What are they

Siphonophores are a relatively small and diverse sub-group of 187 species within the Class Hydrozoa, which is in the phylum Cnidaria. Like all cnidarians, siphonophores possess stinging cells with which they capture and subdue prey. Siphonophores possess 10 different types of nematocysts, which is more than any other cnidarian. Siphonophores are unique amongst the planktonic jellyfish as they are a colonial animal composed of many polyps or zooids. In this regard, they are like corals although unlike corals, siphonophores have different types of polyps that have unique functions within the whole animal. For example, some polyps are for movement while others are for prey capture and digestion. The most recognisable siphonophore is the Portuguese man of war which is characterised by a large colourful float that is propelled by the wind as it floats on the ocean surface. It’s distinctive vibrant colour and easy visibility undoubtedly explains why it was the first siphonophore to be described scientifically by Carl Linnaeus in 1758.


Siphonophores are found at various depths and are thought to be one of the most abundant invertebrate animals in our oceans. They are voracious predators consuming a wide variety of prey species, including fish, crustaceans, molluscs and even other jellyfish. Some species bioluminescence and use this ability to capture prey. For example, Erenna species use luminescent lure to mimic a crustacean species, which draws small fish towards its tentacles.

Life history

Most species are dioecious and possess either male or female gonads, however, some are monoecious and possess both. The gonad bearing polyps are called gonophores and usually remain attached to the animal, although in some species (e.g. Muggiaea atlantica) they become independent free swimming polyps. Each fertilized egg turns into a planula which in turn develops into a larval siphonophore. The larval stage of many species is unknown and the development of each animal from egg to mature colony is dependent on food availability and temperature.

By Emily Mangan