Leviathan Class Organisms

Leviathan is a classification given to extremely large creatures of the Subnautica universe. Any fauna species large enough to lack natural predators can be classified as Leviathan; they need not be predatory. Although the term "Leviathan" is used to refer to a large mythical aquatic creature in the real world, there is no confirmation on whether the Leviathan classification is restricted to 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.

Another pair of Leviathans that seem to be related are the Shadow Leviathan and the Ice Worm. Both of which have elongated serpentine bodies lined with several parapodia, have large distinct blade-shaped heads, blackened skin, and feed with monstrous mandibles. This indicates that they had a common ancestor with a body plan much like that of a centipede crossed with a worm.

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 predecessor; the three-million-year-old Gargantuan Fossil at full length is estimated to be 6-8 times longer than an adult 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 of colossal proportions.

The one-million-year-old Ancient Skeleton is enormous, with the gaps left by its missing teeth being 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 is extremely bizarre 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 more body mass than anything alive on the planet 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).

The Crater

 * Sea Treader Leviathan: Around 20 meters3; smallest known Leviathan
 * Ancient Floater: - ~35 - 40 meters
 * 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 67 meters
 * Sea Dragon Leviathan: 112 meters
 * Sea Emperor Leviathan: 160 - 200 meters
 * Sea Emperor Leviathan Juvenile: Around 60 meters
 * Sea Emperor Leviathan Baby: Around 1.5 meters

Sector Zero

 * Glow Whale - 30 meters
 * Chelicerate - 40 meters
 * Void Chelicerate - 65 meters
 * Shadow Leviathan - 60 meters
 * Ice Worm - 95 meters
 * Ventgarden - 110 meters in height
 * Juvenile Ventgarden - ~22 meters in height

The Squidshark is stated as "a leviathan-class predator at the smaller end of the scale" by its databank entry, though it is not included within the Leviathan tab in the databank.

Crater

 * Ancient Skeleton: Exposed skeleton around 95 meters, full size indeterminate but considered significantly larger.
 * Gargantuan Fossil: Around 1,100 - 1,500 meters.
 * Research Specimen Theta: - ~21 meters

Sector Zero

 * Frozen Leviathan - ~70 meters excluding tail, around 150 meters including tail.

Size Comparisons
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Trivia

 * 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.

Classe Léviathan