Comb Jellies: I Can’t Believe it is not a Jellyfish!
Comb Jelly Background
When people encounter a comb jelly, often the reaction is the same as if a jelly fish had been discovered, “Eww! Will it sting me?!”. Comb jellies are mistaken for jellyfish due to their clear, gelatinous form. In reality jellyfish and comb jellies come from two different phylum. The comb jelly is from the Ctenophora phylum and the jellyfish comes from the Coelenterate (Cnidaria) phylum, which includes hydroids, sea anemones, and coral. The huge difference between these two phyla is that the Ctenophora does not contain stinging cells (nematocysts) nor do they have alternation of generations like the Coelenterate (Coulombe, 1984).
The very small Ctenophora phylum has ~90 members, all of which are comb jellies. The two classes of comb jellies are Tentaculata (with two tentacles, each branched on one side) and Nuda, without tentacles. There are six orders within the Tentaculata, which are as follows: Cydippida, Lobata, Ganeshida, Thalassocalycida, Cestida, Platyctenea. Within Nuda there is one order, Beroida that contains only two genera (Kozloff, 1990).
Comb jellies are transparent and very delicate so they are hard to spot. They are marine plankton often found near the surface of coastal waters, but can at be found further out in the ocean and at great depths (Barnes, 1980: Meglitsch, 1972). Warmer tropical waters are favored, but they can be found in the Artic (Mills, 2001). Their small size, 4 inches or less, helps add to their invisibility, with one exception, the Venus’s-girdle whose ribbon shaped body can be five feet long (Comb Jellies, 2001). When being caught for study, plankton nets are two rough on fragile comb jellies, so divers often collect them in glass jars (Coulombe, 1984).
Comb Jellies Versus Jellyfish
Paul Meglitsch in his book, Invertebrate Zoology, clearly goes over the differences and similarities of jellyfish and comb jellies. The following are 4 of the 7 traits (the ones clearly visible) comb jellies have that jellyfish do not have: 1. Radially arranged rows of ciliated plates for swimming, 2. Tentacles not in whorls around the mouth, 3. Adhesive cells for food capture and the absence of nematocysts. 4. An aboral sense organ. A few of the similarities outlined by the same author were 1. Having a strong biradial symmetry and an oral-aboral axis, 2. A gelatinous medusa-like body, 3. One body cavity.
Comb Jelly Structure
The phylum name, Ctenophora, literally means “comb-bearers” (Introduction, 2001). In general, a comb jellies’ body can be considered spherical and having bilateral symmetry, but there are exceptions such as the ribbon shaped Venus’s-girdle, which was mentioned earlier. The body is best described if divided into two hemispheres. On the oral pole, the lower half, is the mouth and the other half, the aboral pole, contains the statolith, which will be described later in this paper. All comb jellies possess eight rows of comb rows/bands, which divide the body into eight equal sections. The comb row/bands are made up of cilia and are used to propel the comb jelly through the water (Coulombe, 1984).
Comb jellies are planktonic, meaning the ocean currents move them long distances, but they can move up and down in the water column using their cilia. The cilia move in a sweeping motion from the oral end to the aboral end, which propels the animal forward. They usually move mouth first through the water and if they come upon an object they can reserve their direction of movement by moving their cilia in the opposite direction (Barnes, 1980). Comb jellies are the largest animals to possess cilia (Mills, 2001).
Most comb jellies have two tentacles that they use for feeding. Jellyfish tentacles are attached to the surface of the animal, but this is not so for comb jellies (Barnes, 1980). The tentacles on comb jellies come from within deep sheaths (tentacular sheath/pouch) on each side of the body near the aboral end. The tentacles are long with additional branching tentacles on just one side. In some species, such as the Pleurobrachia (sea gooseberry), the tentacles can extended to a length of 3 to 4 centimeters. The tentacles can be extended or contracted because they are made of smooth muscle (Barnes, 1980).
The tentacles do not contain nematocysts (stinging cells) like jelly fish, but rather contain colloblasts. There is one exception and that is the Euchlora rubra, which does not contain colloblasts. Euchlora obtains nematocysts in a interesting fashion. When they eat trachyline medusae, they store the unused nematocysts from the jellyfish in their own tentacles. These nematocysts are not used to catch prey, but for protection (Kozloff, 1990).
The nervous system of the comb jelly is not made up of true nerves, but a subepidermal nerve network that is best developed under the comb rows (Barnes, 1980: Meglitsch, 1972). On the aboral end of the comb jelly their only sense organ is found, the statolith. This organ helps the comb jelly keep balanced and moving in the right direction. The statolith is in a pit sitting on top of four tufts of balancer cilia and each tuft is connected to a comb row. Say a wave comes along and causes the comb jelly to tilt to the left side. The tilting action would cause pressure on the balancer cilia on that side. That pressure signals to the cilia on that side to beat faster in order to make the animal move in the correct direction once again (Barnes, 1980).
In laboratory experiments, one of the balancer cilia was cut. When this was done the comb row connected to the cut balancer cilia started moving independent of the other comb rows (Barnes, 1980). If the statolith was completely removed, all rows moved independently. If the comb row was cut in half the top and bottom cilia acted independently of each other, but the top was coordinated with itself and the bottom coordinated with itself (Meglitsch, 1972).
Food: What and how they eat
Comb jellies are carnivorous, feeding on plankton including copepods, small crustaceans, larval fish and fish eggs (Mills, 2001: Voss, 1976: Kozloff 1990). When it comes to catching food, the colloblasts mentioned earlier come in handy as well as having a mucus covered surface and a big mouth. The comb jellies with colloblasts, such as the Pleurobrachia, put their tentacles out into the water to form a large net. A food item will move into the tentacle net and become caught on one of the sticky colloblasts. Then the tentacle is pulled up to the mouth and wiped off (Barnes, 1980: Meglitsch, 1972).
Some comb jellies use just surface mucus or a combination of surface mucus and tentacles to catch food. If the food item is caught on the surface mucus, it is moved to the mouth by the cilia. This is usually done by lobate comb jellies such as Mnemiopsis and Leucothea. If you are a Beroe comb jelly you use your big mouth to catch food. Beroe often eats other comb jellies with a gulping movement that pulls the prey in their month (Barnes, 1980: Meglitsch, 1972). The food starts to be digested extracellularly in the pharynx and is moved on to the stomach and canal system where it is fully digested. When done, the wastes are passed out through the mouth and anal pores (Barnes, 1980).
Comb jellies can often be found in large groups and after they leave an area there is often very little plankton left behind. (Coulombe, 1984). On the other hand, comb jellies are preyed upon by: jellyfish, fish (e.g. mackerel, ocean sunfish), sea turtles, birds, sharks (e.g. spiny dogfish), and other various marine organisms (Mills, 2001).
What really created an interest in comb jellies for me was their bioluminescence. When I worked on the coast of Georgia, students and I would catch comb jellies during dock study. When we got back to the classroom to look at what we caught, the comb jellies would usually steal the show (only skeleton shrimp created more of a stir). The students would watch the comb jellies’ cilia on the video cam, but the best part came when the lights were turned off. If it was dark enough and someone disturbed the comb jelly it would glow! The best was if the students saw comb jellies bioluminescence in shallow pools during their night walk.
The bioluminescence (cold light) on comb jellies comes from the gastrodermis cells in the walls of the meridional canals. This makes it look like the light is coming from the comb rows and giving an overall appearance of a pulse or blob of light. When disturbed, the comb jellies as well as many other marine organisms (jellyfish, siphonophores, deep sea squid, etc), glow with bioluminescence. Some of those mentioned (unclear if comb jellies do) lose their glowing tentacles or release a glowing cloud to distract predators (Waller, 1996).
A few more random things about comb jellies: Comb jellies are hermaphroditic, the majority simultaneously, but a few have protandry, first being a male and then a female. Fertilization usually takes place in the seawater, but several species brood their eggs (Barnes, 1980). Self-fertilization can occur (Kozloff, 1990). Unlike jellyfish, the comb jellies do not have alternation of generations, the larva grows right into a comb jelly (Coulombe, 1984).
The comb jellies found on the east coast are: Pleurobranchia pileus, Sea gooseberry (Range: Maine to North Carolina); Mnemiopsis mccradyi, Lobate comb jelly (Range: Florida and Caribbean); Bolinopsis infunbulum, Common northern comb jelly (Range: north of Cape Cod); Mnemiopsis leidi, Leidy’s comb jelly (Range: south of Cape Cod); Beroe, Beroe’s comb jelly (Range: south of Cape Cod) (Coulombe, 1984). Hopefully we will have the opportunity of seeing some bioluminescence while were are on our trip to the Keys and the Bahamas.
Barnes, R.D. Invertebrate Zoology 4th edition. Philadelphia, PA: Saunders College.
Comb Jellies. MicrosoftŮEncartaŮ Online Encyclopedia 2001. Available from the
Internet, http://encarta.msn.com, accessed 5/13/01.
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http://www.ucmp.berkeley.edu/cnidaria/ctenophora.html, accessed 5/13/01.
Kozloff, Eugene N. Invertebrates. New York, New York: Saunders College Publishing.
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Mills, Claudia. Comb Jellies. Available from the Internet,
http://fp.redshift.com/pelagia/ctenophores.htm, accessed 5/13/01.
Voss, Gilbert. Seashore Life of Florida and the Caribbean. Miami, Florida: E.A.
Seemann Publishing Inc. 1976.
Waller, Geoffrey ed. Sea Life. Washington D.C.: Smithsonian Institution Press. 1996.
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