Jellyfish are amongst the most prolific creatures to inhabit the marine waters in the world. Spread across 2000 species, they can be found right from the surface of the water to the depths of it. They can be found in every continent, in every sea and every ocean of the world. They usually live in swarms and some swarms can be as large as a billion jellyfish. With new species of jellyfish being discovered every year, and with the invasion of many near coastal areas by large swarms of jellyfish, it is difficult to determine the number and variety of jellyfish existing in our oceans.
Jellyfish are amongst the longest surviving species in the world. This does not refer to the life span of individual jellyfish, which can be as short as a few hours in the case of some species, but to the number of years jellyfish have existed on the surface of the world. It is a well documented fact that jellyfish have existed for at least 650 million years, since even before the dinosaurs did! But, new studies reveal an astonishing fact that perhaps jellyfish was the first creature of the animal kingdom to have existed, and not the sponge, as is commonly believed. One of the reasons that jellyfish can be as prolific as they are is the distinct lack of predators.
Jellyfish are equipped with a very sophisticated system of self preservation. They have long extending tentacles, or in the absence of tentacles, they have long oral arms. These tentacles can be tens of meters in length and are spread out around the jellyfish. These tentacles are lined with thousands of nematocysts, which are capsule-like structures, lining their tentacles. These nematocysts are, in fact, the venom apparatus of the jellyfish. When the tentacles come in contact with anything, the nematocysts get activated and inject a large amount of toxins in the flesh of the predator. If the predator is small, it can be killed by the sting of the jellyfish. But, even if it is too big to be killed, it will be paralyzed by the toxins in the jellyfish's venom. This gives the jellyfish sufficient time to escape.
This defense mechanism is the primary reason for the distinct lack of jellyfish predators. Few marine creatures can escape the reach of the tentacles and even fewer can withstand the fiery venom in the tentacles of the jellyfish. There are however some animals who do prey on jellyfish. Some of the most common and important jellyfish predators include tuna, shark, swordfish, and at least one species of Pacific salmon, as well as sea turtles, also known as leatherback turtle. Strangely enough, some of the predators of jellyfish include jellyfish of other species. One such jellyfish is the Arctic Lion's Mane or winter jelly, which can also grow to be the biggest species of jellyfish.
Jellyfish in symbiotic relationships with other sea creatures, notably crabs, also get preyed upon by the predators of those they have a symbiotic relationship with. For instance, sea birds prey upon crabs and invariable end up feeding upon the host jellyfish as well.
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