Merfolk

Merfolk are one of the two major races that inhabit the sea (the other being the fishmen). Merfolk seem to be ruled by the Great Knight of the Sea, King Neptune.

Appearance
Like the mermaids and mermen of folklore, their upper half is that of a human while the lower half is that of a fish. As demonstrated by Kokoro, mermaids may walk normally on land as when they are thirty years old, their tails split into two. Like the fishmen, they are based on the different fish of the sea; for example, Kokoro is an icefish mermaid and Hyouzou of the New Fishman Pirates is a poisonous blue-ringed octopus merman.

As of now, it seems the mermen can keep more fish-features in the upper body than the mermaids as shown by the royal princes or the two Ryugu Palace Ministers of the Right and Left. For example, the Minister of the Right has grown his hair to resemble a seahorse's ridged back, his mouth however protrudes outward like a seahorse's mouth. Additionally, from what has been seen of obviously older mermen like King Neptune, it seems that the tails of male merfolk don't split with age like those of their female counterparts.

Biology
They are capable of breeding with humans as Kokoro was known to have had a human husband and bore a son with him. However, as seen with her granddaughter, Chimney, just one generation after the initial interbreeding, continuing to breed with humans may result in all merfolk features being lost. However, despite losing the merfolk features, they retain their ability to swim well. Humans, fishmen, and merfolk also share the same blood types. Unlike fishmen, merfolk do not eat meat and fish of any kind, aside from shellfish, so they remain primarily vegetarian.

Pappug explained that merfolk (mermen and mermaids) can have children with fishmen (fishmen and fishwomen). However, the child's race is up to chance between fishmen and merfolk.

Even with merfolk and fishmen in the same family, the species and appearances may differ. The reason is that they still carry the genes of their ancestors, so the species is not determined only by the parents, but it can be any one type among all the species of the ancestors.

Mermaids split their tail when they turn thirty as shown by Kokoro, Otohime, and an unknown mermaid Luffy saw while riding on a fish taxi. However, this does not apply to their male counterparts as their tails won't split even after the age of thirty.

So far, all known merfolk fish-halfs have been real world species, and not any of Oda's made-up species.

Known Types of Merfolk

 * Kokoro: Icefish
 * Keimi: Kissing gourami
 * Medaka Mermaid Quintuplets: Medaka
 * Fukaboshi: Shark
 * Ryuboshi: Oarfish
 * Manboshi: Opah
 * Hyouzou: Blue-ringed octopus
 * Shirley: Shortfin mako shark
 * Neptune: Coelacanth
 * Minister of the Right: Sea horse
 * Minister of the Left: Catfish
 * Shirahoshi: Giant smelt-whiting
 * Den: Bering wolffish
 * Otohime: Goldfish
 * Aladdin: Goatsbeard brotula

Inter-species Relationships
Unlike the fishmen, the merfolk are peaceful, and prefer their race to remain a secret. They often are rumored to aid drowning sailors, plucking them from the sea and taking them to safety. However, some can be violent and hateful, such as the individuals who joined the New Fishman Pirates, who look down on humans.

Mermaids and mermen are friends to the fish of the sea, fun loving, and usually get along with most people, including fishmen (even though fishmen can often take advantage of the merfolk's good nature).

Like fishmen, merfolk face dramatic discrimination in certain areas due to past history between them and humans. While the World Government has attempted to bridge the gap, with Jinbe's appointment into the Shichibukai being an attempt to strengthen the bonds between the races (but due to his resignation, that ended in failure), they are still considered valuable in areas where slavery is still legal in this world. A captured female mermaid under 30 (when their tail splits and they can walk on land) that is sold through auction in the Sabaody Archipelago can go for a starting price of 70,000,000. Parted females (those over 30 years old) are worth 10,000,000, and males are worth 1,000,000. Saint Charloss of the World Nobles spent as much as 500,000,000 for Keimi, who is younger than 30. The wealthy who buy them usually keep them as a trophy.

When three of the mermaids disappeared in Mermaid Cove, the Straw Hats quickly became suspects even though the mermaids who personally met them say that they're nice people. Though the guards were not quick to jump to conclusions, the conversation with the citizens present echoed the amount of mistrust in humans, especially pirates, and their greediness when faced with the selling prices of mermaids. Caribou quoted a Mermaid being sold for 10,000,000 and called the Fishman Island a "treasure trove", showing his lack of respect towards the race.

Because of their huge biological differences, they fail to understand completely the human view of family and habit of categorizing individuals. Humans also fail to understand the fishmen and merman differences.

After the death of Queen Otohime, Prince Fukaboshi told his people the time to repair the damage was too soon for the people to cope with. However, he noted that time will heal the pain they have experienced and that one day they will be able to do what she set about to do and destroy the circle of hate between them and the humans. He told all the people of fishmen island to dream of the sun. To the horror of the island years later, while they had been working towards this goal, the Fishman District had breeding the very opinion their were trying to overcome.

Overall Strength
While merfolk seem to vary as much as humans do, many of the male merfolks carry tridents, and Hyouzou's strength was acknowledged by Luffy, as well as Kokoro being shown to use her tail-fin as a whip-like kick. Merfolk are the fastest swimmers in sea, as demonstrated by Keimi, surpassing even the flying fish and fishmen. The mermaids have the ability to talk to fish and call out to them with a sound wave, which is apparently only audible to them. Jinbe, despite being a fishman, also appears to have a similar ability, leading Crocodile to note the uniqueness. Despite this, however and for reasons as of yet unknown, mermaids are unable to talk with Sea Kings with the exception of an ancient mermaid princess of legend and the current princess Shirahoshi.

Hyouzou looked down upon fellow swordsman Hatchan and considered the fishman's skills sub-par to his own. Thus a merman can surpass a fishman in terms of overall battle prowess. As of note, the fishmen race have many powerful individuals and to capture Vander Decken IX, Neptune had to resort to allowing his sons and the Neptune army to arrest him, which they were unable to do until Decken's defeat during the battle for Fishman Island. Despite the power of the Fishmen race, the Merfolk have a much stronger hold over Fishman Island. To overthrow the royal family, the New Fishman Pirates had to plot and form an alliance to achieve their goal. Mermen also seem to have developed a fighting style unique to their race (like Fishman Karate) known as Merman Combat which seems to involve the manipulation of water.

Early One Piece
An early setting of One Piece, as revealed in One Piece Green: Secret Pieces, said that if merfolk were to soak in enough water, they would return to their once beautiful forms.

Translation and Dub Issues
Often the term "fishmen" is mistranslated from Japanese to "mermen," however the two species are very different. This is because the Kanji for fishmen (魚人) is just the reverse of mermen (人魚). Their translated name, "manfish" is how the Japanese describe the mermen and mermaids of legend, they do not have a word for it. This mistake was carried over to the 4Kids dub.

Trivia

 * When Sanji wondered if Nami was really a mermaid, Luffy drew what he thought she would look like as a mermaid. Surprisingly, that mermaid actually appeared in Chapter 626.
 * The mermen so far are notably a lot less attractive then their female counterparts and so far it seems to reflect the traditional views of mermen in folklore, who are usually considered much wilder and less attractive than their female counterparts.
 * In folklore, Melusine is sometimes despicted as a two-tailed mermaid, although as noted a twin-tailed mermaid appears in other forms, usually holding one of her tails in each hand. This form is the basis of the logo for the coffeeshop chain Starbucks.