For these purely aquatic crabs you don't need a land part in the aquarium, you also don't need to set up an aquaterrarium for them - they live completely and all the time submerged in the water.
For a group of the tiny micro-crab Limnopilos nayanetri, which grows to a carapace width of only 10 mm, a cube with a capacity of 10 liters is already sufficient. Larger crabs like Syntripsa matanensis, the purple Matano cra b from Lake Matano on Sulawesi, the panther crab Parathelphusa pantherina, also from Lake Matano, and also the Towuti crab Parathelphus ferruginea, which also comes from the island of Sulawesi in Indonesia, naturally need larger aquariums. For a pair of these very territorial animals it is best to choose an aquarium at least 100 cm long. The Chinese panther crab Heterochelamon tesselatum is also a pure water crab and needs a similarly large aquarium, especially if it is to be kept in pairs. You can let a piece of root protrude above the water surface and thus offer the crabs a place to sit above the water surface, but the water crabs definitely do not need a land part.
You cansocialize the very peaceful and harmless micro crab with almost all animals that do not attack the tiny creatures, i.e. with dwarf shrimps and snails such as post horn snails or trumpet snails. In aquariums of appropriate size, socialization can also work with very peaceful fish, but the small, delicate crabs quickly feel disturbed. A species tank is definitely recommended for the micro crab.
The larger aquatic crabs are generally carnivores and hunters. Snails, fish and shrimp are considered and eaten as welcome supplementary food. Keeping them in a species tank is preferable for panther crab, matano crab and co. and definitely preferable. The crabs from the Old Lakes on Sulawesi can reproduce in freshwater and can also be bred in the aquarium.
Not really among the crabs, but also among the purely aquatic crustaceans is Aegla platensis, the crab crab. It belongs to the Anomura family and looks like a cross between a crayfish and a crab. The animals also live exclusively aquatic.
The African water hermit crab Clibanarius africanus also does not need a land part, but brackish water or even seawater in the aquarium.