Geothelphusa dehaani, commonly known as the Japanese freshwater crab or sawagani (meaning "creek crab"), is a species of crustacean in the family Potamidae. It is endemic to the Japanese archipelago, found in clear, unpolluted mountain streams and surrounding rice paddies across Honshu, Shikoku, and Kyushu. The genus Geothelpusa combines geo meaning land and thelpusa, a traditional taxonomical name for freshwater crabs, while the species epithet dehaani honors the 19th-century Dutch zoologist Wilhem de Haan, who was instrumental in the early systematic classification of Japanese fauna. Historically, the sawagani has held a firm place in Japanese rural culture and seasonal cuisine, recognized not only as an indicator species for pristine freshwater quality but also as a distinct, crunchy culinary ingredient enjoyed in mountainous regions where fresh saltwater seafood was traditionally inaccessible.

The crab is physically modest in size, typically reaching a carapace width of about 2.5 to 3 centimeters. It possesses a smooth, dark brown-to-rust-colored carapace that allows it to blend perfectly with the rocky, leaf-strewn bottoms of freshwater streams. A defining behavioral trait of Geothelphusa dehaani is its direct development; unlike most coastal crabs that release larvae into the sea, these crabs hatch directly from the egg as miniature versions of the adults, spending their entire life cycle in freshwater environments. They are semi-aquatic, frequently emerging from the water to forage along the banks for detritus, small insects, and aquatic plants. Their presence is highly sensitive to environmental degradation, making them a "living barometer" for the health of local riparian ecosystems.

As a food source, sawagani is a unique specialty often served as karaage (deep-fried) in traditional Japanese mountain gastronomy. Because of their small size and relatively thin, soft shells, they are typically prepared whole. The crabs are either simmered in a savory broth of soy sauce, mirin, and sake (known as tsukudani) or deep-fried until the shell becomes crispy and brittle, allowing the entire crab—including the legs and claws—to be consumed. This preparation style yields a distinct, savory, and mineral-rich flavor profile often described as having an intense "umami" quality, similar to roasted shrimp or soft-shell crab. In some regional traditions, they are even served alive in sake (ganizake), though this is less common today due to food safety concerns regarding potential parasites like lung flukes.

Nutritionally, Geothelphusa dehaani is an excellent source of high-quality animal protein, essential amino acids, and minerals, particularly calcium and phosphorus, derived from the edible shell. The meat and hepatopancreas (often referred to as "crab butter") provide a dense source of lipids, including beneficial omega-3 and omega-6 fatty acids. The vibrant orange-to-red color of the cooked shell is primarily due to astaxanthin, a potent carotenoid pigment that the crab acquires through its diet of aquatic vegetation and detritus. In traditional folk medicine, the crushed meat or broth was sometimes used as a tonic to address fatigue or skin ailments, though these uses have largely been superseded by modern nutritional understanding.