Sea King

Sea Kings (海王類, Kai Ō Rui, literally translated as Sea King Species; known as Neptunians in the English manga) are sea monsters in the anime and manga One Piece, which are generally much larger than normal sea creatures. They are found in great numbers along the Calm Belt around the Grand Line, although in rare occasions they can be found in other places throughout the world.

Appearance
Sea Kings are serpent-like in appearance, however they vary from one Sea King to another in exact appearance and size. Some may have arms and legs and one particular sea king even was noted to look like an oversized frog.

Personality
They are classified in the One Piece world as a type "C" creature, which is the "Big Savage" group of creatures. They will attack anything that comes within reach of them and are savage predators.

Abilities and Powers
Sea Kings make trade and travel difficult for anyone trying to cross the water where they are found. Cases have arisen where some islands in the Grand Line close to the edge of the Calm Belt (their natural breeding ground) have ended up getting almost completely cut off from the rest of the world. In Water 7 and its surrounding islands, this led to the eventual creation of the Sea Train as a means to travel safely to and from each of its surrounding islands. Use of Seastone has led to the Marines discovering a safe method of crossing the Calm Belt unharmed.

Only the strongest of ships or captains can take on a Sea King. Most other creatures in the ocean fear the Sea King, with the notable exception of Bananawani (their only natural predator), Yuda (who are too ferocious even for them), Blue Gorillas (Impel Down guards who are extremely good swimmers and super strong), and Giants (who are incredibly strong themselves). Confrontation with one by all but a few humans is inadvisable and is considered nothing short of suicide. The fishman Jinbei also acknowledged the danger Sea Kings proved when he summoned Whale Sharks into the calm belt, thus proving at the very least Fishmen are also aware of the potential threat they are. Many characters, such as Franky, see the act of killing one a sign of strength and achievement.

The Sea King Menace
Sea Kings occupy the Calm Belt. In the past their reputation and numbers have been the sole protection for Amazon Lily located inside the Calm Belt itself. However elsewhere, islands close to it such as Water 7 remained cut off.

Instances Throughout History

 * During Norland's return to Jaya Island, the two ships led by his King encountered a Sea King themselves. Terrified of such a beast, the King left Norland to handle it by himself.
 * Sea Kings were the reason behind Dorey and Brogy's contest to see who was the stronger warrior. They caught one each and began arguing over who caught the bigger Sea King.
 * Yorki is believed to have perished when he attempted to flee the Grand Line via the Calm Belt.
 * Franky began his attempt to best one of these beasts as a child. He began building the Battle Frankys. Ten years before the storyline, he successfully kills a Sea King.  However his ships for killing Sea Kings would later become of a plan hatched by Spandam.
 * Also ten years ago, Higuma is killed by a Sea King and Shanks loses an arm to the same Sea King while saving the life of Luffy.

Changing Times
After centuries of ruling the sea, the Sea King's control over the Calm Belt has been lessened. Dr. Vegapunk discovered that Seastone emits the same aura as the sea, resulting in the Marines to underline all their ships with it. For the first time, ships move in and out of the Grand Line freely. The Sea Train is invented to create trade and travel between Water 7 and its neighbouring islands.

Trivia

 * Very few Sea Kings end up with names, only those who commonly inhabited the same waters receive names. The Sea King that ate Shanks' arm is known as The Lord of the Coast, and Long Ring Long Land Island had a Sea King called Master of the Waters.
 * Sea King meat is apparently considered a delicacy as Luffy has claimed that food made out of Sea King meat is "crazy good stuff."
 * Though nameless, some of the seakings seen early in the series often crop up in intros and outros even though Oda himself has never reused them for the storyline. In particular note, the white puffy one with a birds beak has been in more then one introduction to the series.
 * "Sea-King" was a term given to powerful Viking pirates.