Phylum Ctenophora: The Sea Walnuts

Ctenophora (te-nof´o-ra) (Gr. kteis, ktenos, comb, +phora, pl. of bearing) is composed of about 150 species. All are marine forms occurring in all seas but especially in warm waters. They take their name from eight rows of comblike plates used for locomotion. Common names for ctenophores are “sea walnuts” and “comb jellies.”

Except for a few creeping and sessile forms, ctenophores are free-swimming. Although these feeble swimmers are more common in surface waters, ctenophores sometimes occur at considerable depths. They are often at the mercy of tides and strong currents, but they avoid storms by swimming into deeper water. In calm water they may rest vertically with little movement, but when moving they use their ciliated comb plates to propel themselves mouth-end forward.

From an examination of Pleurobrachia, it is clear that biradial symmetry results from the presence of two tentacles on the body. There is no head, but an oral-aboral axis is present. The mouth leads from a pharynx into a branched digestive tract ending with an anal pore. The body is transparent and has a gelatinous layer, derived from embryonic ectoderm and endoderm, between the two adult tissue layers. The gelatinous layer contains an extensive set of muscle fibers; fiber patterns are radial, as well as in meridional and latitudinal bands around the body. Muscle fibers are also present in the extensible tentacles.

Ctenophore tentacles capture small planktonic organisms, typically crustaceans such as copepods, from the surrounding waters. Extended tentacles trail in the water, and passing prey are caught by epidermal glue cells called colloblasts. Colloblasts contain an adhesive material discharged on contact with prey; the adhesive binds to the prey and the rest of the colloblast cell remains attached to the tentacle. Food-laden tentacles are wiped across the mouth.

Ctenophores with short tentacles may collect food on the ciliated body surface. Ctenophores without tentacles may feed on other gelatinous animals such as medusae, salps, or other ctenophores. Entire prey may be consumed or small parts, such as tentacles, removed. Some ctenophores that feed on cnidarians collect undischarged cnidocytes from their prey and incorporate them into epidermal tissue as a defense mechanism. The ctenophore Haekelia rubra (named after Ernst Haeckel, nineteenth-century German zoologist) consumes hydromedusae tentacles in this way.

Ctenophores were previously divided between two classes: Tentaculata and Nuda. Based on evidence that the classes are not monophyletic groups, most biologists discuss ctenophore diversity using seven orders below the class level. Morphological and molecular evidence suggest that one common order (Cydippida) is polyphyletic. One family within Cydippida appears related to members of the order Beroida, whereas others may form new clades. Until new classes have been formulated, we will not discuss ctenophore subgroups.

A fundamental understanding of the ctenophore body plan can be gained from consideration of Pleurobrachia and a few other examples.

Characteristics of Phylum Ctenophora

• Eight rows of combs (ctenes) arranged radially around body
• Colloblasts, adhesive cells used in prey capture, present in most
• Entirely marine
• Symmetry biradial; arrangement of internal canals and position of the paired tentacles change the radial symmetry into a combination of radial and bilateral
• Body ellipsoidal or spherical in shape with oral and aboral ends; no definite head
• Adult body with gelatinous middle layer containing muscle cells; derivation of middle cellular layer controversial (ectodermal vs. endodermal) affecting status as diploblastic or triploblastic
• Complete gut; mouth opens into pharynx; gut with a series of branching gastrovascular canals; gut terminates at anal pore; wastes exit via anal pore and mouth
• Extracellular digestion in pharynx
• Two extensible tentacles occur in most
• Muscular contractions via muscle fi bers (cells), not epitheliomuscular cells
• Nervous system consisting of a subepidermal plexus concentrated around the mouth and beneath the comb plate rows; an aboral sense organ (statocyst)
• Reproduction monoecious in most; gonads (endodermal origin) on the walls of the digestive canals, which are under the rows of comb plates; mosaic or regulative cleavage within embryos; cydippid larva
• No respiratory system
• No coelomic cavity

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Useful External Links 

• Sea Walnut by Vedantu
• Sea Walnut (Mnemiopsis leidyi) by Smithsonian
• Comb Jelly (Sea Walnut) by AQUARIUM OF THE PACIFIC