Merfolk are magical sea creatures who have an appearance of an upper human body and a tail of a fish. Mermaids and mermen are the feminine and masculine merfolk.
In Season One, the merfolk appeared as normal-looking people with sea green fish tails and wearing normal human clothes. In Season Two, merpeople have green or bluish skin, and black, blue, green or (in the case of Silver) gray hair. Many of the mermaids wear seashell tops while some mermen wear Greek-style armor.
Being aquatic beings merfolk are expert swimmers and are capable of breathing underwater (with the notable exception of Silver who ironically was terrified of water). While mostly adapted for being underwater, they also show no ill effects being on land for prolonged periods of time (as was the case for Dolores Derceto ). On land, their tails seem to be strong enough to support them in an upright position, and even allow them to ambulate efficiently (albeit slowly) by wriggling along the ground without need of help from their arms.