New discovery and redescription of Hondoniscus kitakamiensis Vandel, 1968 (Crustacea: Isopoda: Trichoniscidae) from scree-covered slope, Iwate-ken, Northern Japan
アイテムタイプ
紀要論文 / Departmental Bulletin Paper
言語
英語
キーワード
ホンドワラジムシ, 地下浅層からの発見, 再記載
キーワード(英)
Hondoniscus kitakamiensis, discovery from upper hypogean zone, redescription
Introduction
Vandel (1968) established a new genus Hondoniscus and described Hondoniscus kitakamiensis on a few specimens collected from Ryusendô cave, Iwate-ken. But in recent year, there has been no additional report of this species. After that, two species of the genus have been known as valid, H. mogamiensis and H. ureirensis from the northern Honshu, Japan, respectively from Tohoku District, Northern Japan.
Among them, H. kitakamiensis has been known only from the limestone cave Ryusendô, the type locality of the species, but the environment of the cave gradually dried to ensure the increasing tourists’ one's safety secure a tunnel-shaped device through which air is blowing along the natural waterway. Caused by this tunnel, condition of air and water changed habitat became worse.
So in recent years, some biological surveys were held by some scientists; Koumori-ana branch hole of Ryusendô (Fujii et al., 2010), which seems worth considering as possible. Uchimagi-dô cave (Fujii et al., 2011), near from Ryusendô. But the occurrence of this species has not been confirmed. This species has designated as an endangered species by Ministry of the Environment in 1991 (Uéno and Nunomura, 1993; Hondoniscus kitakamiensis).
Therefore, the first author (TK) made a series of research in the zone of scree-covered near Ryusendô cave and collected individuals, and they were sent to the second author (NN) and we made a redescription on the specimens.
Hondoniscus kitakamiensis Vandel, 1968 (Figs.1–2)
Material examined: 2♂♂ (2.5–2.6 mm in body length) and 1♀ (approximately 3.0 mm in body length, posterior part broken) from the zone of scree‐covered slope 200 m in altitude, Iwaizumi, Funai, Iwaya-chô, south from the mouth of limestone cave Ryusendô, type locality of Hondoniscus kitakamiensis Vandel, 1968, Iwaizumi-chô, Iwate Ken, 13 June, 2018, coll. Takashi Komatsu. These specimens will deposite as follows: 1♂ (NSMT Cr-xxxxx) at National Museum of Nature and Science, Tokyo 1♂ (TOYA Cr-23766) and 1♀ (TOYA Cr-23767) at Toyama Science Museum.
Description: Body (Fig. 1A) 2.4–2.5 times as long as wide.
Color white. Cephalon with a low and round medial process of antero-lateral angle. Eyes absent, dorsal surface of pereonal somite with 3 rows of small tubercles horizontally. Each lateral margin of many setae on lateral margin. Pleon abruptly narrower than pereonal somites.
Each pleonal segment subequal in length. Posterior part of pleotelson truncated.
Antennule (Fig. 1B and C) with 3 segments: terminal segment with 4–6 aesthetascs at the tip. Antenna (Fig. 1D) with 5 peduncular segments: segments 1–3 rather short, segment 4 longer than the segment 3, segment 5 a little longer than peduncular segment 4 and setose. Flagellum, a little longer than peduncular segment 5, setose and almost inarticulate: only a faint suture line at one-third from the basal margin.
Clypeus (Fig. 2A) rectangular. Labrum (Fig. 2A) semicircular. Labium (Fig. 2B) fusiform, with many short setae. Left mandible (Fig. 1E): pars incisiva with 3 teeth; lacinia mobilis with 3 teeth; 2 setae and a seta; processus molaris wide, with a long seta. Maxillula: mesial lobe: (Fig. 1F) with 3 plumose setae, lateral endite (Fig. 1G) with 10 simple teeth. Maxilla (Fig. 1H): slender, with weaker setae. Maxilliped (Fig. 1I): endite slender, tapering towards the tip, with a seta the tip and many setae; palp with long robust, epipodite elliptical.
A part of appendages were already fallen out the body when observing and unfortunately pereopods 2 and 4 were not confirmed.
Pereopod 1 (Fig. 2C): basis 2.8 times as long as wide; ischium with 2 setae on inner margin; merus with 2 setae on inner margin and a seta at outer distal angle; carpus with 5–6 setae on inner margin and many setae on lateral margin.; propodus with 2 setae on inner margin, many setae on outer margin and many setae on lateral margin.
Pereopod 3 (Fig. 2D): basis 2.8 times as long as wide, with a seta at outer distal angle; ischium with a seta on outer margin; merus with 2 setae on inner margin and a seta on distal outer angle: carpus with 6 setae on inner margin and many short setae on distal part of outer margin; propodus with a longer seta on middle part of inner margin and many setae on outer margin; dactylus with a dactylar seta.
Pereopod5 (Fig. 2E): basis 2.1 times as long as wide, with a seta near inner distal angle; ischium with 2 setae on inner margin and a seta at outer distal angle; merus with 3 setae on inner margin; carpus with 4 setae on inner margin and many setae on outer margin; propodus with 2 setae on middle area of inner margin and many setae on outer margin and many setae on outer margin many setae; dactylus with a well-developed dactylar seta.
Pereopod 6 (Fig. 2F): basis 2.5 times as long as wide, with a seta near inner distal angle; ischium with 2 setae on inner margin; merus with 4 setae of inner margin and a seta at outer distal area; carpus with 7 setae on inner margin and many setae on outer margin and many setae on outer margin and many setae on distal part of outer margin; propodus with a seta on middle area of inner margin and many setae on outer margin; dactylus with a well-developed dactylar seta.
Pereopod 7 (Fig. 2G): basis 2.3 times as long as wide; ischium with 2 setae on inner margin and 2 setae on distal margin; merus with 4 setae on inner margin and 2 setae at outer distal angle: carpus with 4 setae on inner margin and a group of short setae on outer distal area many setae on distal part of outer margin; propodus with a seta on middle area of inner margin and 3 shorter setae on inner margin and many setae on outer margin; dactylus with a well-developed dactylar seta.
Penes (Fig. 1I) fusiform and rather long, 4.6 times as long as wide.
Pleopod 1 in male (Fig. 1I): sympod rectangular, 2.3 times as wide as long, distal area with a seta: endopod 2-segmented, distal segment slender, with many fine setae on distal half; exopod round, with a seta on distal area.
Pleopod 2 (Fig. 1J) in male: sympod 6 times as wide as long, with a seta at outer distal angle; endopod 2-segmented; distal segment tapering towards the tip; exopod small and reniform, 1.3 times as long as wide.
Pleopod 3 (Fig. 1K): endopod smallr, with sinuate outer margin; exopod orbiculate.
Pleopod 4 (Fig. 1L): similar to pleopod 3, but exopod a little smaller, with a seta at the tip.
Pleopod 5 (Fig. 1M): endopod triangular with sinuate outer margin; exopod orbiculate, with a seta at the tip, with microtrichs sparsely.
Uropod (Fig. 1N): sympod stout, almost as long as wide; endopod conical and slender, 4 times as long as the basal width; exopod conical, 1.4 times longer than endopod and 3 times as long as the basal width.
Environments and ecological notes: The sampling site is the right bank of Shizugawa-river, 700 m southeast of the entrance of Ryusendô cave. Most areas of the right bank of the river are covered by hard and high limestone cliff while there is a little of overed by soft soil and scree. The environment around sampling area was forest consisted of planted cedar and natural broad leaved deciduous trees. The ground surface around river bank was dusky and moist. TK dug the collapse area of the river bank by iron bar (40 cm long). As a result, three individuals of H. kitakamiensis were found from rock crack (about 50 cm underground), together with another terrestrial trichoniscid species, Haplophthalmus danicus. The geological features where the specimens collected were composed of quite fragile rock so many cracks (within a few millimeters wide). The surface of the rocks was quite wet (to the extent that waterdrop can no longer drip) and covered by a little of red clay. When the individuals were found, they walked positively on the rock surface and tried to escape downwards though the movement was very sluggish. TK successfully collected three individuals of H. kitakamiensis, and let more five specimens escape.
In addition to the above-mentioned sampling site, TK tried to find by digging ground along a few streams on the mountain massif: the opposite side of the entrance the Ryusendô cave. But, none specimens of H. kitakamiensis were found there.
Remarks: Hondoniscus kitakamiensis was described on a few of specimens collected from the limestone cave Ryusendô (Vandel, 1968) and then Nunomura (1983) redescribed this species. But many features including maxillula, all the pereopods, pleopods 3–5 and some other important appendages have not been described nor figured. Fortunately, some additional specimens were placed at our disposal by TK; we redescribed many hitherto undescribed features: some pereopods 1, 3, 5–7, pleopods 3–5, clypeus, labrum, labium maxillula and maxilla.
The present specimens examined show the following slightly differences from the original description: (1) more numerous aesthetascs of antennule, 4–6 in the present specimens, (2) shorter fourth and fifth peduncular segments of antenna, (3) presence of a faint suture line on antenna, (4) more setose palp of maxilliped, (5) presence of setae n distal half of exopod of pleopod 2 and (6) less numerous setae at the tip of uropod. But these are considered to be slight and they are individual variations.
Ecologically, the discovery from the subterranean environment means the habitat of this species is not limited to cave but also more extensive environments. In addition, the fact that two related species, H. kitakamiensis and H. ureirensis, coexist within Ureira mountain massif is quite important in a viewpoint of biogeography and evolution of troglobites.
Acknowledgments
We thank to Dr. Chiharu Fujii of former Iwate Prefectural Museum and to Dr. Shuji Watanabe of Iwate Prefectural Museum and get some new information on this species.