Best Hydrozoa

Turritopsis dohrnii is a very small hydrozoan jellyfish associated originally with the Mediterranean but now recorded in several marine regions. Under stress or after reproduction, its medusa can transform its specialized cells and return to a colony-forming polyp stage, effectively restarting the l...

The Portuguese man o' war, Physalia physalis, is a pelagic hydrozoan composed of specialized individuals functioning together as a colony, rather than a single jellyfish. Its gas-filled float remains at the sea surface and acts like a sail, while long tentacles trail beneath it to capture fish and o...

The bluebottle, commonly identified as Physalia utriculus in Indo-Pacific waters, is a colonial hydrozoan rather than a true jellyfish. Its visible body is a gas-filled float, while specialized individuals within the colony perform feeding, defense, and reproduction. Long tentacles carry stinging ce...

Halitrephes maasi is a deep-sea hydromedusa in the family Halicreatidae, known from the open ocean at substantial depths. Remotely operated vehicles have recorded specimens whose radial canals reflect artificial light in vivid colors, producing a wheel-like appearance sometimes described as biolumin...

The flower hat jellyfish, Olindias formosus, is a hydrozoan medusa found in coastal waters of the western Pacific, particularly around Japan. Its translucent bell bears radiating colored bands, while numerous tentacles curl upward when they are not extended, producing its flower-like appearance. The...

Turritopsis nutricula is a small hydrozoan jellyfish originally described from the western Atlantic. It has often been associated in popular accounts with the ability to return from the adult medusa stage to a polyp-like juvenile state through cellular transformation, but much of the research behind...

Velella velella, commonly called the by-the-wind sailor, is a colonial hydrozoan that lives at the ocean surface rather than a true jellyfish. Each colony has a flat, blue float and an upright, rigid sail that allows wind to carry it across the water, while feeding polyps capture plankton below. Win...

Aglantha digitale, commonly called the rocket jelly, is a small hydromedusa in the phylum Cnidaria and class Hydrozoa. It occurs in cold and temperate ocean waters and spends its life in the water column, where it feeds on small planktonic animals. The species is notable for having two swimming mode...

Craspedacusta sowerbii is a small freshwater hydrozoan commonly called the freshwater jellyfish. Native to the Yangtze River basin in China, it has been introduced widely and recorded on every inhabited continent, often in lakes, reservoirs, ponds, and slow-moving water. Its life cycle includes a ti...

Colobonema sericeum is a deep-sea hydromedusa in the order Trachymedusae, occurring in the dim mesopelagic waters of the ocean. When disturbed, it can detach luminous portions of its tentacles, leaving glowing fragments behind while the animal moves away. This combination of bioluminescence and auto...

Nanomia bijuga is a species of colonial pelagic hydrozoan belonging to the order Siphonophorae, closely related to corals and jellyfish. Like other siphonophores, it is composed of specialized individual organisms called zooids that work together as a single functioning entity. As a physonect siphon...

Porpita porpita is a small colonial hydrozoan of tropical and subtropical oceans, resembling a blue button up to 3 cm across, whose central float surrounds a colony of feeding and reproductive polyps.

*Aequorea forskalea* is a species of hydrozoan in the family Aequoreidae, commonly found in the Mediterranean Sea and the eastern Atlantic Ocean. As a hydromedusa, it has a delicate, transparent bell and exhibits bioluminescence. The species is closely related to *Aequorea victoria*, the Pacific jel...

Gonionemus vertens, known as the clinging jellyfish, is a small hydrozoan native to the North Pacific Ocean that has since established populations in the North Atlantic. Reaching roughly the size of a coin, this species attaches itself to underwater structures like seagrass and kelp using specialize...

Aequorea macrodactyla is a species of hydrozoan, specifically a large hydromedusa, found in the marine waters of the western Pacific Ocean. The jellyfish is notable for its transparent, gelatinous bell and its inherent bioluminescent capabilities, producing flashes of light along its marginal tentac...

Obelia geniculata is a colonial marine hydrozoan in the family Campanulariidae. Its branching polyp colonies commonly grow attached to kelp and other large algae in coastal waters, where feeding polyps capture small suspended organisms and reproductive structures produce tiny medusae. The species ha...

Tetraplatia volitans is a small, unusual marine hydrozoan adapted to a wholly pelagic existence in the open ocean. Its flattened body bears four wing-like lobes used in swimming, giving it a form unlike the bell-shaped medusae commonly associated with jellyfish. It is the only recognized species in...

Muggiaea atlantica is a calycophoran siphonophore, a colonial hydrozoan whose specialized individual units function together as one pelagic organism. It occurs in Atlantic waters and can become abundant in coastal seas when temperature, currents, and food conditions are favorable. Dense populations...

Aequorea coerulescens is a marine hydrozoan in the family Aequoreidae, commonly described as a hydromedusa or jellyfish. It occurs in Pacific waters and has a transparent, saucer-shaped bell bordered by numerous fine tentacles, a body plan characteristic of the genus Aequorea. Like related species,...

A bioluminescent hydromedusa found in the Mediterranean and eastern Atlantic, notable for its complex tentacle arrangement and belonging to the order Limnomedusae.

A small leptomedusa native to the Black and Caspian seas, now invasive in numerous estuaries worldwide, including San Francisco Bay and Chesapeake Bay.

A narcomedusa notable for a parasitic larval stage that develops inside the bells of other medusae such as Turritopsis before becoming free-swimming.

A hydromedusa that lives commensally on the tubes of polychaete worms in tropical and subtropical seas, making it one of the few hydromedusae with such a lifestyle.

A trachymedusa of warm Atlantic and Mediterranean waters, notable for an exceptionally long manubrium (proboscis) that extends well beyond the bell margin.

Solmundella bitentaculata is a narcomedusa jellyfish bearing only two tentacles, widely distributed throughout the world's open oceans from tropical to temperate zones.

Solmissus albescens is a narcomedusa jellyfish distributed across tropical and subtropical oceans, notable for actively preying on gelatinous zooplankton including other medusae.

A cosmopolitan hydromedusa in the order Trachymedusae distributed across tropical and subtropical open oceans worldwide, recognized by its four-lobed radial symmetry.

A hydromedusa of the northeastern Pacific coast, distinctive for its many radial canals and its ability to both swim actively and rest on the seafloor.

A deep-sea hydromedusa in family Crossotidae described from Arctic and Pacific deep-water samples, distinguished by its pink-red coloration and numerous tentacles.