New subspecies of miyairi snail discovered by Okayama University on Iriomote Island

A joint research group consisting of Associate Professor Hiroshi Fukuda of Academic Research Assembly, Graduate School of Environmental, Life, Natural Science and Technology (Faculty of Agriculture) at Okayama University, Project Researcher Naoto Sawada of Graduate School of Science, the University of Tokyo, and Lecturer Masashi Kirinoki of Graduate School of Medicine, Dokkyo Medical University has discovered an unknown species of coiled shellfish in a waterfall in the mountains of Iriomote Island, Okinawa Prefecture. The group has identified it as a new subspecies of the miyairi (Katayama) snail, which was officially described as Iriomote Miyairi snail (Oncomelania hupensis iriomotensis Fukuda & Sawada in Sawada, Kirinoki & Fukuda, 2026). The findings were published in Malacologia.

The miyairi (Katayama) snail is a coiled shellfish found on wet mud at the water's edge and at the roots of plants in a slow-streaming area or a stagnant water environment such as rice paddy canals and wetlands in lowlands and basins. This can be an intermediate host of Schistosomiasis japonicum, similarly to the subspecies Oncomelania found in China.

If humans become infected by this snail and remain untreated, it can cause severe symptoms such as cirrhosis of the liver, and possibly death. At a time when the cause of the disease was unknown and no specific remedy was available, many people became ill or died over many years in the Kofu Basin, the Fukuyama City area of Hiroshima Prefecture, and the lower reaches of the Chikugo River in Fukuoka Prefecture, where populations of the miyairi snail existed. The disease was considered a serious problem.

After the miyairi snail was found to carry the disease, it was aggressively eradicated as a preventive measure until the 1970s, when it became extinct in most of the country. In Japan, the cases of schistosomiasis japonica completely ceased by the 1980s.

In 1996, Yamanashi Prefecture, the largest epidemic area in the past, "declared eradication of the disease." The schistosomiasis japonica no longer exists in the wild, and only a few miyairi snails survive in Yamanashi Prefecture. It is classified as an endangered species category I in the Red List of the Ministry of the Environment.

The subspecies Oncomelania exists in mainland China as well as in Taiwan, the Philippines, and Sulawesi Island in Indonesia, but no subspecies other than the miyairi snail had been known in Japan.

In 2020, an unknown species of this coiled shellfish was discovered around a waterfall in the mountains of Iriomote Island, Okinawa Prefecture, on a bedrock that is constantly wet from splashing water and among the ferns that thrive there.

A coiled shellfish existing in a similar environment had not been found in the Nansei Islands. At first glance, the external morphology of the shell was rather similar to that of the genus Fukuia (Shibukitsubo), which is endemic to the heavy snowfall area on the Japan Sea side of Honshu (Akita to Shimane Prefecture), far from the Nansei Islands.

However, observation of the movement of the snail revealed that the soft-bodied head and gastropods closely resembled those of the miyairi snail. Phylogenetic analysis based on partial mitochondrial DNA sequence determined that it is included in the clade (monophyletic group) consisting of the miyairi snail and Oncomelania.

The anatomy of the reproductive, digestive, and urinary circulatory organs, as well as the central nervous system is also consistent with the phylogenetic analysis. It was shown to be a homologous separate subspecies of the miyairi snail. Therefore, the research group officially named and described this new subspecies as the Iriomote Miyairi snail.

The geographically closest similar subspecies to this new subspecies are O.h. chiui and Formosan mizomametanisi (Formosan katayama snail) from Taiwan, but the shell morphology of the Iriomote Miyairi snail differs from that of these species.

Environmental DNA was analyzed for river water collected in and around the area of origin to determine if it was an intermediate host for S. japonicum, but its DNA was not detected.

In addition, when many raw shellfish were brought back to a laboratory and reared to test whether they were infected, no infective larvae migrating out of the shellfish were found. Therefore, at present, there is no evidence that the Iriomote Miyairi snail is a potential intermediate host for the parasite. However, the group said continuing research is necessary to assess any potential risks.

It comes as a surprise that another subspecies of the miyairi snail existed in Japan completely unnoticed. Iriomote Island is located more than 260 kilometers from northern Taiwan, which is the closest distribution area and where O.h. chiui exists, and they are separated from each other by the East China Sea.

The new subspecies is a geographically isolated relict taxon and is extremely valuable because the area is located 1,250 kilometers from the lower reaches of the Chikugo River in Fukuoka and Saga prefectures, which had been the southernmost edge of the former distribution range of the miyairi snail. Moreover, the subspecies has been found in only two very small areas in Iriomote Island and is endangered because of its extremely high rarity.

The Iriomote Miyairi snail has been able to survive until now seemingly because of its unique habitat, i.e., around waterfalls deep in the mountains that are far from human settlements, and because it is not easily affected by direct human activities. However, in recent years, impact is feared due to active resort development in the Yaeyama Islands including Iriomote Island, leading to excessive sightseeing which causes the movement of many people.

Fukuda said, "The discoverers of this new subspecies are Project Researcher Sawada and his colleagues. I received the actual snail from them, and while observing it, I noticed that the face looked just like the miyairi snail, so I told them, 'Check the DNA right away, it might be the miyairi snail,' and it turned out I was right. I am modestly proud of the fact that Project Researcher Sawada was surprised, saying, 'How can you tell that much just by looking at the external form?' Considering the nature and the matter, we have to be extremely cautious. We asked Dr. Kiriki, an expert in the research into schistosomiasis japonica, to conduct an experiment."

Journal Information
Publication: Malacologia
Title: Oncomelania hupensis iriomotensis n. ssp. (Caenogastropoda: Truncatelloidea: Pomatiopsidae) from Waterfalls on Iriomote Island, Okinawa, Southern Japan
DOI: 10.4002/040.068.0104