Jooldom’s from the Ocean too (SPOILERS)
So the various species of eel in the fish market have not been able to be bred in captivity. This has led to massive decline in their wild numbers, so as to be considered an unsustainable food-source. Babies are taken from the wild and grown up in captivity until they reach the size to be eaten.
I think this is what happened to Jooldom. Rather than him being caught as an adult, it’s more likely he was caught as a youngster and grown up in captivity, hence not having the same drive to escape as Padak, even though he stands the best chance at escaping out of all the fish. He also seems to lack Padak’s extensive knowledge of the sea, since he was caught from a young age, he wouldn’t have learned and remembered much.
This, imo, adds a layer of depth to his character.
He hardly interacts with the other fish, yet he has taken a position of second-in-command beside the supposed wise “ocean-caught” Master that no-doubt reminded him of home. Teaching him things he should’ve been taught had he not been captured as a child, like how starfish have 50 fins! He respected him. When he hears that Padak is from the ocean, the camera focuses on him during the reveal, opening his eyes from a nap with clear interest in what he was hearing. He also seemed to have developed a creepy “fondness” for Padak, insisting that she share her knowledge of the ocean, even singing a song about it.
He also maliciously toys with old Master’s feelings when Spotty is killed, taunting him for lying about being from the ocean, probably harboring feelings of betrayal that someone he looked up to as a “fellow ocean fish” that can teach him was lying the whole time. Then there’s Master calling him a “sea eel” and he suddenly stops.
Still figuring precisely what species he is. I’m leaning towards the Japanese eel (Anguilla japonica), primarily due to its prevalence in those waters and in the market, and also to his distinctly Japanese name (Anago) in the original Korean version. Could also be a conger eel. *shrugs*