Complex sentence: They primarily host the "green crab" which is native to the Eastern Atlantic Ocean but these crabs have spread to to other bodies of waters but isn't believed "Sacculina" has migrated along with them to these places too.
Split into simpler sentences: They primarily host the "green crab" which is native to the Eastern Atlantic Ocean. Though these crabs have spread to other bodies of waters, it isn't believed "Sacculina" has migrated along with them to these places too.