Olindias formosa

Geographic Range

Flower hat jellies (Olindias formosa) have largely been observed in the shallow waters of the Pacific and Atlantic Oceans. They have often been found along the coasts of Brazil, Japan, and Argentina, where long sea grasses, seaweeds, and kelp are common.


Flower hat jellies are semi-benthic coastal dwellers that live around 35 to 55 m below sea level. They often attach to seagrass or rocks during the day, or remain close to the seafloor. At night, they leave the seafloor to hunt near the surface of the water.

Both adult and juvenile flower hat jelly medusas will split their day between the ocean floor and open water, while polyps spend all of their time attached to substrate near or on the seafloor.

  • Range depth
    55 to 35 m
    180.45 to 114.83 ft

Physical Description

Flower hat jellies vary in size depending on the time of year, as well as food availability within their environment. In December, most individuals will either be juvenile medusae or still in early hydroid growth stages. Hydroid colonies that produce medusae range from 3 to 5 mm long, the attached polyps are around 278 μm (micrometers) long, and the polyp tentacles are around 1,102 μm long. Newly budded medusae are about 2 cm in diameter. It is also common for flower hat jellies to have less access to food in the winter months, which contributes to their slow winter growth. In March and April they can grow to be 12 to 15 cm in diameter as they reach adulthood and more prey fish become active.

The appearance of adults does not change drastically throughout the year. Flower hat jellies can be identified by a large translucent bell that encapsulates six radial canals, though some individuals have been found with only four. These canals are often a light pink/coral color, and can easily be seen through the epidermis. The bell itself is covered in darkly-pigmented, stringlike growths that protrude off of the body on one end. These are called velar tentacles and they have bioluminescent tips, which appear bright pink under natural light and glow bright green in dark environments. The feeding tentacles lining the lip of the bell also have these glowing tips, but they are much longer and more widely spaced out. An individual can have 100 to 300 velar tentacles, but generally only have 10 to 15 feeding tentacles. Feeding tentacles grow with age and are used to sting and pull prey towards their mouth parts. There is no polymorphism between individuals with different environments, diets, or other defining lifestyles.

Since the bells of flower hat jellies are often translucent, the colors of individuals are often expressed in the canals inside their bells, in the lips of their bells, and in their tentacles. It is unknown what causes the variation in flower hat jelly pigmentation, but most individuals seem to have either white, pink, or orange hues pigmenting the structures listed above. The dark pigment of velar tentacles appears the same on all flower hat jellies.

Adult flower hat jellies can be distinguished from closely related species like Olindias diego and Olindias phosphorica (both in the family Olindiasidae) largely by two factors. The feeding tentacles of flower hat jellies are much shorter and more sparse than other species, and the multitude of highly pigmented velar tentacles is also uncommon for their genus. There are other species with velar tentacles (O. diego is one example), but flower hat jellies have enough to cover their entire bells.

  • Sexual Dimorphism
  • sexes alike
  • Range length
    2 to 15 cm
    0.79 to 5.91 in


Flower hat jellies begin their life cycle after gametes released from adults are either externally fertilized by another adult gamete or left alone to attach to a solid surface and form hydroid structures with feeding polyps. These sessile hydroids attach to rocks, the ocean floor, grass, or kelp and produce multiple polyps that branch off of a single structure. These polyps are able to filter-feed with a single tentacle that extends to many times their length and is in constant movement. Polyps gather nutrients by waving this tentacle, until the hydroid they are on has enough nutrients to produce juvenile medusae, which then bud off of the base or stem of the structure and become motile.

Polyp tentacles are extremely important for hydroid survival, as it is the only way for them to feed while sessile. After a medusa buds off, it instead relies on predatory methods for food. Adult flower hat jellies do not care for their offspring at any point in their lives, so both the hydroid clusters and medusae are completely self-sufficient.

Juvenile medusa flower hat jellies look somewhat similar to adults, with the largest differences being a smaller bell size, fewer velar and feeding tentacles, and shorter feeding tentacles. Juveniles have four main tentacles around the lips of their bells and two smaller ones closer to the center of their bells, but these tentacles will continue to multiply and grow longer as medusae mature. Newly-budded medusae are about 1 to 2 mm across and have no velar tentacles.

The behaviors of juvenile and adult medusae are almost identical. They both cycle through high and low activity periods throughout the day and are more active at night, but juveniles spend more time on the seafloor or other solid surfaces and less time swimming than adults do.


Flower hat jellies do not always reproduce sexually, but when they do, fertilization occurs externally as two individuals release gametes that settle on the surrounding seafloor or flora. If a male and female gamete bump into each other, a fertilized hydroid structure with polyps starts to form, usually in non-cooperative colonies of 2 to 8.

Flower hat jelly reproduction has only been observed in captive adults, so there is still much to learn about when and how gamete release is stimulated in the wild. Polyp and young medusa survival could be reliant on water temperature, since the highest survival rate in captivity has been recorded to have occurred at around 15°C, as opposed to less successful experiments in warmer water (20-25°C). Whether flower hat jellies decide to reproduce sexually or asexually depends on if they have enough gametes. If gamete fertilization is not possible, asexual reproduction will proceed.

  • Breeding interval
    It is still widely unknown what spurs the reproductive processes of flower hat jellies, but in captivity, gamete release has occurred in May, which coincides with the time of year that most flower hat jellies observed in the wild are adults.
  • Breeding season
    March to May
  • Range number of offspring
    2 to 7

Since adult flower hat jellies can reproduce asexually or sexually, the only parental involvement they take on is the release of gametes into the surrounding environment, which can then be fertilized externally and form self-sufficient hydroid clusters. After fertilization, adults expend no further energy on their offspring.

  • Parental Investment
  • no parental involvement


Flower hat jelly lifespans have been primarily observed in captive, healthy individuals, which live up to six months. However, much of the wild observations of adults match this trend, based on the time of year and the location in which those observations were made. Most individuals caught in an observational study during December were small and immature medusae, and most caught in May were adults, many being injured or dead near the surface of the water. This suggests that the lifespan of flower hat jellies in the wild is around the same as it is in captivity - around 5 to 6 months.

  • Range lifespan
    Status: captivity
    5 to 6 months
  • Typical lifespan
    Status: wild
    4 to 6 months
  • Typical lifespan
    Status: captivity
    5 to 6 months


Flower hat jellies remain solitary for the majority of their lives, the exception being the occasional small colonies of polyps that form and grow together. In terms of cooperation, individuals do not display behavior like sharing food, caring for their young, or catching prey in groups. However, in the warmer months of April and May, large blooms of these jellyfish will sometimes congregate near beaches or other relatively shallow water. It is not yet known why or how these blooms form because the jellies do not appear to cooperate, but since these blooms are a threat to beachgoers and coastal fisherman, many citizens of these coastal towns are pushing for researchers to investigate them.

In their daily lives, both adult and juvenile flower hat jelly medusae are largely nocturnal; they rest on the seafloor or attach to substrates during the day and actively swim nearer to the surface to hunt at night. Immature hydroids are sessile and do not have an active schedule; polyps continuously filter feed regardless of the time of day.

Home Range

The most noticeable populations of flower hat jellies are found off the coasts of southern Japan, but they have also been found near Brazilian coasts.

Communication and Perception

Flower hat jellies have not been observed to communicate with others in the same species or even other Cnidarians. Since adults do not choose mates and fertilize their gametes externally, if at all, there is no observed social communication between medusae. It is unknown how adults organize gamete release. Similarly, hydroid clusters in captivity have not been observed cooperating or communicating with other clusters, even if they are attached to the same substrate.

Flower hat jellies are known to perceive their environment using touch and communicate photically. They use bioluminescence to attract prey fish in darker environments. They can also feel when fish have brushed against their hanging tentacles and will react by releasing venom and retracting those tentacles towards their mouths.

Food Habits

The diets of flower hat jelly polyps and medusae differ substantially; polyps are strictly filter feeders and use their single tentacle to sweep the surrounding water for floating nutrients. Juvenile and adult medusae are active predators, with many more tentacles that they use to capture prey. Flower hat jelly medusae do not specialize their diets, instead using their hanging tentacles to trap and paralyze any animal that swims into them. These animals are usually small, shallows-dwelling marine fish. Medusae also consume smaller marine organisms like zooplankton and diatoms. In large jellyfish blooms, flower hat jellies are known to eat smaller jellies due to overcrowding or accidental capture.


The most effective deterrent flower hat jellies have are their venomous tentacles, which are fatal to many marine animals and extremely painful to humans. They also avoid predation by being active at night and remaining near the ocean floor during the day, which prevents detection from diurnal fish. In their hydroid form, jellies do not have any anti-predator adaptations beside their small size. Interestingly, the largest predators of flower hat jellies are other jellies, in which case these adaptations are not effective.

  • Known Predators
    • Olindias formosa

Ecosystem Roles

Flower hat jellies do not host or parasitize other species, and act as solitary predators that regulate fish populations in normal circumstances. However, when these jellies start to bloom, they can predate their own species, other jellyfish species, and large amounts of fish. This puts a strain on the balance of coastal ecosystems and is getting increasingly harder to regulate.

Economic Importance for Humans: Positive

The beauty of flower hat jellies is leading to an increase in capture for their display in aquariums, both commercial and personal. Their bioluminescence is being researched in studies of biomimicry and in medicinal research for utilizing their powerful venom for chronic pain management.

Economic Importance for Humans: Negative

Flower hat jellies have caused considerable harm to Japanese, Brazilian, and Argentinian coastal towns in which swimming, fishing, and power plant activity are staples of the community. When flower hat jellies bloom, they can severely harm swimmers, clog fishing nets, over-predate native fish populations, invade marine fish farms, and clog power plant ducts, limiting water flow and inhibiting power production. There has even been a human death reported as a result of exposure to a large flower hat jelly bloom, and those who rely on fish for food and business are struggling where the blooms are common.

  • Negative Impacts
  • injures humans

Conservation Status

Flower hat jellies are not listed as an endangered or threatened species by the following site lists at this time: IUCN Red List, CITES, U.S. Federal List, State of Michigan List.


Carmen Salpekar (author), Colorado State University, Brooke Berger (editor), Colorado State University, Galen Burrell (editor), Special Projects.


Atlantic Ocean

the body of water between Africa, Europe, the southern ocean (above 60 degrees south latitude), and the western hemisphere. It is the second largest ocean in the world after the Pacific Ocean.

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living in the southern part of the New World. In other words, Central and South America.

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Pacific Ocean

body of water between the southern ocean (above 60 degrees south latitude), Australia, Asia, and the western hemisphere. This is the world's largest ocean, covering about 28% of the world's surface.

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living in the northern part of the Old World. In otherwords, Europe and Asia and northern Africa.

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reproduction that is not sexual; that is, reproduction that does not include recombining the genotypes of two parents


Referring to an animal that lives on or near the bottom of a body of water. Also an aquatic biome consisting of the ocean bottom below the pelagic and coastal zones. Bottom habitats in the very deepest oceans (below 9000 m) are sometimes referred to as the abyssal zone. see also oceanic vent.


an animal that mainly eats meat


the nearshore aquatic habitats near a coast, or shoreline.

colonial growth

animals that grow in groups of the same species, often refers to animals which are not mobile, such as corals.


a substance used for the diagnosis, cure, mitigation, treatment, or prevention of disease


animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature

external fertilization

fertilization takes place outside the female's body


union of egg and spermatozoan


a method of feeding where small food particles are filtered from the surrounding water by various mechanisms. Used mainly by aquatic invertebrates, especially plankton, but also by baleen whales.


having the capacity to move from one place to another.


active during the night


generally wanders from place to place, usually within a well-defined range.

pet trade

the business of buying and selling animals for people to keep in their homes as pets.


generates and uses light to communicate


an animal that mainly eats fish


an animal that mainly eats plankton


the kind of polygamy in which a female pairs with several males, each of which also pairs with several different females.

radial symmetry

a form of body symmetry in which the parts of an animal are arranged concentrically around a central oral/aboral axis and more than one imaginary plane through this axis results in halves that are mirror-images of each other. Examples are cnidarians (Phylum Cnidaria, jellyfish, anemones, and corals).

saltwater or marine

mainly lives in oceans, seas, or other bodies of salt water.

seasonal breeding

breeding is confined to a particular season


non-motile; permanently attached at the base.

Attached to substratum and moving little or not at all. Synapomorphy of the Anthozoa


reproduction that includes combining the genetic contribution of two individuals, a male and a female


lives alone


uses touch to communicate


that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).


the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.


an animal which has an organ capable of injecting a poisonous substance into a wound (for example, scorpions, jellyfish, and rattlesnakes).


animal constituent of plankton; mainly small crustaceans and fish larvae. (Compare to phytoplankton.)


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Dunn, C. 2015. Fluorescent proteins function as a prey attractant: experimental evidence from the hydromedusa Olindias formosus and other marine organisms. Biology Open, Volume 4/Issue 9: 1094-1104. Accessed February 10, 2020 at https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4582119/.

Randall, I. 2015. "Slideshow: Glowing predators of the deep" (On-line image). Science. Accessed February 10, 2020 at https://www.sciencemag.org/news/2015/08/slideshow-glowing-predators-deep.

Tanimoto, M., R. Minemizu. 2019. Olindias deigo sp. nov., a new species (Hydrozoa, Trachylinae, Limnomedusae) from the Ryukyu Archipelago, southern Japan. ZooKeys, Volume 900: 1-21. Accessed February 10, 2020 at https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6946721/.