Up to 135 feet long and 30 tons, the jasconius is a gigantic bony fish. They resemble giant coelacanths, but with no dorsal fins. Their coloration is usually blue, but albinos are not uncommon. Male jasconiuses are bulkier than females. Another difference is their naked skin, with no scales.
Strangely, these fish often have colonies of algae, coral, sponges and marine fungus on their backs and stomachs. Within these are many small crustaceans, tardigrades, fish, nematodes and annelids.
The jasconius is the only chordate on Thanatos known to eat anything other than other animals. While they are mostly predatory, they will sometimes eat algae and fungus to supplement their diet. This means that they are immune to the deadly, bone-neutralizing bacteria inside most Thanatonian plants and algae, which would kill any other vertebrate. In turn, many marine predators such as the tusked leviathan often prey on them.