 87906, ZMBN 87907, ZMBN 87908.

Type locality

The material was found in a large wood fall of Siberian pine at the south-eastern Knipovich Ridge in the Norwegian Sea. 73 ◦ 33.19 ′ N, 08 ◦ 16.9 ′ E, 2830 m depth.

Material examined

In addition to the four type specimens 38 additional specimens were found in the same piece of wood (see Table 1).

Morphological description

Adult size from 15 to 20 mm. Pereon elongate and smooth, with metasome and urosome bearing dorsal teeth (Figure 2).

Body. All segments of approximately equal length. Normal proportions and shapes of coxae, with coxa 2 slightly overlapping coxa 1 and coxa 4 with excavate posterior margin. Head and pereon dorsally smooth, pleon with small teeth at each segment of both metasome and urosome, metasome segments 2 and 3 with one medium and one small tooth at each side of dorsal midline, segment 3 also with a very small roundish median tooth (Figure 2); urosome segments 1 and 2 each with two small teeth, segment 1 also with minute simple setae next to the tooth.

Head (Figures 1B, 2). Slightly longer than pereonite 1, with small, rounded rostrum one-tenth of head length; lateral lobe rounded, with small notch ventrally; no apparent eye.

Antenna 1 (Figure 3). Almost double length of antenna 2 (18: 10) and four-fifths of body length (rami not included); all articles of peduncle elongate, article 2 slightly longer than article 1, article 3 half length of article 2; flagellum 1.5 × length of peduncle, 18–30 articles, the most distal bearing few small and simple setae; accessory flagellum 0.7 × length of peduncle article 3, five-articulate, articles 1–4 equal length, fifth article very small, all bearing short simple setae.

Antenna 2 (Figure 3). Peduncle articles 4 and 5 subequal in length, both bearing a few short simple setae, article 2 with gland cone ventrally; flagellum 12-articulate, subequal in length to peduncle articles 4+5, first article three times longer than the other flagellum articles, all bearing thin short setae.

Mouthparts (Figures 1, 3). In lateral view forming a quadrate bundle.

Labrum (Figure 3). Rounded, curved over front of mouthparts, ventral margin weakly indented, densely covered by minute setae.

Mandible (Figure 3). Incisor crenulate; lacinia mobilis on right mandible, serrate; raker setae plumose, as long as incisor; molar cylindrical triturative, chewing plate striate, accessory setae along outer margin of molar; palp three-articulate, third article rectilinear and tapering, slightly thinner than articles 1 and 2, two simple setae at apex and two simple setae along distal part of article 2, the percentage of total length of the articles of the palp are 25% (article 1), 30% (article 2) and 45% (article 3).

Labium (Figure 3). Inner lobes small but well defined, rounded; outer lobes rounded at distal margin, thin, short simple setae along mediodistal margin.

Maxilla 1 (Figure 3). Inner plate subtriangular, somewhat fleshy, inner margin lined with a row of simple setae that become longer at the apex; outer plate elongate, with crown of 12 mitten-shaped cuspidate spinal-teeth; palp two-articulate with article 2 longer than article 1, article 2 slightly bent and with a row of six nearly simple setae – all bearing one denticulation near the apex (pointed mitten-shape) and a parallel row of 10 simple setae.

Maxilla 2 (Figure 3). Inner plate slightly shorter and slightly broader than outer plate, inner margin with row of thin serrate setae and row of thin plumose setae (setules fairly long), tip covered with simple setae, short and thin simple facial setae at upper 20%; outer plate with same rows of simple serrate and thin plumose setae, but only on the tip.

Maxilliped (Figure 3). Compact; inner plates large, fully cleft, distal margins with long setae, small simple facial setae; outer plates reaching three-quarters of way along palp article 2, blade-like, tip with long simple setae; palp four-articulate, article 2 three times as long as article 1, article 1 short and cube shaped; article 3 few simple setae at tip; article 4 inner margin serrate at distal quarter.

Pereon. Dorsally smooth.

Pereopod 1 (Figure 4). Coxa rounded distally, skewed hingeline and a small protuberance at posterior margin; basis straight with groups of long simple setae along both margins; ischium subquadrate, a row of simple setae along posterodistal margin; merus subrectangular, posterodistal margin with a cushion of flat serrate setae, distal margin with a row of long simple setae; carpus almost half length of basis, subtriangular, four oblique rows grouping four to six long serrate setae along outer posterior margin, posterior margin cushioned in flat serrate short setae, inner posterior and anterior margins with cushions of flat serrate setae; propodus subrectangular, four groups of two or three short simple setae along outer posterior margin, cushion of flat serrate setae reaching half of hind-margin, inner anterior and posterior margins with cushions of flat serrate setae reaching half length of margins and a patch of long simple setae proximal to palmar corner, palm minutely serrate, palmar corner rounded; dactylus as long as palm, curved, smooth.

Pereopod 2 (Figure 4). Coxa suboval; basis straight, longer than coxa, a few simple setae; ischium and merus subrectangular with quite acute distal margin; carpus triangular, shorter than propodus, three or four groups of four or five long serrate setae along posterior margin; propodus subrectangular, palm oblique with palmar corner well defined by a double notch, posterior margin with an elongate patch of flat serrate setae, two strong cuspidate setae on inner surface near palmar corner, palm with small simple setae, outer face naked, palm with five crenulations outside palmar setae; dactylus as long as oblique part of palm, curved to the inside of propodus, reaching stopping cuspidate setae, smooth inner margin, tip of dactylus with nail.

Pereopod 3 (Figure 5). Coxa similar size as coxa 2, subrectangular; leg straight, simple with few simple setae, all short except along proximal posterior edge of basis where they are longer; dactylus short (one-third of propodus), with notch and a small seta at posterior distal margin.

Pereopod 4 (Figure 5). Coxa as long as coxa 3, posteriorly excavate, broader than coxae 2 and 3 along distal margin; leg straight, simple and smooth, few simple setae mainly along posterior margin, long at proximal part of basis, short elsewhere; dactylus short (one-third of propodus), with simple setae and a small notch at distal posterior margin.

Pereopod 5 (Figure 5). Coxa small and oval, basis straight, broader than coxa, with posterior border slightly crenulate and posterodistal corner rounded; ischium subquadratic; merus, carpus and propodus straight and slim, approximately same length on all articles; dactylus one-third of propodus length, with nail and simple setae.

Pereopod 6 (Figure 6). Coxa size as coxa 5, bilobed with pronounced, small hindlobe; basis straight and very weakly crenulate; ischium subquadratic; merus, carpus and propodus slim and elongate, length approximately 1: 1: 1.2, few very short simple setae; dactylus short one-third of propodus length, with nail and a small simple seta.

Pereopod 7 (Figure 6). Coxa smaller than coxae 5 and 6, almost round; basis straight, margins slightly crenulated, small simple setae along anterior margin; ischium subquadratic; merus, carpus and propodus slim and elongate, length approximately 1: 1: 1.5, few very short simple setae mainly along anterior margins; dactylus short, one-quarter of propodus length, smooth save for a short simple seta.

Pleopods. Normal, no sexual dimorphism.

Epimeral plate 3 (Figure 2). Posterodistal corner acute, with small marked upturned tooth, distal margin with three very small simple setae.

Urosome. Small teeth on urosomite segments 1 and 2 (Figure 2).

Uropod 1 (Figure 2). Peduncle with strong and curved inter-ramal spur, reaching one-fifth of way along rami; rami subequal in length, shorter than peduncle length; both peduncle and outer ramus with tiny simple setae along outer edge, both rami with a single tiny simple seta at tip.

Uropod 2 (Figure 2). Peduncle shorter than rami (0.75 × rami length), with very short blunt inter-ramal spur; rami subequal in length (inner ramus slightly shorter), few simple setae along outer margin; uropod 2 shorter than uropod 1.

Uropod 3 (Figure 2). Peduncle slightly shorter than peduncle of uropod 2, three short simple setae at distal margin; strongly elongate two-articulate outer ramus (seven times length of inner ramus, three times longer than urosome), with small simple stout setae spaced along the length of article 1, article 2 minute, barely distinguishable from the crowning setae; inner ramus short and ovoid, articulating from inner margin of peduncle, bearing two tiny simple setae at tip.

Telson (Figure 2). Fully cleft, formed by two ovoid slightly pointed parts; a small single simple strong seta at the inner distal margin of each of the parts; telson as long as peduncle of uropod 3.

Colour of living animals. White.

Sexual dimorphism. No observed morphological sexual dimorphisms.

Variations. None in the examined specimens.

Etymology

Named lignicola from the Latin words lignum = wood, cola = living on / inhabiting; lignicola is a noun in apposition.

Molecular identification

Partial COI sequences (DNA barcodes) were obtained from three specimens. There was no variation between the sequences. The sequences are available in GenBank under the accession numbers JQ775390, JQ775391 and JQ775392. The holotype is the paragenophore of these sequences (Pleijel et al. 2009).

Ecological considerations and in situ observations

All specimens of E. lignicola were found living in channels and grooves inside the wood fall, and no specimens could be observed on the outer surfaces of the wood in situ.

Partly digested wood was found in the intestine of this species and it therefore seems to fill the same niche as another melitid / maerid genus in warmer and more southerly waters – Bathyceradocus Pirlot, 1934 – where Barnard (1961) found wood particles in the intestine of Bathyceradocus stephenseni found in the Pacific during the Galathea expedition, and Larsen and Krapp-Schickel (2007) found that Bathyceradocus wuzzae was the most common species in the samples of wood droppings around the Juan de Fuca vent system. The third species of this genus, Bathyceradocus iberiensis, has also been reported from wood falls off the Iberian Peninsula (Andres 1977). The wood fall was found approximately 16 km to the east / north-east of the Loki Castle hydrothermal vent field where the only other (and vent-adapted) Exitomelita (E. sigynae) was found (Tandberg et al. 2012). The same wood fall was also densely populated by the small gastropods Pseudosetia griegi (Friele, 1879) and Skenea profunda Friele, 1879, species that are also known from vents and seeps in the northernmost Atlantic and Arctic Oceans (Schander, Rapp, Kongsrud et al. 2010; Pedersen, Rapp et al. 2010; German et al. 2011).