Mer Creatures are around 4.5 metres long and 2 metres tall. The Mer seem intelligent, by locking up their human prisoners in a sewer, which acts like a pantry. Mer seem to have a society ruled by one large, red "MerQueen", with the other, much smaller animals acting as her subordinates. The MerQueen displays similarities to elephant seals, as she (perhaps accidentally) crushes other Mer while trying to get to her enemies. The elephant seal, a type of huge amphibious sea mammal, does this when fighting rivals for mating.
The Mer did not at any point kill any of their supposed victims, showing this may have been a way of keeping their prey fresh to eat in harder times. At one point, the MerQueen tried to grab Abby Maitland, although Abby must have escaped through the Anomaly. When she was found, she was not near the MerQueen, and was unharmed. Only when the team tried to rescue her did the MerQueen react aggressively by charging at the team.