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 physonect siphonophore whose coordinated jet propulsion by multiple nectophores has been studied as a model for energy-efficient underwater locomotion.

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.

A hydromedusa of the family Aequoreidae found in the Mediterranean and eastern Atlantic, closely related to Aequorea victoria, the species from which GFP was first isolated.

A small hydrozoan of the North Pacific and North Atlantic that clings to seagrass using adhesive pads and delivers a sting disproportionately potent for its centimeter-scale size.

A large bioluminescent hydromedusa of the western Pacific Ocean, related to Aequorea victoria, the species that yielded the Nobel Prize-winning green fluorescent protein.

A colonial hydroid whose tiny free-swimming medusae are widely studied in marine biology; the polyp colony grows on macroalgae in coastal waters worldwide.

An unusual holopelagic hydrozoan with a flat, wing-like body adapted for open-ocean drift, constituting the sole species in family Tetraplatidae.

Muggiaea atlantica is a calycophoran siphonophore of the eastern Atlantic that forms dense blooms in European coastal waters, causing significant mortality in farmed salmon.

A bioluminescent hydromedusa of the North Pacific that emits blue-green light; a close relative of Aequorea victoria, the original source of green fluorescent protein.

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.