Sheikh `Abd al-Rahmân al-`Ajlân, lecturer at the Grand Mosque in Mecca
Marine animals that also live on land and that have normal blood that runs off when slaughtered must be slaughtered before their flesh becomes lawful to eat. This includes animals such as sea turtles, sea otters, and marine birds.
It does not matter what percentage of their lives they live on land and what percentage of their lives are spent in the water. they must be slaughtered in the same manner as land animals.
As for marine animals like crabs that do not have such blood but that can also manage to spend some time on land, they do not have to be slaughtered. Their flesh is lawful like any other seafood.
And Allah knows best.
Source: Islam Today