A strange,never-before-seen jellyfish with a distinctive stomach that looks like a red cross has been discovered inside a volcanic structure off the coast of Japan.

Researchers spotted the jellyfish, which has been named the St. George's cross medusa (Santjordia pagesi), 2,664 feet (812 meters) below the Pacific Ocean's surface near Japan's Ogasawara Islands. It was floating around the Sumisu caldera, a 6.2-mile-wide (10 kilometers) hydrothermally active volcano that sits on a volcanic arc, or chain of volcanoes, known as the Ring of Fire.

The umbrella-shaped jellyfish has a diameter of around 4 inches (10 centimeters) and is transparent, except for a bright red cross, which is visible when the creature is viewed from above.

The jellyfish is so rare it has only ever been spotted twice. It was first seen in 2002 by a remotely operated vehicle (ROV) during a dive in the Sumisu caldera. However, researchers could not describe a newfound species based on a single specimen, as it could have been a mutated individual belonging to an already-known species, according to a statement from the Japan Agency for Marine-Earth Science and Technology (JAMSTEC).

Related: City-size seamount triple the height of world's tallest building discovered via gravitational anomalies

In 2020, scientists revisited the region and spotted the mysterious jellyfish again. While they were unable to collect a sample, they filmed the jellyfish swimming around.

This second encounter enabled them to describe the jellyfish as a unique species. The team reported its findings in a study published Nov. 20, 2023, in the journal Zootaxa.

The unusual cross shape at the center of S. pagesi is the jellyfish's stomach. The color helps to disguise the jellyfish and the food it consumes from predators in its dark, deep-sea home. Much of the creature's diet consists of bioluminescent organisms that glow in the dark — having a red stomach diminishes the light they emit, protecting the jellyfish from predators after it has eaten its meal.

S. pagesi differs a great deal to its closest relatives, which include large, deep-sea jellyfish such as Tiburonia granrojo and the giant phantom jelly Stygiomedusa gigantea, according to the statement. The newfound species is much smaller and, unlike its relatives, has tentacles as well as arms, which it uses for feeding. Because of its unusual traits, researchers believe S. pagesi may have a never-before-seen type of venom that could be used in genetic research, for example, in medicines.

The caldera where the jellyfish lives is rich in minerals and could potentially be commercially developed for deep-sea mining. The researchers published information about the jellyfish to try to protect the area and its marine life.

"Maybe it holds secrets more valuable than all the mineral wealth that could be extracted from that place. All this with the advantage of keeping the species and the site intact," André Morandini, a professor of zoology at the University of São Paulo's Institute of Biosciences and part of the research team, said in a statement.

Scientists in Hong Kong have discovered tiny, cube-shaped box jellyfish in a brackish shrimp pond that are completely unknown to science.

The diminutive jellies have a completely transparent and colorless body, or bell, as well as 12 tentacles ending in small, paddle-like structures that enable the critters to speed through water faster than most other jellyfish species. 

Like other box jellies — a group of Cnidarians that includes the Australian box jellyfish (Chironex fleckeri), the world’s most venomous marine animal, according to the National Ocean Service — the newly described jellies have 24 eyes arranged in clusters of six around its cubic bell.

"This box jellyfish connects the base of its tentacles and its bell with a flat base that looks like a boat paddle, making it distinct from other common jellyfish," Qiu Jianwen, a professor in the Department of Biology at Hong Kong Baptist University who led the research, said in a video. "Another feature of the box jellyfish is that it has six eyes located on each side of its body." 

Researchers named the newfound species Tripedalia maipoensis after Mai Po Nature Reserve in Hong Kong, where they found it. They describe its features and relationship to other box jellies in a study published March 20 in the journal Zoological Studies.

Related: Alien-like giant phantom jellyfish spotted in frigid waters off Antarctica 

T. maipoensis is the first-ever box jelly to be found in Chinese waters. It is unclear whether the half-inch-long (1.5 centimeters) animal can sting humans, but it may be venomous enough to stun tiny shrimp called Artemia. "It seemed to paralyze Artemia offered in the lab," Qiu told Live Science in an email. "But we did not touch the animal to feel the sting."

The researchers first noticed the unusual creatures in samples collected from an intertidal shrimp pond, known as a "gei wai" locally, during the summers of 2020 to 2022. The jellyfish were "quite abundant," Qiu told Live Science, numbering "up to 400 individuals in an area of the pond." A tidal channel from the brackish pool means that the species could also be present in the adjacent waters of the Pearl River estuary, but no work has been done yet to confirm this, the researchers wrote in the study.

Box jellyfish, which are also known as sea wasps, move by allowing water to enter canals that run along a muscular membrane on the underside of their bodies and then expelling it. The researchers found that, unlike closely related species, T. maipoensis has forked canals separating into multiple branches. The newly discovered species is the third known member of a group of box jellies characterized by tentacles ending in flat, paddle-like structures, called Tripedalia. 

The scientists also noted that each cluster of six eyes on the jellies' cubic bell includes a pair of eyes with lenses that enable image-forming, as well as four eyes that can only sense light.

The species probably feeds on small crustaceans called copepods, which were abundant in the samples taken from the shrimp pond, Qiu told Live Science.

"We are thrilled with this discovery," Qiu said in the video. "Finding a new species in Mai Po, where extensive research has been conducted, highlights the potential for more marine life discovery in the Hong Kong and even the Chinese coastal waters."

Lurking in the deep sea is a marine creature thought to be one of the world's largest sea anemones. But the animal, which has tentacles measuring more than 6 feet (2 meters) long, isn't an anemone but rather the first known organism in a new order of animals, according to new research.

In the four-year study, researchers created a "tree of life" for sea anemones, which are sometimes called "flowers of the sea" but are actually stationary meat-eating animals. In doing so, they examined the DNA of Boloceroides daphneae — discovered in 2006 in the deep Pacific Ocean — and found the creature stood out as not fitting on the sea anemone tree of life at all.

Researchers have now renamed the species Relicanthus daphneae, placing it into a new order (the equivalent of Carnivoria for mammals, Crocodilia for reptiles or Actiniaria for sea anemones) within the subclass Hexacorallia, which also includes anemones, black corals and stony corals.

"The discovery of this new order of Cnidaria — a phylum that includes jellyfish, corals, sea anemones and their relatives — is the equivalent to finding the first member of a group like primates or rodents," Estefanía Rodríguez, an assistant curator in the American Museum of Natural History, said in a statement. "This amazing finding tells us that we have so much more to learn and discover in the ocean," added Rodriguez, who led the research. [See Images of the New Creature & Wacky Sea Anemone]

Not an anemone

So why does the oddball look like a sea anemone?

Its similarity to anemones is an example of convergent evolution, which means that two different branches of the tree of life form features that look the same, the researchers said.

"Both groups of animals lack the same character[istic]s, but our research shows that while the anemones lost those character[istic]s over millions of years of evolution, R. daphneae never had them," Rodriguez said in a statement.

The group hopes other members of the same order will be found soon, which will help provide more information on how the tree of life is structured.

From characteristics to DNA

In addition to giving the boot to Boloceroides daphneae, the new study sheds light on sea anemones, a group of animals that are difficult to classify because they have few distinctive structures.

"Anemones are very simple animals," Rodríguez said. "Because of this, they are grouped together by their lack of characteristics — for example, the absence of a skeleton or the lack of colony-building, like you see in corals. So it wasn't a huge surprise when we began to look at their molecular data and found that the traditional classifications of anemones were wrong."

The four-year study aimed to classify the known species of anemones based on their evolutionary relationships with one another. DNA and structural comparisons of more than 112 species worldwide revealed there are only two suborders of anemones (not four, as previously thought).

The findings were published online May 7 in the journal PLOS ONE.

Follow Elizabeth Howell @howellspace. Follow Live Science @livescience, Facebook & Google+. Original article on Live Science.

Jellyfish Swarms

Jellyfish are simple, successful and, occasionally, deadly creatures. The Australian box jellyfish has enough toxin in each of its tentacles to kill 60 people. Some jellyfish create spectacular blooms when fields of polyps, their stationary life stage, simultaneously bud off into free-floating medusae. These blooms are blamed for the deaths of swimmers, clogged fishing nets and power plant intakes, and in certain parts of the world, like Japan, they appear to be on the rise.

But these problems are associated with only a subset of the creatures we call "jellyfish" – a catch all term for an amazingly diverse group of organisms. It includes corals, true jellies and others with stinging cells, as well as the stingless comb jellies, which swim using tiny hairs, called cilia.

What they also are is beautiful, as the following images attest.

Red Giant

This giant red-hued jellyfish called Tiburonia granrojo was described by American and Japanese researchers in 2003. It grows up to 3.3 feet (1 meter) in diameter and lives at depths of 2,000 to 4,800 feet (650 to 1,500 meters) in the ocean. First seen during submarine dives in 1993, the jellyfish is distinct in that it uses four to seven fleshy arms to capture food, rather than fine tentacles like other jellyfish.

Haunting Eyes

Tropical-dwelling box jellyfish have a cube-shaped body, and four different types of special-purpose eyes: The most primitive set detects only light levels, but another is more sophisticated and can detect the color and size of objects. The Australian box jellyfish is also deadly; each of its up to 60 tentacles carries enough toxin to kill 60 people.

Ocean Mixers

Mastigias jellyfish flood Jellyfish Lake, a marine lake in Palau, an island nation in the Pacific Ocean. Here, researchers found that pulsating jellyfish stir up the oceans with as much vigor as tides and winds, making them major players in ocean mixing.

Monster Jellyfish

The Australian spotted jellyfish, Phyllorhiza punctata, have invaded the Gulf of Mexico. With plenty of food, they grow as big as dinner plates and weigh up to 25 pounds (11 kilograms), although in their native waters they tend to be much smaller. They are not dangerous to humans, but pose a threat to shrimping and fishing industries.

Oldest Known Jellyfish Fossils

Fossil evidence of jellyfish dates back to the Cambrian Period, 500 million years ago. This fossil jellyfish shows similarity to the modern jellyfish, Cunina (right). It was one of four different types of jellyfish dated back to the Cambrian by researchers in 2007. These ancient jellyfish showed the same complexity as modern jellyfish, meaning they either developed rapidly 500 million years ago, or today’s varieties are much older.

Moon Jellyfish

The saucer-like Aurelia aurita, or moon jellyfish is carnivorous and feeds on small plankton organisms, such as mollusks, crustaceans and even ctenophores. It can be anywhere from two to 15.7 inches (five to 40 centimeters) in diameter and is found in mostly warm and tropical waters.

Jellies from Above

The moon jellyfish is common in many parts of the world, and it appears to have increased dramatically in Japanese waters in recent decades. Seen from a bird's eye view, a bloom of moon jellyfish appears as white swaths in a Japanese bay. In Japanese waters, its blooms have interfered with fishermen and power plants.

Nomura's Jellyfish

Nemopilema nomurai, known as Nomura's jellyfish, can grow up to 6.6 feet (2 meters) in diameter. It is edible, though it hasn't caught on widely. When Nomura's jellyfish bloomed in 2005, some Japanese coped by selling souvenir cookies flavored with jellyfish powder, according to the New York Times.

Jellyfish Menaces?

Blooms of Nomura's jellyfish have created serious problems in Japanese waters, including clogging fishing nets and stinging fishermen. Blooms have been recorded as far back as 1920, but they were rare events. But beginning in 2002, blooms have occurred nearly every year.

Mating Ritual

This image captures the courtship behavior of the box jellyfish Copula sivickisi. The male (top) and female (bottom) engage in a complex mating ritual unique among cnidarians (jellyfishes, hydroids, anemones, corals and their kin).

Jellyfish Introductions

The moon jellyfish is believed to have been introduced into many new environments by ships, when the jellyfish's stationary developmental stage, called a polyp, attached to their hulls or came in via the ballast water, which ships dump once they arrive at their destination.

Stealthy Predator

The stealthy predator Mnemiopsis leidyi, also known as the sea walnut, uses tiny hairs, called cilia, to create a current which prey don't notice until they are sucked into its mouth region, surrounded by two large oral lobes. The sea walnut swims using fused cilia, which diffract light in many colors in this photo.

Efficient Feeders

A salp bloom off the coast of New Zealand. These bloblike creatures are not true jellyfish, but instead another group of free-swimming invertebrates. The 5-inch (13-centimeter)-long , barrel-shaped organisms resemble streamlined jellyfish and live in mid-ocean waters where they filter the seawater for food particles.

Moon Jellies

Here, a moon jellyfish bloom off the coast of Japan. Moon jellyfish (Aurelia aurita), are carnivorous and feed on the ocean's zooplankton (tiny floating animals).

Fried Eggs?

Fried Egg Jellyfish (Cotylrhiza) from Alicante, Spain, forms blooms along the Mediterranean coastline.

Giant Jellies

Giant jellyfish (Nemopilema nomurai) clogging fishing nets in Japan. Also called Nomura's jellyfish, these gelatinous creatures can grow up to 6.7 feet (2 meters) in diameter.

Man-O-War

Here a Portuguese Man-O-War (Physalia sp.), a species closely related to true jellyfish, is common in the world's oceans.

The oldest known fossils of jellyfish have been found in rocks in Utah that are more than 500 million years old, a new study reports.

The fossils are an unusual discovery because soft-bodied creatures, such as jellyfish, rarely survive in the fossil record, unlike animals with hard shells or bones.

"The fossil record is biased against soft-bodied life forms such as jellyfish, because they leave little behind when they die," said study member Bruce Lieberman of the University of Kansas.

These jellyfish left their lasting imprint because they were deposited in fine sediment, rather than coarse sand. The film that the jellyfish left behind shows a clear picture, or "fossil snapshot," of the animals.

"You can see a distinct bell-shape, tentacles, muscle scars and possibly even the gonads," said study team member Paulyn Cartwright, also of KU.

The rich detail of the fossils allowed the team to compare the cnidarian (the phylum to which jellyfish, coral and sea anemones belong) fossils to modern jellyfish. The comparison confirmed that the fossils were, in fact, jellyfish and pushed the earliest known occurrence of definitive jellyfish back from 300 million to 505 million years ago.

The fossils also offer insights into the rapid species diversification that occurred during the Cambrian radiation, which began around 540 million years ago and when most animal groups start to show up in the fossil record, Lieberman said.

The complexity of these early jellyfish seems to suggest that either the complexity of modern jellyfish developed rapidly about 500 million years ago, or that jellyfish are even older and developed long before that time.

• Votes: Creepiest Creatures of the Deep!
• 10 Amazing Things You Didn't Know about Animals
• Images: Rich Life Under the Sea