A mermaid has a special "siren song" attack which she will deploy under the same circumstances that a mind flayer will use its psychic blast attack. When initiated, “You hear the wild singing of a mermaid.” If a random number between 2 and the mermaid's level (inclusive) is greater than 1/3 of your Charisma, then: if you are male, “You are irresistibly drawn to [the mermaid]’s side!” (teleports hero to a square next to the mermaid, even on a no-teleport level); if you are not male, you are stunned for 1d10 and confused for 2d10 turns.
If you have a tame mermaid who uses the singing attack, it will target a random hostile monster. Likewise if you are polymorphed into a mermaid and use the attack with the #monster command. In these cases, the chance that the monster will resist is based on its magic resistance. If there are no hostile monsters on the level, you get the message "The song seems quite sad."
Mermaids can wear body armor, gloves, helmets, and shields, but for obvious reasons cannot wear boots.
The male version of a mermaid is slightly less dangerous, lacking the singing attack. They have a 2/3 chance of being generated with a trident.
When killed, there is a 1/4 chance that a giant clam will drop a pearl, unless it has been revived or is a polymorphed monster from a polymorph trap, in which case it has only a 1/20 chance of leaving one.
Thrown spears, javelins, arrows, and daggers that hit a whale enter its inventory. The monster will never put such weapons down, but they can be stolen by a nymph.
Always generated peaceful. When killed in water in such a way that it leaves a corpse, a swordfish will always drop either a longsword or two-handed sword, unless it has been revived or is a polymorphed monster from a polymorph trap, in which case it has only a 1/20 chance of leaving one. The sword will be rustproof and enchanted to a random number between +1 and +3, and will automatically become blessed every time the player wields it. The sword is never recoverable if the monster is killed outside of water. Unlike other randomly generated weapons, there is no chance for the sword to be an artifact.