Cloyster is capable of swimming in the sea. It does so by swallowing water, then jetting it out toward the rear. This Pokémon shoots spikes from its shell using the same system. Its shell is extremely hard. It cannot be shattered, even with a bomb. The shell opens only when it is attacking. Cloyster that live in seas with harsh tidal currents grow large, sharp spikes on their shells.
Pokédex entry #91
cloyster in Pokémon GO
It's possible to hatch cloyster from an egg?
No, cloyster can't be hatched.
Which are cloyster’s strengths and weaknesses?
cloyster is a ice and water type Pokémon. ice type pokémons are strong against grass, ground, flying, dragon pokémons but weak against water, ice pokémons. water type pokémons are strong against fire, ground, rock pokémons but weak against water, grass, dragon, electric pokémons.