Actumnus simplex Rathbun, 1911

deVries, Maaike, McGillis Hall, Linda, Dainty, Katie, Fan, Mark, Tscheng, Dorothy, Hamilton, Michael & Trbovich, Patricia, 2024, Ten Species of Crabs of the Families Acidopsidae, Aphanodactylidae and Pilumnidae (Crustacea: Decapoda: Brachyura) from the Ogasawara Islands, Japan, Bulletin of the National Museum of Nature and Science. Series A, Zoology 50 (4), pp. 155-169 : 160-161

Actumnus simplex Rathbun, 1911 [Jn: Shin-ibotegani]

( Fig. 3A–B )

Actumnus simplex Rathbun, 1911, p. 232 , pl. 16 figs. 10–11. — Naderloo & Ng, 2011, p. 1595, figs. 1–3. — Naderloo, 2017, p. 300, fig. 26.7.

Neoactumnus convexus Sakai, 1964, p. 105 fig. 4; 1965, pp. 153 (in English), 66 (in Japanese), fig. 18, pl. 76 fig. 1; 1976, p. 498 (English vol.), p. 307 (Japanese vol.), fig. 267, pl. 177 fig. 4. — Takeda & Koyama, 1974, p. 114, pl. 11 fig. D. — Miyake, 1983, p 133, pl. 45 fig. 3. — Nagai, 1990, p. 115. — Nomura, 1991, p. 41. — Muraoka, 1998, p. 45. — Minemizu, 2000, p. 282, 1 unnumbered fig. — Kato & Okuno, 2001, p. 131, 1 unnumbered fig. — Marumura & Kosaka, 2003, p. 60. — Okuno, 2004, p. 1. — Lee et al., 2008, p. 291, fig. 1. (Syn. nov.)

Neoactumnus unispina Garth & Kim, 1983, p. 699 , fig. 8. (Syn. nov.)

Material examined. ɹ Chichi-jima Is. —Off Futami Bay, Chichi-jima I., from stomach contents of sidespot goatfish [Jn: Ryukyu-himeji], Parupeneus pleurostigma (Bennett) , 1 ˁ (cb 6.7×cl 5.4 mm), NSMT-Cr 32393, date unrecorded, coll. by Y. Kurata.

Comparative specimen examined. ɹ West of Kameki Reef , Sagami Bay, 40–42 m depth, 1 ˁ (holotype of Neoactumnus convexus Sakai, 1964 ), NSMT-Cr R2406 , 19-XII-1962, coll. by His Majesty the Showa Emperor of Japan .

Remarks. ɹ As shown in the photograph ( Fig. 3A–B ), the specimen found in fish stomach contents is in good condition to know the specific and generic characters. The taxonomic identities of Actumnus simplex Rathbun, 1911 , Neoactumnus convexus Sakai, 1965 , and N. unispina Garth and Kim, 1983 , are still unsolved. Naderloo and Ng (2011) discussed deeply on this problem on a reasonable base, but the systematic validity of the genus Neoactumnus and the specific syn- onymity of three species in question were adjourned until future study probably due to modesty rather than hesitation. The smooth and regularly convex carapace with four-lobed anterolateral margins of both sides ( Fig. 3 ) is quite similar in the three species. In the present specimen, the frontal margin is interrupted medially with a median small notch ( Fig. 3B ), agreeing well with the figure of the holotype of A. simplex given by Naderloo and Ng (2011) and differing from the figure of the holotype of N. convexus in which the frontal margin is entire without median notch. In N. unispina , the frontal margin was described as being low-triangular in shape without a median emargination. The complete frontal margin without a median notch is considered to be the generic character of Neoactumnus , together with the complete absence of a lateral lobule of the frontal margion. However, Naderloo and Ng (2011) showed that the presence or absence of a lateral lobule on the frontal margin is not to be reliable as the generic character. The number of the interruption on the supraorbital margin was not mentioned in the generic diagnosis, but described as one close to the external orbital tooth, but the notch is sometimes indistinct and almost discernible. It may be remarked that the type specimens of N. convexus are small (cb 4.5 and 7.5 mm in the holotype and allotype, respectively). The presence of one spine at the carpus inner angle of the cheliped may be individually variable and not always reliable character to distinguish the species.

The holotype of Neoactumnus convexus Sakai, 1965 , is a subadult male and agrees well with the descriptions and figures of Sakai (1964, 1976). In the original description, the specimen was said that its taxonomic status is close to the genus Actumnus Dana, 1851 in the general formation of the carapace, but considered to be generically distinct in having no median indentation of the frontal margin and the absence of notches on the supraorbital margin. The specimen is, without doubt, not fully matured, and these characters may be referrable to the immature state of the specimen. The original label is typed as Actumnus convexus Sakai , type, but it is not sure whether this label was prepared under the guidance of the late Prof. T. Sakai.

As a result of the examination of the type specimen and the reference to the literature concerned, the genus Neoactumnus Sakai, 1964 , was considered to be synonymous with the genus Actumnus Dana, 1852 , and N. convexus Sakai, 1965 , and N. unispina Garth and Kim, 1983 , were reduced to the synonyms of Actumnus simplex Rathbun, 1911 .

The crabs obtained from stomach of a same individual of sidespot goatfish were identified as Neopalicus jukesii (White, 1847) (family Palicidae ) and Crinitocinus alcocki ( Borradaile, 1902) (family Acidopsidae ). Neopalicus jukesii represented by 2 males (cb 7.6×cl 7.1 mm; cb 5.6×5.0 mm) and 1 female (cb 8.9×cl 7.3 mm) has already been known from the Ogasawara Islands by Castro (2000), Komatsu (2011), and Takeda and Tachikawa (2015). The juvenile specimen of Crinitocinus alcocki is recorded in this paper as new to the Ogasawara Islands.

Special ecology of this species has not been mentioned since the original descriptions of Actumnus simplex in 1911, Neoactumnus convexus in 1964, and N. unispina in 1983, but Nomura (1991) first mentioned the symbioticliving with sea-anemone, Nemanthus nitidus (Wassiliefe) , many individuals of which attach to the stems of gorgonaceans at the shallow-water rocky bottom. Then, in Japan, this species is attracted by the divers, and the fine photographs showing the ecology are published in the books for general use such as Minemizu (2000) and Kato & Okuno (2001).

Distribution. Amirante in the western Indian Ocean (as Actumnus simplex ), Persian Gulf in the northern Indian Ocean (as A. simplex ); Sulu Archipelago, Philippines (as Neoactumnus unispina ); Jejudo I., Korea (as N. convexus ); Japan from Okinawa-jima Island northwards to the Boso Peninsula and Hachijo-jima Island (as N. convexus ).

Figures

Fig. 3. A–B: Actumnus simplex Rathbun, ˁ (NSMT-Cr 32392; cb 6.7×cl 5.4mm) found in stomach contents of sidespot goatfish, Parupeneus pleurostigma (Bennett) from off Futami Bay, Chichi-jima I. C–D: Pilumnopeus pearsei (Rathbun), ovig.˂ (NSMT-Cr 32396; cb 5.2×cl 3.7mm) from Futami Bay, Chichi-jima I. (preserved in 75% ethanol). E–F: Pilumnus trispinosus (Sakai), ovig.˂ (NSMT-Cr 6866; cb 6.6×cl 5.2 mm) from Futami Bay, Chichi-jima I. (E) (preserved in 75% ethanol.); Ovig.˂ (NSMT-Cr 32403; cb 6.3×cl 4.5mm) from Tsuiri-hama, Chichi-jima I. (F).