びわ湖こで新しん発見はっけん「鬼おにの金棒かなぼう」のようなこの貝かいが…

Researchers from the University of Tokyo investigated the habitats of the freshwater snail Kawanina, which tells the story of the biodiversity of Lake Biwa in Shiga Prefecture. They newly discovered that a type of Kawanina with different characteristics from those previously known lives on top of an underwater mountain at the bottom of the lake, isolated from other habitats.

The researchers who conducted the study said, It is possible that they have undergone unique evolution, such as developing rugged shells, in order to survive in harsh environments.

According to the Lake Biwa Museum and others, 17 species of small freshwater snails called Kawanina that are found only in Lake Biwa have been identified, and they are known as creatures that represent the biodiversity of Lake Biwa.

Specially Appointed Researcher Naoto Sawada 29 of the University of Tokyo conducted a boat survey in cooperation with local fishermen to find new habitats for the river snail in Lake Biwa.

Mr. Sawada focused on the underwater mountain located at the bottom of the lake about 5 kilometers offshore in Lake Biwa.

Kawanina are usually found from the shallow areas of lakes to depths of around 30 meters, so it has been thought that they cannot inhabit offshore areas where the water is about 60 meters deep.

However, the mountain at the bottom of the lake is about 40 meters high, and its peak is located around 20 meters below the surface.

Mr. Sawada believed that if the water depth near the summit was at this level, river snails could survive, so he set up traditional fishing gear to catch shrimp with the cooperation of local fishermen.

As a result, some river snails were found mixed in with the shrimp in the fishing gear.

As a result of analyzing the shape of the shells and genetic information, it was confirmed that their characteristics are different from those of other river snails in Lake Biwa.

This river snail has protrusions on the surface of its shell, giving it a distinctive shape reminiscent of the onis iron club that appears in old folktales.

So far, 17 endemic species have been identified, accounting for nearly one-third of all species unique to Lake Biwa.

It is believed that they have adapted to the diverse environments such as rocky areas, mud, and sandy shores along Lake Biwas coastline, and over a long period of time, have evolved into a variety of unique endemic species.

The diversification of the river snails in Lake Biwa is believed to have occurred after about 400,000 years ago, according to genetic analysis conducted by Kochi University and others.

Of these, the mountain at the bottom of the lake where this times survey of river snails was conducted is about 40 meters high, which is equivalent to a 13-story building.

On the other hand, Lake Biwa’s deepest point exceeds 100 meters, and the area around the “underwater mountain” where the survey was conducted is also relatively deep, with a depth of 60 meters.

For this reason, the appearance of the mountain at the bottom of the lake is completely invisible from the surface of the water, and it exists quietly hidden underwater.

So, the person who cooperated in making the survey possible was Mr. Shosei Miyazaki 29, a young fisherman belonging to the Kaizu branch of the Shiga Biwako Fisheries Cooperative.

Mr. Miyazaki, who has experience researching fish in Lake Biwa at the graduate school of Tokyo University of Marine Science and Technology, began assisting with surveys after hearing a lecture two years ago by Naoto Sawada 29, a researcher of the same generation who was conducting investigations on river snails.

Mr. Miyazaki loaded the traditional fishing gear called Ebitatsube, used for catching shrimp, onto the boat and headed to the survey area. He aimed to drop the gear near the summit of an underwater mountain that does not appear on the waters surface, and later conducted a survey to retrieve it.

The fishing gear is designed so that once a creature enters, it is difficult to escape, and Mr. Miyazaki was able to successfully bring up river snails along with shrimp and fish.

The first time I succeeded was two years ago.

According to sources such as the Lake Biwa Museum in Shiga Prefecture, the area where Lake Biwa currently exists was once a landscape of deep valleys and mountains before the lake formed. It is believed that, starting about 400,000 years ago, movements along the fault line on the western side of the lake gradually caused the land to sink, and as the lake expanded from the west to the east, the present-day lake was formed.

It is also believed that, during this subduction process, parts of the former mountain became submerged in the lake, forming what is known as an underwater mountain.

However, it is said that there are still many unknown details at this point, such as when the mountain at the bottom of the lake where the river snails were found became submerged.

Therefore, the genetic information of river snails found near the summit is expected to serve as a clue to unraveling the history of the mountain at the bottom of the lake.

According to Specially Appointed Researcher Naoto Sawada of the University of Tokyo, by analyzing genetic information, it is possible to obtain clues such as which era in Lake Biwas history the species of Semisulcospira have existed since, and from there, it may be possible to infer the living environment of that time.

Based on these factors, we expect that comparing and studying the various species of Semisulcospira snails found in Lake Biwa may provide clues to unraveling the history of Lake Biwa, such as the formation of “mountains on the lakebed.”