The Ogasawara Islands are oceanic islands located 1,000 km south of the mainland of Tokyo (Fig. 1). Due to the geographic isolation, the islands’ fauna boasts numerous endemic species (Karube 2004, Shimizu 2010, Chiba 2010). Regarding the scientific value as the typical examples of adaptive radiation on small oceanic islands, the Ogasawara Islands have been designated as a UNESCO World Nature Heritage Site. In recent decades, however, introduction of invasive alien species such as Anolis carolinensis Voigt, 1832 (Reptilia, Squamata, Iguanidae), Rhinella marina (Linnaeus, 1758) (Amphibia, Anura, Bufonidae) and Platydemus manokwari De Beauchamp, 1963 (Platyhelminthes, Tricladida, Geoplanidae) pose a serious threat to the endemic arthropods and molluscs of these Islands (Karube 2005, Ohbayashi 2006, Uchida et al. 2016, Shinobe et al. 2017), which leads to an increase in the importance of frequent monitoring on the fauna of the Ogasawara Islands.

The spider fauna of the Ogasawara Islands were scarcely known until recent decades (Shimomura 1933). Prior to the 1930s, only two spider species were documented, namely, Pachylomerus mirandus Kishida, 1921 (Araneae, Halonoproctidae) [a junior synonym of Conothele fragaria (Dönitz, 1887), supposed by Ono (2011)] and Cladothela boninensis Kishida, 1928 (Araneae, Gnaphosidae) (Kishida 1921, Kishida 1928). Following several faunistic reports, such as Yaginuma (1970) and Shinkai (1991), the spider fauna underwent its first comprehensive study by Ono (2011). This study, based on specimens deposited in the National Museum of Nature and Science, Tokyo, along with additional field surveys, revealed a total of 81 species belonging to 25 families on the Islands.

As a result of the preceding faunistic survey, it was found that the spider fauna of the Ogasawara Islands was predominantly composed of artificially imported species (70% of the fauna), with a low proportion of natural inhabitants (30%) (Ono 2011). Ono (2011) also noted that some arboreal native salticid spiders were not discovered on Chichi-jima Island during the survey between 2009 and 2010, suggesting possible extinction of these species due to predation pressure by alien predators. Subsequent updates to the fauna were provided by Suguro and Nagano (2015), Baba et al. (2015), Baba and Tanikawa (2018) and Shimano et al. (2018). Currently, the fauna of the Ogasawara Islands recognises approximately 90 species belonging to 25 spider families. However, no comprehensive faunistic data on the spider fauna of the Ogasawara Islands have been reported since Ono's study in 2011.

To update and evaluate the current spider fauna on the Ogasawara Islands, the authors have recently been surveying the spider fauna of various sites on Chichi-jima Island, the largest and most populous island in the Islands. During the survey, we discovered an unknown, 8-eyed, dark-coloured ground-dwelling spider with a scutum on the dorsum of the abdomen, exhibiting characteristics consistent with Oedignatha (Araneae, Liocranidae) and a reddish-orange coloured spider resembling members of palpimanids (Araneae, Palpimanidae). Close morphological examination of these specimens revealed that they can be identified as Oedignatha scrobiculata Thorell, 1881 (Araneae, Liocranidae) and Boagrius qiong Lin & Li, 2022, respectively. Oedignatha scrobiculata is known to be native to South and Southeast Asia and speculated to have been introduced to Madagascar, Seychelles, Reunion and Germany and Boagrius qiong has been recorded exclusively from Hainan, China (World Spider Catalog 2024).

As part of our survey of the spider fauna on Chichi-jima Island, this brief report presents the first records of Oedignatha scrobiculata and Boagrius qiong from Japan, based on specimens collected from the Island. Additionally, we provide photographs and the habitat of these species in Chichi-jima Islands and a brief discussion on the presumed introduction of these species from other regions.

Specimens were collected from litter layers and preserved in 70% ethanol solution. Photographs of these specimens were taken using a digital camera (habitus: LAOWA 50 mm F/2.8 2X ULTRA MACRO APO; genitalia: Olympus OM-System Zuiko Auto-Macro 38 mm and Olympus Auto 65-116 mm attached to Olympus OM-D E-M1) and stacked using an imaging software (Zerene Stacker; Zerene Systems, Washington, USA). A living image was taken using a Canon EOS 60D with a Canon MP-E 65 mm lens and focus-stacked using Zerene Stacker image stacking software. Habitats were photographed with a digital camera (Olympus Tough TG-5). Internal female genitalia was clarified by 10% potassium hydroxide (KOH) solution. Measurements are given in millimetres. Measurements of the legs are given in the following format: femur + patella + tibia + metatarsus + tarsus = total. Specimens used in this study are deposited in the arthropod collection of the Tokushima Prefectural Museum, Tokushima, Japan (O. scrobiculata: TKPM-AR 3191, 3193; B. qiong: TKPM-AR 3219).

Terminology of the genital morphology is in accordance with Deeleman-Reinhold (2001),Zonstein and Marusik (2020), Lin et al. (2022), Sankaran (2022), Lin et al. (2023) and Lu et al. (2023). Abbreviations : ALE: anterior lateral eye; AME: anterior median eye: CO, copulatory opening; DS, dorsal scutum; E, embolus; ES, epigastric scutum; RTA, retrolateral tibial apophysis; S, spermathecae; TA, tegular apophysis; VP, ventral process.

Our survey revealed the presence of two tropical ground-dwelling spiders, Oedignatha scrobiculata and Boagrius qiong on Chichi-jima Island. These spiders represent novel additions to the spider fauna of the Ogasawara Islands and Japan. Amongst them, O. scrobiculata spiders were collected at various developmental stages, ranging from small juveniles (identified, based on the general appearance and excluded from the materials section) to adults. This suggests that the species have been reproducing and establishing a population on Chichi-jima Island. In contrast, we obtained only a single individual of B. qiong from the Island and it remains uncertain whether its population has been established.

Mbo and Haddad (2020) proposed that O. scrobiculata populations in Madagascar, Seychelles and Reunion may have been introduced from Asia, because most species of the genus inhabit South to Southeast Asia and none of the congeners has been discovered from the African continent. The habitat type of this species in Southeast Asia, artificially disturbed environments such as palm plantations (Deeleman-Reinhold 2001), further suggests that human activities may enable O. scrobiculata to expand its distribution range. Contrary to O. scrobiculata which exhibits a broad distribution, B. pumilus was recorded only from Hainan, China before the present study (Lin et al. 2022). Another three species of Boagrius are also known to inhabit relatively restricted regions: Malaysia to Sumatra (B. pumilus), Borneo (B. simoni) and India (B. tenuisus) (World Spider Catalog 2024). Consequently, this marks the first occurrence of Boagrius species on a small, isolated oceanic island in the Pacific Ocean.

On Chichi-jima Island, O. scrobiculata and B. qiong were collected from a litter layer beside a walkway on Mt. Ogami-yama (Fig. 4C and D), along with large numbers of several alien invertebrate species, such as Porcellionides pruinosus (Brandt, 1833) (Isopoda, Porcellionidae), Technomyrmex brunneus Forel, 1895 (Insecta, Hymenoptera, Formicidae) and Satsuma mercatoria mercatoria (L. Pfeiffer, 1845) (also recognised as S. miyakoensis M. Azuma & Y. Azuma, 1987; Kameda (2018)) (Gastropoda, Stylommatophora, Camaenidae). It is unclear whether these populations of invasive invertebrates were introduced simultaneously or prior to the two spider species into the site. Nevertheless, the site is considered to be susceptible to the invasion of invasive species. A similar situation was observed on Hachijo-jima Island in the Izu Islands, Tokyo, Japan. On this Island, a funnel web spider, Coelotes oshimensis (Araneae, Agelenidae), originally from Amami-oshima Island in the Nansei Islands, co-existed sympatrically with a large number of invasive arthropods, such as Opisthoplatia orientalis (Blattaria, Blattodae) and Chamberlinius hualienensis (Diplopoda, Polydesmida, Paradoxosomatidae) (Okumura and Cheng 2023).

Chichi-jima Island is approximately 2,000 km away from Taiwan and the Philippines and 3,400 km away from Hainan, where O. scrobiculata and B. qiong have, respectively, been recorded. Given this geographic distance, it is less likely that these ground-dwelling species reached Chichi-jima Island through natural dispersion, such as ballooning or current dispersion. Notably, both species were not collected on Chichi-jima Island during the field surveys conducted between 2009 and 2010 (Ono 2011, Ono, personal communication). Based on the above information, we hypothesise that the considered species, especially O. scrobiculata, are not native inhabitants of Chichi-jima Island, but were artificially transported to the Island, potentially through shipments of cargo containing seedlings and soil in recent years.

It is also important to consider the potential impact of the two spiders on the fauna of Chichi-jima Island. Oedignatha scrobiculata is known for constructing silken tubular nests with multiple openings in the soil, ambushing prey from within (Koh et al. 2022). This behaviour indicates that O. scrobiculata is likely a predator of soil arthropods. Palpimanid spiders are recognised as araneophages, meaning they feed on other spiders and known to specifically prefer cursorial spiders (Pekár et al. 2022). These observations suggest a potential predatory influence of O. scrobiculata and B. qiong on other ground-dwelling arthropods in Chichi-jima Island. However, as mentioned earlier, the habitats of these two species on Chichi-jima Island are predominantly occupied by invasive ground-dwelling arthropods. This circumstance raises the possibility that prey-predator interactions may be established between alien prey and alien predators.

Our discovery further emphasises the significance of consistently monitoring arthropod fauna to promptly detect alien species. This proactive approach is also crucial for assessing the potential future dispersion of such species into other islands within the Ogasawara Islands and impact to other arthropods.