Comb Jelly (Pleurobrachia sp.)
Comb jellies are part of a small phylum allied to the cnidarians and similar to them in many ways. They are radially symmetrical like a cnidarian medusa, the body is mostly water, and they capture their prey in a somewhat similar manner with tentacles. But there are many differences.
Comb jellies can be seen floating around near the surface of Puget Sound and other Pacific Northwest waters. Pleurobrachia are about 1.5 cm in length and egg-shaped, with a mouth at one end (oral) and anal pores at the other (aboral). Two long (as much as 15 cm) tentacles extend from and can be retracted into sheaths near the aboral end. Along the body axis run eight ciliated bands (comb rows) that provide the comb jelly’s locomotion and give it its name.
Instead of stinging nematocysts on their tentacles, ctenophores have colloblasts, rather complex structures that produce a glue that sticks to the animals they capture for food. This is the same fishing method as that used by cnidarians but obviously independently evolved. However, one kind of ctenophore does use nematocysts rather than colloblasts, making certain the origin of ctenophores from cnidarians.
The digestive tract is a complex series of meridional canals running along the periphery and from them inward to the central digestive canal. Digested food can be excreted through the mouth or anal pores.
Pleurobrachia and other comb jellies have a statocyst, a visible balancing organ at the aboral end. This organ contains a solid structure called a statolith that sits in a pit surrounded by four patches of balancer cilia. When the animal is tilted, the statolith contacts one of the patches, and a neural signal is sent to tilt it back up to its “upright” position by differential beating of the comb rows. The animal also has additional control of its positioning, for example turning the mouth toward a tentacle with captured prey.
Unlike cnidarians, ctenophores are hermaphrodites, with both sexes in the same individual. The gonads are separate, a set of testes and ovaries in each. Eggs and sperm are shed to the outside through the mouth. The larva that develops from fertilized eggs looks much like a small version of the adult in this species.
One of the special features of some comb jellies is the rainbow effect of their comb rows when seen in a bright light. The movement of the cilia scatters light in such a way to cause this effect; it is not bioluminescence.