On a research visit, Prof Ndongo headed for the Moyamba district where sightings had been reported.
With the help of locals he found several Afzelius crabs, which have a brown shell and are about 20mm in size, he said.
He also found the Sierra Leone crab, which hadn’t had a confirmed sighting since 1955.
Camapign group Re:Wild says the lost species are land-living crabs that live in burrows on the rainforest floor, often far from permanent water sources.
They have specially adapted lung-type structures that allow them to breathe air, and some of their close relatives elsewhere in West Africa can even climb trees.