Cyclopoida. Prosome slender, 5-segmented. Urosome 5-segmented in female, 6-segmented in male, first somite with leg 5. Caudal rami elongate, bearing six or seven setae. Antennule 14-segmented in female and 17-segmented in male; geniculation of male antennules between segments 15 and 16. Antenna 4-segmented, without exopod; armature formula 0-1-5-7. Mandible palp biramous, with elongate basis; endopod 2-segmented, first segment with two, second segment with four setae; exopod small, indistinctly 2-3-segmented, with two terminal setae. Maxillulary coxal endite absent. Maxilla with praecoxa, coxa, basis, and 3-segmented endopod armed with four, two and four setae, respectively. Maxilliped 7-segmented, with syncoxa bearing three (1+2) setae, basis with two setae and 5-segmented endopod with setal formula 1, 1, 1, 1, 3. Legs 1-4 biramous, with 3-segmented rami; armature formula as in Table 1. Leg 1: inner margin of basis bearing long flattened setules. Third endopodal segment of leg 3 with three spines and three setae (1,II,I+2); in male with small outer terminal spine near spine-like outgrowth. Middle endopodal segment of leg 4 with distal inner seta modified into spine. Leg 5 consisting of coxa, basis, and exopod, with intercoxal sclerite; endopod absent; setal formula -0; 1-0; I, I+1+I in female and 0-0; 1-0; I, I+1+I or 0-0; 1-0; I, I+1+I, 1 in male.

Barathricola rimensis Humes, 1999. Barathricola thermophilus sp.nov. is the second species of this genus.

The hydrothermal vent field of OVF (11°24'52.97"S, 66°25'25.48"E) on the Central Indian Ridge in the Indian Ocean; sediments at 2022 m in depth.

Holotype (♀, MABIK CR00244723) and paratypes (6 ♀♀, 6 ♂♂, MABIK CR00244724) have been deposited in the MABIK. Dissected paratypes (2 ♀♀, 1 ♂) are retained in the collection of the last author. All type specimens collected on 23 June 2018 from the type locality.

Body (Fig. 1A) slender. Length of dissected and described specimen 776 μm. Other three measured specimens 700, 710, and 715 μm, respectively. Prosome nearly oval, 400 μm long, slightly longer than urosome, consisting of cephalosome and four pedigerous somites. Greatest width of prosome 273 μm across cephalosome. Cephalosome with angular posterolateral corners. First to third pedigerous somites almost equal in length. Fourth pedigerous somite distinctly shorter and narrower than the third. Urosome (Fig. 1B) slender, 5-segmented. Fifth pedigerous somite 38 × 74 μm, broadened distally, with angular posterolateral corners. Genital double-somite 109 × 70 μm, 1.56 times as long as wide, gradually narrowing posteriorly; genital aperture located dorsolaterally at 38% region of double-somite length. Three free abdominal somites 40 × 47, 30 × 42, and 50 × 37 μm, respectively. Anal somite with large anal region and minute spinules along ventrodistal margin. Caudal ramus (Fig. 1C) 116 × 13 μm, 8.92 times as long as wide, more than twice as long as anal somite, armed with six setae and ornamented with row of spinules along ventrodistal margin; outer lateral seta located at 39% region of ramus length; spermatophore (Fig. 1D) attached to female 60 × 27 μm, with thick wall.

Rostrum (Fig. 1E) triangular, with thin-walled lobate distal apex. Antennule (Fig. 1F) 225 μm long, longer than cephalosome, and 14-segmented. Eleventh segment the longest. Armature formula 2-5-4-7-6-(2 + aesthetasc)-0-1-0-1-(2 + aesthetasc)-2-(2 + aesthetasc)-(6 + aesthetasc). Second and third segments each with a trace of one subdivision, and fourth segment with three subdivisions. First segment with two rows of fine spinules. Most of setae naked, except several feebly pinnate ones of proximal two segments. Aesthetascs broad, constricted at region slightly distal to middle, and attenuated distally.

Antenna (Fig. 1G) 4-segmented, consisting of basis and 3-segmented endopod. Basis unarmed, ornamented with several rows of minute spinules. First endopodal segment 36 × 17 μm, with one seta on inner margin and minute spinules proximally and on outer margin. Second endopodal segment narrow proximally and gradually broadened distally, 30 × 18 μm, armed with five setae (three distal and two smaller subdistal) and ornamented with row of minute setules on outer side. Third endopodal segment 23 × 14 μm, armed with seven unequal setae distally, and ornamented with setules on outer side.

Labrum weak, easily destroyed. Mandible (Fig. 1H) consisting of coxa, basis, exopod, and endopod. Coxa with setules on outer margin; cutting margin of gnathobase with six acutely pointed teeth, two thin proximal setae, three setules between distal second and third teeth, and one small, transparent digitiform process bearing fine spinules distally between distal first and second teeth. Basis elongate, 42 × 9 μm, bearing five or six setules subdistally. Exopod small, indistinctly 3-segmented, armed only with two setae on third segment, outer one of these setae sparsely pinnate and slightly longer than inner one. Endopod 2-segmented, armed with two and four setae on first and second segments, respectively, all six setae sparsely pinnate; first segment with several setules on medial margin.

Maxillule (Fig. 1I) with eight setae on praecoxal arthrite; second distal seta spiniform. Coxal endite absent. Epipodite with two unequal setae. Basis with four setae, three proximal and one distal. Exopod with four large setae distally; setae becoming longer from outer to inner margin. Endopod shorter than exopod, armed with five setae, one on medial margin, and four distally.

Maxilla (Fig. 2A) stout, 5-segmented, consisting of syncoxa, basis, and 3-segmented endopod. Syncoxa armed with 11 setae, grouped as four, one, three, and three on first to fourth endites, respectively; third and fourth endites ornamented with two spinules at distal region. Basis armed with three unequal setae (one large, proximally unarticulated, spiniform, one long, and one small setae) and ornamented with one spinule. First endopodal segment with four setae (two proximal and two distal). Second endopodal segment with two long setae; third endopodal segment small, with four setae (one long and three shorter).

Maxilliped (Fig. 2B) slender, 7-segmented, consisting of syncoxa, basis, and 5-segmented endopod. Syncoxa with several scattered rows of minute setules, and armed with three setae, proximal one small and naked. Basis with two setae and rather long setules on medial margin. Endopod armed with one, one, one, one, and three setae on first to fifth segments, respectively; middle seta on terminal segment naked, much longer than other setae, longer than basis and endopod combined. Articulation incomplete between third and fourth endopodal segments.

Legs 1–4 (Figs 2C–E; 3A) with 3-segmented exopod and endopod, lacking inner seta on first exopodal segment; third exopodal segment distinctly broader than proximal segments. All intercoxal sclerites smooth without spinule/setule array along distal margin and on both anterior and posterior surfaces. Endopods of legs 1-3 shorter than exopod, but that of leg 4 distinctly longer than exopod. Leg 1 (Fig. 2C) basis with seven thick setules on inner margin; inner distal spine large, 48 μm long, extending to middle of third endopodal segment, spinulose along both margins. Leg 2 (Fig. 2D) with inner coxal seta characteristically bent at proximal quarter; outer seta on basis shorter than those of legs 1, 3 and 4. Inner distal corner of basis of legs 2-4 with pointed dentiform process. Leg 3 (Fig. 2E) with two distal spines on third endopodal segment (outer spine ca. half as long as inner spine). Leg 4 (Fig. 3A), third endopodal segment elongate, 3.6 times as long as wide; inner distal seta on second endopodal segment and two inner and one outer setae on third endopodal segment transformed to spines. Armature formula for legs 1-4 as in Table 1.

Leg 5 (Fig. 3B) 3-segmented, consisting of coxa, basis and exopod; intercoxal sclerite small, narrow, with pointed outer distal corners and slightly concave distal margin. Coxa quadrate, unarmed, not articulated from somite. Basis also quadrate, armed with one pinnate seta outer distally. Exopod 54 × 24 μm, 2.25 times as long as wide, armed with three spines (two distal and one outer) and one pinnate seta; medial margin spinulose and outer margin setulose.

Leg 6 (Fig. 3C) represented by one spinule and one naked seta on genital operculum.

Body (Fig. 3D) much narrower than that of female, 582 μm long. Prosome 314 × 153 μm, approximately twice as long as wide. First pedigerous somite slightly narrower than cephalosome and second pedigerous somite. Urosome 6-segmented. Fifth pedigerous somite narrower than genital somite. Genital somite (Fig. 3E) 86 × 72 μm, longer than wide, with well-developed genital operculum. Four abdominal somites 25 × 40, 23 × 34, 20 × 31, and 30 × 28 μm, respectively. Caudal ramus 6.1 times as long as wide (61 × 10 μm); arrangement and locations of caudal setae as in female.

Rostrum as in female. Antennule (Fig. 3F) 17-segmented; armature formula (2 + aesthetasc)-(5 + aesthetasc)-4-2-(2 + aesthetasc)-2-2-2-2(2 + aesthetasc)-(1 + spine)-(2 + aesthetasc)-2-[3 + aesthetasc (or 2+aesthetasc)]-[0 (or 1)]-(1 + aesthetasc)-(9 + 2 aesthetascs); eleventh segment with short posterior margin and much longer anterior margin, spine on this segment slender. Antenna as in female.

Mandible and other mouth appendages as in female.

Legs 1, 2, and 4 also as in female. Leg 3 sexually dimorphic; third endopodal segment (Fig. 3G) bearing two spines, three setae, and distally two small specialized elements, one curved, non-articulating, spinule-like element and one straight, distally bifurcate articulating element.

Leg 5 as in female. Leg 6 (Fig. 3E) represented by three naked setae on genital operculum, medial one smaller than other two.

The specific name thermophilus is a combination of Greek words therm (=heat) and phil (=loving), referring to the finding of the new species in a hydrothermal vent field.

Females and males from the type locality dissected by A.G. Humes and marked as Barathricola rimensis in the Zoological Museum of Lomonosov Moscow State University (collection numbers: w.cyc.sch.1.1-1.5). The hydrothermal vent field is at Juan de Fuca Ridge (44°08.6'N, 129°42'W) in the northeastern Pacific, 26 August 1996 at 2254 m depth.

Body as in original description. Differs from Barathricola thermophilus sp. nov. in following features.

Caudal ramus (Fig. 4D) elongate, 99 × 9 mm, ratio of length to width 11:1. Outer lateral seta located approximately at junction of first and second thirds of ramus. Dorsal seta short. Outermost terminal seta also short, placed dorsally. Innermost terminal seta short. All these setae smooth. Two long median terminal setae 117 mm (outer) and 234 mm (inner), both with lateral setules. Few minute spinules at distal outer corner of ramus.

Antennule (Fig. 4A) 14-segmented with numerous subdivisions. Armature formula: 3-8-8-5-3-0-1-0-1-(2 + aesthetasc)-(2 + aesthetasc)-(2 + aesthetasc)-(6 + aesthetasc).

Antenna (Fig. 5A) four-segmented, with coxa, basis, and two-segmented endopod, armed with 0, 1, 5, and 7 setae. Exopod absent. Length 122 mm without setae.

Mandible (Fig. 5B, C) with coxa having medially directed gnathobase armed distally with row of seven or eight slender teeth. Palp biramous. Basis elongate, with minute exopodal process carrying two long setae, and two prominent setae distally on margin of basis; endopod two-segmented, first segment small, trapezoidal, bearing two setae and row of minute spinules, second segment small with four distal setae and row of minute spinules along anterior edge.

Maxillule (Fig. 6A) with large praecoxa bearing arthrite with eight setae; coxa-basis with 3+1 setae; exopod with two short stout setae and two long slender setae; endopod with five setae.

Maxilla (Fig. 6B) with praecoxa having two endites, proximal endite bearing four setae, distal endite represented by single seta. Coxa with two endites, both with three setae. Basis with endite bearing three setae, one short, one long and slender, and one stout and claw-like, and having few minute subterminal spinules. Endopod three-segmented, with first segment having two endites with two setae each, small second segment bearing two setae, and minute third segment with four setae.

Maxilliped (Fig. 6C) with both coxa and basis swollen medially and bearing three and two setae, respectively; endopod slender, consisting of five segments armed with 1, 1, 1, 1, and 3 setae. Coxae of maxillipeds joined ventrally by one sclerotized line.

Legs 1–4 (Fig. 7A, C, E) biramous with three-segmented rami; armature formula for legs 1-4 as in Table 1. Leg 1 (Fig. 7A), inner side of basis with barbed spine and row of eight slender curved setules. Leg 3 (Fig. 7C) with 2 distal spines on third endopodal segment.

Leg 5 (Fig. 7F). Both legs connected by small quadrangular intercoxal sclerite and consisting of coxa, basis, and one-segmented exopod. Coxa and basis with setules along both sides. Basis with outer seta 44 mm long. Exopod 21 mm in greatest dimensions (15.5 mm wide distally) bearing three spines and one seta. Outer marginal barbed spine 57 mm, two terminal spines 58 mm (outer) and 41 mm (inner), both with minute outer spinules and longer inner fringelike setules. Seta between these two spines smooth, 55 mm. Outer margin of segment proximal to spine with setules; distal to spine and along inner side of segment with shorter setules, inner margin with minute spinules.

Differs from Barathricola thermophilus sp. nov. in following features:

Caudal ramus (Fig. 4E) resembling that of female but shorter, ratio 8.5:1. Antennule (Fig. 4B, C) 17-segmented; armature formula (2 + aesthetasc)-(5 + aesthetasc)-4-2-(2 + aesthetasc)-2-2-2-2(2 + aesthetasc)-(1 + spine)-(2 + aesthetasc)-2-[3 + aesthetasc (or 2+aesthetasc)]-[0 (or 1)]-(1 + aesthetasc)-(9 + 2 aesthetascs); eleventh segment with short posterior margin and much longer anterior margin, spine on this segment slender. Legs 1 (Fig. 7B) inner side of basis with barbed spine and row of eight slender curved setules. Leg 3 sexually dimorphic; third endopodal segment (Fig. 7D) bearing two spines, three setae, and distally two small specialized elements, one curved, non-articulating, spine-like element and one straight element. Leg 5 (Fig. 7G) different from that of female in having additional seta on inner margin of exopod (armature formula 0-0; 1-0; I, I+1+I, 1).

Martínez Arbizu (2006) established the family Schminkepinellidae into which he incorporated five genera, Cyclopinella, Muceddina, Barathricola, and his two new genera Einslepinella and Schminkepinella. The family is a monophyletic group of genera distinguished from other cyclopoid families by the reduction of a maxillulary coxal endite and the transformation of the distal inner seta on the middle endopodal segment of leg 3 into a spine (Martínez Arbizu 2006). None of the synapomorphies for the order Cyclopoida (a brush-like seta on the exopod of mandible, a brush-like seta on the exopod of maxillule, one or more flange-like setae on the endopod of swimming leg 4, pores with sensory dendrites laterally on the male cephalosome) proposed by Abiahy et al. (2006) are found in Schminkepinellidae. Karanovic (2008) described Cyclopinella tincanbayensis as a new species and synonymized two monotypic genera Muceddina and Barathricola with Cyclopinella and included these genera within Cyclopinidae based on the two major characters as synapomorphic shared by these nominal genera and Cyclopinella: the third endopodal segment of leg 4 with all armature elements transformed into spines and the three-segmented female leg 5 with an uniform armature and the elongate exopod. Karanovic (2008) recognized the mandibular palp as the most important morphological character differentiating species of Cyclopinella and its reduced segmentation and setation is consistent with reductions in other cephalic appendages and in the maxilliped. Our re-examination of the type species of the genus Muceddina, confirmed the original description and did not reveal the presence of a sexually dimorphic leg 3. This as well as our re-examination of the type specimens of Barathricola rimensis does not provide sufficient support for inclusion of Muceddina multispinosa and Barathricola rimensis in Cyclopinella. Additional data are needed to provide for the proposed taxonomic changes; here Barathricola and Muceddina are considered valid genera with clear distinctive characters separating them from other genera (see Key and Table 2). Cyclopinella tincanbayensis should remain in Cyclopinella although its distinctive characters may be significant enough to consider moving it to a new genus after revision. Barathricola, Cyclopinella, and Muceddina should remain in the Schminkepinellidae as was proposed by Martínez Arbizu (2006) until more data are available.

Data here show that the sexual dimorphism in leg 3 occurs in B. thermophilus and B. rimensis. Thus, the sexually dimorphic leg 3 known from the two species living in the hydrothermal vent environment is clearly the derived character of the genus Barathricola as mentioned by Martínez Arbizu (2006). Barathricola thermophilus sp. nov. shares with B. rimensis the shape of the mandibular palp and a number of other characters, e.g., Humes (1999) described the mandibular exopod of B. rimensis as “a minute process carrying two long setae”, but his illustrations and those here for this appendage show that the exopod is indistinctly 3-segmented, with two setae on the third segment, as in B. thermophilus sp. nov. In addition, the two species share the identical armature formula for the antenna (0-1-5-7), the loss of an inner seta on the basis of the mandible, a two-segmented mandibular endopod bearing two and four setae on the first and second segments, respectively, and elongate caudal rami.

Although the two species of Barathricola are very similar to each other, they cannot be treated as conspecific due to a significant difference in leg 5 of the male. The exopod (terminal segment) of leg 5 is armed with three spines and two setae (formula I, I+1+I, 1) in B. rimensis, in contrast to three spines and one seta (formula I, I+1+I) in B. thermophilus sp. nov. lacking a seta on the inner margin of the exopod. Within the Schminkepinellidae males of six species are known, including B. rimensis and B. thermophilus sp. nov. In these species a sexual dimorphic leg 5, as in B. rimensis, is known in Muceddina multispinosa, Schminkepinella plumifera, and Einslepinella ulrichi. However, Sars (1913) recorded that leg 5 of male Cyclopinella tumidula is of exactly the same appearance as in the female. Thus, the sexual dimorphism in leg 5 appears to be a character differentiating species, but not genera, in the Schminkepinellidae. An additional morphological difference between the two species of Barathricola is the ratio of the length to the width of the caudal ramus is 11.0:1 in the female and 8.5:1 in the male of B. rimensis, which is 8.9:1 in the female and 6.1:1 in the male of B. thermophilus sp. nov.