The following synopsis is based on the scheme proposed by Harbison (1996), in particular for adult epipelagic forms from the North Atlantic, and Mills (pers. comm.). The main diagnostic characters are those suggested by Mayer (1912), Liley (1958), Greve (1975), Harbison (1985, 1996), O’Sullivan (1986) and Harbison and Madin (1982). Many descriptions are complemented with personal observations. As pointed out above, this classification is provisional, very likely to change as more species and phylogenetic data are obtained. Genera in blue characters are those reported from SE and SW Atlantic waters, and therefore treated in detail here. See also page Ct. 1 Introduction for more about Ctenophora.
Phylum Ctenophora Eschsholtz, 1829
Class Tentaculata Mills, 1998
Order Cydippida Gegenbaur, 1856
Family Pleurobrachiidae Chun, 1880.
Genus Pleurobrachia Fleming, 1822
Genus Hormiphora L. Agassiz, 1860
Genus Tinerfe Chun 1898
Family Mertensiidae L. Agassiz, 1860.
Genus Callianira Péron and Lesueur, 1808
Genus Mertensia Lesson, 1836
Family Lampeidae Krumbach, 1925.
Genus Lampea Stechow 1921
Family Bathyctenidae Mortensen, 1932
Genus Bathyctena Mortensen 1932
Family Haeckeliidae Krumbach, 1925.
Genus Haeckelia Carus, 1863
Genus Aulacoctena Mortensen, 1932
Family Euplokamidae Mills, 1987.
Genus Euplokamis Chun, 1879
Family Dryodoridae (sensu Harbison, 1996)
Genus Dryodora L. Agassiz, 1860
Order Platyctenida Mortensen, 1912
Family Ctenoplanidae Willey, 1869
Genus Ctenoplana Korotneff, 1886
Order Thalassocalycida Madin and Harbison, 1978
Family Thalassocalycidae Madin and Harbison, 1978.
Genus Thalassocalyce Madin and Harbison, 1978
Order Lobata L. Agassiz, 1860
Family Bolinopsidae Bigelow, 1912.
Genus Bolinopsis L. Agassiz, 1860
Genus Mnemiopsis L. Agassiz, 1860
Family Leucotheidae Krumbach, 1925.
Genus Leucothea Mertens, 1833
Family Eurhamphaeidae L. Agassiz, 1860.
Genus Eurhamphaea Gegenbaur, 1856
Genus Kiyohimea Komai and Tokioka, 1940
Genus Deiopea Chun, 1879
Family Bathocyroidae Harbison and Madin, 1982.
Genus Bathocyroe Madin and Harbison, 1978
Family Ocyropsidae (Lesson, 1843).
Genus Ocyropsis Mayer, 1912
Order Cestida Gegenbaur, 1856
Family Cestidae Gegenbaur, 1856.
Genus Cestum Lesueur, 1813
Genus Velamen Krumbach, 1925
Class Nuda Mills, 1998
Order Beroida Eschscholtz, 1829
Family Beroidae Eschholtz, 1829.
Genus Beroe Browne, 1756
(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 walnuts, sea gooseberries, cat’s eyes, and all other comb jellies belong to Phylum Ctenophora. Ctenophoran bodies are flexible and mobile, the consistency of soft jelly in a membrane bag. Paddlelike comb plates (ctenes) unique to ctenophorans sweep these translucent, biradially symmetrical invertebrates through the sea. Ctenophores—like nudibranch molluscs and turbellarian flatworms—retain cilia as locomotor organs throughout life. Thousands of individual cilia fused together at their bases make up a comb plate; eight rows of comb plates extend along the length of the ctenophore. Each comb plate (ctene) mechanically activates the next to lift in sequence from mouth to the opposite—aboral—end to propel the animal mouth end forward. Many comb jellies swim with a combination of comb action and muscle movements; some also swim like jet-propelled cnidarian medusae. Benthic comb jellies crawl over other organisms on the seafloor. Comb jellies are weak swimmers. Currents, tides, and wind carry planktonic ctenophores through open seas from the Antarctic to the Arctic. Tides and currents sometimes concentrate comb jellies in vast numbers. When beached, the fragile bodies collapse and quickly dry.
Comb jellies range in size from 0.4 cm to more than 1 m in length. These predators compete with commercial fish for copepods (crustaceans) and are themselves food for sea turtles, fish, medusae, and other ctenophores. Multicolored flashes in the night sea often originate from bioluminescent comb jellies. By day, comb jellies are iridescent; their plates refract light. When touched, Eurhamphaea releases sparkly blue green ink. Tropical comb jellies are tinted delicate violet, rose, yellow, or brown by symbiotic algae. Deep-sea comb jellies may be purple or red.
About 100 comb jelly species have been described in two classes, Tentaculata and Nuda. Most are grouped in Class Tentaculata and have two tentacles that retract into pits in larval stages and generally in adulthood. The twofold symmetry of the tentacles is superimposed on the comb jelly’s radial symmetry. The solid tentacles are usually extended into sinuous fishing nets as much as 100 times the length of the body. Most lobate comb jellies—such as Bolinopsis—have elaborate large tentacles that are less apparent because their tentacles are hidden by the lobes. Tentacles snare living prey but are not used in swimming. However, creeping ctenophores extend their tentacles and raft with sea currents.
Not all of these ocean floaters bear tentacles; comb jellies lacking tentacles are grouped into the other class of ctenophores, Class Nuda. The North Sea thimble jelly, Beroë gracilis, has no tentacles at any stage in its development. Beroë gulps in comb jellies and salps (soft urochordates, Subphylum Urochordata) by rapidly opening its mobile, muscular lips lined with “macrocilia.” Under the electron microscope, macrocilia are actually multicilia—composed of thousands of interconnected cilia arising from a single cell. Macrocilia shaped like sawteeth take bites from or trap prey. After feeding, Beroë zips its lips shut with reversibly adhesive strips of cells having highly folded membranes; closed lips probably reduce drag as Beroë swims mouth-forward to seek prey.
Comb jellies of both classes have the same general body plan; a comb jelly can be sliced through its long axis—from mouth to aboral end—to produce mirror images. A cut through the long axis rotated 90 degrees results in different identical halves. Ctenophores have a biradial body plan. The central cavity is lined with gastrodermis. Interposed between epidermis and gastrodermis, mesoglea provides buoyancy, stores food reserves, and contains ameboid cells that develop into smooth muscle within the mesoglea. Oxygen diffuses in and carbon dioxide out, across the body wall. Comb jellies lack any special circulatory or respiratory organs.
The comb jelly nervous system is a diffuse epidermal net that coordinates without a central nervous system. An aboral apical sense organ (a statocyst) determines the orientation of the animal. Tilting the comb jelly brings several hundred calcareous stones called statoliths, each 5 to 10 µm in diameter, to bear on one of four fused groups of cilia called balancers, which support the statocyst as on a spring. From each balancer, a ciliated groove branches to two comb rows and controls them. The pair of comb rows corresponding to each balancer beat faster or slower, depending on the tilt, and thus restore the animal to an upright position. The apical sense organ and the nerve net synchronize swimming.
Adhesive cells called colloblasts or lasso cells stud the tentacles and—in some species—oral lobes. Colloblasts entangle live fish, crustaceans, fish eggs, and other zooplankton. The sticky head of the colloblast attaches to two filaments, one of which spirals around the other and acts as a spring. This design prevents struggling prey from tearing free. Lobate comb jellies, such as Bolinopsis, gather prey with oral lobes covered with colloblasts and mucus; a pair of ciliated flaps called auricles guides food into the mouth. Whatever the food—live larvae, eggs, tiny fish, arrow worms (Phylum Chaetognatha), copepods—cilia and mucus carry it into the digestive cavity. Ciliated gastrodermis lines the digestive cavity of all comb jellies. Comb jellies with tentacles wipe their tentacles across their mouths to unload captive food. Perhaps poisonous or anesthetic mucus such as that secreted by the colloblast head facilitates prey capture. Unlike nematocysts of cnidarians, colloblasts do not penetrate prey.
Enzymes secreted into the digestive cavity break down food. The resulting nutrients are distributed along canals that run through the mesoglea, the thick jelly that lies between the epidermis and the digestive cavity. Some of the dead-end canals run beneath the comb plates. Undigestible food is voided through the mouth and through two anal pores beside the statocyst. No functional excretory system has been demonstrated.
Ctenophores are sequential or simultaneous hermaphrodites. Ovaries and testes develop under the comb rows along each digestive canal. Gametes, shed into the digestive cavity, are released through the mouth or through gonopores between comb plates. In most, fertilization takes place externally; creeping nonplanktonic comb jellies have internal fertilization. Fertilized eggs develop into free-swimming cydippid larvae. Cydippids of all tentaculate ctenophores are oval with two tentacles; those of Class Nuda lack tentacles. Cydippids develop into sexually mature forms in some species; they metamorphose into adults gradually in other species. Tjalfiella, a viviparous comb jelly, is an exception that broods its young in special pouches and then releases free-swimming young. Tjalfiella bear comb plates only as juveniles and live as commensals (benefiting from a relation with another species without either benefiting or harming the host) on the seafloor. Mnemiopsis can reproduce asexually by regeneration from fragments. Tentacles, combs, and colloblasts regenerate readily in most ctenophores. In a process similar to pedal laceration of sea anemones (Phylum Cnidaria), platyctenes—flat benthic ctenophores of Order Platyctenida, Class Tentaculata—reproduce asexually.
Large—as much as 2 m long—ribbonlike Venus’s girdle (Cestum veneris and Velamen) undulate through the water, rolling up and unrolling by contraction of muscle fibers in the mesoglea. They feed with small tentacles along the ribbon’s edge. Ocyropsis swims both with comb plates and by flapping its oral lobes. Ocyropsis surrounds prey with an oral lobe, drawing food to its mouth. Some comb jellies live at depths of several kilometers. Not all comb jellies are planktonic. Leaflike Ctenoplana creeps on the water surface or on the floor of shallow tropical seas and swims in the plankton. Coeloplana, a flattened, tentacled creeper found in the Indo-Pacific Ocean, loses its combs as an adult. Lampea pancerina—a free-living bottom dweller—is the only parasitic (symbiotrophic) ctenophore described so far. Its juvenile form, which has a different name, Gastrodes parasiticum, bores into and feeds on the floating tunicates Cyclosalpa and Salpa (Subphylum Urochordata).
Because ctenophora have such soft and fragile bodies, their potential for preservation as fossils is very low, and clues to their evolution must be inferred primarily from living forms. X-rays have detected a single fossil comb jelly 400 million years old in Devonian slate, complete with comb rows, statocyst, and tentacles. Some suggest that ctenophores evolved from medusa-shaped cnidarians (Phylum Cnidaria), but the similarity of medusae to ctenophorans seems only superficial. Comb jellies and hydrozoan cnidarians are more generally thought to share a common ancestor. Limited sequence data portray sponges and ctenophores as having branched from protoctists, with cnidarians branching later. Cnidarians and ctenophores have evolved similar features: mesoglea interposed between epidermis and gastrodermis, carnivory, bioluminescence, a netlike nervous system, and the ability to regenerate body parts. However, in fundamental morphology, cnidarians and ctenophores differ: the smooth muscle fibers within the mesoglea of ctenophores are lacking in cnidarians; colloblasts of comb jellies differ from nematocysts of cnidarians; and only ctenophores bear ctenes. Ctenophore muscle fibers develop from ameboid cells in mesoglea rather than from epidermal and gastrodermal cells as in cnidarians. Most ctenophores swim with a combination of comb action and muscle movements; a few comb jellies use the jet propulsion of medusae. Some cnidarians are polymorphic; ctenophores are monomorphic, lacking a polyp form. Cnidarians are radially symmetrical; ctenophores are biradial.
Comb jellies are underrepresented in descriptions of the open-sea ecosystem because their delicate bodies are difficult to net and preserve. With new tools—such as the remotely operated vehicles—comb jellies are revealed to be abundant and diverse. Diving in submersibles, marine biologists gather ctenophores by using a slurp gun, a gentle vacuum cleaner used to collect plankton. A slurp gun consists of a collecting cylinder connected at one end to a flexible hose, all mounted on a submersible vehicle. The free end of the hose is positioned near an animal by the robot arm of the submersible and the comb jelly is sucked into the cylinder. From field observations, fragile comb jellies are considered the most abundant planktonic animals between 700 and 400 m below the surface.