Dubinectes infirmus, a new species of deep-water Munnopsidae (Crustacea, Isopoda, Asellota) from the Argentine Basin, South Atlantic Ocean

Abstract Dubinectes infirmus sp. n., Munnopsidae, is described from the Argentine Basin, southwest Atlantic, at depths between 4586–4607 m. The new species is distinguished by a narrow rim of the pleotelson posterior margin which is not raising over its dorsal surface; article 3 of the antennula is subequal in length to article 2; distomedial lobes of male pleopod 1 are of same size as distolateral lobes; stylet of male pleopod 2 is subequal in length to protopod; uropod exopod is more than a half of endopod length. Some generic characters which are weakly pronounced in the new species or have different state are defined more precisely in the revised diagnosis of Dubinectes. The modified diagnosis of the genus, a key to the species of Dubinectes as well as the distribution of the genus are presented.


Introduction
Isopods of the family Munnopsidae were collected in 2009 during the DIVA 3 (Diversity of the Abyssal Atlantic) expeditions onboard RV Meteor in 2009 in the southwest Atlantic (M 79-1). Some known, rare munnopsid species originally described from ANDEEP (Antarctic benthic DEEP-sea biodiversity: colonization history and recent community patterns) collections, e.g. species of Microcope Malyutina, 2008 and Dubinectes Malyutina & Brandt, 2006, were identified in the DIVA 3 samples taken from the Argentine Basin. The genus Dubinectes was erected for four species from the southeast Atlantic and Weddell Sea: D. nodosus (Menzies, 1962) and D. acutitelson (Menzies, 1962), which were formerly placed in Eurycope Sars, 1864and D. intermedius Malyutina & Brandt, 2006and D. acutirostrum Malyutina & Brandt, 2006. Along with the known species, D. intermedius and D. nodosus a new species of Dubinectes was found in the DIVA 3 samples. Though the new species possesses the main diagnostic characters of the genus, some of these characters have different state in contrast to other species of Dubinectes and this allows to define more precisely the generic diagnosis. In this paper we describe a new species, the fifth species of the genus, present the revised diagnosis of Dubinectes, a key of the genus, new records of two known species, as well as the distribution of the genus.

Material and methods
Specimens were collected using an epibenthic sledge (Brenke 2005) and then fixed in pre-cooled 96% ethanol. The material was sorted in the laboratory and identified using a Leica MZ 12 stereomicroscope equipped with a camera lucida, and drawn using an Olympus SZX7 compound microscope, also equipped with a camera lucida.
The terminology and measurements mostly follow Wilson (1989). Total body length was measured medially from the tip of the rostrum to the posterior tip of the pleotelson. The dorsal view was used for measuring the width, while the length of body segments was measured in lateral view. For the description of the body and pereopods the holotype was used, for mouthparts and pleopods a male paratype was dissected and for some details that the paratype lacked or were different from male, a female paratype was used. Scale bars for total body figures are 1 mm.
Remarks. Before finding the D. infirmus sp. n. the key characters which helped to easily determine known species of Dubinectes without dissection were the strongly calcified integument of the body and the special shape of the pleotelson rim which is well projected ventrally and rises over the dorsal surface. These external features are only weakly pronounced in the new species. Nevertheless, it possesses these characters as well as the main set of diagnostic characters of Dubinectes. The new species has a rim of a pleotelson even if it is narrow, a rostrum which is longer than antennula article 1, a reduced distomedial lobe of the antennula article 1. The head and the ambulosome of the new species are of equal width, the pereonites are progressively shorter from 1 to 4; the coxa of pereopod 4 is broadest in lateral view; the mandible possesses a high sharp ridge between a short spine row and a broad molar; the condyle is longer than the molar; the distal margin of the male pleopod 2 protopod is truncated and the exopod is inserted close to the distal margin and emerged posteriorly. There are some characters which differ the new species from others. These are the shape of the distal margin of male pleopod 1, the length of the stylet of male pleopod 2, the length of the antennula article 3 as well as the length of uropod exopod. These characters in different states were suggested earlier as generic, but seem to be just specific characters. Therefore, the description of D. infirmus sp. n. allows to revise the diagnosis of the genus.
Distribution of the genus Dubinectes (Fig.10). The genus Dubinectes occurs in the Southern hemisphere only in the South Atlantic Ocean. Species of the genus were known from the southeast Atlantic and the Weddell Sea, Southern Ocean, from depths ranging between 1121 and 4960 m. New records the DIVA 3 expedition from the abyss of the Argentine Basin expand the geographical range for the genus to the southwest Atlantic until 27° S latitude in the north. Geographically, D. nodosus is the most widely distributed species of the genus. It is most eurybathic and has been sampled between 1121 to 4976 m. It was collected in the Weddell Sea at almost every station during the ANDEEP expeditions (Malyutina and Brandt 2007), but it was much less abundant in the DIVA collections. All other species were recorded only from abyssal depths between 4586 to 4976 m. D. acutirostrum and D. infirmus sp. n. are known only from their type localities. Diagnosis. Rostrum of head with rounded apex; antennula article 3 subequal in length to article 2; anterolateral margins of pereonites 6, 7, pleotelson and posterior rim of pleotelson weakly pronounced, not raised dorsally; pleopod 1 distomedial lobes of the same size as distolateral lobes; endopod stylet of male pleopod 2 subequal in length to protopod. Uropod exopod more than half of endopod length.
Antenna 1 of male (Fig. 4) article 1 1.05 times width, setation: 1 dorsal and 1 distolateral broom seta and 1 simple seta, 1 stout unequally bifid and 2 broom setae distomedially; article 2 0.6 times length and 0.4 width of article 1, with 1 simple and 2 distal broom setae; article 3 0.7 length of article 2 with 2 simple setae, article 4 0.3 times length of article 3, article 5 twice as long as article 4, following 11 flagellar articles subequal in length to article 4, some of the distal articles with an aesthetasc. Antenna 1 of female ( Fig. 9) article 2 length 0.65 times length of article 1, article 3 slightly longer (1.1) than article 1, article 4 length 0.35 times article 3 with broom seta and aesthetasc distally; flagellum of 5 articles, last one with 1 aesthetasc and 5 setae.
Antenna 2 of female ( Fig. 8) articles 1-4 subequal in size, length together 0.6 times body length, scale on article 3 nearly half the length of article 4. Article 5 almost as   wide and 2.1 times as long as articles 1-4 together, with 3 stout medial setae and 2 small simple distolateral setae, article 6 narrower and 1.1 times as long as article 5, with simple setae along article. Flagellum broken off, articles subequal in length to article 1. Mandibles (Fig. 4) longitudinal outer keel of body weakly pronounced, incisor process with 2 acute and 1 weak cusps; lacinia mobilis of left mandible triangular with many denticles, 0.8 times length of incisor process; spine row with 5 and 6 spines on left and right mandibles respectively; molar process 0.5 times as long and as broad as incisor process on proximal part, ventral margin of triturative surface with a few denticles and 3 setulose setae; ridge between spine row and molar with 1 pick; condyle 0.3 times body length, longer than molar; palp 0.9 times body length; article 1 with 2 distal setae, article 2 broadened at midlength, length 2.2 times article 1 length, with 3 stout distal setae and small marginal setae, article 3 0.75 of length and 1.15 of width of article 2, with 3 long simple distal setae and long comb-like marginal setae. Hypopharynx (Fig. 5) outer lobes 3.7 times as wide as inner lobes with dense distal fine setae and four lateral setae each.
Maxilliped (Figs 5,9) basis 2.7 times width, endite with 3 coupling hooks (2 in immature female), distal margin with 4-5 large fan setae and numerous simple slender setae; palp article 2 margins almost straight, lateral margin 1.3 times longer than medial margin; article 3 medial margin slightly convex, with low denticles and 5 setulose setae, 1.2 times longer than medial margin of article 2; article 4 2 times longer than articles 3 laterally, medial lobe almost not produced, with 4 distal setae, article 5 0.9 of article 3 laterally, with 7 distal setae. Epipod subequal in length to basis length 1.6 times width, lateral extension rounded, almost as broad as distal angle.
Pleopod 2 of female as long as wide, with medial keel, 2 long simple setae proximolaterally and 10 small simple setae on posterior margin (Fig. 3).
Uropod (Fig. 7) length 0.3 times pleotelson length; protopod as long as wide, with 5 long whip setae on medial projection and several distal setae, endopod 0.33 times as wide and slightly longer than protopod, with 1 lateral and one distal broom and 6 whip setae; exopod length 0.7 times endopod length, with 4 setae distally.
Distribution. Southwest Atlantic: northern Argentine Basin at depths between 4586-4605 m.
Etymology. The epithet infirmus (Latin) refers to the state of the generic characters of the species, which are less obvious than in other species.
Remarks. Dubinectes infirmus sp. n. differs from the four other species of the genus by weakly pronounced generic characters: a rim, surrounding the pleotelson posterior margin is narrow, not raising over the dorsal surface; lateral margins of the head are without an obvious ledge, anterolateral angles of pereonites 5-7 and pleotelson not produced dorsally. Article 3 of male antennula in the new species is shorter than article 2, in females and juveniles it is slightly longer than article 2 in contrast to other species which have article 3 nearly twice as long as article 2; pleopod 1 distolateral lobes are subequal in size to the distomedial lobes (in other species they are broader and longer); the endopod stylet of male pleopod 2 is subequal in length to the protopod (0.9) in contrast to state in other species which is approximately two times the length of the protopod; both endopod and exopod of the uropod are rather stout, shorter than in other species, the exopod is 0.7 times the endopod length (0.35 -0.5 in other known species). The new species is the most similar to D. nodosus in body shape and size of the rostrum, but the new species has a less calcified cuticle, a rounded tip of the rostrum, a narrow posterior rim of the pleotelson, not raising dorsally, shorter article 3 of antennula and another configuration of distal margin of pleopod 1 than in D. nodosus.