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Monday, February 8, 2010

Freshwater Jellyfish

Freshwater jellyfish or Craspedacusta sowerbyi belongs to the phylum Cnidaria. It belongs to the class Hydrozoa. However, they are somewhat different from their marine counterparts. One obvious difference is that unlike marine jellyfish, C. sowerbyi has a structure called a velum on the ventral surface. This thin, shelf-like membranous structure extends inward from the circular edge of the jellyfish's bell. The manubrium, which ends in a mouth, extends down through a hole in the velum.

The Freshwater jellyfish originated in the Amazonian forests of South America and are currently found on all continents of the world, except perhaps Africa. The freshwater jellyfish usually gets transported along with shipments of plants and animals and finds itself in a new habitat. This unwilling transportation is possibly the reason freshwater jellyfish are found in abundance in most parts of the world.

Freshwater jellyfish can be found in freshwater lakes, reservoirs, man-made impoundments, water-filled gravel pits, rock quarries, algae-filled ponds, and rivers. Unlike their marine counterparts, freshwater jellyfish do not like currents and tides and prefer standing water. This is why they are not usually seen in fast flowing streams or rivers.

The freshwater jellyfish are somewhat flatter than their marine counterparts and extremely delicate. Almost 99% of their bodies are made of water. They grow to about 25 mm in diameter in size. The body of the freshwater jellyfish is transparent and has a whitish or greenish tinge. Their gonads are usually opaque and white in color and are visible through their gelatinous body. They can have as many as 400 tentacles at the margins of their bodies. The tentacles have a number of nematocysts, or stinging cells, that are capable of injecting venom into a prey to kill or paralyze it. Their prey mostly consists of zooplanktons including capepods. However, these nematocysts are not powerful enough to break into human skins, and so, humans don't feel any sting from the freshwater jellyfish.

The reproductive cycle of the freshwater jellyfish is similar to that of marine jellyfish. They too, go through the polyp and the medusa stage. However, the polyps themselves are capable of forming "frustules" which move freely in the water, although very slowly. After a few days of free floating, the frustules attach themselves to a suitable substrate and become polyps themselves. A newborn polyp formed in this way is about 0.3 mm long. Each new polyp usually forms a colony of 2-3 polyps by creating new polyps at the base of itself.

Freshwater jellyfish can be kept in captivity in aquariums as long as they are kept without any fish. This is because their sting could kill or paralyze small fish. Also they are very fragile and if the fish collides with them, they could die. Also, the water in the aquarium should only be slowly circulated as they cannot withstand strong currents and may die by dashing against the wall of the aquarium. The aquarium should have suitable places for the polyps to attach themselves such as rocks or ceramic pots. Young freshwater jellyfish in aquariums can be feed rotifers while older specimens can be fed desalted brine shrimp.

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