Leviathans are made of both biological and mechanical systems, and are therefore biomechanoid. They have a thick outer skin capable of keeping a pressurized atmosphere inside, and also generate a bioelectric field that can keep small openings in the hull from venting atmosphere. Holes in the hull are repaired by symbiotic organisms called Hodian trill bats, whose excrement is a sealant. Other maintenance details are handled by mechanical DRDs, terrier-sized droids that the Leviathan manufactures.
Inside, the Leviathan consists of countless corridors and compartments. Each Leviathan grows differently depending on the needs of its crew. At the very core of a Leviathan is an area known as the "Pilot's Den". This is where a member of the Pilot species symbiotically bonds to the Leviathan. While Leviathans can operate unbonded, most functional Leviathans have a Pilot bonded to them. A Relgarian can also be bonded to a Leviathan, although it has yet to be seen in the series. Lacking a Pilot, Leviathans can be manually steered by the crew from the control deck.
Because of their nature, Leviathans are not able to communicate well with their passengers. Pilots play the role of translator and liaison between the ship and the crew. They are also responsible for maintaining life support functions, minor internal repairs and other detail jobs that are not related to the Leviathan's well-being but important for the passengers. When a Pilot dies or is incapacitated, most Leviathans lose some degree of control over these internal systems (especially life support).
Leviathans have a vast knowledge of space, and their senses are specifically adapted to be used as sensors for space travel. Leviathans have the unique ability to perform starbursts, a superluminal form of travel that they use for defense and for long-range travel. A Leviathan enters starburst by energizing a matrix of sorts on its outer hull, beginning at the front of its body and going to the tips of its three tails that come together at their ends. Once the matrix has been completely energized, the ends of the three tails separate from one another, and the Leviathan enters the rift in space-time that opens ahead of it, which promptly seals itself as soon as the Leviathan is through. While in starburst, the Leviathan is "riding the seams between dimensions", essentially sliding around the outside of the universe to get to (roughly) its destination. Navigation is less of a science and more of an art, as one never knows exactly where one will come out when leaving starburst.
Leviathans possess no natural weaponry, as they were designed as transport ships (the exception being Talyn, who is a hybrid). They rely on their ability to Starburst to escape danger. Leviathans also have a docking web they use to "pick up" objects and small ships and bring them into their docking bay. The docking web is apparently powerful enough to immobilise and "reel in" small ships (like a prowler or transport pod), although only at very short range. Because of their large size and usefulness, Leviathans are often enslaved by Peacekeepers by means of control collars.
DRDs (Diagnostic Repair Drones) are small robots that are built by the Leviathans as it grows. The DRDs are extensions of the leviathan's consciousness when the Pilot is not in control of them. The DRDs perform various functions, including ship repairs, searching for lost items, and defensive combat using built-in lasers.
They are ovoid, approximately 14 inches long, 10 inches wide and 8 inches tall, with two flexible black eyestalks with lights. They contain multiple types of tools and sensors, including a plasma welder which can double as a weapon.
The most notable DRDs on Moya are: 1812, a DRD painted red, white, and blue, and named for the 1812 Overture by Tchaikovsky which John Crichton taught it to "whistle"; DRD Pike, named after Captain Christopher Pike of Star Trek, who communicated through the use of blinking lights,and sacrificed himself (unintentionally) to destroy an energy rider; and One-Eye, a DRD who randomly surfaced throughout the series, recognizable by his one broken eyestalk, which was damaged in the first episode when John arrived, and repaired (badly) with tape by John at the end of the first episode out of guilt for breaking it.
There are hundreds of DRDs on a leviathan, and each leviathan shown in the series has its own variation on the DRD design. Moya's DRDs are yellow and largely benign; Talyns (Moya's offspring) are red and contain powerful weapons.
These small, unarmed, crafts are the standard issue Leviathan cargo and personnel transports. They are capable of atmospheric flight, and fast enough to allow interplanetary travel.
Leviathan transport pods are bioships. A Leviathan can grow new transport pods as needed, and as such they are somewhat expendable. Unlike the Leviathans themselves the transport pods are not sapient, or even intelligent enough to fly on their own. They require at least one crewmember to pilot them.
If a transport pod is kept away from its Leviathan host for an extended period of time, it will go inactive and become unusable.
One is ultimately used to destroy the Peacekeeper Gammak Base and another is used in a daring mission by Pilot, John and Aeryn to destroy the only known wormhole leading to Earth.
Very little is known about Leviathan reproduction or mating rituals. Leviathans females can become pregnant, gestate a fetus, and give birth. The pregnancy is usually a long and complicated process. The child is conceived via a biological process and the embryo nests in one of the innermost chambers (usually on utility tiers which are inaccessible to the passengers). The embryo is tended to and defended by DRDs. This is an instinctive behavior that can override the usual DRD control channels. The DRDs may attack crew members who get too close to the developing child if they are perceived as a threat.
Side effects of pregnancy include involuntary starbursts, malfunctioning of life support and various other internal systems, as well as erratic DRD behavior. Crews of pregnant leviathans should be prepared for many discomforts and are usually well-advised to abandon the ship for the remainder of her pregnancy.
Peacekeepers have a Leviathan breeding research program attempting to produce a hybrid Leviathan warship that can take orders directly from a Sebacean captain (rather than interact with the crew via a bonded pilot). They successfully artificially impregnated a Leviathan called Moya, who gave birth to the first (and only) hybrid Leviathan ship Talyn. All other breeding attempts resulted in the death of the mother and the child, attributed to the fact that all other attempts were with enslaved leviathans wearing control collars.
Talyn had a very powerful external retractable gun turret and wall-mounted security guns on the inside, and didn't need a Pilot to control him; however Talyn's architecture still offered a vestigial pilot's den. He could bond with a Sebacean pilot via a special neural implant at the top of the spine. Talyn chose to bond with rogue Peacekeeper captain Bialar Crais; however — using the Pilot's den — Talyn was able to temporarily, yet successfully, bond with Stark while Crais was incapacitated.
Leviathans normally live over 300 cycles. When old age approaches, they try to travel to their sacred burial grounds to rest among the remains of other members of their species. Bonded pilots die along with their host, though their natural lifespan is over 3 times longer than that of a Leviathan. Although it is theoretically possible to "transplant" a pilot to another ship, most Pilots do not even consider it as an option. In fact, the Pilot society forbids young individuals to bond with Leviathans until they reach a certain age.
The sacred Leviathan burial ground is in a remote area of the uncharted territories, and the exact location is a secret.
Leviathans have many predators, from Peacekeepers who try to enslave them, to the Grudeks, who harvest their toubray (neural) tissue. The Patriqu-ro - a race of ship builders that compete with the Builders - also rank among the enemies of Leviathans. In the episode Meltdown, they had employed the services of an alien, Mu-Quillis, who created a Siren Star to bait and, ultimately, destroy Leviathans.