Lecture Notes on Invertebrate Zoology

Content Module 2. Origin of Multicellular Organisms and Their Phyla: Flatworms, Roundworms, Segmented Worms

Lecture Topic 6. Phylum Ctenophora (Comb Jellies)

Formation of three germ layers, departure from radial Symmetry. Key morphological features. Planktonic and benthic forms. Taxonomy of the phylum.

Ctenophores, or comb jellies, are an extremely primitive phylum of exclusively marine animals (even more primitive than Flatworms). Today, about 200 species are known, most of which inhabit tropical seas, while some occur in polar regions where certain species form massive aggregations. The Black Sea and the Sea of Azov are inhabited by only a single species, Pleurobrachia rhodopis, which invaded these waters in the 1980s.

Ctenophores are predominantly free-swimming pelagic predators, although benthic detritivorous species that crawl or attach to the substrate are also known. Their size ranges from 2–3 mm (e.g., Tinerfe cyanea) to up to 2.5 meters (e.g., Venus' girdle, Cestus veneris).

The body of ctenophores is sac-like, oval, spherical, pear-shaped, or ribbon-like. The oral pole bears the Mouth opening, while the opposite, aboral pole contains the aboral organ. The main body axis runs between both poles. They exhibit a radial type of symmetry, meaning that two or more planes of symmetry can be passed through the body (whereas The Human Body has only one). Furthermore, they are diploblastic animals with a rudimentary third germ layer (mesoderm) and, unlike Cnidarians, they lack stinging Cells (nematocysts) and do not have a polyp stage in their life cycle. Ctenophores display two basic structural body plans: eight-rowed and biradial symmetry, the latter approaching bilateral symmetry, similar to humans. The main elements of symmetry include a flattened Pharynx, Digestive System canals, and tentacles positioned perpendicularly to the pharyngeal plane.

Additionally, eight meridional ridges, or comb rows (hence the Ukrainian name), run along the body surface; these bear transverse plates formed by fused cilia adapted for locomotion. These are the longest cilia in the animal kingdom, measuring several millimeters in length. As they move, the plates refract light into a spectrum, creating the illusion of shimmering rainbow colors rippling across the ctenophore's body.

Most ctenophores possess two tentacles, sometimes significantly longer than the body, which can be retracted into specialized tentacular sheaths. One side of the tentacles is branched and covered with adhesive cells (colloblasts), which are unique to ctenophores. Each Cell is hemispherical and connected to the tentacle by an elastic straight filament (a modified Nucleus) and a spirally coiled filament (a modified flagellum). The Cell produces a sticky substance that captures planktonic organisms, allowing the tentacle to reel the prey in toward the mouth. If the prey struggles to escape, the filaments cushion the shocks felt by the ctenophore. Ctenophores feed on various small planktonic organisms, especially crustaceans, and prey on salps, small fish, and other ctenophores. Some species are commensals—living at the expense of other animals without harming them—and inhabit the surfaces of starfish, corals, or Sponges, where they feed on the prey captured by their hosts. The ctenophore body consists of 90% Water and is filled with a crystal-clear mesoglea, making the animal virtually invisible in the water. Their presence is given away only by a bright Bioluminescence. Transparent Muscles are scattered throughout the mesoglea, but the musculature is most highly developed in the tentacles to ensure efficient prey capture.

At the aboral pole, ctenophores possess a unique sensory Structure known as the aboral organ. It determines the animal's spatial orientation, indicating whether it is swimming upward or downward. In addition to its equilibrium function, it regulates the beating of the comb rows. Ctenophores are responsive to light. For instance, the coloration of benthic comb jellies (Beroida) shifts from milky-white to pinkish-purple under illumination due to the action of specialized chromatophore-like cells in their epidermal layer.

Many ctenophores are capable of bioluminescence, which is associated with specialized photocyte cells localized in the gut. Among the most well-known are Venus' girdle (Cestus veneris), Mnemiopsis, Beroe, and Pleurobrachia. Certain species of the genus Beroe exhibit the most intense luminescence—the light emitted by a single specimen is bright enough to read by! Bioluminescence in ctenophores is coordinated with their defensive response. Upon disturbance, ciliary motion halts, the body undergoes a sharp contraction, and a flash of light is produced.

Ctenophores are hermaphrodites; they have neither separate females nor males, as each individual Functions simultaneously as both. Some pelagic species exhibit disogony—a unique form of reproduction at the larval stage (neoteny)—where the larva, shortly after hatching from the egg, begins to produce small eggs that develop into similar larvae, which subsequently grow and transform into normal-sized adults. Some crawling forms exhibit parental care, developing brood chambers within the maternal body where the eggs mature. Asexual reproduction via body fragmentation, as seen in Hydra, does not occur in ctenophores, with rare exceptions among certain benthic forms (Coeloplana, Planoctena, Vallicula).

The anatomy of ctenophores shares striking similarities with the tornaria larva of hemichordates and the dipleurula larva of Echinoderms, which may suggest that deuterostomes (echinoderms, hemichordates, and Chordates) evolved from ancient ctenophore-like ancestors.