Merfolk

Merfolk are shown to be one of the two major races that inhabit the sea, the other being the fishmen, and seem to be ruled by the God of 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 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 Hyozo of the New Fishman Pirates is a poisonous blue-ringed octopus merman.

As far as 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 seahorses ridged back, his mouth however pertrudes outwards like a seahorse's mouth. Additionally, from what has been seen of obvoiusly 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.

According to Den, there is a reason why even in family, the species and appearance 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 one among all the species of the ancestors.

Known Types of Merfolk

 * Kokoro: Icefish
 * Keimi: Kissing gourami
 * Medaka Mermaid Quintuplets: Medaka
 * Fukaboshi: Shark
 * Ryuboshi: Oarfish
 * Manboshi: Opah
 * Hyozo: Blue-ringed octopus
 * Madame Shirley: Shortfin mako shark
 * Neptune: Coelacanth
 * Minister of the Right: Sea horse
 * Minister of the Left: Catfish
 * Shirahoshi: Giant smelt-whiting

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, as one of them joined the New Fishman Pirates, who looks 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 3 of the mermaids disappeared in Mermaid Cove, the mermaids were quick to jump to conclusions based on their disappearance accusing the Straw Hats of kidnapping them. Though the guards were not quick to judge the Straw Hats Pirates without evidence, 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 Mermaid being sold for 10,000,000 and called the Fishman Island a "treasure trove", showing his lack of respect for the race.

Overall Strength
While merfolk seem to vary as much as humans do; many of the male merfolks carry tridents, and Hyozo's strength was acknowledged by Luffy, and Kokoro has been 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. Some 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.

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

 * 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.