[5] Zooids are multicellular units that develop from a single fertilized egg and combine to create functional colonies able to reproduce, digest, float, maintain body positioning, and use jet propulsion to move.

[8][9] Siphonophores are colonial hydrozoans that do not exhibit alternation of generations but instead reproduce asexually through a budding process.

[7] Zooids typically have special functions, and thus assume specific spatial patterns along the stem.

[7] Siphonophores typically exhibit one of three standard body plans matching the suborders: Cystonectae, Physonectae, and Calycophorae.

[11] They use a pneumatophore, a gas-filled float, on their anterior end and drift at the surface of the water or stay afloat in the deep sea.

[11] Smaller, warm-water siphonophores typically live in the epipelagic zone and use their tentacles to capture zooplankton and copepods.

[11] Larger siphonophores live in deeper waters, as they are generally longer and more fragile and must avoid strong currents.

A siphonophore is a complex aggregate colony made up of many nectophores, which are clonal individuals that form by budding and are genetically identical.

The smaller individuals are concentrated towards the top of the siphonophore, and their function is turning and adjusting the orientation of the colony.

[17] Every individual is key to the movement of the aggregate colony, and understanding their organization may allow us to make advances in our own multi-jet propulsion vehicles.

[17] This means that even if some individual nectophores become functionally compromised, their role is bypassed so the colony as a whole is not negatively affected.

[17] The velum, a thin band of tissue surrounding the opening of the jet, also plays a role in swimming patterns, shown specifically through research done on the previous mentioned species N.

[18] The siphonophore Namonia bijuga also practices diel vertical migration, as it remains in the deep-sea during the day but rises during the night.

[19] Similar to many other organisms in the phylum of Cnidaria, many siphonophore species exhibit nematocyst stinging capsules on branches of their tentacles called tentilla.

[5][20][21] The nematocysts then shoot millions[19] of paralyzing, and sometimes fatal, toxin molecules at the trapped prey which is then transferred to the proper location for digestion.

[5] Some species of siphonophores use aggressive mimicry by using bioluminescent light so the prey cannot properly identify the predator.

[21] Heteronemes are the largest nematocysts and are spines on a shaft close to tubules attached to the center of the siphonophore.

In addition, siphonophores in a group denoted Erenna have the ability to generate bioluminescence and red fluorescence while its tentilla twitches in a way to mimic motions of small crustaceans and copepods.

[8] Siphonophores of the deep-sea genus Erenna (found at depths between 1,600–2,300 metres or 5,200–7,500 feet) are thought to use their bioluminescent capability for offense too, as a lure to attract fish.

[11] The species Resomia ornicephala uses their green and blue fluorescing tentilla to attract krill, helping them to outcompete other organisms that are hunting for the same prey.

[3] The phylogenetic relationships of siphonophores have been of great interest due to the high variability of the organization of their polyp colonies and medusae.

Early analysis divided siphonophores into three main subgroups based on the presence or the absence of two different traits: swimming bells (nectophores) and floats (pneumatophores).

[15] Eukaryotic nuclear small subunit ribosomal gene 18S, eukaryotic mitochondrial large subunit ribosomal gene 16S, and transcriptome analyses further support the phylogenetic division of Siphonophorae into two main clades: Cystonectae and Codonophora.

Suborders within Codonophora include Physonectae (consisting of the clades Calycophorae and Euphysonectae), Pyrostephidae, and Apolemiidae.

[11] On April 6, 2020, the Schmidt Ocean Institute announced the discovery of a giant Apolemia siphonophore in submarine canyons near Ningaloo Coast, measuring 15 m (49 ft) diameter with a ring approximately 47 m (154 ft) long, possibly the largest siphonophore, and longest animal, ever recorded.

[11] Ernst Haeckel described numerous siphonophores, and several plates from his Kunstformen der Natur (1904) depict members of the taxon:[27]