Order Siphonophorae Eschscholtz, 1829

diagnosis

Colonial, nearly always pelagial, freely movable Hydrozoa, composed of medusoid and polypoid zooids that are morphologically and functionally specialized. The integration of the zooids is so strong that the colony attains the character of an individual. Medusae are only occasionally released as medusoids, mostly they remain part of the colony; individual medusoids are not radially symmetric, often bilaterally symmetric or irregular, radial canals curved. Skeleton absent. Sizes 2 mm to 30000 mm, usually 5-20 mm. Cnidome can include desmonemes and stenoteles.

higher classification

Cnidaria, Medusozoa, Hydrozoa, Leptolinae

species

about 199

organization

Siphonophores are extraordinarily complex colonial organisms. The individuals composing  the colony - modified polyps and medusae - are highly polymorphic and adapted to their specific function. The integration of the individual zooids into one system is so much advanced, that we have to consider the siphonophore colony as superorganism that shows all characteristics of an individual.

The above scheme of a physophorid siphonophore explains their anatomy. The siphonophores are best imagined (and likely they were derived like this) as a reversed solitary polyp that forms the axis (stem). The oral pole is below, the aboral pole (the basal region of a polyp in normal orientation) is at the top, where some forms develop an air-filled chamber that acts as an buoy (pneumatophore, for righting reaction). Below the pneumatophore is a region where medusae bells develop. These nectophores allow the colony to move by their propulsions. There are usually several nectophores, their ensemble is called the nectosome. The nectophores offer taxonomically very important traits. Below the nectosome there is a budding region from where the the elements following below are being produced. The whole region below, also called siphosome, is composed of repeated units, the cormidia.

At the top of each cormidium there are one or more bracts, modified medusae that act as protective shields. Another element are the palpons (dactylozooids).  These polyp-like (manubrium-like?) zooids with a filiform tentacle serve as defensive zooids. The nutritive function is provided by the gastrozooids. The have usually one tentacle provided with characteristic side-branches, the tentillae. The tentacles can be extremely expanded, and because there  are great number of goastrozooids per colony, a large number of tentillae with their powerful nematocysts can be distributed in a extensive body of water, and hence  fishing efficiently for prey. The reproductive function is provided by medusoid gonophores that originate from branched structures, the gonodendra. The gonads are produced on the manubrium. The medusoids are NOT liberated like in in other hydrozoans (e. g. Velella). However, the entire cormidium can detach at the end of the stem, thus allowing the colony to disperse reproductive units (e. g. eudoxids of the Calycophorae).

Note: this generalized "bauplan" has been modified extensively in the various orders and families.

some expressions:

Cormidium: an organized group of siphosomal elements, usually including a gastrozooid, tentacle, palpons (in physonects), blastostyle, and bracts.

Eudoxid: reproductive stage of calycophorans that usually becomes detached from polygastric stage.

Hydroecium: ventral cavity of calycophoran nectophore into which the siphosome may be wholly or partly retracted.

Palpon: a cormidial element, probably a reduced gastrozooid, which may have sensory or excretory functions.

Pneumatophore: apical gas-filled float, present in Cystonectae and Physonectae.

Polygastric stage: the complete animal bearing both asexual and reproductive elements.

Somatocyst: prominent extension of the stem in nectophores of the Calycophorae, may contain an oil droplet at its apex.

Classification

Suborder Physonectae

Siphonophorae with an apical pneumatophore and beneath it, a series of nectophores (swimming bells), except in the Athorybiidae which lack nectophores or have a reduced nectophore. Two budding zones present in most species, one under the pneumatophore giving nectophores and the other at the basal end of the nectosome giving the cormidia that form the siphosome. Cormidia of the siphosome with bracts, palpons, gastrozooids and gonophores.

Suborder Calycophorae

Siphonophorae without pneumatophore, highly polymorphic, with a nectosome typically formed by one or two nectophores. A single bract is developed on each cormidial unit, with the exception of the family Hippopodiidae where bracts are absent. Generally the cormidial units are detached successively from the stem and become eudoxid, or sexual stage.

Suborder Cystonectae

Siphonophorae with a budding zone on only one side of the base of the aboral float; without bracts and without swimming bells; tentacles are generally simple, the nematocysts are not concentrated in batteries on the side branches, when these are developed.

Remarks

The genera Velella and Porpita (Family Porpitidae) do not belong to the Siphonophorae, but the Anthomedusae, Capitata.

Although they are highly modified colonies, the Siphonophorae show close affinities with the Anthomedusae (manubrial gonads, desmonemes).

References

Kirkpatrick, P. A., & Pugh, P. R. 1984. Siphonophores and velellids. Synopses  of  the British Fauna (New Series) 29: 1-154.

Mackie, G. O., Pugh, P. R., & Purcell, J. E. 1987. Siphonophore Biology. Advances in Marine Biology 24: 98-262.

Pages, F., & Gili, J.-M. 1992. Siphonophores (Cnidaria, Hydrozoa) of the Benguela Current (southeastern Atlantic). Scientia Marina  56: 65-112.

Totton, A. K. 1965. A synopsis of Siphonophora - British Museum (natural History), London: 230 pp.