Leviathan Class Organisms

Leviathan is a classification given to extremely large creatures. Although the wording of the Reaper Leviathan's databank entry would seem to imply that the leviathan classification is exclusive to predatory life forms, this is not the case; any fauna species of a suitable size can be classified as a Leviathan. Although the term "Leviathan" is used to refer to an aquatic creature in the real world, there is no confirmation on whether or not the Leviathan classification is restricted to aquatic fauna in the Subnautica universe, or is simply the term used for any species of a sufficient size.

Relation to Other Leviathans
Leviathan is not part of a taxonomic classification so does not imply any of the species classified as such are related. In fact, the only two that appear to have any close relation are the Sea Emperor Leviathan and Sea Dragon Leviathan who share several features including an almost identical body shape, seven identically positioned tentacles dotted with glowing spots, two forelimbs and a pair of light-producing organs atop their snouts. These two most likely shared a common ancestor that has since diverged into a smaller, cave-dwelling predatory species and a larger, planktivorous species residing in the open ocean.

Leviathans of the Past
Despite being enormous when compared to the fauna of Earth, the leviathans inhabiting Planet 4546B in the present day are dwarfed by their predecessors; the three-million-year-old Gargantuan Fossil at completion measured an estimated 6-8 times longer than a fully grown Sea Emperor Leviathan specimen. To support a predator of such titanic proportions, many of the other creatures of that time period must have also been similarly sized or larger.

The one-million-year-old Ancient Skeleton too is enormous, with the gaps left by its missing teeth being more than large enough for Ryley Robinson to swim through comfortably. The exact size of this creature cannot be determined as the body form of the creature cannot be determined and it is unknown if the skeleton visible is the entire skeleton. What is available is around 100m in length, but it is also extremely bulky, giving it far more body mass than anything living today.

Known Leviathan Class Organisms

 * Sea Treader Leviathan: About 20mx20mx20m; smallest Leviathan
 * Reaper Leviathan: 55m
 * Reefback Leviathan: Around 70m in length yet also extremely broad, making it one of the largest creatures
 * Ghost Leviathan Adult: 107m
 * Ghost Leviathan Juvenile: Around 65m
 * Sea Dragon Leviathan: 112m
 * Sea Emperor Leviathan: 160 - 200m
 * Sea Emperor Leviathan Juvenile: Around 60m
 * Sea Emperor Leviathan Baby: Around 1.5m

Extinct Leviathan Class Organisms

 * Ancient Skeleton: Exposed skeleton around 95m, full size indeterminate
 * Gargantuan Fossil: Around 1100 - 1300m

Trivia
Classe Léviathan
 * Several whales of Earth such as the fin whale and the blue whale fit the classification of Leviathan.
 * The Sea Emperor Leviathan is suggested to be the largest identified species in the Subnautica universe, at around 160-200 meters in length. Despite this, it is massively out-sized by the Gargantuan Fossil which would measure well in excess of one kilometer at completion. Due to the massive size disparity between the largest confirmed Leviathan and the Gargantuan, as well as the lack of any mention in the databank entry, it is unknown whether or not the Gargantuan would be given the classification of Leviathan, or perhaps a new classification for a creature of such extreme size that it exceeds the size of a Leviathan.