Stygiopontius lomonosovi Ivanenko and Martínez Arbizu, 2006

Ivanenko, Viatcheslav N., Arbizu, Pedro Martínez & Stecher, Jens, 2006, Copepods of the family Dirivultidae (Siphonostomatoida) from deep­sea hydrothermal vent fields on the Mid­Atlantic Ridge at 14 ºN and 5 ºS, Zootaxa 1277, pp. 1-21 : 6-16

Stygiopontius lomonosovi Ivanenko and Martínez Arbizu , new species

Figs 3–9

Type material. Dissected holotype Ψ mounted on 3 slides ( SMF 31035), dissected allotype ♂ mounted on 5 slides ( SMF 31036), 2 paratype ΨΨ mounted on 2 slides ( SMF 31037and SMF 31038), and 1 paratype ♂ mounted on 1 slide ( SMF 31039).

Type locality. Mid­Atlantic Ridge, Logachev­1 hydrothermal field, IRINA II site, station 35­ GTV, 14°45.19’N, 44°58.75’W, depth 3019 m. Periphery of the smoker complex covered by Bathymodiolus puteoserpentis ( Fig. 1 A). Collected 25 January 2004.

Etymology. The specific epithet is derived from the family name of the outstanding Russian scientist Michail V. Lomonosov (1711–1765), one of the founders of the Moscow State University established in 1755. It is the Latinized, masculine, genitive form of Lomonosov.

Female holotype. Body ( Fig. 3 A) with broad flattened prosome. Total length, excluding caudal setae, 0.93 mm, greatest width 0.50 mm. Prosome with ventrolateral folds as shown for male in Fig. 7 B. Urosome ( Fig. 3 B) 5­segmented, consisting of fifth pedigerous somite, genital double­somite and 3 postgenital somites. First somite of urosome trapezoidal in dorsal view. Lateral expansions in anterior part of genital doublesomite with dorsolateral genital fields bearing small seta ( Fig. 3 B–C). Posteroventral margin of the genital double­somite with lateral spiniform processes ( Fig. 3 B & D). Anal opening near posterior margin of the last abdominal somite ( Fig. 3 E).

Rostrum weakly developed, oral cone short and robust as in male ( Fig. 7 B).

Caudal ramus ( Fig. 3 B, E & F) short, with 6 setae, outermost terminal seta smooth, innermost terminal seta with setules on inner side (2 dorsal setae lost in holotype).

Antennule ( Fig. 4 A) 12­segmented: first segment with 1 seta; second segment the longest, with 14 unequal setae; segment 3 subdivided, with 10 unequal setae, 2 setae very small; segment 4 short with 2 setae; segment 5 double, with 4 (2 pairs) of setae; segments 6–9 each with 2 setae, 1 seta near the middle, and another seta near the distal edge; segment 10 with 2 setae and 1 aesthetasc near the distal edge; segment 11 bearing six setae; segment 12 with 8 setae. All setae, except distal seta of segment 5, smooth.

Antenna ( Fig. 4 B): small coxa without ornamentation, elongate basis with setules on outer and inner margins. Exopod small, 1­segmented, with 3 smooth setae. Endopod 2­ segmented; segment 1 elongate and unarmed, with setules on outer and inner margins; segment 2 short, ornamented with setules, armed with 5 setae: 4 terminal setae (3 elongate of different lengths and 1 short) and 1 short subterminal seta.

Mandible ( Fig. 4 C): gnathobase stylet­like and flattened; terminal teeth of unequal size, proximoventral margin with a row of small teeth.

Maxillule ( Fig. 4 D–E): inner lobe with a row of slender setules on inner margin and 5 terminal setae, 1 short and 4 long; outer lobe articulate, with 3 long, terminal and 1 short, subterminal setae. All long setae ornamented with setules of different lengths.

Maxilla ( Fig. 5 A) 2­segmented: segment 1 wide; segment 2 elongate, with flattened spoon­like tip ornamented with setules. Long inner seta near juncture of segments; its distal part ornamented with setules.

Maxilliped ( Fig. 5 B): syncoxa with 1 long, inner, bipinnate seta; basis with shorter, inner seta along inner margin of the segment, ornamented with spinules. Endopod 3­ segmented: segment 1 with 2 short, posterior setae; segment 2 with 1 long, posterior seta ornamented with spinules; segment 3 bearing 2 terminal setae with setules on inner margin, 1 seta very long and thick.

Swimming legs 1–4 ( Figs 5 C–D, 6A–C) with 3­segmented rami, except for leg 4, with 2­segmented endopod. Formula for the armature of legs 1–4 in Table 1 . Distal endopodal segment of legs 1–3 and distal exopodal segment of leg 3 ornamented with groups of slender posterior setules as shown in the figures. Leg 1: inner seta of basis slender and smooth; outer spine of exopodal segment 2 small and curved proximally; 1 outer spine of first exopodal segment and 3 outer spines of third ornamented with long subterminal setule; this setule absent on spines of other swimming legs. Leg 4: inner seta of exopodal segment 1 smooth in proximal part and ornamented with small setules in distal part ( Fig. 6 C). Intercoxal sclerites of legs 3–4 expanded. Distal exopodal segment of leg 4 with 2 outer spines.

Leg 5 ( Fig. 6 D) indistinctly 2­segmented; segment 1 with 1 long outer seta; segment 2 armed with 3 setae of different lengths (lost seta arrowed).

Leg 6 ( Fig. 3 C) represented by a flap covering gonopores and bearing 1 small seta. Color of living specimens unknown.

Egg sacs not observed.

Male allotype. Differs from female as follows:

Body ( Fig. 7 A–B): total length of allotype, excluding caudal setae, 1.03 mm, greatest width 0.56 mm. Shield of cephalothorax and tergites of 2 anterior metasomites with small pointed posterolateral processes, also present in female, but not visible in dorsal view.

Urosome ( Fig. 7 C–E) 6­segmented, consisting of fifth pedigerous somite, genital somite, and 4 postgenital somites. Genital somite with pair of posterior genital flaps on ventral side ( Fig. 7 D) and posterodorsal prominence visible from lateral view ( Fig. 7 E). First abdominal somite corresponding to abdominal somite included in the female genital double­somite and possessing posterolateral spiniform processes.

Antennule ( Fig. 8 A–D) 12­segmented, geniculate between segments 10 and 11; formula of setation as follows: 1, 2, 12, 8, 2, 2, 4, 2, 2, 4, 4+aesthetasc, 12. Segments 2 and 3 correspond to segment 2 of female antennule. Distal part of segment 3 with long curved process bearing subterminal seta ( Fig. 8 B). Segment 4 with 3 incomplete arthrodial membranes indicating a segment complex. Segment 5 with ventral process and curved stout spine on it ( Fig. 8 C). Two setae on segment 10 and 3 setae of segment 11 modified to short flattened element ( Fig. 8 D, arrowed).

Maxilliped ( Fig. 8 E–F). Syncoxa prolonged medioventrally into terminal clawlike process with subterminal barbed seta. Inner seta of basis shorter than that of female. Segment 2 of endopod with 2 posterior setae, 1 distal and 1 middle (the last is absent in female).

Distal endopodal segment of leg 1 ( Fig. 9 A) and distal exopodal segment of leg 3 ( Fig. 9 C) with group of slender posterior setules. Leg 2 ( Fig. 9 B): distal endopodal segment of leg 2 with 1 outer, 2 terminal, and 1 distomedial spine (formula I,II,I,2), outermost terminal spine curved inwardly; coxal seta short.

Leg 5 ( Figs 7 D, 9D) located ventrally. Basis fused with somite represented by 1 outer seta, exopodal segment articulate, armed with 3 setae.

Leg 6 ( Fig. 7 D–E) represented by genital flap bearing 2 long posterior setae.

Colour unknown.

Remarks. Stygiopontius lomonosovi n. sp. can be easily assigned to the genus Stygiopontius by the combination of the following characters: the endopod of leg 1 is 3­ segmented; the endopod of leg 4 is 2­segmented, its first segment unarmed and its second segment with two setae (one terminal and one inner) ( Ivanenko & Ferrari 2003). The new species is similar to S. mirus Humes, 1996 and S. latulus Humes, 1996 (described from the Snake Pit (23ºN) of the MAR) sharing with the new species the unusual medioventrally prolonged syncoxa of the maxilliped. The new species can be distinguished from both S. mirus and S. latulus by the absence of one spine on the distal exopodal segment of leg 4: armature formula II,I,4, instead of III,I,4 as found in S. mirus and S. latulus . Five other congeners possess only two outer spines on the third exopodal segment of leg 4, viz. S. cinctiger Humes, 1987 ; S. mucroniferus Humes, 1987 ; S. teres Humes, 1996 ; S. rimivagus Humes, 1997 ; S. verruculatus Humes 1987 . In this group, only females are known for the first three species and only males are described for the last two species. The following attributes distinguish these species from S. lomonosovi n. sp.: the maxilliped of S. rimivagus and S. verruculatus lacks an elongation of the syncoxa; leg 5 of S. teres is short and 1­segmented, its prosome moderately slender; the coxa and basis of maxilliped of S.

mucroniferus are armed with a stout seta; the second abdominal somite of S. cinctiger is shorter.

Males of 12 valid species included in the genus Stygiopontius have been described so far. Only males of S. lomonosovi sp. n. and five other species ( S. mirus , S. latulus , S. brevispina Humes, 1991 , S. lauensis Humes, 1991 , and S. rimivagus ) were recorded in the Lau Basin of the West Pacific and the MAR. They are characterized by reduction of two inner setae on the exopod of leg 5 (the segment is fused with the sixth thoracic somite in S. latulus ). The exopod is armed with three setae instead of a maximum of five setae present in males of congeners. Further investigations of unknown males of dirivultid species are necessary to clarify if the reduction of the setae on the exopod of leg 5 can be used as a character to define a monophyletic lineage of Stygiopontius distributed in both the West Pacific and the MAR.

Figures

FIGURE 1. Mid­Atlantic Ridge: A, Logachev hydrothermal field at 14 ° N, IRINA II site, musselbed of Bathymodiolus puteoserpentis, at the southwest from the smoker­complex; B, Turtle Pits hydrothermal site at 5 ° S, active chimney “ Tower ” with alvinocaridid shrimps of different size. (Photos courtesy by MARUM © 2005, University of Bremen.)

FIGURE 3. Stygiopontius lomonosovi n. sp., holotype Ψ: A, habitus, dorsal; B, urosome, dorsal; C, right genital field, seta of right leg 6; D, ventro­lateral outgrowth near posterior edge of genital double­somite; E, anal somite and caudal rami, dorsal; F, anal somite and left caudal ramus, lateral. Arrows indicate missing setae.

FIGURE 4. Stygiopontius lomonosovi n. sp., holotype Ψ: A, antennule, arrows indicate 1 missing and 2 short seta; B, antenna, arrow indicates short seta; C, mandible; D, maxillule, arrow indicates short seta; E, outer lobe of maxillule, arrow indicates short seta.

FIGURE 5. Stygiopontius lomonosovi n. sp., holotype Ψ: A, maxilla; B, maxilliped, posterior view; C, leg 1, anterior; D, leg 2, anterior.

FIGURE 6. Stygiopontius lomonosovi n. sp., holotype Ψ: A, leg 3, posterior; B, distal exopodal segment of leg 3, posterior; C, leg 4, anterior, arrows indicate missing spines; D, leg 5, left, lost seta arrowed; E, leg 6, left.

FIGURE 7. Stygiopontius lomonosovi n. sp., allotype ɗ: A, habitus, dorsal; B, ventral view of cephalothorax, position of cephalic appendages marked by dashed circles; C, urosome, dorsal, arrow indicates missing seta; D, urosome, ventral; E, urosome, lateral.

FIGURE 8. Stygiopontius lomonosovi n. sp., allotype ɗ: A, antennule; B, antennule, outgrowth of segment 3; C, modified seta of segment 5; D, 6 distalmost segments of antennule, modified setae arrowed; E, maxilliped, anterior; F, maxilliped, posterior.

FIGURE 9. Stygiopontius lomonosovi n. sp., allotype ɗ: A, distal endopodal segment of leg 1, posterior; B, endopod, inner margin of coxa and basis, intercoxal sclerite of leg 2, anterior; C, distal exopodal segment of leg 3, anterior; D, leg 5, ventral.

Tables

TABLE 1. Stygiopontius lomonosovi n. sp., holotype Ψ, armature formulae of the swimming legs 1 – 4. Roman numerals indicate spines, Arabic numerals, setae.

	Coxa	Basis	Exopod	Endopod
Leg 1	0­1	1­1	I­1; I­1; III,4	0­1; 0­2; 1,2,3
Leg 2	0­1	1­0	I­1; I­1; III,I,4	0­1; 0­2; 1,2,3
Leg 3	0­0	1­0	I­1; I­1; III,I,5	0­1; 0­2; 1,I,3
Leg 4	0­0	1­0	I­1; I­1; II,I,4	0­0; I,1