Dive Dry with Dr. Bill

067: Ctenophores or Comb Jellies

I entered the water during the middle of the week without any specific focus for my dive, just happy to have the park to myself (although a mermaid would have been a welcome buddy!). Sometimes you just have to let Mother Nature give it its best shot... most of the time, in fact. I was not disappointed. While winding my way to one of my favorite areas in the park, I noticed that there was quite a group of different fish species gathered under the kelp canopy. As I approached, I noticed they were feeding on small, transparent, jelly-like animals. Then I looked up and noticed these creatures were all over just below the water's surface.

No, I hadn't stumbled on a "school" of jellyfish. These were comb jellies or ctenophores. Like many jellyfish, comb jellies also drift in the water currents as large plankton. Early marine biologists wrongly grouped comb jellies with jellyfish because they look superficially similar. Today scientists aren't quite sure exactly where ctenophores fit into the grand scheme of animal life on this planet, so they have their own group.

These "jellies" couldn't sting me if they wanted to. The tentacles on these critters do not contain the stinging cells (nematocysts) of jellyfish and their relatives. Instead they are sticky due to specialized cells known as colloblasts, and are used to capture and eat plankton, eggs, and planktonic fish or invertebrate larvae. Some, like the species I observed, prey on other species of comb jellies. Their mouths have cilia that can "chew" other gelatinous prey. Comb jellies are strictly carnivorous and skip the vegetables (such as phytoplankton). Perhaps they are following the high protein Adkins diet. In turn they may be eaten by true jellyfish, fishes, sea turtles and the ocean sunfish (Mola mola).

Because ctenophores lack advanced organs, digestion begins in a simple stomach cavity. The partially digested food is then distributed through the organism's tissues by a series of "canals" located largely under the rows of combs. They have a structure known as a statocyst at the rear end that helps them maintain proper orientation and direction. // One distinguishing feature of comb jellies is the eight rows of "combs," hair-like cilia, that surround the body. The cilia beat almost continuously and are the main form of locomotion for these animals. The plates start moving at the end opposite the mouth and move towards it. Unlike the pulsing movement of jellyfish, this causes a smooth, continuous motion. Some species can also move by flapping their lobes or undulating their bodies. Direction is regulated by the statocyst.

Reproduction can be accomplished in several ways. Comb jellies are hermaphrodites, containing both male and female active sex organs at once. They are capable of fertilizing themselves, although scientists are unsure how. Perhaps they know of a position not found in the Kama Sutra. Eggs and sperm are typically released through the mouth into the water column, and unite externally. The fertilized eggs develop into larvae which are also planktonic. Even very young ctenophores are capable of reproduction, producing large numbers of offspring in a short time.

Although somewhat innocuous, ctenophores can have destructive effects when introduced into foreign ecosystems. An American ctenophore introduced through ship ballast into the Black Sea destroyed a fishery of economic importance to the six countries bordering that body of water within just 10 years. The alien invader matured earlier than the target fish's eggs, and ate all the zooplankton food that the fish larvae needed to survive. Strangely, another introduced ctenophore helped reduce their impact because it fed on the first.

There are about 100 different species of modern-day ctenophores. They are strictly marine. Most are found in the upper water column near the surface, although a few are bottom dwellers. They are more common in the tropics, but large numbers of a single species may be seen in the Arctic. We know little about their evolution since their soft bodies do not form good fossils, although a few specimens have been found.

Often when the light catches the ctenophore's beating combs just right, a beautiful rainbow effect can be seen due to diffraction. Some comb jellies are bioluminescent, producing their own "cold light" through chemical processes not unlike the "fire flies" I used to capture as a child on warm, summer Chicago nights. Although the days are now much shorter, I didn't stick around long enough to observe whether these comb jellies glowed on the dark.

The ctenophores were all gone the next day when I went back for more footage. Ctenophores tend to be concentrated into large aggregations (like the one I observed) by currents. Therefore they are very patchy in distribution. You may see them in one location one day, and another the next. I have only seen these congregations of large numbers of ctenophores a few times in the Dive Park.

© 2003 Dr. Bill Bushing. Watch the "Dive Dry with Dr. Bill" underwater videos on Catalina Cable TV channel 49, 10:00 AM and 5:00 PM weekdays.

Comb jellies or ctenophorees at ocean's surface; close-up showing rows of "combs,"
comb jelly backlit by the sun; comb jelly seen end-on

This document maintained by Dr. Bill Bushing.
Material © 2003 Star Thrower Educational Multimedia