Elaphognathia kikuchii (Nunomura, 1992)

Ota, Yuzo, 2013, Redescription of five gnathiid species from Japan (Crustacea: Isopoda), Zootaxa 3737 (1), pp. 33-56 : 35-41

publication ID	https://doi.org/ 10.11646/zootaxa.3737.1.3
publication LSID	lsid:zoobank.org:pub:CE23AED0-EBA7-422D-AB3C-47DC2D3FB4D
DOI	https://doi.org/10.5281/zenodo.6160795
persistent identifier	https://treatment.plazi.org/id/412987E8-FFBD-3316-FF3C-FEDBBFAA2B82
treatment provided by	Plazi
scientific name	Elaphognathia kikuchii (Nunomura, 1992)
status

Treatment

Elaphognathia kikuchii (Nunomura, 1992) View in CoL

(Japanese name: Futokuwa-umi-kuwagata) ( Figs 2–4 View FIGURE 2 View FIGURE 3 View FIGURE 4 )

Gnathia kikuchii Nunomura, 1992: 61 –63, fig. 2.—Nunomura, 1995: 208, 209, fig. 21-196E.—Saito et al. 2000: 17.

Caecognathia kikuchii .—Cohen & Poore 1994: 288.—Smit & Davies 2004: 299—Nunomura & Shimomura 2012: 585, 588, fig. 103.

Gnathia amakusaensis Nunomura, 1992: 63 –65, fig. 3.—Saito et al. 2000: 17.

Caecognathia amakusaensis .—Cohen & Poore 1994: 288, table 4.—Smit and Davies 2004: 295, table 1.—Nunomura & Shimomura 2012: 585, 587, fig. 101. [New synonymy]

Gnathia saikaiensis Nunomura, 1992: 67 –69, fig. 5.—Saito et al. 2000: 18.

Caecognathia saikaiensis .—Cohen & Poore 1994: 288, table 4.—Smit & Davies 2004: 297, table 1.—Nunomura, 2012a: 69, fig. 7.—Nunomura & Shimomura 2012: 585, 587, fig. 102. [New synonymy]

Caecognathia sp. (aff. kikuchii Nunomura, 1992 ).—Nunomura 2012a: 71, fig. 8.

Gnathia azumai Nunomura, 2012a: 58 –60, fig. 1.—Nunomura & Shimomura 2013a: 142, 143, 145, fig. 117. [New synonymy]

Gnathia quadricephala Nunomura, 2012a: 60 –62, fig. 2.—Nunomura & Shimomura 2013a: 142, 143, 145, 146. fig. 118. [New synonymy]

Gnathia recticornata Nunomura, 2012a: 68 –69, fig. 6.—Nunomura & Shimomura 2013a: 142, 144. fig. 116. [New synonymy]

Material examined of type series. Male adult, holotype of Caecognathia kikuchii (Nunomura, 1992) , TOYA Cr- 11165, 3.7 mm, from off Kita-iwasaki, Reihoku-cho, Kumamoto Pref., Kyushu, western Japan, 15 Jul. 1978, coll. Taiji Kikuchi. Male adult, holotype of Caecognathia amakusaensis (Nunomura, 1992) , TOYA Cr-11166, 5.1 mm from 30–35 m depth, off Use, Reihoku-cho, 30 Apr. 1964, coll. Taiji Kikuchi. Male adult, holotype of Caecognathia saikaiensis (Nunomura, 1992) , TOYA Cr-11169, 3.1 mm, off Tomoezaki, Reihoku-cho, 5 Jul. 1978, coll. Taiji Kikuchi. Male adult, deposited as paratype of Gnathia nasuta , TOYA Cr-11168, 4.0 mm, from sandy bottom, 8.5 m depth, off Tomioka, Reihoku-cho, 22 May 1980, coll. Akira Hirayama. Nunomura (1992) did not show latitude and longitude but all of these type materials were collected around the coast of Tomioka Peninsula, western Japan (32°20–22′N, 130°01–03′E).

Male adult, holotype of Gnathia azumai Nunomura, 2012 a, TOYA Cr-23349, 4.1 mm, from 21 m depth of Shijiki Bay, Nagasaki Pref., western Japan, 10 May 1984, coll. Mikio Azuma. Male adult, holotype of Gnathia quadricephala Nunomura, 2012 a, TOYA Cr-23351, 8.4 mm in original description but actually 5.0 mm, from 44 m depth of Shijiki Bay, 10 May 1984, coll. Mikio Azuma. Male adult, holotype of Gnathia recticornata Nunomura, 2012 a, TOYA Cr-23361, 4.3 mm, from 35 m depth of Shijiki Bay, 10 May 1984, coll. Mikio Azuma. Nunomura (2012a) did not show latitude and longitude but all of these type materials were collected at Shijiki Bay (33°11′– 13′N, 129°24′–21′E). All materials from Shijiki Bay were collected by Nagasaki University Sledge (NUS) net or Smith-McIntyre grab.

New material. Two male adults, KMNH IvR-500,703, 4.4 and 3.9 mm, from 95 m depth, off north Yakushima Island, southwestern Japan (30°27.80′N, 130°35.30′E), sledge net, 3 Jun. 1996, coll. Takeo Yamauchi, TR /V Toyoshio-maru. Three male adults, KMNH IvR-500,704, 3.8–4.8 mm, from 47 m depth, off west Tangegashima Island, southwestern Japan (30°33.50′N, 130°53.30′E), dredge, 26 May 2001, coll. Michitaka Shimomura, TR /V Toyoshio-maru. Three male adults, KMNH IvR-500,705, 4.6–5.8 mm, from 78–83 m depth off east Koshiki Islands, off Kyushu, western Japan (30°50.91′N, 130°01.46′E), dredge, 25 May 2011, coll. Yukiko Narahara and Hiroshi Yamasaki, TR /V Toyoshio-maru. Seven and 2 male adults, KMNH IvR-500,706 and 500,707, 3.9–5.4 mm, from 63.1–75.9 m and 24.8–29.2 m depth, muddy sediments, off Tateyama, Boso Peninsula, central Japan (34°59.47′N, 139°46.98′E and 34°59.66′N, 139°48.50′E), dredge, 16 Apr. 2013, coll. Yuzo Ota, TR /V Hosakamaru.

Distribution ( Fig. 1 View FIGURE 1 , Table 1). From western to central Japan (8.5–105 m depth).

No. Samples Depth (m) Note

Elaphognathia kikuchii (Nunomura, 1992)

k1 1♂ not stated Holotype of " Elaphognathia kikuchii (Nunomura, 1992) " k2 2♂ 30–35 Type series of " Caecogntahia amakusaensis (Nunomura, 1992) " k3 5♂ not stated Type series of " Caecognathia saikaiensis (Nunomura, 1992) " k4 1♂ 8.5 Paratype of " Gnathia hirayamai Nunomura, 1992 " k5 3♂ 35 Type series of " Gnathia recticornata Nunomura, 2012 " k6 7♂ 21–44 Type series and 5 non types of " Gnathia azumai Nunomura, 2012 " k7 1♂ 44 Holotype of " Gnathia quadricephala Nunomura, 2012 " k8 7♂ 35–41 Reported as " C. saikaiensis " by Nunomura, 2012 k9 1♂ 105 Reported as " Caecognathia aff kikuchii " by Nunomura, 2012 k10 2♂ 95 New materials

k11 3♂ 47 New materials

k12 1♂ 78–83 New materials with 2 ♀ and 4 larvae

k13 2♂ 63.1–75.9 New materials including one damaged specimen k14 7♂ 24.8–29.2 New materials

Gnathia nasuta Nunomura, 1992

n1 1♂ 8.5 Holotype of Gnathia nasuta Nunomura, 1992 n2 3♂ 8.5 Type sereis of " Gnathia hirayamai Nunomura, 1992 " n3 4♂ 23–31 Type sereis and one non type of " Gnathia nagasakiensis Nunomura, 2012 " n4 9♂ 31–35 Type sereis and 5 non types of " Gnathia shijikiensis Nunomura, 2012 " n5 15♂ 21–31 Type series and 7 non types of " Gnathia brevicephala Nunomura, 2012 " n6 1♂ 180 New materials with 3 larvae

n7 6♂ 126 New materials with 4 ♀ and 6 larvae

n8 1♂ 163–170 New materials

n9 2♂ 71–75 New materials with 1 larva

n10 1♂ 403–412 New materials with 3 larvae

n11 2♂ 130 New materials with 2 larvae

n12 2♂ 142 New materials with 1 ♀ and 13 larvae n13 17♂ 213 New materials with 8 larvae

n14 9♂ 205 New materials with 1 ♀ and 2 larvae

n15 6♂ 63.1–75.9 New materials

n16 1♂ 24.8–29.2 New materials

Gnathia sanrikuensis Nunomura, 1998

s1 7♂ 42 Type series with 2 ♀ and 4 larvae s2 1♂ 46.5 New materials with 1 larva in Otsuchi Bay Gnathia mutsuensis Nunomura, 2004

m 1♂ intertidal Holotype of Gnathia mutsuensis Nunomura, 2004 Gnathia bungoensis Nunomura, 1982

b 1♂ estuary Holotype of Gnathia bugoensis Nunomura, 1982 Redescription ( Figs 2–4 View FIGURE 2 View FIGURE 3 View FIGURE 4 ). Body 3.9–5.8 mm (mean ± SD; 4.7 ± 0.6 mm, n = 17, new materials) without tubercles. Color of fixed specimens white or light brown; digestive organs brown due to congealed host blood. Cephalosome ( Figs 2 View FIGURE 2 A–D, 4A–F) sparsely covered with setae, rectangular with slight concave or almost straight posterior margin. Frontal border deeply concave with long setae extending half length of mandibles. Frontal process absent. Ventral frontal margin slightly visible on central frontal border in dorsal view. Dorsal sulcus wide and shallow, and often pentagonal shape or U-shape. Oval-shaped translucent region sometimes visible anteromedianly on dorsal sulcus. Eyes composed with 49 ocelli. Paraocular ornamentation absent. Supraocular lobe not prominent with smooth apex.

Pereonite I ( Figs 2 View FIGURE 2 A–D, 4A–F) short, not fused or medially fused with cephalosome, reaching lateral margin of cephalosome. Widths and lengths of pereonites II, III, and IV almost same. Pereonite II anteriorly concave. Pereonite III almost straight. Pereonite IV with constriction anteriorly and anterolateral lobes. Areae laterales and lobi laterals slightly visible on pereonites V and VI, respectively. Pereonite VII short and narrow, overlapping pleonite I.

Pleonites I–V ( Fig. 2 View FIGURE 2 A) almost equal length and narrower posteriorly, with 1 or 2 setae on lateral margin of each segment. Epimera not prominent.

Pleotelson ( Figs 2 View FIGURE 2 E, 4G–K) almost triangular with convex lateral margin, covered with scales. Three or 4 pairs of seta on dorsal surface and 1 pair of seta on distal apex.

Mandible ( Figs 2 View FIGURE 2 A–D, 4A–F,) almost straight with smooth apex, approximately half-length of cephalosome. Inner part of apex fine dentate blade and medial inner part rough dentate blade and inner lobe visible in dorsolateral view. One mandibular seta presents near armed carina on mid-dorsal surface. Basal neck not prominent. Erisma visible.

Antennule ( Fig. 2 View FIGURE 2 F) composed with 3 peduncle and 5 flagellar articles. Three, 3, and 1 feather-like bristles on distal margins of peduncle article I, II, and III, respectively. Three feather-like bristles on flagellar article I. One aesthetasc on flagellar article III, IV, and V, respectively. Flagellar article V terminating in 2 setae.

Antenna ( Fig. 2 View FIGURE 2 G) composed with 4 peduncle and 7 flagellar articles. One and 4 feather-like bristles on peduncle III and IV, respectively. 1–3 setae on distal margins of flagellar articles I–VI. Article VII terminates in 3 setae.

Maxilliped ( Fig. 3 View FIGURE 3 A) composed with 1 basis and 4 palp articles. Endite reaching palp article II. Four, 8, 5, and 7 plumose setae on external margins of palp articles I–IV, respectively. Two setae on internal margin of palp article IV.

Pylopod ( Fig. 3 View FIGURE 3 B) 3-articled. Article I semicircular with posterior corner; 3 areolae visible, 36 plumose setae on internal margin, 3 setae near basis, and 7 setae on distal margin. Article II circular with 3 setae on distal margin. Article III minute.

Pereopod II ( Fig. 3 View FIGURE 3 C) sparsely covered with setae. Basis oblong with 2 feather-like bristles. Ischium shorter than basis, becoming larger distally. Merus about half-length of ischium. Carpus slightly larger than merus with 1 denticulate compound spine on distal margin. Propodus rectangular, bearing 2 denticulate compound spines on inner-mid and inner-distal margins. Dactylus terminating in unguis.

Pleopod II ( Fig. 3 View FIGURE 3 D) covered with scales on peduncle. One seta on outer distal corner. Coupling hook on inner margin. Both rami oval and equal in length, fringed with fine setae. Appendix masculina of pleopod II absent. Eight or 9 plumose setae on exopods. Seven or 8 plumose setae on endopods.

Uropod ( Figs 2 View FIGURE 2 E, 4G–K) fringed with fine setae laterally. Both rami subequal in length. Exopod not extend beyond apex of pleotelson and endopod slightly extend apex. Exopod with 6 simple and 4 plumose setae and endopod with 3 simple and 6 plumose setae, and several feather-like bristles.

Penes ( Fig. 3 View FIGURE 3 E) fused with 2 contiguous papillae.

Remarks. Re-examinations based on holotype specimens of Caecognathia kikuchii (Nunomura, 1992) , C. amakusaensis (Nunomura, 1992) , C. saikaiensis (Nunomura, 1992) , Gnathia azumai Nunomura, 2012 a, G. quadricephala Nunomura, 2012 a, and G. recticornata Nunomura, 2012 a revealed that their morphologies shared several characteristics as follows: almost straight mandibles, excavated frontal border without processes, several setae extending up to half the length of the mandibles, almost pentagonal or U-shaped dorsal sulcus, and a triangular pleotelson with convex lateral margins. The morphology of the mandible, frontal border, and pleotelson, as well as the structure of the cephalosome surface, such as the presence of setae, dorsal sulcus, and paraocular ornamentation, are important characteristics for gnathiid identification.

The present study revealed numerous differences from the drawings between Nunomura (1992) and the present study, as follows. Caecognathia kikuchii : (1) the number of flagellar articles in the antennule was seven in the original description but only five in present study; (2) plumose setae visible on the inner margin of the pylopod where actually absent; (3) the uropodal peduncle on the pleotelson was actually located underneath. Caecognathia amakusaensis : (1) the endite of the maxilliped and article III of the pylopod were not present; (2) flagellar article IV of the antennule was presented as a long segment but it was actually not. Caecognathia saikaiensis : (1) pereonite I was not described; (2) long setae on the frontal border were not drawn; (3) the uropodal peduncle was on the pleotelson but it was actually underneath; (4) the long flagellar article I was actually short. In Nunomura (1992), the mandible structures such as mandibular seta, dentate blade, and basal neck were not clearly drawn, whereas pereonite VII was not described.

The following are the differences between the drawings of Nunomura (2012a) and the present study. Gnathia azumai : (1) the straight frontal border with a frontal process was actually excavated without processes; (2) several long setae on the dorsal surface of the mandible were actually absent, except for one mandibular seta; (3) article I of the pylopod was actually semicircular and not oval-shaped; (4) article III of pylopod was not drawn; (5) appendix masculina was described as a “stylus” but was actually absent. Gnathia quadricephala : (1) the straight frontal border with a frontal process was actually excavated without processes; (2) the number of flagellar articles on each of the antennule and antenna was mentioned as six but there were actually five and seven, respectively; (3) a dentate blade was not observed but was actually present; (4) the endite of the maxilliped was not shown. Gnathia recticornata : (1) the straight frontal border with a frontal process was actually excavated without processes; (2) pereonite I was not described; (3) appendix masculina was shown but was actually absent.

These six species were reported from almost the same localities, i.e., C. kikuchii , C. amakusaensis , and C. saikaiensis were reported from the coastal waters off the Tomioka Peninsula, Amakusa, while G. azumai , G. quadricephala , and G. recticornata were reported from Shijiki Bay (Nunomura 1992, 2012a). Considering the similar characteristics and localities among these six species, it is adequate to determine these six species are the same species. Thus, the present study determined that C. amakusaensis , C. saikaiensis , G. azumai , G. quadricephala , and G. recticornata are junior synonyms of C. kikuchii .

According to the key to Gnathiidae genera in Cohen & Poore (1994), the genus Gnathia is distinguished from other genera by the following characteristics: large pylopod comprising two or three articles, ii) five-articled maxilliped, iii) a frontal process on the frontal border, iv) cephalon possibly to possess paraocular ornamentation and/or a dorsal sulcus, v) frontal border not deeply excavated, and vi) mandibles not elongated. In addition to characteristics i)–iv) of Gnathia , the genus Elaphognathia has the following characteristics: frontal border excavated and long mandible lacking a dentate blade. In addition to characteristics i)–iii) of Gnathia , the genus Caecognathia has the following characteristics: frontal border without a frontal process (often rounded) and cephalon lacking a paraocular ornamentation and a dorsal sulcus.

The genus affiliation of the present species was reported as Gnathia (Nunomura 1992) but later Cohen & Poore (1994) transferred them to the genus Caecognathia (Cohen & Poore 1994) . However, the present species clearly did not belong to Caecognathia because the frontal border was deeply excavated. The present species had an excavated frontal border but no mediofrontal process and mandibles with a dentate blade. According to the results of a cladogram analysis based on the species-characteristics of Gnathiidae by Cohen & Poore (1994), nine species belonging to Elaphognathia clustered in a monophyletic clade that shared several synapomorphies: excavated frontal border, mandible with an internal lobe, and the absence of a dentate blade. Other characteristic changes were reversed in some species: mediofrontal process and mandibular seta were absent. Thus, they concluded that Elaphognathia with a deeply excavated frontal border was immediately distinguishable from Gnathia . Thus, the present species belonged to Elaphognathia .

To date, 21 Elaphognathia species have been reported worldwide. The present species, E. kikuchii , was easily distinguishable from the other Elaphognathia species by its almost straight mandibles with a dentate blade, although Cohen & Poore (1994) indicated that a dentate blade on mandible did not apply to the features of Elaphognathia . However, E. cornigera (Nunomura, 1992) , E. nunomurai Ota, Tanaka, Hirose , & Hirose, 2010, and E. australis Svavarsson & Bruce, 2012 have a dentate blade on the tip of their mandibles. In addition, presence of several long setae on the frontal border is a remarkable characteristic in E. kikuchii , whereas the presence of a few short setae or none characterizes most Elaphognathia species (e.g., see Holdich & Harrison 1980; Cohen & Poore 1994; Svavarsson & Bruce 2012). Several long setae are present on the frontal border in Elaphognathia rimifrons (Holdich & Harrison, 1980) , but they are present in the central area (Holdich & Harrison 1980). In E. kikuchii , long setae are present between the center and the base of the mandibles.