73Freshwater and Brackish-water Macroinvertebrates
in the Ogasawara Islands
Kiyoshi SATAKE1*, Ryuhei UENO1 and Tetsuro SASAKI2
1

Center for Environmental Biology and Ecosystem Studies, National Institute for Environmental Studies
16-2 Onogawa, Tsukuba, Ibaraki 305-8506, Japan
2
Institute for Boninology
Nishi-machi, Chichi-Jima, Ogasawara-mura, Tokyo 100-2101, Japan*Email: satanii@nies.go.jp

Abstract
During the past twenty years several surveys have been conducted on the freshwater and brackish-water
macroinvertebrates of the Ogasawara Islands. As a result, several species of macroinvertebrates have been
described as new species endemic to these islands. Subsequent investigation has shown that some of theseendemic species are on the verge of extinction. Consequently, the species are treated as threatened orendangered species in the Red List by the Ministry of the Environment, Japan.
Key words : brackish-water, endemic species, freshwater, macroinvertebrates, oceanic islands

1. Introduction
The Ogasawara Islands are known as oceanic islands
that consist of small islands such as Chichi-Jima andHaha-Jima. During the past twenty years several surveys
were conducted on the freshwater and brackish-water
macroinvertebrates of the Ogasawara Islands by several
research groups. As a result, macroinvertebrates species
such as a caridean shrimp, mitten crab, isopod andcaddisflies were described as new species endemic tothese islands (Komai et al., 2006; Ito et al., 2011;
Nunomura & Satake, 2006; Satake & Cai, 2005; Satakeet al., 2005). Subsequent investigation showed that someof these endemic species were on the verge of extinction.
At present, some of these species are treated as threatenedor endangered species in the Red List of the Ministry ofthe Environment, Japan.

2. Fauna Identified
2.1 Aquatic Insects
The distribution of EPT (typical aquatic insect faunaof the three orders Ephemeroptera, Plecoptera andTrichoptera) among oceanic islands is extremely rare
anywhere in the world. In the Ogasawara Islands, aquaticinsects of the orders Ephemeroptera and Plecoptera seemto be absent. However, it has been confirmed that twospecies of Trichoptera or caddisflies inhabit streams ofthe Ogasawara Islands. One species of the genus Goerathat inhabits only the headwaters of Chichi-Jima wasdescribed as a new species, Goera ogasawaraensis,
Global Environmental Research
23/2019: 073-075
printed in Japan

endemic to the Ogasawara Islands (Satake et al., 2005).
Another caddisfly species of the genus Hydroptila wasconfirmed in the rivers of Chichi-Jima and Haha-Jima,
and was also described as a new species, Hydroptila
ogasawaraensis, endemic to these islands (Ito et al.,2011).
Regarding the other aquatic insect orders, several
studies were also conducted on the freshwaters of theOgasawara Islands.
Recently, a new species of aquatic coleoptera of thegenus Ochthebius was described as a new species. This
record is the first from the family Hydraenidae in theOgasawara Islands. This coleopteran species usually foundin sheeting freshwaters, but after a drought in 2018–2019
the area of sheeting freshwater was markedly reduced(Yoshitomi et al., 2019).
As for dipterans, chironomid fauna have beenstudied on Chichi-Jima and Haha-Jima during the lasttwenty years. A total of 17 species have been recorded,among which Dicrotendipes yaeyamanus, Glyptotendipes
tokunagai and Tanytarsus magnihamatus were new tothose islands. The chironomid fauna of these islandsdiffer markedly from those of the Japanese mainland, andsome species seem to be endemic to the islands, but asother species also occupy the Northern Marianas, furtherstudies are needed to clarify the endemism of this groupof insects (Ueno & Satake, 2009).
2.2 Crustaceans
Decapod crustaceans are typical fauna of thefreshwater and brackish-water habitats of the Ogasawara©2019 AIRES

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K. SATAKE et al.

Islands.
As for freshwater and brackish-water shrimps, ninespecies from three families have been confirmed, ofwhich there are three species of the family Palaemonidae,and five species of the family Atyidae. In addition, onespecies of the genus Metabetaeus has been discovered in
an anchialine pool, but its taxonomic position is still
uncertain. At present, two species of shrimps are regarded
as endemic species to the Ogasawara Islands: Palaemon
ogasawaraensis, which lives only in the brackish watersof Chichi-Jima (Kato & Takeda, 1981), and one speciesof the genus Paratya inhabiting the upstream stretches ofsome streams on Chichi-Jima and Haha-Jima, which has
been described as Paratya boninensis (Satake & Cai,2005). As for this species, compared with its congener
shrimps living in Japan and neighboring countries, thebody size is smaller, and the egg size is larger. At present,
these two endemic shrimp species are regarded as
endangered or threatened species by the Ministry of theEnvironment, Japan and are at risk of extinction due tohabitat degradation, drought, etc. in their remaining
habitat.
One species of mitten crab has been described as a
new species endemic to the Ogasawara Islands (Komai etal., 2006). Distribution patterns and life cycles of thismitten crab were revealed by Kobayashi and Satake(2009). Kobayashi and Satake (2017) comparedmorphological differences and growth patterns between
this mitten crab and its congener, Eriocheir japonica, andconcluded that the mean carapace width of this island
species is 1.6 times larger than that of mainland species.
In addition to this mitten crab, a sesarmid crab and afiddler crab have also been described as new species
endemic to the Ogasawara Islands (Komai & Ng, 2013;
Shi et al., 2013). These three endemic crab species areregarded as endangered, threatened or vulnerable species
by the Ministry of the Environment, Japan.
An endemic species has also been found from theIsopoda. One species of the genus Gnorimosphaeroma,
which was collected from the upper reaches of a stream
on Haha-Jima, was described as a new species (Nunomura& Satake, 2006). A new species of the genus Ligia wascollected from steep streams on Chichi-Jima and AniJima (Nunomura et al., 2011). These two isopod species
are regarded as species endemic to the Ogasawara Islands.
As the habitat of these species is extremely unstable,
these species seem to be at risk of extinction.
2.3 Gastropods
Although Stenomelania boninensis is known as athiarid snail endemic to the Ogasawara Islands, an alien
thiarid snail, Melanoides tuberculata was recorded from
Chichi-Jima in 2005. Melanoides tuberculata wasoriginally widely distributed from tropical to subtropicalregions, such as East to South Asia, the Middle East andNortheast Africa, but has become an invasive alienFig. 1 An ovigerous female of the freshwater shrimp, Paratyaboninensis, endemic to the Bonin Islands.

species in North America, South America, New Zealand,
Europe and elsewhere, undergoing parthenogenetic
reproduction. It is causing problems such as replacementof native gastropods. In the case of the Ogasawara
Islands, there is a possibility of it replacing the endemic
species Stenomelania boninensis through competition, someasures to reduce its effects are being sought (Sasaki etal, 2009; Satake et al., 2006).

3. Conclusions
In recent years, the extinction of various species andlocal populations has been reported occurring in theterrestrial habitats of the Ogasawara Islands. Freshwaterand brackish-water endemic species are also at the risk ofextinction possibly due to stream renovations, droughts
and invasions of alien species (Sasaki et al., 2009; Satake& Ueno, 2012). Protective measures and management
plans are needed to conserve these species in theOgasawara Islands, otherwise these endemic species may
go extinct one after another.
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Kiyoshi SATAKE
Kiyoshi Satake, B.Sc., is a senior researcher at theNational Institute for Environmental Studies(NIES). His major research fields are stream
ecology and benthology. In recent years, he has
conducted field studies on aquatic
macroinvertebrates of oceanic islands off Japanto find endemic species of those islands. He is
also extending his efforts to conserve those endemic species.
(Received 9 December 2019, Accepted 13 December 2019)