Tethylembos cavatus, Ariyama & Kodama, 2024

Ariyama, Hiroyuki & Kodama, Masafumi, 2024, Three species of the family Aoridae Stebbing, 1899 (Crustacea: Amphipoda) collected from remote islands in southern Japan, with a key to all Japanese species of the family, Zootaxa 5433 (4), pp. 500-528 : 512-517

Tethylembos cavatus sp. nov.

[Japanese name: Hekomi-maeashi-yokoebi, new]

( Figs 9–13 )

Type material. Holotype:male, 5.4 mm (OMNH-Ar-12901), Tatsune, Miyake Island, Tokyo Prefecture, 34°02 ’ 55 ’’ N, 139°31 ’ 29 ’’ E ( Fig. 1E ), using SCUBA, 8–10 m depth, among volcanic lapilli (diameter: 1–8 mm) and algae ( Zonaria diesingiana and Amphiroa beauvoisii ) ( Fig. 8 ), 31 July 2020, coll. S. Tadokoro. Paratypes: 3 males, 6.7, 5.8, 5.0 mm (OMNH-Ar-12902–12904), and 2 females, 8.2, 7.3 mm (OMNH-Ar-12905, 12906), same data as holotype.

Type locality. Tatsune , Miyake Island in Tokyo Prefecture, Japan .

Etymology. From the Latin cavatus , - a, - um (= cavate), referring to the shape of the pereopod 5 basis in male.

Description of male. Based on holotype, male, 5.4 mm (OMNH-Ar-12901).

Head ( Figs 9 , 10 ). Rostrum small, anteroventral corner projected. Eyes relatively small, about 0.25 times as long as head. Antenna 1 slender; length ratio of peduncular articles 1–3 1:1.25:0.3, article 1 with 3 ventral robust setae; accessory flagellum long, with 6 long and 1 short articles; primary flagellum with 21 articles, first article long, last article minute, articles 4–19 each with aesthetasc distally. Antenna 2, peduncular articles 4, 5 and flagellum lost. Upper lip produced ventrally, ventral margin with many thin setae. Mandibles with length ratio of palp articles 1–3 1:1.6:2.3, article 1 bare, article 2 with 7 setae, article 3 almost straight, narrowed distally, ventral margin with dense short setae and many long setae, medial and lateral surfaces setose dorsally; body small, with 5-toothed incisor, 4- toothed lacinia mobilis, 7 serrated accessory setae and 3 plumose setae in left, and with 6-toothed incisor, 6-toothed lacinia mobilis, 6 slender accessory setae and 2 plumose setae in right; molar large, developed, surface with oblique lamellae in left. Lower lip, outer lobes with relatively long mandibular lobes, setose mediodistally, ventral surface with many thin setae, apical parts of inner lobes covered with thin setae. Maxilla 1, inner plate short and rounded, bearing a long plumose seta; outer plate with 10 distal robust setae; palp article 1 bearing a long seta, article 2 with 7 apical robust setae and 5 slender setae. Maxilla 2, inner plate narrower than outer plate; inner plate with mediodistal setae and mediofacial row of setae; outer plate bearing distal long setae and lateral short setae. Maxilliped, basis without triangular flange; inner plate with 3 stout robust setae distally and 1 inward robust seta; outer plate bearing 15 long-to-short robust setae mediodistally; palp article 2 long, exceeding outer plate, setose medially, article 3 setose distally, article 4 narrow, with slender nail on tip.

Pereon ( Fig.11 ). Ventral surface lacking sternal processes. Gnathopod 1 enlarged, subchelate;coxa,anteroventral corner distinctly projected; basis wide, with hollow for receiving carpus anterolaterally, distal end of anterolateral margin strongly expanded anterodistally, lateral surface and posterior margin with 1 and 2 long setae, respectively; ischium short, posterodistal corner with several setae; merus posterior margin and medial surface setose distally; carpus longish triangular, posterior margin and medial surface setose; propodus enlarged, about 1.6 times as long as carpus, posterior margin heavily setose, with quadrate and triangular projections distally, medial surface heavily setose anteriorly and weakly setose posteriorly; dactylus relatively short, slightly curved, posterior margin minutely serrate. Gnathopod 2 smaller than gnathopod 1, subchelate; coxa roundish quadrate; basis slender, almost straight, anterodistal corner expanded, anterior margin lined with several short setae, posterior margin bearing a long seta; merus posterodistal margin with many setae; carpus long, posterior margin densely setose, medial surface bearing many setae distally; propodus about 0.95 times shorter than carpus, palm slightly oblique, defined by a robust seta, anteromedial surface and posterior margin with dense setae; dactylus short, curved posteriorly, posterior margin minutely serrate. Pereopods 3 and 4 similar in size and shape, longer than gnathopod 2; coxae roundish square; bases slender, posterior margins with 2 long setae; meri, carpi and propodi relatively narrow, weakly setose posteriorly; dactyli short, slightly curved posteriorly. Pereopod 5 shorter than pereopod 4; coxa bilobed, posteroventral corner with a robust seta; basis unique-shaped, distal 1/3 of posterior margin excavated, posteroproximal corner projected, anterior margin with 3 robust setae, posterior margin bearing several slender setae; merus posterodistal corner slightly projected, with a thick seta; carpus and propodus with 1+1 and 1+1+1+1 posterior robust setae, respectively; dactylus short, curved. Pereopod 6, coxa short, bilobed, anterior margin with 3 plumose setae; basis–dactylus lost. Pereopod 7, coxa short, oval, anterior margin with 3 plumose setae; basis–dactylus lost.

Pleon ( Fig. 12 ). Epimeral plates 1–3, posteroventral corners each with small notch and minute seta, lateral surfaces with ridge, ventral margins bare. Pleopods almost same lengths; each peduncle bearing several plumose setae and 2 coupling hooks; outer rami shorter than inner rami, both rami with 11 articles. Uropod 1, peduncle with a basofacial robust seta, dorsal surface bearing 2 lateral and 3 medial robust setae, mediodistal corner acutely projected, ventrodistal end with long inter-ramal process (about half length of peduncle proper); both rami longer than peduncle, outer ramus subequal in length to inner ramus, former with 2 dorsolateral, 2 dorsomedial and 5 terminal robust setae, latter bearing 1 ventrolateral, 1 dorsolateral, 4 dorsomedial and 5 terminal robust setae. Uropod 2 about 0.7 times as long as uropod 1; peduncle with long inter-ramal process at ventrodistal end (about 0.75 times length of peduncle proper), dorsal surface with 1 lateral and 1 medial robust setae distally; outer ramus about 1.3 times as long as peduncle, outer ramus shorter than inner ramus, former with 2 dorsolateral, 2 dorsomedial and 4 terminal robust setae, latter bearing 2 dorsolateral, 4 dorsomedial and 5 terminal robust setae. Uropod 3 short, about half length of uropod 2; peduncle slightly longer than wide, swollen medially, ventrodistal end with short inter-ramal process, mediodistal corner pointed, distal margin bearing ventrolateral, dorsal and dorsomedial robust setae; outer ramus about 1.2 times length of peduncle, with 3 dorsolateral setae, tip bearing tiny second article and 6 long and 1 short terminal setae; inner ramus longer than outer ramus, with 1 lateral seta and 2 medial robust setae, tip bearing 1 robust and 4 slender setae. Telson fleshy, slightly wider than long, with a pair of hooked cusps dorsodistally, dorsal surface with pairs of 3–4 setae and 2 short sensory setae.

Description of female. Based on paratype, female, 8.2 mm (OMNH-Ar-12905). Almost similar to holotype, male, 5.4 mm (OMNH-Ar-12901) except for gnathopods and oostegites. Gnathopod 1 ( Fig. 12 F-RG1) subchelate, smaller than that of male; coxa roundish rhomboidal; basis stout, with hollow for receiving carpus anterolaterally, distal end of anterolateral margin roundly expanded anterodistally, posterior margin and medial surface each with a long seta; ischium short, posterodistal corner setose; merus posterior margin with many setae; carpus triangular, setose posteriorly; propodus long, about 1.4 times as long as carpus, posterior margin with rounded projection and triangular projection bearing a robust seta, posterior margin and anterodistal corner moderately setose, medial surface with many long setae anteriorly; dactylus long, curved, posterior margin minutely serrate. Gnathopod 2 ( Fig. 12 F-RG2) slightly longer and more slender than gnathopod 1, subchelate; coxa wide, roundish; basis straight, anterodistal corner slightly expanded, anterior margin with several short setae, posterior margin bearing a long seta; merus posterodistal margin setose; carpus longish triangular, anterior margin with a few short setae, posterior margin densely setose; propodus almost same length as carpus, palm weakly oblique, smooth, defined by a robust seta, anterior and posterior margins weakly and strongly setose, respectively, medial surface setose anteriorly; dactylus short, curved posteriorly, posterior margin minutely serrate. Pereopods 5–7, bases – dactyli lost.

Variation. Paratype, male, 5.0 mm (OMNH-Ar-12904): gnathopod 1 ( Fig. 12M 2-G 1 ), propodus posterior margin with rounded projection and triangular projection bearing a robust seta.

Coloration of preserved specimens. Paratype, male, 5.8 mm (OMNH-Ar-12903, Fig. 13A ): head light brown with brown patches, eyes dark brown; pereonites and pleonites also light brown with brown patches; gnathopods, pleopods and uropods faintly-brownish white. Paratype, female, 7.3 mm (OMNH-Ar-12906, Fig. 13B ): almost same as male.

Remarks. Shape of the mandibles indicates that this species belongs to Tethylembos . The pereopod 5 basis with posterior excavation is a unique character among the species of Tethylembos and the related genus, Bemlos .

Habitat. Among volcanic lapilli and algae in the subtidal zone (8–10 m depth).

Distribution. Japan: Miyake Island in Tokyo Prefecture (present study).

Figures

FIGURE 1. Map showing collecting sites of the examined specimens.

FIGURE 2. Protolembos ryukyuensis sp. nov. Holotype, male, 8.8 mm (OMNH-Ar-12888). Habitus.

FIGURE 8. In situ photograph of the collecting point of Tethylembos cavatus sp. nov. by S. Tadokoro.

FIGURE 9. Tethylembos cavatus sp. nov. Holotype, male, 5.3 mm (OMNH-Ar-12901). Habitus.

FIGURE 10. Tethylembos cavatus sp. nov. Holotype, male, 5.3 mm (OMNH-Ar-12901). Scales: 0.1 mm.

FIGURE 11. Tethylembos cavatus sp. nov. Holotype, male, 5.3 mm (OMNH-Ar-12901). Scales: 0.2 mm.

FIGURE 12. Tethylembos cavatus sp. nov. All except for M1, M2 and F: holotype, male, 5.4 mm (OMNH-Ar-12901); M1: paratype, male, 5.8 mm (OMNH-Ar-12903); M2: paratype, male, 5.0 mm (OMNH-Ar-12904); F: paratype, female, 8.2 mm (OMNH-Ar-12905). Scales: 0.2 mm.

FIGURE 13. Tethylembos cavatus sp. nov. Photograph of preserved specimens by H. Ariyama. A: paratype, male, 5.8 mm (OMNH-Ar-12903); B: paratype, female, 7.3 mm (OMNH-Ar-12906).