Even among jellyfish, the flower hat stands out. Its bell is laced with opaque stripes and ringed by a tangle of tentacles, some curling up over the top of the bell like the brim of a fancy hat, each tipped in fluorescent colour. It was rarely seen outside its native waters until public aquariums began displaying it, and it remains one of the least-studied showpiece jellies.

Where does the name come from?

The species was described in 1903 by the Japanese zoologist Seitaro Goto, who placed it in a new genus as Olindioides formosa; the species name means “beautiful”. It was later moved into the genus Olindias, created by Fritz Müller in 1861 for a South American species, and today it is usually written Olindias formosus, although the older spelling formosa still appears in many aquarium labels and books. The English name refers to the way the coiled, brightly tipped tentacles sit over the bell like the flowers on a festival hat.

The genus belongs to the family Olindiidae in the order Limnomedusae, a small group of about 60 hydrozoan species. Many limnomedusae are tiny, and some of their relatives even live in fresh water, including the freshwater jellyfish Craspedacusta. Olindias species are among the giants of the order, with bells that can reach 10 cm or more. The World Register of Marine Species lists several: O. formosus in Japan, O. sambaquiensis on the Atlantic coast of South America, O. phosphorica in the Mediterranean and eastern Atlantic, O. tenuis in the western Atlantic, and O. malayensis and O. singularis in the Indo-Pacific. At the end of 2019 Japanese researchers added another, O. deigo, from Okinawa, named after the red-flowered coral tree that is Okinawa’s prefectural flower.

What does a flower hat jellyfish look like?

A translucent bell up to about 15 cm wide with gold, brown or pink bands, ringed by dozens of tentacles that coil both above and below the rim and end in neon pink, yellow or green tips.

The tentacles that sprout from the top of the bell are a distinctive feature: many jellies have tentacles only at the rim. When the jelly rests, its tentacles fan out over the seabed like a flower.

The arrangement of tentacles is the key to the genus. Olindias medusae have two kinds. A small number of long primary tentacles grow not from the rim but from part-way down the outside of the bell. They carry adhesive pads and bands of stinging cells, and each has a pair of balance organs, or statocysts, at its base. Around the margin there is a much larger number of short tentacles armed with rings of stinging cells, which immobilise prey, and between them small club-shaped buds that can develop into new tentacles. The bell itself is clear, with four radial canals, many shorter canals running in from the rim, and opaque, pinstripe-like bands radiating from the top. The tentacle tips contain fluorescent pigments that glow pink, green or yellow under blue or ultraviolet light, which is why the jellies look so vivid in aquarium lighting.

Where do flower hat jellyfish live?

Off central and southern Japan and Jeju Island in South Korea, in shallow coastal water near the seabed close to seaweed, rocks and seagrass. The similar jellies of Brazil and Argentina are a different species.

It is an unusual jelly in that it spends the day resting on or near the bottom and rises to hunt at night. That habit, and the limited range, help explain why it is seldom seen by divers.

Many popular sources, including some aquarium fact sheets, say the flower hat jelly also lives off Brazil and Argentina. That is a confusion with its close relative Olindias sambaquiensis, a similar-looking species that is common in spring and summer along the coasts of southern Brazil, Uruguay and Argentina. Taxonomic references treat the true flower hat jelly as a northwestern Pacific species, found off central and southern Japan and around Jeju Island in South Korea. In Japanese waters the medusae are typically seen from about December to July, with a peak in April and May. Further south, in the subtropical Ryukyu Islands, a third species, O. deigo, appears in shallow water of 3 to 10 m in winter and spring, and divers there may easily mistake it for O. formosus; it can be told apart by the number and colour of its tentacles.

How fast does a flower hat jelly grow?

The medusa stage is short and seasonal. When the young medusae first appear in Japanese waters around December they are only about 2 cm across. They grow slowly through the cold winter months, when prey is scarce, and then much faster in spring as small fish become more active, reaching 12 to 15 cm by about March and April. By midsummer the adults have spawned and disappeared. In aquariums the medusa typically lives for about four to six months. This “boom in spring, gone by summer” pattern is common among coastal hydromedusae, which survive the rest of the year as tiny, inconspicuous polyps on the seabed.

Its behaviour also changes with the time of day. By day the flower hat jelly rests on or close to the bottom, often among rocks, seaweed or seagrass, with its tentacles coiled close to the bell. At night it lifts off and drifts or swims slowly with its tentacles extended, ready to snag passing fish. The long primary tentacles with their adhesive pads probably help it hold onto the seabed as well as onto prey, so it behaves partly like a bottom-dwelling sea anemone and partly like a free-swimming jellyfish. The closely related O. deigo in Okinawa behaves the same way, resting on sandy or sloping bottoms with good water movement during the day and drifting with tentacles extended at night.

What does the flower hat jelly eat?

Mostly small fish, which it catches with its sticky, stinging tentacles.

Fish-eating is unusual among hydromedusae, many of which feed on tiny plankton. The flower hat lies in wait near the bottom, with its tentacles spread to snare prey. See how do jellyfish eat?

Catching fish is demanding for an animal made mostly of water. The marginal tentacles are packed with nematocysts, microscopic harpoon-like capsules that fire on contact and inject venom, paralysing small fish within seconds. The tentacles then contract and bend towards the mouth at the centre of the underside of the bell, where the prey is digested in the stomach and its nutrients are carried around the bell through the radial canals.

Why do the flower hat jelly’s tentacles glow?

The glow is fluorescence, not bioluminescence. The tentacle tips contain fluorescent proteins, relatives of the green fluorescent protein first isolated from the crystal jelly, which absorb blue light and re-emit it as green, yellow or pink. Deep blue is the dominant colour of daylight a few metres underwater, so a tentacle tip that turns it into a warmer colour stands out against the background. For years this was simply a striking feature with no known purpose. In 2015 Steven Haddock and Casey Dunn tested an idea in the journal Biology Open: that the glowing tips work as lures. They offered juvenile rockfish the chance to approach flower hat jelly tentacles under different lighting. Under blue light, where the tips fluoresced visibly above the background, the young fish were strongly attracted to them. Under yellow or white light, which did not produce visible fluorescence, they showed no significant interest.

The researchers also watched wild fish respond to bright points of light underwater and found similar lure-like structures in other animals, including the siphonophore Rhizophysa eysenhardti. Their conclusion was that fluorescent proteins can act as prey attractants, offering one of the first experimentally supported functions for these widespread molecules. For the flower hat jelly, which waits for small fish rather than filtering plankton, a set of glowing “bait” tips around the bell makes good sense.

Is the flower hat jellyfish dangerous?

Its sting can cause a painful rash, and stings have been reported within its range, but it is not considered deadly.

Aquarium sources report that blooms of Olindias off Brazil, almost certainly the South American species, have caused problems for shrimpers by clogging nets and driving shrimp away.

The best-studied stings in the genus come from the South American species, O. sambaquiensis, which drifts into bathing areas in spring and summer. At the Argentine resort of Monte Hermoso, a study of jellyfish injuries in 2016 and 2017 found that this species caused 57 of the 79 stings recorded in 2017, more than any other jelly that year. Every patient stung by it reported pain and reddening of the skin, and the injuries typically took the form of whip-like lines where the tentacles had touched. Researchers in Brazil and Argentina have since isolated several cytolysins, proteins that damage cells, from its stinging capsules. Stings from the Japanese flower hat jelly are generally described as painful with a rash rather than dangerous, but anyone with a severe or spreading reaction, breathing difficulty or chest pain after a sting should get medical help.

If you are stung, leave the water, lift away any tentacle fragments with a gloved hand or the edge of a card rather than bare fingers, and avoid rubbing the area or rinsing it with fresh water, which can make remaining stinging cells fire. Follow local first-aid advice, since the best treatment differs between jellyfish groups; for many stings, soaking the area in hot water that is comfortable to the touch helps relieve the pain.

How does the flower hat jelly reproduce?

Like most hydrozoans, it alternates between a tiny seabed polyp and the free-swimming medusa. The polyp was found only in 2012, in an aquarium.

Adult medusae release eggs and sperm into the water, and the fertilised eggs develop into larvae that settle and grow into polyps. Wild polyps have still never been found. Compare with the true-jellyfish cycle in jellyfish life cycle.

For more than a century nobody had seen the polyp stage of any Olindias species. Monterey Bay Aquarium first collected flower hat jellies in Japan in 2002 for a jellyfish exhibit, and its aquarists, led by Wyatt Patry, spent years trying to make them breed. They eventually built artificial habitats in the tanks to encourage spawning and settlement, and in November 2012 they found the hydroid colony by using the animals’ own fluorescence: under blue light, tiny glowing polyps showed up on wire mesh in the tank. Their description, published in 2014 in the Journal of the Marine Biological Association of the United Kingdom, showed that the polyps are minute, each with a single very active tentacle, and form small colonies a few millimetres long.

The experiments also revealed how temperature controls the cycle. The polyps grew and budded new polyps only at about 15 °C, not at 20 or 25 °C, while the warmer temperatures triggered the release of more medusae, each only about a millimetre across when it detached. That matches the seasons in Japan, where cool water favours polyp growth and warming water releases the young jellies. In the same study, researchers at the Okinawa Churaumi Aquarium documented the full life cycle of O. deigo as well, collecting thousands of fertilised eggs from the tank floor in the early morning and raising them through the polyp stage. The same fluorescence trick may help scientists find wild Olindias polyps, which would show where and when blooms begin.

How is the flower hat different from a true jellyfish?

It is a hydromedusa, so it has a thin velum (a shelf of tissue under the bell rim) used for jet propulsion, and a simpler body than true jellyfish.

Other hydromedusae on TheSea.Org include the crystal jellyfish and the immortal jellyfish.

Can you see flower hat jellies in aquariums?

Flower hat jellies are now displayed by several large public aquariums, including Monterey Bay Aquarium and the Aquarium of the Pacific in California, and by Japanese aquariums that collect them locally. They are demanding to keep. They need cool, full-strength seawater, usually around 15 to 18 °C, a tank designed so that a bottom-resting jelly does not get trapped in currents. Because the medusa lives only a few months, aquariums either collect fresh animals each season or maintain polyp cultures that can be stimulated to release new medusae, which has become possible only since the life cycle was worked out.

The conservation status of the flower hat jelly has not been formally assessed, and it is listed as data deficient by aquarium sources. There is no evidence that it is threatened, but almost nothing is known about the size or trends of its wild populations, the location of its polyps, or how warming seas around Japan might shift the timing of its seasonal appearance. Like many hydromedusae it is far better known from aquarium tanks than from the wild.