Merfolk can psychicly communicate with each other over great distances allowing Sally to stay in contact with her people while at the Sanctuary.
Merfolk live in undersea societies called "pods" and because they rely on each other so much for surrvival they're incredibly peaceful.
After Sally received a distress call,which she described as a series of flashes of anger and fear, on the psychic channel from her people in the Bermuda Triangle, Dr. Magnus and Will Zimmerman set out on a submarine expedition to help where they encountered what appeared to be a massacre among the merfolk. They later discover that a parasite that infested their brains had caused the creatures to tear each other apart.
There are also hints given in the opening credits of Season 1 and 2 that merfolk were present at the sinking of the Titanic.