'Mermadens' are a legendary species of the modulate family thought to live in the hydrocarbon smog layer of Impendia. There is significant doubt that the smog layer is dense enough to support such creatures, which are purported to 'swim' through the smog as their namesakes from Earth, mermaids, are depicted swimming through water.
Further doubt is cast when examining accounts of sightings of the creatures. Legends are kept alive by ships caught at shadowtide when the hydrocarbon smog is at its highest, but thus far it hasn't been proven that said ships have reached depths where the smog is dense enough to support the weight of swimming modulates. Some scientists also point out that certain types of structi which can weather hydrocarbon tides may take on shapes which look similar to the described mermadens.