1. Introduction
The well-known Cavernacmella kuzuuensis (Suzuki, 1937), a minute species of Assimineidae land snails, is one of only a few cave-dwelling snails inhabiting Japan. This species is reportedly found only in limestone caves from karst areas in which carbonate rocks are widely distributed (Suzuki, 1937; Habe, 1942; Itoh, 1967; Kawana and Matsumoto, 1985; Fukuda and Mitoki, 1995; Kashiwagi, 2012; Kashiwagi et al., 2015), and it inhabits the surface of cave walls kept moist by high humidity within the caves.
Cavernacmella kuzuuensis has been designated as “Critically Endangered and Endangered (CR+EN)” by the Ministry of the Environment in Japan (Ministry of the Environment, 2019). In addition, among the 47 local governments in Japan, 17 have categorized this species as either “Critically Endangered and Endangered (CR+EN),” “Endangered (EN),” “Vulnerable (VU),” “Near Threatened (NT),” or “Data Deficient (DD).” Due to the habitat requirements of troglobites as an obligate cave-dwelling species, C. kuzuuensis is extremely sensitive to environmental changes within and outside caves. Therefore, to effectively conserve the species and its habitat, it is necessary to characterize the status of its present population and its distribution throughout each karst region and evaluate the condition of its critical habitat and that of the surrounding lands.
In the Hokuriku District of Central Japan, C. kuzuuensis occurs in the Shirouma-do Cave in Fukui Prefecture (Itoh, 1967) and in the Saru-ana Cave in Toyama Prefecture (Kashiwagi, 2012). However, no information of its occurrence has been obtained in Ishikawa Prefecture although the Togatani Cave in Ishikawa Prefecture has been known at least 30 years and has been utilized by the Hakusan-roku Children’s Natural Center for teaching nature exploration to elementary school students since 2000. Nevertheless, this cave was only recently recognized by the Hakusan Tedorigawa Geopark Promotion Council as an important geosite in the Hakusan Tedorigawa Geopark.
Sakamoto (2011) recently determined the age of the sediments in the Togatani Cave (based on radiocarbon dating) to discuss relative sea level change in the Tedori River drainage basin, although a map of the cave has not been published. Based on the description of the Togatani Cave by Sakamoto (2011), we expected that C. kuzuuensis likely inhabits the cave. Therefore, we surveyed the Togatani Cave for C. kuzuuensis; herein, we describe the specimens of C. kuzuuensis that we found in the cave and provide SEM photographs of important morphological characteristics. A detailed description of the Togatani Cave and its history from its discovery to latest scientific research have been prepared by the authors.
2. Materials and Methods
The Togatani Cave is located along the left bank of Tedorigawa Dam on the upper Tedori River, Hakusan City, Ishikawa Prefecture in central Japan (Fig. 1). The cave comprises a nearly horizontal passage of up to 80 m in length and a subterranean river flows from the upper end of the cave to near its entrance (Fig. 2). The first author searched for specimens on the cave walls on October 16, 2018. Collected specimens were photographed under a scanning electron micrograph (Hitachi, TM 3030) and measurements were recorded for dimensions of specimens (height and width of shells) using a measurement tool in Adobe Photoshop CS4.
3. Results and Discussion
Four empty shells of C. kuzuuensis were collected from the cave wall along an upward-extending fissure at the end of the cave (Fig. 1); the cave was completely dark and very wet, which is characteristic of karst caves. Moreover, a translucent shell was found on the cave wall (Fig. 3), but it could not be collected. Our survey provides the first recorded occurrence of the species from Ishikawa Prefecture and the third recorded occurrence for the species in the Hokuriku District.
The C. kuzuuensis shell specimens we extracted from the cave were poorly preserved, white, and opaque (Fig. 4). The environment in which we found the specimens confirms that C. kuzuuensis is an obligate troglobite. Few studies have reported that living C. kuzuuensis have translucent shells (Habe, 1942; Itoh, 1967). Although we did not collect any living individuals, we were able to photograph a specimen with a translucent shell that was likely living (Fig. 2).
In Ishikawa Prefecture, the distribution of the carbonate rocks is restricted to an area within the strata of Hida metamorphic rocks, which are composed primarily of gneiss and recrystallized limestone (Kobayashi, 1958; Kaseno, 1978). Hida metamorphic rocks mainly occur in the upper reach of the Tedori River drainage basin where the Togatani Cave is located. A survey of underground resources conducted in 1950 showed the presence of a cave of about 8–9 m in length (Hida, 1951). Thus, the area was considered as likely to support other caves. At present, the Togatani Cave is the only limestone cave known in Ishikawa Prefecture that has been precisely mapped. The fact that we found a possible living specimen of C. kuzuuensis suggests that conditions within the Togatani Cave are suitable for troglobites. Therefore, the cave should be thoroughly surveyed for rare biota and if any more are found, it should be managed as critical habitat. Furthermore, additional field surveys should be conducted to determine if any more undiscovered limestone caves exist in the area and confirm whether additional populations of C. kuzuuensis occur in them.
4. Supplementary descriptions
Family Assimineidae H. Adams and A. Adams, 1856
Cavernacmella kuzuuensis (Suzuki, 1937)
(Common name in Japanese: Horaana-goma-oka-chigusa Habe, 1942)
Morphological characteristics of the illustrated specimens are described as follows: (1) simple ovate-conic outline of shell with relatively rounded apex, (2) tiny shells (height, 1.23–1.54 mm; width, 1.05–1.26 mm), based on four specimens, (3) width and height of each whorl gradually increasing from apex to main body whorl, (4) wrinkle-like sculptures on the surface of protoconch, (5) teleconch whorls convex in outline with faint traces of fine striae, and (6) distinctly constricted sutures between whorls. Collecting site and above-mentioned morphological features support the identification of the specimens from the Togatani Cave as C. kuzuuensis (Suzuki).
The shells of the examined specimens were similar in terms of the overall conical outline of shell to some species of the genera Angustassiminea and Assiminea (Family Assimineidae), although C. kuzuuensis is different as it has a more rounded apex and a much smaller and wider than long shell. Furthermore, the habitat of the latter species is salt marsh, not unlit caves. C. kuzuuensis also resembles the already-known species of the genera Akiyoshia and Bythinella (Family Hydrobiidae) in which the shells have a blunt apex, although the former is distinguishable from the latter in that its shell has a broader in outline. Regarding habitat, C. kuzuuensis are terrestrial, whereas most species of the genera Akiyoshia and Bythinella are aquatic.
5. Acknowledgements
The authors are sincerely grateful to Mr. Ozaki, K. (Hakusan-roku Children’s Natural Center) for the access to the Togatani Cave. We thank Mr. Otsuka, K. (Cultural Properties Division, Board of Education of Hakusan City), Mr. Hibino, T. (Hakusan Tedorigawa Geopark Promotion Council) and Prof. Aoki, T. (Kanazawa Univ.) for helping us contact with owners of the Togatani Cave, Mr. Yamada, S. (Division of Instrumental Analysis, University of Toyama) for the support of the operation of the scanning electron microscope TM3030 (Hitachi), and Dr. Minato, H. (Shirahama Town, Wakayama Prefecture) for thoughtful comments and suggestions. This research was financially supported by Grant-in-Aids for Scientific Research of Japan Society for the Promotion of Science (16H02235) to Prof. Kano, A. (Univ. Tokyo).