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.

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.

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.

Known Types of Merfolk

 * Keimi: Kissing gourami
 * Kokoro: Icefish
 * Medaka Mermaid Quintuplets: Medaka
 * Fukaboshi: Shark
 * Ryuboshi: Oarfish
 * Manboshi: Moonfish
 * Hyozo: Blue-ringed octopus
 * Madame Shirley: Shortfin mako shark

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.

Overall Strength
With merfolk, it is difficult to tell their strength, and they seem to vary as much as humans do. Merfolk are the fastest swimmers in sea, as demonstrated by Keimi. 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.

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 merman and mermaid 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.