Japanese freshwater crab
Refers specifically to Geothelphusa dehaani, a small crab found in mountain streams and rivers across Japan. Often kept as a pet or used in environmental education.
When I was a child, I caught freshwater crabs in the river.
Freshwater crabs can only live in clean water.
Katakana spelling is common for this animal name and matches usual-kana usage.
Kanji spelling exists but is uncommon; the word is usually written in kana.
Compound of 沢 (sawa, 'mountain stream') and 蟹 (kani, 'crab'), describing its habitat. The exact historical derivation is straightforward.