Siphonophores are complex polymorphic pelagic gelatinous cnidarians, sometimes known as ‘string jellies’. At present the group comprises about 170 species.
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.
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.
Mapstone, G. M. 2009. Siphonophora (Cnidaria, Hydrozoa) of Canadian Pacific waters. NRC Research Press, Ottawa, Ontario, Canada. 302 pp including 65 figs. ISBN 978-0-660-19843-9.
Hiscock, K., Mapstone, G.M., Conway, D.V.P. and Halliday, N. 2010. Occurrence of the physonect siphonophore Apolemia uvaria off Plymouth and in south-west England. Marine Biodiversity Records 3: 1-4. doi:10.1017/S1755267210000205.
Mapstone, G. M. 2005. Re-description of Rosacea cymbiformis, a prayine siphonophore (from the Mediterranean Sea), with comments on nectophore designation and bract orientation. Journal of the Marine Biological Association of the United Kingdom 85: 709-721, 5 figs.
Mapstone, G. M. and Pugh, P. R. 2004. Case 3309. Rosacea Quoy & Gaimard, 1827: proposed conservation of usage (Cnidaria, Siphonophora); Desmophyes annectens Haeckel, 1888 and Rosacea plicata Bigelow, 1911: proposed conservation. Bulletin of Zoological Nomenclature 61(3): 149-153.
Mapstone, G. M. 2004. First full description of the large physonect siphonophore Halistemma amphytridis (Lesueur & Petit, 1807). Hydrobiologia 530/531: 231-240, 3 figs.
Mapstone, G. M. 2003. Redescriptions of two physonect siphonophores, Apolemia uvaria (Lesueur, 1815) and Tottonia contorta Margulis, 1976, with comments on a third species Ramosia vitiazi Stepanjants, 1967 (Cnidaria: Hydrozoa: Apolemiidae). Systematics and Biodiversity 1(2): 181-212, 15 figs.
Mapstone, G. M. 1997. Bargmannia lata, an undescribed species of physonect siphonophore (Cnidaria, Hydrozoa) from Canadian Pacific waters. Pp. 141-147, 3 figs. In Commemorative volume for the 80th birthday of Willem Vervoort. Edited by: J.C.den Hartog, A.C. van Bruggen, P.F.S. Cornelius and L.P. van Ofwegen. Zoologische Verhandelingen. Leiden: 323: i-xii, 1-448.
Mapstone, G.M. and Arai, M.N. 1992. Abundance and vertical distribution of siphonophores (Cnidaria) from the central Strait of Georgia, British Columbia, during spring and summer. Contribution to Natural Science 15: 1-8.
Mapstone, G. M. and John C. Ljubenkov. New observations on Dromalia alexandri Bigelow, 1911, a rhodaliid physonect siphonophore from southern Californian waters. Presented as a talk at the 7th Hydrozoan Workshop, Porto Cesareo, Italy on 15th September 2010. For submission to a special volume of Hydrobiologia in February 2011.