Mermen (魚人, Gyojin) are one of the species that dominate the seas (the other being the mermaids and manfishes). In terms of leadership, King Neptune is the king of Mermen Island. Fisher Tiger, Jinbe, and Arlong were said to have the highest respect amongst their people. The slave price would be shown as, 1,000,000 berries. Utan Sonar
Appearance-wise, they are more "Hybrid species" than mermaids, looking like a cross between a man and a fish of the ocean such as a whale shark, manta ray, or sawshark; however, they still have legs (in fact, Decken has four). They also have gills between their shoulders and necks, sometimes covered by their clothing, as well as most having webbed hands. Depending on species, they may have multiple limbs (mainly extra arms).
For fishmen pirates in pirate crews that are primarily fishman based, (Sun Pirates, Arlong Pirates, and New Mermen Pirates) it is customary to have that crew's jolly roger tattooed somewhere on the person's body, showing allegiance to a crew.
Mermen have their gills near their necks which only work when they are in water. On land they can breathe with their lungs like humans do. However, their gills cannot breathe in air, like a normal fish cannot breathe in air through their gills.
Even though mermen are part fish, they are still mammals and give birth to live young. Mermen can also interbreed with other races: as was revealed during the Davy Back Fight Arc with Big Pan, mermen can breed with giants, producing a hybrid who the has biological abilities of a merman and the huge body of a giant (but still smaller than a full one, as related by Robin), they are known as wotans. Pappug explained that fishmen (mermen and merwomen; of female mermen) can have children with other of its sea species (manfish and mermaids); however, the child's race is up to chance between fishmen, mermaid and manfish. There are many merfolk who bear physical traits normally unique to mermen, like Den's webbed hands and Shyarly's fangs. Furthermore, mermen can sometimes demonstrate powers usually unique to merimaids and manfish, such as Jimbei's ability to communicate with whale sharks.
Even with fishman, mermaid and mermen in the same immediate family, the primary species (mermaid, manfish or merman), subspecies (type of fish) 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. It is also possible for humans and mermen to interbreed, as Sapi is a result of such. Humans, mermen, mermaid, and manfish also share the same blood types.
The mermen's subspecies have many variations, just like mermaid and manfish, being many types of fishes. So far, all known mermen fish-halfs are based on real-life fish species and not on any of Oda's made up species. Also, their size may vary greatly, with Wadatsumi being a merman the size surpassing that of an average giants. Unlike mermaids of manfish, mermen eat meat including fish.