Jellyfish

[denise]

Despite their name, jellyfish are not really fish at all, and no close relation of them. They belong to the Cnidarian family, relatives of corals and sea anemones. Jellyfish are simple animals with no head, brain, heart, eyes, or ears (they consist of 95-98% water!), but they are still effective and voracious predators. They come in a wide variety of sizes, shapes and colors; most are semi-transparent and bell-shaped, surrounded by a net of tentacles. These tentacles contain stinging cells called nemanocysts, which are used for defense and capturing food. Each cell contains a tiny barb, and when stimulated by touch or movement, shoots into the prey and delivers a debilitating toxin. The potency of jellyfish sting varies amongst jellyfish, causing reactions in humans ranging from mild rashes to death. Obviously jellyfish don’t set out to harm humans, their preferred food ranges from microscopic animals and fish larvae to other jellyfish. Usually people are stung when jellyfish are carried close to shore, where swimmers inadvertently brush against their tentacles.


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Photo by Robert Palmer

Jellyfish are found in all oceans of the world from icy polar seas to the warm tropics. There are even some freshwater varieties. Currently we know of 350 different species, but more are being discovered all the time (mainly deep sea varieties). Scientists estimate there may be 2,000 species of jellyfish.


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Moon jellyfish, Boynton Beach
Photo by Robert Palmer

Jellyfish are weak swimmers, they use a form of jet propulsion for movement, but this serves mainly to keep them afloat rather than to swim any direction. Their movement is strongly influenced by the ocean’s current. Without a brain or eyes, jellyfish rely on nerve cells to help them react to food or danger. Special sense organs tell them whether they are heading up or down, into the light or away from it. They have sacs around the bell to help then to remain balanced in the water. If the jellyfish turns too far to one side or the other, the sacs stimulate nerve endings and contract muscles that reposition jellyfish in the correct direction (working similarly to the balance organs in the inner ears of human).


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Cluster of Jellyfish showing long tentacles

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Close up of moon jellyfish, Aurelia aurita, showing their four bright circular gonads

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Moon jellies at night

Moon jellyfish are found in most the worlds oceans, ranging from Mexico as far North as the Arctic. They are the most common type of jellyfish in the Atlantic Ocean. Moon jelly fish are usually seen in the summer when the currents carry them close to beaches and harbors.


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Ctenophores swimming at night


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Jellyfish, West Palm Beach
Photo by Debbie Buck