S. gigantea has an umbrella-shaped bell that can grow up to 1 m (3.3 ft).^[6] The bell's pliant tissue allows for the jellyfish to stretch 4 to 5 times its size, presumably to engulf their prey. Their four arms have a "paddle-like" or "kite-like" shape and can grow up to 10 m (33 ft) in length. The arms grow in a "V" shape transversely,^[7] with a wider base and tapering towards the ends. They do not have any stinging tentacles and instead use their arms to trap and engulf their prey which consists of plankton and small fish.^[5][3] From Browne's analysis of a collected S. gigantea, their jelly appears a red-orange color only when there is visible light.^[7] However, since they dwell in the deep ocean, visible light does not penetrate far enough. Thus, the giant jellyfish may appear "invisible" or glow orange very faintly in its surroundings, depending on the depth of the water. Furthermore, their bodies being made of either spongy tissue or jelly allows for the species to withstand the enormous deep ocean pressure of 40,000 kPa (5,800 pounds per square inch). The circular stomach contains canals that travel to the surface of the sub-umbrella. It is inferred that the lower stomach is thick to ensure the species has the strength to carry their long arms. Its four genital openings are also small to avoid weakening the stomach.^[7] Since there are no gastric pouches nor radial canals, the jellyfish is determined to be a part of the Ulmaridae family.^{[citation needed]}

Sightings of giant phantom jellyfish in the Antarctic Ocean: Although Stygiomedusa is not native to the Arctic Ocean, there have been sightings of the jellyfish in the Antarctic Ocean, with the help of submersibles.

Daniel M. Moore, a marine biologist with Exeter University in the U.K. and chief scientist for Viking Cruises, states that the reason Antarctic waters below 160 feet haven’t yet been well explored is that they are so difficult and expensive to reach. However, the frequent encounters with this animal are from tourism expeditions in the Antarctic that are increasingly offering personal submersibles to guests to take photos. This leads to the sighting of the giant phantom jellyfish hundreds of feet underwater off the coast of Antarctica's Rongé Island. The Viking Expeditions cruise ship had shown those photos taken with the private submersibles to Daniel M. Moore.

There have been observations of the rarely encountered Stygiomedusa gigantea at depths of 80–280 m (the mesopelagic and lower epipelagic zones) around the Antarctic Peninsula coastal waters. The Norwegian Polar Institute and an international peer-reviewed journal called Polar Research, which corresponds with Daniel M. Moore of Viking Expeditions, found Stygiomedusa gigantea in the Antarctic Peninsula at Georges Point, Rongé Island, Fournier Bay, Anvers Island, and Paradise Harbour.

Giant phantom jellyfish live in every ocean except for the Arctic Ocean. However, because they typically swim deep below the surface, they are hardly seen by humans. Daniel M. Moore noted that one potential explanation is that the jellyfish swim higher up to expose themselves to ultraviolet radiation, which will rid them of parasites. Another hypothesis is that the upwelling deep water found around the Antarctic continent just carries them upward.

Known to be one of the largest invertebrate predators in the deep sea, the giant phantom jellyfish's typical prey consists of plankton and small fish. The S. gigantea tends to be more dominant in locations with a low productivity system, which in turn deters other predatory organisms, like fish, to high productivity systems (coastal, upwelling zones). However, the jellyfish remains an important predator for the deep sea, often competing with squids and whales.^[8]

Larger S. gigantea have also been observed to be in the immediate vicinity of hydrothermal vents where large proportions of zooplankton are abundant. This is in mesopelagic and bathypelagic depths. The further away from hydrothermal vents, the smaller the medusae are—indicating that zooplankton are an important resource for the species. Due to this, the medusae are well off during early spring to early summer when zooplankton biomass is enhanced.^[8]

Evidence has been collected to support the first-ever documented symbiotic relationship for an ophidiiform fish, Thalassobathia pelagica. Scientists have observed that the large umbrella-shaped bell of S. gigantea provides food and shelter for T. pelagica, while the fish aids the giant phantom jelly by removing parasites. The S. gigantea's jelly providing shelter for T. pelagica is essential for the fish, considering the lack of shelter resources at such extreme ocean depths. Studies to further support this symbiotic relationship have shown that the two species reassociate with one another even if separated. It was inferred that T. pelagica is able to find its way back to the giant phantom jelly due to neuromasts that increase the sensitivity of low-frequency water movements—which the bell of the jellyfish emits.^[6]

The first S. gigantea specimen, weighing in at ninety pounds (40 kg), was collected in 1899, but it was not recognized as a species until 1959.^[7] Despite having discovered only 118 individuals within 110 years (1899–2009), gelatinous mucus from the medusa have been found covering vents, indicating they may travel in swarms. Similar large jelly Schyphomedusae were observed traveling in swarms off the West coast of North America. However, there are instances in which the species is spotted alone, such as the S. gigantea identified at a depth of 1,200 m (3,900 ft) in the San Clemente Basin just off of California.^[8]

The giant phantom jelly occurs all around the world with the exception of the Arctic Ocean. They are typically found 61°N–75°S and 135°W–153°E. In areas of high latitude in the Southern ocean, the depth at which the species may be found are at the mesopelagic and epipelagic levels. However, in areas of mid to low latitude, the species are typically found at bathypelagic and mesopelagic levels. This is due to the variability of the ocean's temperature and light distribution.^{[citation needed]}

Determining the reproduction of the S. gigantea is difficult considering how rare sightings are. It has been noted that young captured S. gigantea looked like an exact miniature of the adult.^[7] However, researchers have analyzed the jellyfish's structure and anatomy enough to understand how it may reproduce. The S. gigantea has four brood chambers that protrude into the stomach in folded narrow ridges and epithelium that covers the gastric side. Its lower periphery has frills along the folds, creating a band about 20 millimetres (0.79 in) high.^[9] Above this band, there is a germinal line that forms a shallow groove with different epithelial cells that are more cubical in shape with large, rounded nuclei.^[9] Irregular placement of the cells in small pits (small clumps of cells, similar to cyst) along the germinal line produce a multiplication of epithelial cells that create a deep invagination. This is the first stage leading to the reproduction of S. gigantea. The cyst grows with a pointed end on the subumbrella side. As its size increases, it pushes out the brood chamber wall and into the cavity of the chamber. Simultaneously on the opposite end, two outgrowths develop horizontally, making the cyst into a "T" shape.^[9] This protrudes more and more as size increases, taking the brood chamber with it. Eventually, a thin membrane forms and the cyst enters the stomach cavity.^{[citation needed]}

Within the cyst, a scyphistoma—a single developing medusa—forms and is now called a chorion. Once the chorion grows into about 2 mm (79 mils) long and 2–3 mm (79–118 mils) in diameter with teat-shaped distal ends (which are basal outgrowths), it begins to be pushed out of the chamber. Within the chorion capsule, differentiation and formation begins. The inner epithelial wall is directly from the parent tissue and is pocketed into its distal tips that will eventually become the S. gigantea's arms. As the "baby" medusa grows, it takes the shape of the capsule.^[9]

In order to escape, the well-developed "baby" medusa will detach from the subumbrella wall where it was already slightly protruding. It then exits through the gastric cavity and out the parent's mouth.^[9]

The baby medusa soon become free-swimming planules, then polyps or scyphistomae that reproduce asexually through budding or podocysts. These are what become larval medusae that feed on plankton. Eventually, it will grow into the size of an adult. It is inferred that reproduction of S. gigantea is continuous with one parent estimated to produce fifty to one hundred medusa.^[9]

• Giant phantom jelly - Monterey Bay Aquarium Research Institute
• Giant, cryptic ocean dweller filmed roaming the deep sea - "Mashable"
• Rare monster jellyfish caught on tape – Discovery Channel
• Distribution of Stygiomedusa gigantea – Ocean Biodiversity Information System