Back To CourseHigh School Biology: Help and Review
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Marta has taught high school and middle school Science and has a Master's degree in Science Education.
You might not know them as cnidarians, but you are probably more with jellyfish, corals and sea anemones, which all belong to this group of animals. They are simple organisms, which have been around for millions of years, and have remained relatively unchanged.
Although in show business they are often only given guest or minor roles in shows featuring sarcastic sponges or movies starring adorable clownfish, in real life cnidarians are extraordinary organisms that play a critical role in marine ecosystems. They are beautiful, graceful and sometimes extremely dangerous animals, but what makes them truly fascinating is that they can be so simple and yet so diverse (there are over 10,000 species of them!)
Cnidarians are invertebrate animals (meaning they have no backbone, like insects, worms and sponges,) which live mainly in the ocean at a wide range of depths and temperatures. Their name comes from the Greek word for stinging nettles, 'cnidos', because they have thousands of stinger cells on their tentacles. Each stinger cell releases a harpoon-like structure, which injects poison into their prey, which can hurt, paralyze or even kill them.
Cnidarians can have two basic body forms: polypoid and medusoid. Anemones and corals generally exist as polypoids (the mouth and tentacles of the organism facing up, the other side attached to a rock or other surface), while true jellyfish have medusoid forms (free swimmers with the mouth and tentacles hanging downwards).
Some cnidarians go through both stages in their life cycle. A particularly interesting case is that of the Immortal Jellyfish (Turritopsis nutricula) pictured here, which reaches sexual maturity as a medusoid and is then able to revert back to its immature polyp stage after it reproduces. Basically, it would be the equivalent of becoming an adolescent, having a baby and then reverting back to being a preteen and going through puberty all over again, and then again and again, thus staying forever young. This 'turning back the clock' can go on for many generations, in theory making the jellyfish 'immortal', although in reality competition, predation or disease would eventually kill it.
In addition to the two possible body forms, all Cnidarians share certain common features:
Cnidarians are classified into four main classes according to their body forms and characteristics, all of which you're looking at on screen:
Anthozoa (flower-like animals) are strictly polypoids that attach to solid surfaces such as rocks, shells and sometimes other living organisms. This class includes true corals, anemones and sea pens. Corals are polyp colonies which feed on plankton they trap with their tentacles. They form reefs, which are among the world's most productive ecosystems because they host photosynthetic algae. Fish and other invertebrates also live among the coral, hiding from predators among the coral's stinging tentacles in exchange for food scraps. Anemones may look like beautiful, exotic underwater flowers, but they are vicious predators that use their tentacles to lure and trap small fish and shrimp passing by.
Hydrozoa (water animals) go through both medusoid and polypoid stages and use a combination of sexual and asexual reproduction to ensure a large number of offspring. This is a varied group ranging from the tiny hydra (smallest cnidarians of all at 0.4mm) to the notorious, menacing Portuguese Man-O-War (with tentacles that can measure up to 50 meters, the length of an Olympic size swimming pool). The latter, seen in the picture on screen, has an air bladder that floats above the water surface resembling the sails of 16th century Portuguese war ships. It is not a single organism but a colony of thousands of tiny polyps forming a carnivorous mass, which paralyzes fish with its stingers and causes red welts and great pain to humans (although rarely death, contrary to popular belief).
Scyphozoa (cup-shaped animals) are also known as the 'true jellyfish.' They are the cnidarians people are most familiar with, the ones you are likely to come across while swimming at the beach or walking along the shore. They range from the size of a nickel to 2 meters (7 feet) in diameter and go through both medusoid and polypoid stages in their life. The largest member of this class, and also holding the record among all cnidarians, is the giant lion's mane (pictured here). An encounter with this monster can be rather unpleasant, given its hundreds of sticky tentacles can measure up to 30m (six cars end-to-end). Scyphozoans are a favorite snack among fish, larger invertebrates, turtles, marine mammals and sea birds.
Cubozoa (box or cube-like animals) are similar to true jellyfish, but have a distinct box-like body shape and usually only four tentacles. They are fast and agile and have specialized 'eye' structures, which contribute to their abilities as predators. The most infamous of all cubozoans is the Australian box jellyfish, which can kill a human in less than four minutes. Unlike other poisonous jellyfish, which drift around, box jellies seek prey actively. Because they are transparent, they are almost impossible to detect. In the Philippines alone, where they are abundant, 20-40 people die every year of box jelly stings.
Cnidarians are simple organisms, which have been around for millions of years and have remained relatively unchanged. They have no true organs or brains and owe their evolutionary success to an incredibly sophisticated defense mechanism, the firing of thousands of tiny harpoons filled with poison, called nematocysts, which has earned them the respect of small and large prey alike.
They come in two body types: : polypoid, the mouth and tentacles of the organism facing up, the other side attached to a rock or other surface; and medusoid,free swimmers with the mouth and tentacles hanging downwards and have radial symmetry, meaning they have top and bottom but no sides.
There are four main classes of cnidarians you should remember: anthozoa, which polypoids that attach to solid surfaces, such as rocks, shells and sometimes other living organisms; hydrozoa, which go through both medusoid and polypoid stages, and use a combination of sexual and asexual reproduction to ensure a large number of offspring; scyphozoa, which are the true jellyfish that you likely imagine; and cubozoa, which are similar to true jellyfish but have a distinct box-like body shape and usually only four tentacles.
So, next time you come across a jellyfish in the beach, instead of panicking, impress your friends with your knowledge of cnidarians!
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Back To CourseHigh School Biology: Help and Review
36 chapters | 570 lessons
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