Ctenophora (redirected from phylum Ctenophora)

Ctenophora (tĭnŏf`ərə), a small phylum of exclusively marine, invertebrate animals, commonly known as sea walnuts or comb jellies. Because they are so delicate that specimens are difficult to collect, little was known about them until the advent of blue-water scuba and submersible collecting. Ctenophores are characterized by eight rows consisting of ciliated plates called ctenes (combs), which are radially arranged on the spherical body surface. The animals swim weakly, powered by those structures. The two hemispheres of the ctenophore body are marked by a mouth, or oral pole, on the underside, and an opposite aboral pole, on which is located the statocyst, a unique sense organ controlling equilibrium. Most ctenophores resemble biradially symmetrical (see symmetry, biological) jellyfish (phylum Cnidaria) but lack the cnidarian whorl of tentacles around the mouth. They lack the specialized stinging cells (nematocysts) found in coelenterates, but one species (Haeckelia rubra) incorporates those of its jellyfish prey for its own defense. Ctenophores, which are all carnivorous, have specialized adhesive cells called colloblasts, used to capture planktonic animals on which the ctenophores feed. Approximately 50 species are known, but many become locally abundant and are ecologically significant. They vary from less than 1-4 in. (0.6 cm) to over 1 ft (30.5 cm) long. Most are transparent, but pale pinks, reds, violets, and oranges are also known in some species. Most ctenophores are also bioluminescent, the production of light originating in the walls of the eight canals. Most ctenostomes are hermaphrodites, developing through a cydippid larval stage to adults. They can also regenerate lost parts.

Class Tentaculata

Members of this class typically have two feathery tentacles that can be retracted into specialized sheaths. In some, there are smaller, secondary tentacles, and the primary tentacles are reduced. This class includes the small, oval sea gooseberries (genus Pleurobrachia), common on both Atlantic and Pacific coasts. The more flattened species of the genus Mnemiopsis, about 4 in. long (10 cm), is common on the upper Atlantic coast; it has a large mouth and feeds mainly on larval mollusks and copepods. This species is brilliantly luminescent. The similar, but larger, genus Leucothea is abundant on the Pacific coast. Venus's girdle (genus Cestum) is a flattened, ribbonlike form reaching over 1 yd (91 cm) in length, and found in tropical waters.

Class Nuda

This class includes species that have no tentacles. Typical is the large-mouthed genus Beröe, which feeds on jellyfish and other ctenophores.

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Ctenophora [tə′näf·ə·rə]

(invertebrate zoology)

The comb jellies, a phylum of marine organisms having eight rows of comblike plates as the main locomotory structure.

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Ctenophora

A phylum of exclusively marine organisms, formerly included in the jellyfish and polyps as coelenterates. These animals, the so-called comb jellies, possess a biradial symmetry of organization and have eight rows of comblike plates as the main locomotory structures. Most are pelagic, but a few genera are creeping. Many are transparent and colorless; others are faintly to brightly colored. Almost all are luminescent. Many of these organisms are hermaphroditic. Development is biradially symmetrical, with a cydippid larval stage. Five orders constitute this phylum: Cydippida; Lobata; Cestida; Platyctenida; Beroida.

The body is gelatinous and extremely fragile; its form may be globular, pyriform, or bell- or helmet-shaped. Some species resemble a ribbon. Their size ranges from to 20 in. (3 mm to 50 cm). The axis of symmetry is determined by the mouth and the organ of equilibrium, or statocyst (see illustration). The mouth leads into the flattened, elongated pharynx. The sagittal plane is thus referred to as the stomodeal plane. The other plane of symmetry is perpendicular to the sagittal plane and is marked (Beroida excepted) by tentacles. Eight meridional comb-plate rows or ribs stretch from the ab­oral pole on the surface of the body. The comb plates, which are used as oars during locomotion, are the most characteristic structure of the ctenophores, possessed by all members of the phylum. Each plate consists of a great number of very long related cilia.

Structure of a cydippid ctenophore

Fertilization usually occurs in seawater. Pelagic ctenophores are self-fertile, but cross-fertilization might also take place inside a swarm of ctenophores. Ctenophores have high powers of regeneration. Asexual reproduction (in a few platyctenid genera) is by regeneration of an entire organism from a small piece of the adult body.

The ctenophores feed on zooplankton. They are themselves important plankton organisms. They are quite common and have a worldwide distribution in all seas, where they can appear in enormous swarms. Some genera stand great changes in the seawater salinity. Because of their voracity as predators of zooplankton, they play an important role in plankton equilibrium and in fisheries.