Craspedacusta sowerbyi
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| Craspedacusta sowerbyi Lankester 1880 |
Craspedacusta sowerbyi (alternatively C. sowerbii), or freshwater jellyfish, are freshwater animals in the Class hydrozoa. Contrary to what the common name implies, C. sowerbyi is not a true jellyfish, but a hydrozoan, making it more closely related to hydras. Unlike true marine jellyfish, C. sowerbii has a structure called a velum on its ventral surface. C. sowerbii is found throughout the world in freshwater bodies of water.
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[edit] Form
Freshwater jellyfish are about 20 - 25 mm in diameter when fully grown. They are shaped like an umbrella and have a whorl of tentacles around their ring canal. Most of the body is translucent with a whitish or greenish tinge, with the exception of large flat sex organs called gonads. The tentacles contain hundreds of cells called Cnidocytes, which contain nematocysts, and are used to capture prey.
[edit] Habitat
C. sowerbii is usually found in calm, freshwater reservoirs, lakes, impoundments, gravel pits or quarries. They have also been seen in river systems such as the Allegheny River, the Ohio River, and the Tennessee River. They prefer standing water, and are not generally seen in fast flowing streams or rivers.
[edit] Feeding
C. sowerbii is a predator on zooplankton including daphnia and copepods. Prey is caught with their stinging tentacles. Drifting with its tentacles extended, the jellyfish waits for suitable prey to touch a tentacle. Once contact has been made, nematocysts on the tentacle fire into the prey, injecting poison which paralyzes the animal, and the tentacle itself coils around the prey. The tentacles then bring the prey into the mouth, where it is digested.
[edit] Natural history
C. sowerbii begins life as a tiny polyp, which lives in colonies attached to underwater vegetation, rocks, or tree stumps, feeding and asexually reproducing during spring and summer. Some of these offspring are the sexually reproducing medusae. Fertilized eggs develop into small ciliated larvae called planula. The planula then settle to the bottom, and develop into polyps. However, the majority of C. sowerbii populations existing in the United states are either all male or all female, so there is no sexual reproduction in those populations. During the cold winter months, polyps contract and become resting bodies, called podocysts. It is believed that podocysts are transported by aquatic plants or animals to other bodies of water. Once conditions become favorable, they develop into polyps again. The medusa appearance is sporadic and unpredictable. Often, they will appear in a body of water where they had never been documented before in very large numbers, and they may be even reported on the news.
