Therefore, when slaughtering an animal on land in a halal way (zabiha) we have to let all the blood flow out of the body. This is because blood carries many pathogens, diseases, toxins, bacteria and germs. If we ingest this blood into our bodies it could cause us harm and make us sick. So as Muslims we let all the blood flow out of the animal after slaughter. Then we wash the meat carefully before eating. The meat that we eat slaughtered the Islamic way is much more fresh, hygienic and pure.
Fish and other seafood such as crabs and lobsters do not have flowing blood when they die. Fish have prothrombin in their blood that causes a clot reaction when they pass out and die. This means that when the fish dies the blood clots in the heart of the fish and does not flow through the body. So there is no need to slaughter it the Islamic way (Zabiha) to make it halal.
{Lawful to you is the pursuit of water-game and its use for food …}[Quran 5:96]
of Abu Hurayrah
who said, “The Messenger of Allaah 
said (concerning of the ocean): ‘Its water is pure and its dead animals are permissible (to eat).‘”
Allah and his Messenger (Qur’an 6:7)
Qur’an 6:115)
“He has only forbidden to you dead animals (carrion), blood, the flesh of swine (pig). And that (food) which has been dedicated to other than Allah” – [Qur’an 2:173]
Prima Quran 