Ligia exotica Roux, 1828

Ariyama, Hiroyuki & Hiki, Kyoshiro, 2024, A morphological and molecular study of Ligia exotica Roux, 1828 (Crustacea: Isopoda: Ligiidae) from Japan, with descriptions of two new species, Zootaxa 5453 (4), pp. 451-486 : 460-466

Ligia exotica Roux, 1828

[Japanese name: Torai-funamushi, new]

( Figs 3–7 )

Lygia exotica Roux, 1828: 61 (unnumbered), pl. 13, fig. 9.

Ligia exotica .— Milne Edwards, 1840: 157.— Budde-Lund, 1885, 266 (in part).—? Nunomura, 1975: 34 (in part).

Ligia baudiniana ?.— Miers, 1877, 670 [not Ligia baudiniana Milne Edwards, 1840 ].

Ligia shinjiensis Tsuge, 2008: 51 , figs 1, 3-1.— Nunomura & Shimomura, 2017b: 643, fig. 293.

Ligia exotica artificial-coastal form Yamanishi, 2021: 167, figs 2, 3.

Material examined. One male, 32 mm (OMNH-Ar-12585), 1 female, 26 mm (OMNH-Ar-12588), 3 females, 26, 25, 25 mm (OMNH-Ar-12594*, 12595, 12596*), and 1 ovigerous female, 25 mm (OMNH-Ar-12597*), Nishinomiyahama, Nishinomiya City , Hyogo Prefecture, 34°42'38"N, 135°19'30"E ( Fig. 1B ), 28 June 2022, coll. R. Yamanishi ; 2 males, 29, 27 mm (OMNH-Ar-12586, 12587), 5 males, 27, 25, 23, 22, 18 mm (OMNH-Ar-12589– 12593*), and 1 ovigerous female, 23 mm (OMNH-Ar-12598*), same locality, 21 June 2022, coll. R. Yamanishi . Three males, 31, 31, 29 mm (OMNH-Ar-12608–12610), 1 ovigerous female, 24 mm (OMNH-Ar-12611), 3 males, 33, 30, 25 mm (OMNH-Ar-12612, 12613, 12614*), 1 ovigerous female, 24 mm (OMNH-Ar-12615*), 1 female with juveniles, 24 mm (OMNH-Ar-12616*), and 1 female, 22 mm (OMNH-Ar-12617), shore of Lake Shinji, Tamayucho, Matsue City , Shimane Prefecture, 35°25'50"N, 132°58'57"E ( Fig. 1D ), 5 August 2022, coll. S. Morihisa. Asterisks indicate specimens for molecular analysis .

Type locality: Cayenne in French Guiana ( Roux 1828) [this designation follows Article 76.1.1 of the International Code of Zoological Nomenclature ( International Commission on Zoological Nomenclature 1999)].

Description of male: Based on male, 32 mm (OMNH-Ar-12585).

Body ( Figs 3A , 6 Pt) oblong oval, width 45% of length; dorsal surface granular. Pereonites 1–7 gradually shorter, sutures indistinct; lateral margins of pereonites 1–7 projected posteriorly, with angle progressively sharpened. Pleonites 1 and 2 short and narrow; pleonites 3–5 gradually shorter, posterolateral corners of pleonites 3–5 acutely produced. Pleotelson much broader than long; distal margin with rounded median tooth, pair of small intermediate tooth and pair of triangular lateral tooth.

Head ( Figs 3A , 4 ) narrow, about 0.4 times of body width; eyes situated anterolaterally, diameter ca. 0.4 times of head width. Antennula 3.5% of BL; article 1 wide, with 3 minute setae at ventrodistal corner; article 2 slender, ventral margin with 3 setae, distal part of dorsal margin lined with minute setae, ventrodistal corner with 3 setae, mediodistal margin bearing 3 short aesthetascs; article 3 small, tip rounded. Antenna longer than body (about 1.1 times), tip exceeding pereonite 7; peduncular articles 3–5 with length ratio of 1:2.85:3.85, laterodistal corners of articles 3–5 each bearing small robust seta; flagellum with 32 articles. Upper lip large and broad, separated into two parts by asymmetrical suture; lower part covered with fine setae. Mandibles stout, both incisors and left lacinia mobilis each with 3 rounded cusps, right lacinia mobilis with 1 large and 8 small triangular projections; setal rows composed of ca. 25 long thick setae; molar large, with dense fine setae. Lower lip fleshy, with triangular median lobe; several parts covered with dense fine setae. Maxillura flattened; outer plate apically with 5 thick and 8 slender robust setae and a slender pectinate seta; inner plate bearing 3 large plumose setae mediodistally, medial margin with rounded projection in middle. Maxilla relatively slender; outer plate wide, rounded distally, covered with dense fine setae; inner plate small. Maxilliped stout; epipod rounded, lateral and distal margins setose; palp partly coalesced with basis, articles 1–2, articles 2–3 and articles 3–4 also partly coalesced, but articles 4–5 slightly coalesced, distal end of basis and palp articles 1–4 with several robust setae; endite barely reaching palp article 2, with ca. 20 denticles and dense fine setae.

Pereon ( Fig. 5 ). Pereopod 1 short; basis parallel-sided; ischium slightly widened distally, about 0.65 times length of basis, laterodistal corner bearing a long robust seta; merus wide, almost same length as ischium, ventral margin strongly swollen; carpus a little longer than merus, narrow, ventral margin slightly swollen; propodus narrow, anterodistal end with distinct rounded projection; dactylus short. Pereopods 2, 3 similar in shape each other, longer than pereopod 1; basis slightly widened distally; ischium widened distally, ca. 0.6 times length of basis, laterodistal corner with 1 and 2 long robust setae, respectively; merus shorter than ischium, ventral margin straight; carpus long and broad, about 1.7 times length of merus, ventral margin swollen; propodus narrow, proximal part strongly curved, ventral margin without robust setae; dactylus short. Pereopod 4 longer than pereopod 3; basis weakly widened distally, ischium widened distally, ca. 0.6 times as long as basis, laterodistal corner with a long robust seta; merus slightly longer than ischium, ventral margin not swollen; carpus slender, about 1.25 times length of merus, ventral margin not swollen; propodus straight and narrow, about 1.1 times as long as carpus; dactylus short. Pereopods 5–7 similar in shape one another, longer than pereopod 4, pereopod 6 longest; basis expanded at distal 1/5 point of lateral margin; ischium widened distally, ca. 0.7–0.8 times length of basis, laterodistal corner with a long robust seta; merus shorter than ischium, ventral margin slightly excavated; carpus slender, about 1.5 times length of merus, ventral margin not swollen; propodus straight and narrow, ca. 1.1–1.25 times length of carpus; dactylus short, with a strap-like seta in pereopods 6, 7.

Pleon ( Fig. 6 ). Penile papillae simple, narrowed distally, but distal parts slightly wider, lateral margins with fine setae. Pleopod 1, exopod reniform, endopod roundly projected posteromedially, posteromedial and posterior margins setose. Pleopod 2, protopod with oblong outer branch; exopod oval, attached ventrally on protopod, anterior–medial–posterior margins setose; endopod slender, lengthened posteriorly, article 2 about 1.7 times length of exopod; appendix masculina with minutely spinous swelling on dorsal surface, lateral and medial surfaces lacking projection, distal margin rounded. Pleopods 3–5 similar in shape one another; protopod bilobed, outer lobe long, inner lobe short, heavily setose medially and posteriorly; exopod wide, attached dorsally on protopod, posteromedial corner weekly projected, medial margin heavily setose, posterior margin weakly setose; endopod also wide, semiquadrate. Uropod very long (ca. 0.9 times of BL); peduncle slender, medial margin bearing 10 robust setae, distal margin with lateral projection; both rami very slender, inner ramus ca. 1.95 times as long as peduncle, tip of inner ramus with slightly curved second article, tip of outer ramus lost.

Description of female [based on female, 26 mm (OMNH-Ar-12588); Figs 3B , 7 ]: Body almost as wide as that of male, width 48% of length; sutures present on pereonites 2–4. Antenna longer than body (ca. 1.1 times). Pereopod 1, basis parallel-sided; ischium widened distally, about 0.6 times length of basis, laterodistal corner bearing a long robust seta; merus longer than ischium, straight, ventral margin not swollen; carpus narrow, shorter than merus, ventral margin not swollen; propodus slender, anterodistal end lacking projection; dactylus short. Pereopods 2, 3 similar in shape each other, longer than pereopod 1; basis slightly widened distally; ischium widened distally, ca. 0.6 times length of basis, laterodistal corner with a long robust seta; merus longer than ischium, ventral margin straight; carpus long and broad, about 1.3 times length of merus, ventral margin slightly swollen; propodus slender, proximal part strongly curved, ventral margin without robust setae; dactylus short. Pleopod 2, exopod oval, medial and posterior margins sparsely setose; endopod very small (probably regenerated), roundly projected posteromedially. Uropod ca. 0.6 times of BL. Other parts almost similar to those of male except for penile papillae and oostegites.

Variation: Pleotelson ( Fig. 6M 1 -Pt), dorsal margin with acute median tooth. Antenna ca. 1.0–1.1 times of BL in Nishinomiya and ca. 1.0–1.2 times in Lake Shinji, flagellum with 30–35 articles in Nishinomiya and 32–39 articles in Lake Shinji. Right mandible, lacinia mobilis with 1 large and 7–11 small triangular projections. Male pereopod 1, ventral margin of merus weakly swollen in most individuals, ventral margin of carpus not swollen in part. Male pleopod 2, appendix masculina ( Fig. 6M 2 -AM, M3-AM) almost similar to that of male, 32 mm (OMNH-Ar-12585) in spite of various degree of twisting. Uropod about 0.6–0.9 times of BL, tip of outer ramus lost in all individuals.

Coloration in life ( Fig. 3 ). Male: antenna brown, eyes black; body and uropod light brown, but median part of body dark brown with a few small brown marks. Female: antenna brown, eyes black; body and uropod brown, but median part of body dark brown, with several brown marks and many indistinct gray spots.

Diagnostic molecular characters (16S rRNA): A distinguishing nucleotide position for Ligia exotica is found at position 175, where the nucleotide is G, contrasting with A in the other species.

Remarks: As stated above, no genetic differences among the Ligia exotica specimens collected from Nishinomiya, Lake Shinji and many localities in the world are detected by the molecular analysis. In terms of morphology, the present specimens closely resemble the habitus of the original description by Roux (1828) in proportion of the body, antenna and uropod. Morphological characters of the specimens are generally similar to the descriptions by Budde-Lund (1885) and Miers (1877). The characters also correspond to the description and figures of Ligia shinjiensis by Tsuge (2008); however, the following points are different: (1) the antenna in Tsuge’s (2008) figure is shorter (less than 0.9 times of BL), although the number of the flagellar articles is almost the same [32–38 in Tsuge (2008)], (2) the lacinia mobilis on the right mandible with 1 large and 4 small teeth (1 large and 7–11 small projections in the present study), (3) the outer plate of the maxillura with 12 teeth (13 robust setae and 1 slender seta in the present study), (4) the palp articles of the maxilliped are fully separated (partly coalesced in the present study), and (5) the spination of the pereopods. Because we also examined the specimens from the type locality of L. shinjiensis , these differences are probably due to his overlooking, and L. shinjiensis can be synonymized with L. exotica .

Several papers on the Ligia exotica species complex showed a figure of its appendix masculina relatively similar to that of this species: figures in Verhoeff (1928), Reca (1972), Roman (1977), Kersmaekers & Verstraeten (1990), and Taiti et al. (1992) resemble Fig. 6M 3 -AM, and figures in Barnard (1932) and Green (1962) resemble Fig. 6 AM, M2-AM. To confirm identity with the true Ligia exotica , detailed comparison of the other morphological characters is also required.

Habitat: Supralittoral, on rocks.

Certain distribution in Japan: Lake Shinji in Shimane Prefecture ( Tsuge 2008; present study); Lake Jinzai in Shimane Prefecture ( Tsuge 2008); from Kobe City in Hyogo Prefecture to Hannan City in Osaka Prefecture ( Yamanishi 2021; present study).

Figures

FIGURE 1. Map showing collecting sites of the examined specimens. A, Misaki Town in Osaka Prefecture; B, Nishinomiya City in Hyogo Prefecture; C, Enoshima Island in Kanagawa Prefecture; D, Lake Shinji in Shimane Prefecture.

FIGURE 2. Maximum likelihood phylogenetic tree based on 16S rRNA nucleotide sequences of Ligia species. Ligia hawaiensis and L. perkinsi were used as the outgroups. Values at nodes represent bootstrap values/Bayesian posterior probabilities. Vertical bars represent assignments by species delimitation analyses, including ASAP, bPTP and mPTP. The scale bar represents the number of nucleotide substitutions per site. The complete list of used nucleotide sequences is shown in Table S1. The detailed phylogenetic tree is shown in Fig. S2.

FIGURE 3. Ligia exotica Roux, 1828. A, male, 32 mm (OMNH-Ar-12585); B, female, 26 mm (OMNH-Ar-12588). Scales: 10 mm.

FIGURE 4. Ligia exotica Roux, 1828. Male, 32 mm (OMNH-Ar-12585). Scales: 0.3 mm.

FIGURE 5. Ligia exotica Roux, 1828. Male, 32 mm (OMNH-Ar-12585). Posterior views (posterolateral to posteromedial). Scales: 1 mm.

FIGURE 6. Ligia exotica Roux, 1828. Male, 32 mm (OMNH-Ar-12585); male, 29 mm (OMNH-Ar-12586) [M1]; male, 27 mm (OMNH-Ar-12589) [M2]; and male, 30 mm (OMNH-Ar-12613) [M3]. Scales: 0.5 mm.

FIGURE 7. Ligia exotica Roux, 1828. Female, 26 mm (OMNH-Ar-12588). Scales: 1 mm.