Petrobiellus akkesiensis Mtow & Machida 2024, comb. nov.

Petrobiellus akkesiensis (Uchida, 1949) comb. nov. [Japanese name: kojima-ishinomi] (Figs. 2, 3) Halomachilis akkesiensis Uchida, 1949:110; Uchida 1955:32; Tadauchi 1989:18; Machida 1996:58; Machida 2020:77 Halomachilis kojimai Uchida, 1950:4; Uchida 1965:1; Uchida 1988:529 Petrobius kojimai: Machida 1999:799; Machida 2015:1549. Petrobiellus kojimai: Machida 2008:6 General features similar to those described by Uchida (1949, 1950). Body length of males 11–12 mm, that of females 12–13 mm. Antennal length approximately 15 mm. Cerci strongly curving outwards (Figs. 2A, B) and approximately 5 mm long; caudal filament approximately 15 mm long. The scale pattern on the dorsal body surface as shown in Figs. 2A, B. Scales absent from the antennae (Fig. 2C), maxillary and labial palpi (Figs. 2D, E), thoracic legs (Figs. 2F–H), and coxal and abdominal styli (Figs. 2G–J). Compound eyes in living specimens army-green in color; dark pigmentation present on the scapus (Fig. 2C), pedicellus (Fig. 2C), maxillary and labial palpi (Figs. 2D, E), and thoracic legs (Figs. 2F–H). Ovipositor extremely long (Figs. 2B, J). Remarks The ovipositor is much longer than indicated in Uchida (1950), who had described the ovipositor of the species as short, with its apex slightly surpassing the end of coxite IX. This difference could probably be attributed to the specimens Uchida examined being younger females: the body length in Uchida’s description was 9–10 mm, whereas that of females we examined was 12–13 mm. Petrobiellus is an enigmatic, halophilic genus of Archaeognatha found only in Japan and the Russian Far East. Absence of scales on the appendages is one of the most significant diagnostic features of Petrobiellus (Silvestri 1943; Sturm & Machida 2001; Mtow 2021). The genus contains of four validly described species: Petrobiellus takunagae Silvestri, 1943 [from Shirahama, Wakayama, Japan (Silvestri 1943) and from Shimoda, Shizuoka, Japan (Machida 1999, 2020)], P. curvistylis Uchida, 1954 [from Hachijo Island, Tokyo, Japan (Uchida 1954)], P. kusakini Kaplin, 1980 [from Simushir Island, Chishima Islands, Japan (Kaplin 1980)], and P. sachalinensis Kaplin, 2020 [from Sakhalin Island, Russia Kaplin (2020)] (Sturm & Machida 2001; Mtow 2021). Ma et al. (2015) detected two “ Petrobiellus ” species from Yunnan Province, China, i.e., P. bannaensis (Mengla County, Xishuangbanna Dai Autonomous Prefecture) and P. puerensis (Ning’er Hani and Yi Autonomous County), based only on molecular data. Their descriptions (and proposed names) are apparently unavailable as Kaplin (2020) mentioned, for the following reasons: 1) no morphological descriptions were included; 2) no types were designated; 3) the descriptions were made without designating types, and 4) the concerned species were not declared to be new species [cf. International Commission of Zoological Nomenclature (2000), Arts. 13.1.1., 16.1., 16.4.]. Petrobiellus was originally described by Silvestri (1943), based only on females. Later, Uchida (1954) described P. curvistylis, including information on males, and the junior author (RM) succeeded in collecting the males of P. takunagae, which is the type species of Petrobiellus (Sturm & Machida 2001; Machida 2008; Klass & Matushkina 2018): Machida (2020) had suggested P. curvistylis could possibly be a synonym of P. takunagae. As for P. akkesiensis, no male was identified (Uchida 1950, 1965; Machida 2008), but the senior author (SM) recently successfully collected and identified males (Figs. 2A, I). The males of P. akkesiensis share several remarkably similar features of the genitalia with the males of P. curvistylis and P. takunagae (Uchida 1954; Sturm & Machida 2001; Klass & Matushkina 2018): 1) the distal part of the penis is sclerotized and hook-shaped, 2) coxites IX are medially strongly concave, 3) styli IX are robust with the concaved medial side, on which dark strong setae are densely distributed, and 4) the paramerae, which are present only in abdominal segment IX, are slender (Fig. 2I). Biological notes Petrobiellus akkesiensis was collected by us on September 1, 1990, and June 6–7, 2022, from the supralittoral cliffs or from under rolling stones along the rocky coasts, around the Akkeshi Marine Station, Field Science Centre for Northern Biosphere, Hokkaido University, Aikappu, Akkeshi Town, Akkeshi County, Kushiro Subprefecture, Hokkaido, Japan (Figs. 1A–D). Uchida (1965) had previously reported that P. akkesiensis also inhabited the Kaki Islands in Lake Akkeshi, but the Kaki Islands now no longer exist due to subsidence (the approximate area where the islands were is depicted with a dotted line in Fig. 1C). Petrobiellus akkesiensis individuals live close to each other in high density (Fig. 3A), similar to halophilic archaeognathan Petrobius spp. (Larink 1968). The sex ratio of P. akkesiensis seems strongly biased towards females. On June 6, 2022, 75 individuals were collected, among which seven were males and 68 were females; and among 122 exuviae found in a crevice of the cliff eight were males and 114 were females (Fig. 3B).