Comb jellies look superficially like jellyfish and were once lumped with them in the phylum Coelenterata. Closer examination revealed that they are quite different structurally from jellyfish so comb jellies now have their own phylum, the Ctenophores. One important difference is that comb jellies do not have stinging cells, which jellyfish and their close relatives, sea anenomes and corals, all have. Ctenophores are also called sea gooseberries. They are quite pretty little creatures as can be seen in pictures on the internet. Ctenophores are an ancient group and fossils are known from the Cambrian Period. They may be the oldest multicellular organisms, older perhaps than even the sponges.
Like jellyfish, most ctenophores are part of the marine zooplankton, spending their lives floating around in the oceans. They are most common in tropical and shallow seas but some occur in polar regions and a few are found down to several thousand meters in the ocean depths. They are small (a few mm to a few cm in length), transparent and hard to see, so most people are unaware of them. Worldwide there are about 100-150 species so it isn't a large group. A very few comb jellies have become benthic dwellers and crawl around like slugs instead of swimming.
Comb jellies get their common name from their most noticeable characteristic: eight rows of "comb's", thousands of cilia fused together at the base. These combs beat the water, giving the ctenophore its mode of locomotion. As the cilia beat and the ctenophore moves through the water, rainbows of colour are produced which make these little animals very pretty to look at. This is caused by diffraction of light through the cilia. In addition, most ctenophores are bioluminescent, using chemicals to create light and colours in the dark at night or at depth.
To sense their environment, ctenophores have a nerve net. They have a sensory organ near their mouths that also acts as a gyroscope and helps them keep their balance in the water. All ctenophores are carnivorous and use either sticky tentacles or oral lobes to catch their planktonic prey: animals like copepods and ostracods. One group of comb jellies are cannibals, using their large mouths to swallow other comb jellies whole. Other predators of ctenophores include fish, sea turtles and jellyfish and of course they may be consumed by baleen whales when they swallow giant mouthfuls of zooplankton.
Comb jellies are organised at the tissue level. They have ectoderm on the outside, endoderm lining their stomachs and some mesodermal muscle tissue to help them move. Other than the mouth, stomach and some canals to get the digested food to their cells, they do not have organ systems. Ctenophores also have a pretty simple, straight forward life cycle. The adults are hermaphroditic and shed the eggs and sperm into the water. The larvae are planktonic and develop into planktonic adults so the whole life cycle occurs in the plankton.
Ctenophores have not had much of an impact on humans, except in the last few years due to human introduction of a couple of species to new habitats where they have become aggressive invaders. Ship's ballast water brought an American species of ctenophore, Mnemiopsis leidyi, to the Black Sea in the 1980's, where it caused the collapse of native fisheries. It has since spread to the Caspian, North and Baltic Seas and it may soon reach the Mediterranean. It has no native predators in these seas and aggressively competes for food with fish, while reproducing quickly, like a marine rabbit. This is a good example of why we must be so careful not to introduce species to new habitats. The way we have indiscriminately released ballast water around the world has unfortunately had some very unpleasant side effects including this ctenophore, an animal that is harmless in its own ecosystem but a pest in others.