Verrill, 1865
Editor's note: two of the three chapters (Nr. 8 and Nr. 9) of Zooplankton of the South Atlantic Ocean involved with cnidarians are not concordant with each other as to the classification of taxa at class level and higher. Below an overview of the classifications as provided by the concerned authors.
¥ Classification as in Chapter 8. Hydromedusae (by Jean Bouillon).
The phylum Cnidaria can be classified as follows (Bouillon, 1985a, 1995; Bouillon et al., 1992):
Phylum Cnidaria Verrill, 1865
Subphylum Anthozoaria Petersen, 1979 (devoid of medusa phase)
Superclass Anthozoa Ehrenberg, 1833
Subphylum Medusozoa Petersen, 1979 (medusa stage important in the life cycle, although it may be secondarily lost)
Superclass Scyphozoa Goette, 1887
Superclass Cubozoa Werner, 1973
Superclass Hydrozoa Owen, 1843
Class Siphonophora Eschscholtz, 1829
Class Hydroidomedusae Bouillon, Boero, Cicogna, Gili and Hughes, 1992
Subclass Anthomedusae Haeckel, 1879
Order Filifera Kühn, 1913
Suborder Margelina Haeckel, 1879
Family Bougainvilliidae Lütken, 1850
Family Clavidae McCrady, 1859
Family Cytaeididae L. Agassiz, 1862
Family Eucodoniidae Schuchert, 1996
Family Hydractiniidae L. Agassiz, 1862
Family Rathkeidae Russell, 1953
Suborder Tiarida Haeckel, 1879
Family Calycopsidae Bigelow, 1913
Family Niobiidae Petersen, 1979
Family Pandeidae Haeckel, 1879
Family Protiaridae Haeckel, 1879
Family Russelliidae Kramp, 1957
Order Capitata Khün, 1913
Suborder Moerisiida Poche, 1914
Family Moerisiidae Poche, 1914
Family Polyorchidae A. Agassiz, 1862
Suborder Tubulariida, Fleming, 1828
Family Corynidae Johnston, 1836
Family Corymorphidae Allman, 1872
Family Eleutheriidae Russell, 1953
Family Euphysidae Haeckel, 1879
Family Margelopsidae Uchida, 1927
Family Tubulariidae Fleming, 1828
Suborder Zancleida Russell, 1953
Family Porpitidae Goldfuss, 1818
Family Zancleidae Russell, 1953
Family Zancleopsidae Bouillon, 1978
Subclass Leptomedusae Haeckel, 1886
Family Aequoreidae Eschscholtz, 1829
Family Blackfordiidae Bouillon, 1984
Family Cirrholoveniidae Bouillon, 1984
Family Eirenidae Haeckel, 1879
Family Eucheilotidae Bouillon, 1984
Family Laodiceidae Agassiz, 1862
Family Lovenellidae Russell, 1953
Family Malagazziidae Bouillon, 1984
Family Mitrocomidae Haeckel, 1879 (part); Torrey, 1909
Family Orchistomidae Bouillon, 1984
Family Phialellidae Russell, 1953
Family Tiarannidae Russell, 1940
Family Tiaropsidae Boero, Bouillon and Danovaro, 1987
Order Proboscoida Broch, 1910
Family Campanulariidae Jonhston, 1836
Subclass Laingiomedusae Bouillon, 1978
Family Laingiidae Bouillon, 1978
Subclass Limnomedusae Kramp, 1938
Family Olindiidae Haeckel, 1879
Family Proboscidactylidae Hand and Hendrickson, 1950
Subclass Narcomedusae Haeckel, 1879
Family Aeginidae Gegenbaur, 1857, emend. Maas, 1904
Family Cuninidae Bigelow, 1913
Family Solmarisidae Haeckel, 1879
Subclass Trachymedusae Haeckel, 1866
Family Geryoniidae Eschscholtz, 1829
Family Halicreatidae Fewkes, 1886
Family Ptychogastriidae Mayer, 1910
Family Rhopalonematidae Russell, 1953
¥ Classification as in Chapter 9. Siphonophorae (by Phil Pugh).
The classification adopted here is basically that used by Totton (1965). That taxonomy is well established, although a few more recent authors (e.g. Alvariño, 1981) still use a few outmoded names. Other useful references include Bigelow (1911), Kirkpatrick and Pugh (1984) and Totton (1954). The only change that has been adopted here is the re-establishment of the Siphonophorae as a subclass of the class Hydrozoa (see Bouillon et al., 1992), and the consequent raising of the 3 old suborders to order status. Totton (1965) recognised approximately 130 species and since then about 70 other species and subspecies either have been described or resurrected. However, it is certain that several of Totton's "doubtful" species, and many of the subsequent new ones, are not valid. This is because many aberrant forms have been described as new species, or the reasons given for distinguishing the new material from extant species are insufficient. Nonetheless, it is clear from submersible collections that there are still many species yet to be described.
Phylum Cnidaria Verril, 1865
Class Hydrozoa Owen, 1843
Subclass Siphonophorae Eschscholtz, 1829
Order Cystonectae Haeckel, 1887
Family Physaliidae Brandt, 1835
Family Rhizophysidae Brandt, 1825
Order Physonectae Haeckel 1888
Family Apolemidae Huxley, 1859
Family Agalmatidae Brandt, 1835
Family Pyrostephidae Moser, 1925
Family Physophoridae Eschscholtz, 1829
Family Athorybiidae Huxley, 1859
Family Rhodaliidae Haeckel, 1888
Family Forskaliidae Haeckel, 1888
Order Calycophorae Leuckart, 1854
Family Prayidae Kölliker, 1853
Subfamily Amphicaryoninae Chun, 1888
Subfamily Prayinae Haeckel, 1888
Subfamily Nectopyramidinae Bigelow, 1911
Family Hippopodiidae Kölliker, 1853
Family Diphyidae Quoy and Gaimard, 1827
Subfamily Sulculeolariinae Totton, 1954
Subfamily Diphyinae Moser, 1925
Subfamily Giliinae Pugh and Pagès, 1995
Family Clausophyidae Totton, 1954
Family Sphaeronectidae Huxley, 1859
Family Abylidae L. Agassiz, 1862
Subfamily Abylinae L. Agassiz, 1862
Subfamily Abylopsinae Totton, 1954.
The only family not dealt with herein is the Rhodaliidae whose species are benthic.
¥ Classification as in Chapter 10. Cubomedusae and Scyphomedusae (by Hermes Mianzan and Paul Cornelius).
The following classification is largely based on the work of Kramp (1961). The position of the Cubomedusae remains problematic. Some authorities regard the group as equivalent in rank to the Scyphozoa, calling them Cubozoa, while others include them within the Scyphozoa, as the Cubomedusae. Arguments are balanced, and the ranks given to the taxa of the Hydrozoa have also to be considered. Kramp (1961) and several authors before and since have recognized newly proposed groupings within the Rhizostomeae at various ranks above family level. The validity of some has been debated, and rather than include a lengthy discussion we prefer to mention none of the higher divisions here. Genera in bold are those Cubozoa or Scyphozoa reported from South Atlantic waters.
Phylum Cnidaria Hatschek, 1888
Class Cubozoa Werner, 1975
Order Cubomedusae Haeckel, 1877
Family Carybdeidae Gegenbaur, 1856
Genus Carybdea Péron and Lesueur, 1810
Genus Tamoya Müller, 1859
Genus Tripedalia Conant, 1897
Genus Carukia Southcott, 1967
Family Chirodropidae Haeckel, 1892
Genus Chirodropus Haeckel, 1880
Genus Chironex Southcott, 1956;
Genus Chiropsalmus L. Agassiz, 1862
Class Scyphozoa Goette, 1887
Subclass Scyphomedusae Lankester, 1877
Order Coronatae Vanhöffen, 1892
Family Atollidae Bigelow, 1913
Genus Atolla Haeckel, 1880
Family Atorellidae Vanhöffen, 1902
Genus Atorella Vanhöffen, 1902
Family Linuchidae Haeckel, 1879
Genus Linuche Eschscholtz, 1829
Family Nausithoidae Bigelow, 1913
Genus Nausithoe Kölliker, 1853
Genus Palephyra Haeckel, 1880
Family Paraphyllinidae Maas, 1903
Genus Paraphyllina Maas, 1903
Family Periphyllidae Haeckel, 1880
Genus Nauphantopsis Fewkes, 1885
Genus Pericolpa Haeckel, 1880
Genus Periphylla Haeckel, 1880
Genus Periphyllopsis Vanhöffen, 1900
Order Semaeostomeae L. Agassiz, 1862
Family Pelagiidae Gegenbaur, 1856
Genus Chrysaora Péron and Lesueur, 1810
Genus Pelagia Péron and Lesueur, 1810
Genus Sanderia Goette, 1886
Family Cyaneidae L. Agassiz, 1862
Genus Cyanea Péron and Lesueur, 1810
Genus Desmonema L. Agassiz, 1862
Genus Drymonema Haeckel, 1880
Family Ulmaridae Haeckel, 1879
Subfamily Aureliinae L. Agassiz, 1862
Genus Aurelia Péron and Lesueur, 1810
Subfamily Deepstariinae Larson, 1986
Genus Deepstaria Russell, 1967
Subfamily Poraliinae Larson, 1986
Genus Poralia Vanhöffen, 1902
Subfamily Sthenoniinae Mayer, 1910
Genus Phacellophora Brandt, 1835
Subfamily Ulmarinae Kramp, 1961
Genus Diplulmaris Maas, 1908
Genus Discomedusa Claus, 1877
Subfamily Stygiomedusinae Russell, 1959
Genus Stygiomedusa Russell, 1959
Order Rhizostomeae Cuvier, 1799
Suborder Kolpophorae Stiasny, 1921
Family Cassiopeidae L. Agassiz, 1862
Genus Cassiopea Péron and Lesueur, 1810
Family Cepheidae L. Agassiz, 1862
Genus Cephea Péron and Lesueur, 1810
Genus Cotylorhiza L. Agassiz, 1862
Family Mastigiidae Stiasny, 1921
Genus Mastigias L. Agassiz, 1862
Genus Mastigietta Stiasny, 1921
Genus Phyllorhiza L. Agassiz, 1862
GenusVersuriga Kramp, 1961
GenusThysanostoma L. Agassiz, 1862
Suborder Daktyliophorae Stiasny, 1921
Superfamily Inscapulatae Stiasny, 1921
Family Lychnorhizidae Haeckel, 1880
Genus Anomalorhiza Light, 1921
Genus Lychnorhiza Haeckel, 18
GenusPseudorhiza von Lendenfeld, 1882
Family Catostylidae Gegenbaur, 1857
Genus Acromitoides Stiasny, 1921
GenusAcromitus Light, 1914
Genus Catostylus L. Agassiz, 1862
Genus Crambione Maas, 1903
Genus Crambionella Stiasny, 1921
Family Lobonematidae Stiasny, 1921
Genus Lobonema Mayer, 1910
Superfamily Scapulatae Stiasny, 1921
Family Rhizostomatidae Cuvier, 1799
Genus Eupilema Haeckel, 1880
Genus Rhizostoma Cuvier, 1799
Genus Rhopilema Haeckel, 1880
Family Stomolophidae Haeckel, 1880
Genus Stomolophus L. Agassiz, 1862
Incertae sedis
Genus Tessera Haeckel, 1880
Genus Tetraplatia Busch, 1851
(To complete all classifications ETI has added the Kingdom and the Phyla of all the different taxa treated on this DVD-ROM without higher classification descriptions. Texts from Lynn Margulis and Karlene V. Schwartz, Five Kingdoms. CD-ROM Copyright 2002 ETI / Freeman & Co Publishers)
Sea anemones, jellyfish, hydras, and corals are among the 9400 species of Cnidaria. These radially symmetrical invertebrates are the least morphologically complex members of Subkingdom Eumetazoa, the true metazoa. The term coelenterate is used only in reference to both the Cnidaria and the Ctenophora phyla; it is no longer used as a synonym for Cnidaria. All cnidarians are aquatic and nearly all are marine. The four classes of cnidarians are the Hydrozoa (hydras), the Scyphozoa (true jellyfish), the Anthozoa (most corals and sea anemones), and the Cubozoa (sea wasps and several other genera named for their cube-shaped bodies). Cnidarian tentacles and oral arms are replete with stinging cells called cnidoblasts, each containing an intracellular nematocyst, unique to this phylum.
Cnidarians have two basic body patterns—polyp and medusa; because they have more than a single body pattern, cnidarians are said to be polymorphic. Polyps, such as Hydra, are cylindrical animals that live mouth upward Some polyps are sedentary; others glide or somersault or employ their tentacles as legs. Medusae—Craspedacusta, for example—usually swim free, the Frisbee-, umbrella-, or box-shaped bell pulsating, mouth downward, with tentacles trailing. The tentacles of medusae resemble the snaky locks of the mythical Medusa. Some cnidarian species are exclusively polyps; others are medusae only; and still others alternate between polyp and medusa forms.
Cnidarian cells are assembled into tissues, in comparison with placozoans, which lack tissues. Between a cnidarian’s outer layer of epidermis and its inner layer of gastrodermis lies an intermediate layer called the mesoglea. This mesogleal layer contains translucent secretions and often loose cells but is not organized as a tissue. The gastrodermis lines the gastrovascular cavity, or stomach.
Cnidarians have nerve nets but no central nervous system. The pacemaker of the nerve net maintains the swimming rhythm of medusae. Motion and light-sensitive cells on the edges of many medusae enable them to detect light and orient themselves. The nerve fibers of cnidarians are the only truly naked nerves in the animal kingdom—all others are covered by sheaths of insulating material, such as myelin. Most cnidarian nerve junctions transmit impulses in two directions, whereas nerve junctions of all other animals transmit impulses in only one direction.
The contractile system of polyps consists in part of a layer of epitheliomuscular cells; at the base of the epitheliomuscular cells are contractile fibers that run longitudinally and are anchored in the mesoglea. In addition, cnidarians have nutritive-muscular cells on the inside (below the epithelium) with contractile fibers that run circularly; nutritive-muscular cells contract, taking up and digesting food particles from the gastrovascular cavity. A medusa has muscle fibers in its swimming bell. Although cnidarians have no bones, polyps and medusae are stiffened by fluid pressure in the gut itself and by the mesoglea with its collagen matrix. Hydrocorals (Hydrozoa) and true corals (Anthozoa) secrete calcium carbonate exoskeletons within which their soft polyps shelter. Medusae lack a carbonate skeleton.
Cnidarians are usually carnivores. An herbivorous soft coral (Dendronephthya) has recently been described; this reef inhabitant lacks symbiotic algae and has poorly developed nematocysts but feeds almost exclusively on phytoplankton. Plant-digesting enzymes such as laminarinase and amylase are present in the soft coral Alcyonium. The cubozoans may pursue fish prey. Cnidarians sting when they contact active prey—worms, crustaceans, fish, comb jellies, diatoms, and other protoctists. Cnidoblasts triggered by touch or by chemical stimulation or by both forcibly discharge their stings—nematocysts—for food getting or defense. Unlike sponges (Phylum Porifera), which lack a stomach, cnidarians digest food within their single digestive sac, the gastrovascular cavity, or stomach, which opens through the mouth. The mouth squirts out waste and serves as an anus—which cnidarians lack. Radial canals distribute dissolved oxygen and carbon dioxide as well as nutrients from the stomach to the periphery of the medusa.
Within the cells of most shallow-water corals, horny corals, some sea anemones, and a few medusae, algae are symbiotic partners. The algae sustain the cnidarian partner with photosynthate and oxygen. Symbionts are not common in scyphozoans.
Hydrozoans, with about 3100 described species, include the freshwater hydras, marine colonial hydroids such as Portuguese man-of-war, and fire corals. A velum—characteristic of most hydrozoan medusae—forms a rim around the umbrella margin. Psammohydra is interstitial in sea sand, completely cilia covered, and less than 1 mm in length. Hydras are named for the nine-headed dragon slain by Hercules in Greek myths. Hydrozoans can be polyps; many have small or abortive medusae or lack medusae altogether. Hydrozoan polyps usually reproduce by budding daughter polyps to form polyp colonies and medusae, whereas hydrozoan medusae reproduce sexually, releasing eggs and sperm from gonads along the radial canals. Hydrozoan polyps make either eggs or sperm in most species; polyps are hermaphrodites in a few species. The zygote develops from a fertilized egg into a microscopic blastula and then into a free-swimming, ciliated, solid mouthless larva called a planula. Planula larvae metamorphose into polyps.
All 200 or so species of Scyphozoans are marine—most are free-swimming medusae. Haliclystus and a few other scyphozoans are sessile medusae. Medusae of scyphozoans and hydrozoans are frequently called jellyfish because their mesoglea is thick relative to that of other cnidarians. Scyphozoan medusae do not have vela. Sexual reproduction by scyphozoan medusae produces zygotes that grow into planulae and then usually into sessile polyps that reproduce asexually, giving rise to stacks of tiny, swimming, incipient medusae called ephyra. Ephyra eventually develop into adult medusae. Planulae of some open-ocean scyphozoans bypass the polyp stage, metamorphosing directly into medusae. Many scyphozoans brood their larvae in pouches or in oral arms.
Cubozoa consists of the sea wasps, bearing one or a group of tentacles at each of the four corners of their glassy medusae. Tripedalia, Chironex, and other sea wasps are active swimmers in tropical and subtropical seas. In human encounters, nematocysts usually cause just nasty stings, although sea wasps have been fatal to people. Lenses, light-sensing pigments, and retinas make sea wasp eyes among the most complex invertebrate eyes. The cubomedusan planula gives rise to a polyp; the polyp subdivides into additional polyps (rather than into multiple medusae, as in scyphozoans). Cubozoan medusae reproduce sexually, but few life cycles of these venomous cnidarians are described.
Anthozoans, as solitary or colonial marine polyps, include about 6500 species—sea anemones, sea pens, sea fans, sea pansies, stony (true or hard) corals, and soft corals. Anthozoans form polyps exclusively, never medusae. Some anthozoan species are hermaphroditic, whereas others have separate sexes. Fertilized anthozoan eggs usually develop into planula larvae that settle, attach, and then metamorphose into polyps, cemented by secretions of their pedal disc. Some anemones are viviparous: offspring of sexually mature polyps are “born.” Anemones may also reproduce asexually by splitting in two, by budding, or by pedal laceration—splitting off part of the pedal disc. Without their symbionts, most anthozoans can survive, but they grow faster in sunlight with partners and corals deposit limestone faster.
Coral reefs—underwater limestone ridges in shallow tropical seas—usually form by combined secretions of several species of cnidarians and other carbonate-precipitating organisms, such as chlorophytes (Phylum Chlorophyta) and rhodophytes (Phylum Rhodophyta). Soft corals predominate in Atlantic reefs; a soft coral does not lay down an exoskeleton, only internal spicules. Hard corals are more important in the Pacific. Below a depth of 60 m, corals do not form reefs because the shortage of light limits photosynthesis by the algal symbionts. Most symbionts of anthozoans are dinomastigotes (Phylum Dinomastigota); for example, Gymnodinium adriaticum, also called zooxanthellae, are yellow in color. Zooxanthellae inhabit coral polyp tissue in densities as high as 5 million/cm2. Corals that lack algae can live to a depth of about 3000 m. Reefs support life on land by serving as barriers that reduce waves. Reefs provide a sea haven for protoctists, fish, and other marine animals. Coral reefs form certain islands, such as Bermuda, the Bahamas, and St. Croix.
Cnidarians are eaten in Korea, Japan, and China. Jewelry has been carved from the internal limestone skeletons of the red coral Corallium rubrum since pre-Roman times, from black coral Antipathes, and from the blue coral Heliopora. Overcollecting of corals prompted the United States to forbid the importation of coral. Biocoral, a biomaterial derived from natural coral, is being used for jaw and face bone grafts in Europe and the United States; the porous structure of coral facilitates movement into the substitute bone graft by cells that form bone (osteoblasts). The biocoral graft is partly replaced by normal bone when the graft is resorbed.
Many of the newly discovered deep-sea medusae bioluminesce, sparkling blue green. One species sheds its bioluminescent tentacles on attackers; the bell then pulses off into the wine dark sea and eventually regenerates tentacles. Branchiocerianthus, a hydrozoan polyp, may reach 2 m in length. This giant is a seafloor-deposit feeder. The sea blubber, or lion’s mane, Cyanea, is the largest medusa—its bell is more than 3.6 m wide with tentacles more than 30 m long. The tiniest cnidarians, such as Cryptohydra, are polyps and jellyfishes smaller than 2 mm in diameter.
Cnidarians are among the oldest fossil metazoans. Ediacara, a well-known South Australian fossil found as a sandstone imprint, has been interpreted to be a hydrozoan—it is nearly 700 million years old. Imprints of scyphozoans, hydrozoans, and medusae are found in Burgess shale, Cambrian rocks 500 million years old. Fossil corals abound from the Ordovician through the Devonian (from 500 million to 430 million years ago.)
Major differences between cnidarians and ctenophores (Phylum Ctenophora) are in tentacle morphology (hollow tentacles in cnidarians, solid tentacles in ctenophores), pattern of development, alternation of generations (cnidarians are polymorphic, whereas ctenophores are monomorphic, lacking polyps), and the presence of colloblasts (adhesive, nonstinging cells) but not nematocysts in ctenophores. Whereas cnidarians are radially symmetrical, ctenophores are biradially symmetrical. The middle layer (mesoglea) differs between cnidarians and ctenophores: cnidarians have loose cells in mesoglea, epitheliomuscular cells, and nutritive-muscular cells; ctenophores have smooth muscle fibers. The two phyla are no longer believed to derive from common ancestors; similarities of cnidarians and ctenophores appear to be due to convergence rather than close phylogenetic relationships. Although at least one theory of the origin of the rest of the phyla of metazoan animals has them evolving from Cnidaria, ribosomal RNA data suggest that sponges and ctenophores branched from protoctist ancestors first, followed by trichoplaxes and cnidarians.
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