Leviathan Class Organisms

Leviathan is a classification given to extremely large creatures of the Subnautica universe. 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, at least despite their evolutionary status. In fact, the only two that appear to have any close current 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 absolutely dwarfed by their predecessor/ancestors; the three-million-year-old Gargantuan Fossil at completion measured an estimated 6-8 times longer than an oversized Sea Emperor Leviathan specimen, and is unknown if the specimen is even fully grown. To support a predator of such titanic proportions, many of the other creatures of that time period must have also been similarly sized or possibly even 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 100 meters in length, but it is also extremely bulky, giving it far more body mass than anything living today.

In order to support creatures of these magnitudes (if any are still alive currently in 4546B), it is mandatory for these creatures to reside tens (if not hundreds) of kilometers away from the Crater Edge for their space along with food supply to be satisfied enough for sustenance and survival. As well as this, they would have to feed on fully grown leviathans that feed on smaller leviathans or microscopic creatures (such as Adult Ghost Leviathans or other organisms of the same caliber as the latter).

Known Leviathan Class Organisms

 * Sea Treader Leviathan: Around 203 meters; smallest known Leviathan
 * Reaper Leviathan: 55 meters
 * Reefback Leviathan: Around 70 meters in length yet also extremely broad, making it one of the largest creatures
 * Ghost Leviathan Adult: 110 meters
 * Ghost Leviathan Juvenile: Around 65 meters
 * Sea Dragon Leviathan: 112 meters
 * Sea Emperor Leviathan: 160 - 200 meters
 * Sea Emperor Leviathan Juvenile: Around 60 meters
 * Sea Emperor Leviathan Toddler: Around 1.5 meters

Extinct Leviathan Class Organisms

 * Ancient Skeleton: Exposed skeleton around 95 meters, full size indeterminate but considered significantly larger.
 * Gargantuan Fossil: Around 1,100 - 1,500 meters

Size Comparisons
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Trivia
Classe Léviathan
 * Some fauna on Earth may fit in the categorization of Leviathans, such as the whale shark, as well as large whales, including the humpback whale and the blue whale.
 * 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 even be given the classification of Leviathan, or perhaps a new rank for a creature of such preposterous size that it exceeds the size of a Leviathan, possibly giving it the classification of "Titan".