Although known as comb jellies and endowed with delicate gelatinous tissue like cnidarian jellies, ctenophores are only very distantly related to hydromedusae and scyphomedusae. Like the cnidarians, they survive quite well without the benefits of teeth and hard skeletal parts. Unlike the true jellies (cnidarians), comb jellies lack stinging nematocysts and have developed other strategies, such as sticky tentacles and oral lobes to capture prey. The ctenophore body exhibits bi-radial symmetry (with an underlying bilateral symmetry), which is a bit different than the radial symmetry typical of cnidarian medusae. A slice through the tentacular plane of Pleurobrachia, for example, yields 2 halves that are somewhat different than the halves that result from a pharyngeal plane slice.
The signature characteristic of ctenophores are the comb rows. Formed by plates of closely-spaced cilia that are fused at the bases, comb rows beat in a wave pattern that moves from the aboral to the oral end. This constantly shifting pattern of rising and falling comb plates is easily visible. Movement of a comb plate triggers the comb next in line to begin its stroke, which in turn leads to a sequential succession of strokes to form the wave. Each comb makes a rapid stroke in the direction of the aboral end, with a slower recovery to the upright position. This serves to propel the animal with a smooth motion quite unlike the cnidarian stop-and-go pattern. Comb jellies are the largest of all animals that utilize the beating of cilia for locomotion. It doesn’t stop there – with lengths up to 2 mm, ctenophore cilia are the longest of any known. Each single comb within a row is made up of several thousand cilia, and each row may have dozens of combs, so an individual ctenophore is endowed with many thousands of cilia. All adult ctenophores possess eight comb rows, which easily distinguish them from any other type of gelatinous animal. A strikingly beautiful shimmering rainbow pattern is produced by the diffraction of light passing between the cilia.
A sensory structure, the apical sense-organ, sits at the end opposite the mouth. This structure holds a statolith that sits on 4 clumps of large sensory cilia (the balancers) and determines orientation of the comb jelly. The pattern of beating of pairs of comb row can then be modified based on stimuli from the balancer cilia to maintain proper vertical orientation or change swimming direction. Sensory information from a diffuse ectodermal nerve net can also influence comb row beating. Most ctenophores are capable of bioluminescence, which is quite faint and only visible in darkened conditions. The dim glow is typically confined to the digestive canals that underlie each of the comb rows.
Ctenophores are strictly carnivorous and prey on a variety of planktonic animals. About 90 species inhabit all types of marine environments. Some, like those in the Order Cydippida (Pleurobrachia and Hormiphora), use a pair of sticky tentacles that can be retracted into tentacle sheaths near the pharynx to ensnare copepods, larval fish, eggs, and other small zooplankton. Cydippid bodies are typically globular or pear-shaped. Those in the Order Lobata (Bolinopsis and Leucothea) utilize large oral lobes, ciliated ribbon-like projections (the auricles), and mucus to harvest similar prey. Lobate comb jellies typically have flattened oval-shaped bodies. Tentacles are present but much reduced in size compared to cydippids. Their close relatives in the Order Cestida (Cestum and Velamen) have a ribbon-like body, flattened in the tentacular plane and greatly elongated in the pharyngeal plane. Comb rows run along the aboral edge. The Cydippida, Lobata and Cestida all pass through a cydippid stage, which possesses a pair of feeding tentacles much like the sea gooseberry (Pleurobrachia). The terrors of the ctenophore world are the various species of Beroe (Order Beroida), which unlike the other orders lack any kind of tentacles at all stages of the life cycle. These vicious beasts have the habit of using a large mouth to consume other comb jellies with reckless abandon.
Like cnidarians, comb jellies are at the tissue level of organization. The body exterior is covered with a thin ectoderm that enters the mouth and lines the pharynx. The ctenophore mouth consists of a narrow slit, which leads into the compressed stomodaeum, or stomach formed by endodermal tissue. Extracellular digestion begins in the pharynx, with further digestion in the stomodaeum. Partially digested food is then distributed throughout the body by complex systems of gastrovascular canals. These canals are lined with endodermal cells that complete digestion of food materials intracellularly. The canals extend into eight meridional canals that lie directly under each comb row, and run through the gelatinous mesoglea that forms the bulk of the body between the ectoderm and endoderm. Connective tissue fibers, muscle cells and wandering amoeboid cells help to add substance to the mesoglea. Muscle cells underlie the ectoderm, and are concentrated in the tentacles, the area around the mouth and pharynx, and surrounding the aboral area.
Certain species, most notably Pleurobrachia, Bolinopsis and several types of Beroe, can at times be incredibly abundant in surface waters of Monterey Bay and other West Coast locations. As with all jellies however, you are unlikely to find them at any particular time. Since they are not particularly strong swimmers, water currents and zones of convergence can concentrate comb jellies in massive aggregations. Unlike the cnidarian jellies, ctenophores have no sessile, polyp-like phase. Most are hermaphrodites, capable of producing eggs and sperm at the same time. Self fertilization is possible, but it’s not clear how often that happens. Eggs and sperm are typically released directly into the water through the mouth from the gonads located on the meridional canals beneath each comb row. The eggs are fertilized externally and develop into tiny ciliated larvae. Cestid and lobate comb jelly larvae develop cydippid-like paired tentacles that eventually become reduced as the adult body form takes hold. Cydippid ctenophores retain these tentacles throughout the lifespan. Beroid comb jellies, on the other hand, never pass through a tentaculate phase.
Comb jellies are common residents of both nearshore and open sea habitats. Densities of open ocean species have probably been greatly underestimated since many delicate species are destroyed by trawls and other collection methods. More ctenophore species favor tropical seas, but Arctic waters may harbor very high densities of types like Pleurobrachia, Beroe and Bolinopsis. Most comb jellies are also near-surface dwellers (down to several hundred meters), but a few reach depths of 2000 to 3000 meters. Comb jellies fall prey to a variety of creatures seeking easily digested gelatinous morsels, including cnidarian jellies, fishes, sea turtles and the ocean sunfish (Mola).