It does chase fish that get too close sometimes but neither fish nor crab have ever been injured. It keeps to itself for the most part, spending its time hiding under plants or rocks. At feeding time the crab can be very entertaining. When it gets wind of a food pellet sinking nearby, it may charge out of hiding and take the food from much bigger fish. It waves its claws around and puts on quite a show, snatches the food and runs for cover.
Female crabs lack it.
Reportedly this species breeds in brackish water but then returns to fresh water to live. Long ago I had an ich outbreak in the crab's tank. I learned that sesarma bidens seems to tolerate malachite green based medications like RidIch quite well. The crab showed no signs of stress while the tank was being treated. Some people have written to me with stories of very aggressive crabs.