Erenna Jellyfish

Erenna Jellyfish

Class             Hydrozoa(hydralike animals, hydroids, and hydrozoans)
Order           Siphonophora
Suborder    Physophorae
Family          Agalmidae
Genus           Erenna

Introduction to Erenna jellyfish

Erenna is a genus comprising several scientifically described species of jellyfish, such as Erenna bedoti, Erenna cornuta, Erenna laciniata and Erenna richardi.

As you can see above, this genus belongs to the order Siphonophora and the Erenna jellyfish are therefore a type of Siphonophores. Siphonophores can appear to be colonies of single-celled organism, but they are actually more of a “superorganism” that grows by budding off specialized polyps and medusae.

Nearly all members of the order Siphonophora are luminous, i.e. they emit their own light which distinguishes them from other shimmering animals who simply reflect the light of the sun.

We still know comparatively little about Erenna jellyfish. They are believed to be common in the ocean, but their fragility makes them difficult to study alive. The great depths at which you find certain Erenna species make it even more complicated to research this genus.


Together with the other members of the order Siphonophora, Erenna jellyfish are important predators in many marine ecosystems. Unlike most other siphonophores, Erenna jellyfish feed on fish instead of crustaceans. Each gastrozooid (A gastrozooid is a nutritive polyp of colonial coelenterates, characterized by having a mouth and tentacles.) is equipped with a tentacle that branches off of it from the base. On this tentacle, you can see tiny side branches which are called tentilla, and it is on the tentilla that you can find the stinging cells of an Erenna jellyfish.   

The new finding from 2005

In 2005, a new species of Erenna jellyfish was found by a research team1 using a deep-diving remotely operated vehicle from the Monterey Bay Aquarium to explore the little known depths between 1600 m and 2300 m.

Most siphonophores catch their food by sitting still and stinging the unfortunate animal that accidently swims into their tentacles, but Erenna jellyfish are not as passive as most siphonophores, and this newfound species seem to be particularly active when it comes to catching prey.

The Erenna jellyfish observed from the remotely operated vehicle held their tentacles close to the body instead of extending them out into the water. This puzzled the researchers, because keeping your tentacles folded don’t seem like a very efficient way of catching prey. They were however offered an important clue by the arrangement and behaviour of the tentacle side braches of the Erenna jellyfish. On this species, the side braches were equipped with a large array of stinging cells attached to a transparent central stalk.  At the end of the central stalk, you will find a red “lure” that starts out small with a white centre. The white centre is made up by bioluminescent material. As the jellyfish lure matures, it grows bigger and bigger and the white bioluminescent material gradually becomes surrounded by red fluorescent material. 

After studying the emission and excitation spectra of the red substance, the researchers concluded that the blue light emitted at the lure centre can be expected to excite fluor and send out an orange-red light. The strange thing about this type of light is that it will not reach very far in the dark waters of the deep sea; centimetres or metres rather than tens of metres. It should also be noted that most organisms known to be present at these harsh depths can not see red light.   

The lure of the Erenna jellyfish does not only light up; the transparent stalk will also contract rapidly which causes the lure to flick back and forth. The researchers have therefore suggested that the Erenna jellyfish may try to imitate deep-sea copepods in order to lure hungry fish close enough to sting. Deep-sea copepods are a favourite food among many species of fish and can often be seen jumping and slowly sinking in the water. Maybe a fish that sees the flicking stalk of the Erenna jellyfish will think it’s a copepod and try to swallow it? As mentioned above, Erenna jellyfish feeds on fish rather than crustaceans.

According to the researchers, the most likely prey item for the newly found Erenna jellyfish are small bristlemouth fish from the family Gonostomatidae. Perhaps one or several of these species are capable of seeing orange-red lights? We still know very little about the organisms that inhabit the deep sea. 

As you can see, this new species of Erenna jellyfish is special in several respects. Not only is it a jellyfish that actively lures in its prey using light; it is also a creature capable of emitting orange-red light in an environment where very few fishes can see red or orange lights. Hopefully, we will learn much more about this Erenna jellyfish in the future.

1 Steven H. D. Haddock, Monterey Bay Aquarium Research Institute, USA; Casey W. Dunn, Ecology and Evolutionary Biology, Yale University, USA; Philip R. Pugh, National Oceanography Centre, UK; Christine E. Schnitzler, Monterey Bay Aquarium Research Institute, USA

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