Freshwater jellyfish or hydromedusae (Craspedacusta sowerbyi)
Freshwater jellyfish or hydromedusae
(phylum Cnidaria, class Hydrozoa, species Craspedacusta sowerbyi)
Like other cnidarians (organisms that have cnidocytes – specialized stinging cells used to capturing prey), Craspedacusta sowerbyi is an opportunistic predator, feeding on small organisms that come within its reach. Both polyp and hydromedusa forms use cnidocytes to capture prey.
They mostly occur in flooded quarries, ponds, backwaters of rivers and other still water bodies. Freshwater jellyfish are now common in temperate climates almost globally.
An up and down swimming motion is accomplished by the contraction and expansion of the bell and a thin circular membrane called a velum located on the underside of the bell.
Craspedacusta sowerbyi hydromedusae are up to 25 mm (approximately 1 in.) in diameter.
Craspedacusta sowerbyi is able to reproduce both sexually and asexually. Mature hydromedusae reproduce sexually by releasing eggs and sperm to the water column. After the fertilization and cleavage, the embryo begins to elongate and grows into ciliated planula (larva) which creeps on the substrate. The planula then settles to the bottom, and develops into a polyp (the term „planula“ is applied to this stage until they settle somewhere and become polyps). New polyps grow and undergo budding. Polyps can produce three types of buds:
- Daughter polyps that remain attached to the parent and form a colony.
- Buds capable of producing frustule larvae (asexual reproduction) which move to new locations before metamorphosing into new polyps.
- Under suitable conditions, when the water temperature is above 20° C long enough, the polyps make special buds to produce medusae.
By most of us, jellyfish are considered as typical inhabitants of the sea. Yet we can meet them more and more frequently in freshwater habitats as well. Craspedacusta sowerbyi was first described from specimens collected in 1880, from water-lily tanks in Regents Park in London, where they were transported along with the exotic plants from Southeast Asia.
Craspedacusta sowerbyi is a hydrozoan (Phylum Cnidaria, Class Hydrozoa), which is the most easily recognized when it takes the form of a small, bell-shaped jellyfish, known as a hydromedusa. The main difference between Craspedacusta sowerbyi and „true“ (scyphozoan) jellyfish is the presence of a velum – thin, circular membrane on the underside of the bell.
Most of the body is transparent or translucent, with a whitish tinge. Hanging down from the center of the inside of the bell is a large stomach structure called a manubrium with a mouth opening. Four large flat sex organs (gonads) are attached to the radial canals, and are usually opaque white. Tentacles of varying lengths are tightly packed around the bell margin, arranged with short tentacles between longer ones. The total number of tentacles varies from 50 to several hundreds. Each tentacle bears thousands of cnidocytes, and each cnidocyte contains nematocyst (stinging organelle).
Craspedacusta sowerbyi is a predator on zooplankton including daphnia and copepods. Drifting with its tentacles extended, the medusa waits for a prey to touch a tentacle. Once contact has been made, nematocysts on the tentacle fire into the prey (C. sowerbyi’s nematocysts are too small to penetrate human skin), inject poison which paralyzes or kills the animal, and the tentacle itself coils around the prey. The tentacles then bring the prey into the mouth, where it is digested. Waste is finally expelled out of the same opening.
Conspicuous swarms of hydromedusae appear sporadically, but are only one part of the animal’s life cycle (described above). Craspedacusta sowerbyi also occur as:
- Planulae – larvae produced sexually by the hydromedusae.
- Frustules – larvae produced asexually by the polyps. Frustules are much bigger than planulae.
- Sessile polyps.
- Microscopic dormant podocysts. During the cold months, polyps contract and enter dormancy as resistant (resting) bodies called podocysts. It is believed that podocysts are unintentionally transported to other locations on bird feathers, animal fur but also on equipment for diving or fishing. Once conditions become favorable, they develop into active polyps again.
The impact of this widespread jellyfish is unclear. In large number could probably influence relative zooplankton species structure. There are no known regulations for this species and crayfish and turtles are considered as the only important predator of the hydromedusae.