Siphonophores may be some of the most unique and fascinating creatures residing in the mesopelagic zone. Instead of being a single organism, they are actually colonial, super-organisms consisting of several individuals that function together as one. These individual units are known as zooids, and are specialized to achieve their tasks. Some zooids are developed for reproduction, some for locomotion and some for catching prey and digesting food. Together, these zooids form the whole siphonophore, usually seen as long and transparent bodies floating in the mesopelagic environment.

3D model of the “Galaxy Siphonophore”, often observed with its stinging tentacles spread out like a spiderweb, to catch passing prey.

As they are gelatinous pelagic animals, they are often mistaken for jellyfish. Although jellyfish and siphonophores belong to the phylum Cnidaria, they belong to different systematic orders. All siphonophores are predators and carnivores. They are usually equipped with stinging cells (nematocysts), akin to their jellyfish relatives, which are used to stun and kill prey. The prey is then brought towards the siphonophores body, where specialized gastrozooids will be able to digest the body and absorb its nutrients. There are two main predatory tactics used by siphonophores – some sit and wait with their tentacles extended (usually large species), while others take a more active approach to swimming around to attempt catching them (usually small species). Some species have also been discovered to use bioluminescense as a lure to attract prey, similarly to deep-sea anglerfish!

The Portugese man ‘o war, a siphonophore often misidentified as a jellyfish! It has a large, gas-filled “pneumatophore”, allowing the siphonophore to keep itself afloat at the sea surface, and stinging cells to catch and kill fish. Other siphonophores use their gas-filled bladder to adjust their bouyancy and movement in the deep sea. (Credit: NOAA)

The diets of siphonophores are varied, usually consisting copepods, other small crustaceans, and small fish. Similarly to the mesopelagic community, many siphonophores perform diel vertical migration, participating in the largest movement of biomass in the whole world. They thus follows their prey towards the sea surface at night, and sink back to the dark depths during daytime. Due to their gas-filled pneumatophores, they are also easily seen by acoustics observation – akin to fish with gas-filled bladders.

 

Siphonophore in the deep sea. Credit: Adobe Stock, Pipehorse

Parts of a siphonophore (Agalma sp.) seen under the microscope. Credit: Kristian Fjeld, SINTEF Ocean

 

Siphonophores can range from lenghts of over 40 meters (Galaxy siphonophore), down to 10-20 cm (left image), all the way to having small bodies only seen in detail in a microscope (right image). With their unique lifestyle including a large colony of individuals helping it live in the deep – the siphonophore is a truly fascinating and extraordinary inhabitant of the ocean twilight zone.

 

Are you interested in learning more about life in the deep sea mesopelagic zone? Take a look at our latest video, a small exhibition of the incredible twilight zone!