Lobatoradarea thermicola Ariyama & Chen, 2025

Lobatoradarea thermicola Ariyama & Chen , sp. nov.

[New Japanese name: Nessui-udenaga-yokoebi]

( Figs 47–51)

Amphipoda indet. 2: Chen et. al., 2024: 10, fig. 5b.

Type material. Holotype: female, 7.3 mm (OMNH-Ar-13253), Sumisu Caldera hydrothermal vent field, northeast of Sumisu Island, Tokyo, 31°28'12.7"N, 140°04'17.0"E ( Fig. 1E), 694 m depth, dacite bottom, by a suction sampler mounted on HOV Shinkai 6500 during dive #1801 on-board R /V Yokosuka cruise YK24-13C, 21 August 2024, coll. Chong Chen. Paratypes: female, 7.5 mm (OMNH-Ar-13254), same data as holotype; female, 7.3 mm (OMNH-Ar-13255), almost same data as holotype from a different diffuse flow location with lat/long 31°28'12.6"N, 140°04'17.1"E ( Fig. 1E), 692 m depth; male, 6.2 mm (OMNH-Ar-13256), Sumisu Caldera hydrothermal vent, 31°28'12.7"N, 140°04'16.4"E ( Fig. 1E), 689 m depth, dacite bottom, using HOV, 22 August 2024, coll. Daisuke Uyeno.

Type locality. Sumisu Caldera hydrothermal vent, Tokyo, Japan .

Etymology. From the Latin thermae (= warm springs) and - cola (= dweller), referring to the hydrothermal vent habitat. Used as noun in apposition.

Description of female. Based on holotype, female, 7.3 mm (OMNH-Ar-13253) and paratype, female, 7.5 mm (OMNH-Ar-13254) only for uropod 1.

Head ( Figs 47, 48). Rostrum developed, lateral cephalic lobe rounded, anteroventral corner slightly projected. Eyes medium-sized (diameter: 33% of head height). Antenna 1, peduncular articles 1–3 with length ratio of 1.0: 0.6: 0.3; accessory flagellum very short, conical, with 5 setae; primary flagellum with 30+ articles, tip lost, article 1 long, 11 articles with aesthetasc. Antenna 2, peduncular articles 3–5 with length ratio of 1.0: 2.7: 3.9, each article bearing several robust setae; flagellum long, with 38+ articles, article 1 long, tip lost. Upper lip relatively long, produced ventrally, ventral margin bearing many setae, anterior surface covered with minute setae. Mandibles, both incisors with 9 cusps, laciniae mobilis 6- and 3-toothed in left and right, respectively, accessory setal rows with 5 thick and 5 slender setae in left and 4 thick and 4 slender setae in right; palp stout, article length ratio 1.0: 3.0: 1.7, article 1 short, bare, article 2 long, with 0–1 proximal and 5 subdistal setae, article 3 relatively short, bearing 1 proximal, 8 ventral and 1 thick and 3 slender distal setae, lateral surface covered with dense thin setae. Lower lip, inner lobes developed, bare; outer lobes with mandibular lobes not-setose distally, mediodistal margins and ventrodistal surfaces setose. Maxilla 1, inner plate pear-shaped, medial margin bearing long plumose setae, tip with a long thick seta; outer plate distally with 11 denticulate robust setae and 8 slender setae; palp article 1 bare, article 2 apically with 5 triangular and 1 long robust setae and 3 slender setae in left and 6 narrow robust setae and 3 slender setae in right. Maxilla 2, inner plate ca. 1.5 times width of outer plate. Maxilliped, inner plate with 3 triangular robust setae distally; outer plate bearing 4 long and 8 short robust setae mediodistally; palp article 3 dorsal surface with 5 thick setae and many fine setae.

Pereon ( Figs 47, 49). Pereonite 7 strongly projected posterodorsally. Coxae 1–7, lateral surfaces covered with minute scale-like furrows. Gnathopod 1 small; coxa roundish rectangular, with a small notch each on anteroventral and posteroventral margins, medial surface bearing a few setae; basis slender, slightly curved anteriorly, anterior margin with several short setae, posterior margin bearing 8 long setae, medial surface with 3 long setae; ischium with a short seta posterodistally; merus acutely projected distally, with several thick setae on posterodistal margin; carpus longish triangular, anterodistal corner with a seta, posterior margin bearing many thick setae; propodus almost same length as carpus, posterior margin setose, palm weakly oblique, defined by 3 (1 lateral and 2 medial) short robust setae; dactylus short. Gnathopod 2 much longer than gnathopod 1; coxa roundish trapezoidal, with a small notch each on anteroventral and posteroventral margins, medial surface bearing a single seta posteriorly; basis slender, straight, anterior margin lined with several short setae, posterior margin and lateral surface with 6 and 3 setae, respectively; merus acutely projected distally, posterior margin with a few setae; carpus long and narrow, posterior margin bearing several bundles of slender setae; propodus a little longer than carpus (ca. 1.1 times), narrow, anterior margin lined with short setae, posterior margin bearing many bundles of slender setae, palm weakly oblique, defined by 4 (2 lateral and 2 medial) short robust setae; dactylus short. Pereopod 3 shorter than gnathopod 2; coxa roundish trapezoidal, with 6 and 3 small notches on anteroventral and posteroventral margins, respectively, medial surface lacking setae; basis slightly curved anteriorly, anterior margin with a few slender setae proximally and a few short setae distally, posterior margin with several slender setae and a posterodistal robust seta; merus anterior and posterior margins with 1 and 4 robust setae, respectively, anterodistal and posterodistal corners each bearing 2 robust setae; carpus anterior margin with several bundles of setae, posterior margin bearing 2+2+1+2 robust setae; propodus longer than carpus, anterior margin with several short setae, posterior margin with 1+1+1+1+2+2 robust seta; dactylus with a minute projection on inner margin. Pereopod 4 almost same length as pereopod 3; coxa ventral margin weakly angular, with 7 and 5 small notches on anteroventral and posteroventral margins, respectively, medial surface bare; basis slightly curved anteriorly, anterior margin with 2 proximal slender setae and a few distal short setae, posterior margin with a proximal slender seta and posterodistal robust seta; ischium–dactylus almost similar to those of pereopod 3. Pereopod 5, coxa anteroventral, posteroventral and posterior margins each with 2 small notches; basis oval, lateral surface with anterior ridge, anterior margin with a few proximal setae and 7 robust setae, anterodistal corner bearing a long robust seta, posterior margin serrated, medial surface with vertical ridge bearing a robust seta; ischium with 2 robust setae at anterodistal corner; merus with 2+3 anterior, 2 anterodistal, 1+1 posterior and 2 posterodistal robust setae; carpus–dactylus lost. Pereopod 6, coxa anterior margin straight, posteroventral margin with 2+1 small notches, medial surface with several setae anteroventrally; basis ovoid, lateral surface with anterior ridge, anterior margin with a few proximal setae and 8 robust setae, anterodistal corner bearing a robust seta, posterior margin serrated, medial surface with vertical ridge bearing a robust seta; ischium with 2 robust setae anteriorly; merus with 2+2+2 anterior, 2 anterodistal, 3 posterior and 4 posterodistal robust setae; carpus–dactylus lost. Pereopod 7, coxa short, semicircular, anterior margin bare, posteroventral margin with 3 small notches; basis wide, posterodistal corner expanded, lateral surface without ridge, anterior margin with 3 proximal setae and 1+1+1+1+2 robust setae, anterodistal corner bearing a robust seta, posterior margin serrated, medial surface with short vertical ridge bearing a robust seta; ischium with an anterodistal robust seta; merus with 1+3+3 anterior, 4 anterodistal, 1+1+1+1+2 posterior and 4 posterodistal robust setae; carpus–dactylus lost.

Pleon ( Figs 47, 50). Pleonites 1, 2 acutely projected posterodorsally. Epimeral plates 1–3, posteroventral corners minutely produced, lateral surfaces with ridge in plates 1, 2, ventral margins of plates 2, 3 each bearing 2 robust setae. Pleopods, pleopod 2 longest; each peduncle bearing several setae and 2 coupling hooks, peduncle of pleopod 3 with 1 lateral and 5 posteromedial robust setae; outer rami slightly longer than inner rami, outer rami bearing 14, 14 and 15 articles, respectively, inner rami each with 13 articles. Uropod 1, peduncle long, dorsal surface with 7 lateral and 5 medial robust setae, ventrodistal end slightly produced; outer ramus ca. 0.6 times length of peduncle, with 5 lateral and 4 (1 lost) terminal robust setae; inner ramus ca.1.7 times as long as outer ramus, with 12 (3 lost) lateral, 6 medial and 4 (1 lost) terminal robust setae. Uropod 2 shorter than uropod 1; peduncle short, with 3 lateral and 1 laterodistal robust setae, ventrodistal end triangularly produced; outer ramus ca. 0.75 times length of peduncle, with 2 lateral and 4 (1 lost) terminal robust setae; inner ramus damaged distally, with 3+ medial robust setae. Uropod 3, peduncle very short, longer than wide, ventrodistal end acutely produced, dorsal surface and lateral margin with several minute setae; outer ramus ca. 1.3 times length of peduncle, with 4 lateral and 3 medial robust setae, tip bearing minute seta; inner ramus lost. Telson small, roundish triangular, dorsal surface with pair of 2+1 sensory setae, each lateral margin bearing many short setae, distal margin rounded, mediodistal margin bearing many minute setae.

Description of male. Based on paratype, male, 6.2 mm (OMNH-Ar-13256). Almost similar to holotype, ovigerous female, 7.3 mm (OMNH-Ar-13253) except for size and oostegites. Gnathopods resembling those of female; but in gnathopod 1 ( Fig. 50 M-G1), coxa with a small notch on posteroventral margin, posterodistal corner of propodus bearing 4 (2 lateral and 2 medial) robust setae; gnathopod 2 coxa narrower, with 1 and 2 small notches on anteroventral and posteroventral margins, respectively ( Fig. 50 M-G2).

Coloration in life ( Fig. 51). Based on holotype, female, 7.3 mm (OMNH-Ar-13253). Eyes silvery orange, other parts white.

Remarks. Lobatoradarea thermicola sp. nov. is characterized by a combination of the posterodorsal projection on the pereonite 7 and the angular ventral margin of the coxa 4. This combination is similar to that of L. angulata sp. nov., but this new species can be distinguished from L. angulata by the smaller eyes and the dactyli of the pereopods 3–7 with a minute projection. In addition, the minute furrows on the coxae are unique in the genus.

Habitat. Hydrothermal vent, dacite bottom between the depth of 689– 694 m. For further details of the Sumisu Caldera vent field see Chen et al. (2024). This is the second record of Lobatoradarea gen. nov. from hydrothermal vents, as Shaw (1989) already recorded L. longimana from hot vents on the Endeavour Segment of Juan de Fuca Ridge in the northeastern Pacific (depth: 1797–2200 m).

Distribution. Japan: Sumisu Caldera hydrothermal vent field, Tokyo (present study).

Key to the species of the genus Lobatoradarea gen. nov.

In the present paper, Lobatoradarea gen. nov. was established based on ten new species from Japan and Oradarea longimana View in CoL in North America was transferred to the new genus as Lobatoradarea longimana comb. nov. These eleven species closely resemble one another, but they can be distinguished by several morphological characters. Main differences among all the species of Lobatoradarea are summarized in Table 2. Besides the characters shown in the table, there are also many further minor differences among the species (e.g., number of the small notches on the coxae, shape of the bases of the pereopods 5–7, setation of the pereopods, and body coloration). A key to the known Lobatoradarea species is provided as follows:

1. Pleonites 1, 2 lacking posterodorsal projection.............................................................. 2

- Pleonites 1, 2 projected posterodorsally.................................................................... 3

2. Anteroventral corner of head slightly produced, eyes large............. L. inermis Ariyama, Kohtsuka & Kodama , sp. nov.

- Anteroventral corner of head acutely produced, eyes medium-sized........... L. subinermis Ariyama & Kohtsuka , sp. nov.

3. Posterodorsal projection on pereonite 7 distinct............................................................. 4

- Posterodorsal projection on pereonite 7 absent or indistinct.................................................... 7

4. Anteroventral corner of head acutely produced...................... L. coccina Ariyama, Kohtsuka & Hoshino , sp. nov.

- Anteroventral corner of head moderately or triangularly produced............................................... 5

5. Dactyli of pereopods 3–7 with a minute projection on inner margins.............. L. thermicola Ariyama & Chen , sp. nov.

- Dactyli of pereopods 3–7 smooth......................................................................... 6

6. Ventral margin of coxa 4 strongly angular.................................. L. angulata Ariyama & Kodama , sp. nov.

- Ventral margin of coxa 4 not strongly angular.............................. L. lignorum Ariyama & Moritaki , sp. nov.

7. Anteroventral corner of head acutely produced, palms of gnathopods strongly oblique................................................................................................... L. obliquua Ariyama & Kodama , sp. nov.

- Anteroventral corner of head moderately or slightly produced, palms of gnathopods weakly oblique.................... 8

8. Dactyli of pereopods 3–7 smooth......................................... L. longimana ( Boeck, 1871) , comb. nov.

- Dactyli of pereopods 3–7 with a minute projection on inner margins............................................. 9

9. Posteroventral corner of epimeral plate 2 minutely produced...................... L. anko Ariyama & Hoshino , sp. nov.

- Posteroventral corner of epimeral plate 2 strongly produced................................................... 10

10. Anteroventral corner of head slightly produced, eyes small............................. L. littoralis Ariyama , sp. nov.

- Anteroventral corner of head moderately produced, eyes large............. L. kagoshimensis Ariyama & Kodama , sp. nov.

Emended diagnosis of the genus Oradarea

Following the new insights gained from the description of Lobatoradarea gen. nov., we here also emend the diagnosis for genus Oradarea as follows: