An ancient stage for evolution
Lake Biwa is one of the world's oldest lakes, with a history stretching back approximately four million years. Located in Shiga Prefecture, it is the largest freshwater lake in Japan. Its immense age and long, uninterrupted history of isolation have created a natural place for evolution. This has allowed for a diversity of life to develop, with over 60 species that are endemic, meaning they are found nowhere else on Earth.
Among these unique creatures is a group of freshwater sculpins in the genus Cottus. These are small, bottom-dwelling fish, typically less than 15 cm long. Originally, a single ancestral species of sculpin colonized the ancient lake. Over millions of years, this ancestor gave rise to a "species flock," a group of closely related species that all evolved from a common ancestor within a single, isolated geographic area. The lake's varied environments, from deep open water to rocky and sandy shorelines, provided different ecological niches that drove this divergence.
A story told in jaws
The speciation of Lake Biwa's sculpins is a example of adaptive radiation. As different populations of the ancestral sculpin became isolated in distinct habitats within the lake, they faced different selective pressures. This led to the evolution of specialized traits, particularly in their feeding anatomy. Each of the eight endemic species developed unique jaw structures and body shapes suited to a specific diet and lifestyle.
For instance, some species evolved larger, more robust jaws for crushing the shells of snails found on rocky bottoms. Others developed smaller, more agile mouths for catching tiny crustaceans in the open water. One species, Cottus reinii, is found in the lake and its surrounding rivers. Another, the Japanese fluvial sculpin (Cottus pollux), also inhabits the area and is closely related to the endemic lake species. These differences in feeding apparatus reduced competition between the emerging species, allowing them to coexist and thrive by partitioning the lake's resources. This process, occurring over the geological timescale of the lake's existence, resulted in the eight distinct sculpin species we see today. The Lake Biwa Museum houses exhibits on these endemic species, showing these evolutionary changes.