Siphonophores are complex polymorphic pelagic gelatinous cnidarians, sometimes known as ‘string jellies’. At present the group comprises about 170 species.
Habitat and life cycle
Siphonophores are marine and typically live offshore below the surface layer. They are fragile and break up easily when disturbed, passing their entire life cycle in the water column, without a benthic stage.
These important pelagic predators feed on zooplankton, particularly copepod crustaceans, and also sometimes on fish larvae and young food fishes. When conditions are suitable, they can reproduce rapidly and may occasionally become the dominant predator.
Most siphonophore species have two or more swimming bells – the nectophores – for propulsion, often on a stem; this stem and its bells form the nectosome.
Attached to the nectosome is the siphosome, or main stem, with numerous iterative units - the cormidia – along its length. A cormidium contains zooids for feeding (gastrozooids), others for buoyancy (bracts) and yet others for reproduction (gonophores).
Gastrozooids are muscular tubes attached to the stem at one end and with a mouth opening at the other end; each gastrozooid bears a single tentacle for prey capture.
Complex stinging batteries arise on side branches of each tentacle, and these contain a very large number of nematocysts which immobilize the prey. The gastrozooid mouth then engulfs the prey and enzymes are secreted onto it from the glandular walls of the gastrozooid.
In one group of siphonophores, the physonects, digestion is completed inside reduced gastrozooids known as palpons.
Species identification and variation
Nectophores are important for species identification. Bracts can also be diagnostic, and the complex stinging batteries on the gastrozooid tentacles of physonects particularly so.
Siphonophore species vary in size from small diphyid calycophorans, such as Muggiaea atlantica with a nectophore only 2-3 mm long, to large physonects with a number of nectophores and a long trailing siphosome up to 30 metres in length.