Rhizophysa filiformis, copyright Dubas.

Belongs within: Cnidaria.
Contains: Staurozoa, Cubozoa, Scyphozoa, Limnomedusae, Narcomedusae, Trachymedusae, Halammohydra, Thecata, Capitata, Filifera, Codonophora.

The Hydrozoa are cnidarians ancestrally exhibiting a life-cycle alternating between a medusoid and a polypoid phase, though one or the other of these phases has been lost in a number of derived subgroups. They may be divided between the Trachylina, in which statocysts are of ecto-endodermal origin, and Hydroidolina, whose statocysts (if present) are of ectodermal origin (Daly et al. 2007).

E pluribus unum
Published 13 August 2008

For many people, the name “Ernst Haeckel” is most associated with slightly dodgy illustrations of vertebrate embryos that have doomed his memory to be quote-mined by people with an agenda to push for all eternity. For others, though, the epitome of Haeckel’s work lies in the many spectacular illustrations of invertebrates and protozoa he produced in such works as his reports on the biological material collected by the HMS Challenger expedition, and his 1899–1904 Kunstformen der Natur (“Artforms in Nature”). With their awe-inspiring detail and spectacular presentation, the plates he produced are more than just technical illustrations, they are true works of art. Perhaps among the greatest of his productions were the plates of siphonophores, an example of which is shown above. Baroque tentacled horrors, they loom out of the page threatening to engulf Dunwich. I wouldn’t be able to tell you whether Lovecraft had ever seen one of Haeckel’s illustrations to inspire him in his descriptions of the twisted hybrid offspring of Yog-Sothoth, but the resemblance is uncanny.

Siphonophores are planktonic cnidarians (the group that includes corals and jellyfish), distantly related to hydras (a good online reference on siphonophores has been put together by Casey Dunn). The most familiar member of the group is Physalia, the Portuguese man of war (so-called because of a supposed resemblance to that form of ship), but on the whole Physalia is not very typical of the order. All siphonophores are colonial, in their way—incomplete budding leads to the production of a colony of generally large numbers of metabolically interconnected zooids that are developmentally homologous to the more independent polyps of other cnidarians. However, the individual zooids of siphonophores are each highly specialised for separate divided functions such as feeding, reproduction or motility, meaning that siphonophore zooids are incapable of living independently of the colony. Perhaps more than any other group of organisms, the siphonophores challenge the question of what defines an individual or a colony, which has led to their description as “superorganisms”.

Siphonophores have been divided into three main groups, the Cystonectae, Physonectae and Calycophorae, but the phylogenetic analysis of Dunn et al. (2005) found calycophores to be nested within physonects, the two together forming a clade they named the Codonophora. Cystonects (which include Physalia) form the sister-group to the codonophores, and share a colony morphology characterised by a division between a terminal pneumatophore (float) and the siphosome, the region of the colony containing feeding and reproductive zooids coming off a central stalk (in Physalia the central stalk is relatively short, but other siphonophores will have exceedingly long colonies). In the “physonects”, the pneumatophore and siphosome are separated by the nectosome, a region of generally bell-shaped zooids called nectophores specialised for motility. In the calycophores, the pneumatophore has been lost and the colony is composed of the nectosome and siphosome. The illustration at the top of the post represents the physonect Physophora hydrostatica – the pneumatophore is the bulb-shaped structure at the top, with the zooids of the nectosome between the pneumatophore and the tentacle-like structures representing the top of the siphosome. These latter structures are not actually tentacles (the tentacles are the filaments radiating from the siphosome) but palpons, zooids whose function remains unknown but has been suggested to be related to excretion or defense. Underneath the palpons are the gonophores, the reproductive zooids, with separate male and female forms (males and females may both be present in a single colony, or there may be colonies of separate sexes). The large funnels like the horn of an old gramophone are gastrozooids, the feeding individuals. The clubbed side-branches on the trailing tentacles are tentilla, and contain concentrations of nematocysts for capturing prey. Most codonophores (but not cystonects) also have shield-like gelatinous bracts protecting the siphosome. Cystonects also have structures called gonodendra, which are concentrations of gonophores, palpons and also specialised nectophores that can propel a detached gonodendron through the water. Many codonophores are bioluminescent—the bracts may contain luminescent cells, and at least one member of the genus Erenna has flashing red tentilla that probably function as lures. The Physonecta illustrated above has only one iteration of the siphosome, but in other forms (such as the one illustrated below in another Haeckel plate) the clusters of palpons, gastrozooids and gonophores may form iterative elements that repeat continuously down the growing stem.

Despite what can only be described as their inherent coolness, siphonophores as a group are poorly known. Like other planktonic cnidarians, their gelatinous structure makes them quite frail and difficult to collect. The entire colony may be only loosely connected by the slender stem, such as in the example just above. Some siphonophores reach spectacularly large sizes—species of Apolemia may be more than 30 m in length, yet only a few centimetres in diameter. Attempts to net such specimens using conventional means would be lucky to retrieve anything more than disassociated mush.

Systematics of Medusozoa
<==Medusozoa [Acraspedae, Aequoreae, Medusinae, Tetrazoa]
    |  |--CubozoaOH17
    |  `--ScyphozoaOH17
    `--Hydrozoa [Craspedotae, Hydroida, Hydroidomedusae, Hydromedusae]OH17
         |  i. s.: Rhabdohydra [Rhabdohydridae]BU02
         |           `--R. tridens Kozłowski 1959NS93
         |         Chaunograptus Hall 1879 [Chaunograptidae]M14
         |           `--*C. novellus (Hall 1883) [=Dendrograptus (Chaunograptus) novellus]B70
         |         Conchopeltidae [Tetradialomorpha]IF02
         |           |--Conchopeltis Walcott 1879KC60
         |           |    `--C. alternata Walcott 1876NS93
         |           `--Conomedusites Glaessner & Wade 1966IF02, G79
         |                `--*C. lobatus Glaessner & Wade 1966G79
         |         Plumaria undulataB79
         |         StephanidiumH15
         |         PerigonimusBK77
         |           |--P. abyssiN79
         |           `--P. cidaridisBK77
         |         CuspidellaBK77
         |         DisjectoporidaeRLS04
         |           |--Disjectopora japonica Yabe & Sugiyama 1935RB93
         |           |--Balatonia kochi Vinassa 1908RB93
         |           `--Radiotrabeculopora Fan, Rigby & Zhang 1991 (see below for synonymy)RLS04
         |                |--*R. xiangboensis Fan, Rigby & Zhang 1991RLS04
         |                `--R. reticulata Fan, Rigby & Zhang 1991RLS04
         |         Cassiniastraea Volz 1896RS01
         |         Parkeria sphaericaS02
         |         TurrisF27
         |           |--T. neglectus Lesson 1837F27
         |           `--T. pileataD56
         |         PhialidiumB26
         |           |--P. hemisphericumD56
         |           `--P. languidumB26
         |         Staurophora mertensii [incl. S. falklandica Browne 1902]B26
         |         CambrohydraEL11
         |         Keramosphaerina Stache 1913 [=Bradya Stache 1889 nec Boeck 1872 nec Carter 1877]LT64
         |           `--*K. tergestina (Stache 1889) [=*Bradya tergestina]LT64
         |         Rhaptapagis cantacuzeneiCS-P86
         |         Ceratella Gray 1868 non Hook. f. 1844 (ICBN)B86
         |           `--C. fusca Gray 1868B86
         |         Dehitella atrorubensB86
         |         Craspedonites Haeckel 1865 [=Craspedites (l. c.) non Pavlow 1892]WCH96
         |         Arctapodema australis (Vanhöffen 1902)BP00
         |         Sminthea eurygaster Gegenbauer 1857BP00
         |         AmphinemaBP00
         |           |--A. rubra (Kramp 1957)BP00
         |           `--A. rugosumPO99
         |         Ventromma halecioidesPP64
         |         Monotheca posidoniaePP64
         |         Polyplumaria secundariaPP64
         |         Mazohydra megabertha Schram & Nitecki 1975NS93
         |         Clavatella proliferaA64
         |         Lyriope catharinensisA64
         |         Garveia nutansA64
         |         AntennulariaA64
         |           |--A. antenninaA64
         |           `--A. indivisa [=Sertularia (Antennularia) indivisa]G20
         |         Dicoryne confertaA64
         |         Parypha croceaA64
         |         Tima bairdiiA64
         |         Mesonema coerulescens [incl. Stomobrachium mirabile]A64
         |         Thaumantias inconspicuaA64
         |         AeginetaA64
         |           |--A. gemmiferaA64
         |           `--A. prolifera [=Cunina prolifera]A64
         |         Stauridia productaA64
         |         Archaeodendrum Obut 1974M14
         |         Dyadodendrum Sennikov 1998M14
         |         Plumalina Hall 1858M14
         |         Coryna Bosc 1802BP02
         |--Trachylina [Trachylinida]DB07
         |    |  i. s.: Velumbrella czarnockii Stasinska 1960NS93
         |    |--LimnomedusaeDB07
         |    |--NarcomedusaeDB07
         |    |--TrachymedusaeDB07
         |    `--Actinulida [Halammohydrida, Halammohydrina]DB07
         |         |--HalammohydraDB07
         |         `--Otohydra Swedmark & Teissier 1958CS-P86 [OtohydridaeDB07]
         |              |--O. tremulans Lacassagne 1973CS-P86
         |              `--O. vagans Swedmark & Teissier 1958CS-P86
              |--Anthoathecata [Anthomedusae, Athecata]DB07
              |    |  i. s.: Gemmaria McCrady 1857H04, RL03
              |    |           `--G. sagittariaH04
              |    |         Tiara pileataH04
              |    |         Stomotoca pterophyllaH04
              |    |         Thamnostylus dinemaH04
              |    |--CapitataP79
              |    `--FiliferaP79
              `--Siphonophorae [Physophorae, Physophorina, Siphonantha, Siphonophora]DB07
                   |  i. s.: DiphyopsisB26
                   |           |--D. disparB26
                   |           `--D. majorB26
                   |         Anthophysa formosaB26
                   |         Arethusa caraveliaG20
                   |         Dimophyes arcticaPO99
                        |--Epibulia ritterianaH04
                        |--Cystalia monogastricaH04
                        |--Salacia polygastricaH04
                        |--Physalia [Physaliidae]DPH05
                        |    |--P. pelagicaC45
                        |    `--P. physalis (Linnaeus 1758) [=Holothuria physalis]DB07
                        `--Rhizophysa [Rhizophysidae]DPH05
                             |--R. eysenhardtiDPH05
                             `--R. filiformisDPH05
Medusozoa incertae sedis:
    |--E. (Eudora) undulosaG20
    `--E. (Berenice) euchromaG20
  Ephyra simplexG20
  Callirhoe basterianaG20
    |--F. bunogasterR26
    `--F. lineolataR26
    |--M. crucigeraG20
    |--M. fasciculataR26
    `--M. quadriloba [incl. Brachionus tubifex]G20
  Berenix thalassinaG90
  Rhacostoma bunogasterG90
  Nubiella mitraG90

Radiotrabeculopora Fan, Rigby & Zhang 1991 [incl. Flabellisclera Wu 1991, Fungispongia Wu 1991, Gigantosclera Wu 1991, Tubulispongia Wu 1991]RLS04

*Type species of generic name indicated


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[CS-P86] Clausen, C., & L. v. Salvini-Plawen. 1986. Cnidaria. In: Botosaneanu, L. (ed.) Stygofauna Mundi: A Faunistic, Distributional, and Ecological Synthesis of the World Fauna inhabiting Subterranean Waters (including the Marine Interstitial) pp. 33–42. E. J. Brill/Dr W. Backhuys: Leiden.

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[DB07] Daly, M., M. R. Brugler, P. Cartwright, A. G. Collins, M. N. Dawson, D. G. Fautin, S. C. France, C. S. McFadden, D. M. Opresko, E. Rodriguez, S. L. Romano & J. L. Stake. 2007. The phylum Cnidaria: a review of phylogenetic patterns and diversity 300 years after Linnaeus. Zootaxa 1668: 127-182.

[D56] Dawes, B. 1956. The Trematoda with special reference to British and other European forms. University Press: Cambridge.

[DPH05] Dunn, C. W., P. R. Pugh & S. H. D. Haddock. 2005. Molecular phylogenetics of the Siphonophora (Cnidaria), with implications for the evolution of functional specialization. Systematic Biology 54 (6): 916–935.

[EL11] Erwin, D. H., M. Laflamme, S. M. Tweedt, E. A. Sperling, D. Pisani & K. J. Peterson. 2011. The Cambrian conundrum: early divergence and later ecological success in the early history of animals. Science 334: 1091–1097.

[F27] Finlay, H. J. 1927. New specific names for austral Mollusca. Transactions and Proceedings of the New Zealand Institute 57: 488–533.

[G79] Glaessner, M. F. 1979. Precambrian. In: Robison, R. A., & C. Teichert (eds) Treatise on Invertebrate Paleontology pt A. Introduction. Fossilisation (Taphonomy), Biogeography and Biostratigraphy pp. A79–A118. The Geological Society of America, Inc.: Boulder (Colorado), and The University of Kansas: Lawrence (Kansas).

[G20] Goldfuss, G. A. 1820. Handbuch der Naturgeschichte vol. 3. Handbuch der Zoologie pt 1. Johann Leonhard Schrag: Nürnberg.

[G90] Goy, J. 1990. Preliminary observations on the zooplankton of Shark Bay, Western Australia, with emphasis on medusae and description of a new rhizostome jellyfish (Cnidaria, Scyphozoa). In: Berry, P. F., S. D. Bradshaw & B. R. Wilson (eds) Research in Shark Bay: Report of the France-Australe Bicentenary Expedition Committee pp. 107–113. Western Australian Museum.

[H04] Haeckel, E. 1899–1904. Kunstformen der Natur. Bibliographisches Institut: Leipzig und Wien.

[H15] Harmer, S. F. 1915. The Polyzoa of the Siboga Expedition—Part I. Entoprocta, Ctenostomata and Cyclostomata. E. J. Brill: Leyden.

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[KC60] Knight, J. B., L. R. Cox, A. M. Keen, R. L. Batten, E. L. Yochelson & R. Robertson. 1960. Gastropoda: systematic descriptions. In: Moore, R. C. (ed.) Treatise on Invertebrate Paleontology pt I. Mollusca 1: Mollusca—General Features, Scaphopoda, Amphineura, Monoplacophora, Gastropoda—General Features, Archaeogastropoda and some (mainly Paleozoic) Caenogastropoda and Opisthobranchia pp. I169–I331. Geological Society of America, and University of Kansas Press.

[LT64] Loeblich, A. R., Jr & H. Tappan. 1964. Sarcodina: chiefly “thecamoebians” and Foraminiferida. In Moore, R. C. (ed.) Treatise on Invertebrate Paleontology pt C. Protista 2 vol. 2. The Geological Society of America and The University of Kansas Press.

[M14] Maletz, J. 2014. The classification of the Pterobranchia (Cephalodiscida and Graptolithina). Bulletin of Geosciences 89 (3): 477–540.

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[NS93] Nudds, J. R., & J. J. Sepkoski Jr. 1993. Coelenterata. In: Benton, M. J. (ed.) The Fossil Record 2 pp. 101–124. Chapman & Hall: London.

[OH17] Ou, Q., J. Han, Z. Zhang, D. Shu, G. Sun & G. Mayer. 2017. Three Cambrian fossils assembled into an extinct body plan of cnidarian affinity. Proceedings of the National Academy of Sciences of the USA 114 (33): 8835–8840.

[PO99] Pagès, F., & C. Orejas. 1999. Medusae, siphonophores and ctenophores of the Magellan region. Scientia Marina 63 (Suppl. 1): 51–57.

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[P79] Petersen, K. W. 1979. Development of coloniality in Hydrozoa. In: Larwood, G., & B. R. Rosen (eds) Biology and Systematics of Colonial Organisms pp. 105–139. Academic Press: London.

[RB93] Rigby, J. K., G. E. Budd, R. A. Wood & F. Debrenne. 1993. Porifera. In: Benton, M. J. (ed.) The Fossil Record 2 pp. 71–99. Chapman & Hall: London.

[RLS04] Rigby, J. K., G. A. Linder & C. H. Stevens. 2004. A new occurrence of the “hydrozoan” Radiotrabeculopora reticulata Fan, Rigby, and Zhang, 1991 in the Permian of California. Journal of Paleontology 78 (2): 410–413.

[R26] Risso, A. 1826. Histoire naturelle des principales productions de l’Europe méridionale et particulièrement de celles des environs de Nice et des Alpes maritimes vol. 5. F.-G. Levrault: Paris.

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[S02] Scott, R. W. 2002. Albian caprinid rudists from Texas re-evaluated. Journal of Paleontology 76 (3): 408–423.

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