Merfolk

Merfolk are shown in the world of One Piece as one of the two major races that inhabit the Sea, the other being the Fishmen, and seemed to be ruled by an individual know for now only as the Mermaid Princess.

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 recently 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.

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 after 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.

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.

Mermaids and Mermen are friends to the fishes 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 Jinbei'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 in auction in the Sabaody Archipelago can go for a starting price of 70,000,000. Saint Charloss of the World Nobles spent as much as 500,000,000 for Keimi. The wealthy who buy one usually keep them as a trophy.

Overall Strength
They appear much weaker then the Fishmen, however it is difficult to tell their strength considering only two so far have made an appearance. Merfolk are also the fastest swimmers in sea and 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. Kokoro has shown to be able to use her tail to kick her enemies with enough force to send them flying a great distance.

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.