What animals eat jellyfish?

Jellyfish are marine invertebrates belonging to the Scyphozoan class, and in turn the phylum Cnidaria. The body of an adult jellyfish is composed of a bell-shaped, jellylike substance enclosing its internal structure, from which the creature's tentacles suspend. Each tentacle is covered with stinging cells (cnidocytes) that can sting or kill other animals: most jellyfish use them to secure prey or as a defense mechanism. Others, such as Rhizostomae, do not have tentacles at all. To compensate for its lack of basic sensory organs and a brain, the jellyfish exploits its nervous system and rhopalia to perceive stimuli, such as light or odor, and orchestrate expedient responses. In its adult form, it is composed of 94–98% water and can be found in every ocean in the world.


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Jellyfish in the sea
Most jellyfish are passive drifters that feed on small fish and zooplankton that become caught in their tentacles. Jellyfish have an incomplete digestive system, meaning that the same orifice is used for both food intake and waste expulsion. They are made up of a layer of epidermis, gastrodermis, and a thick jellylike layer called mesoglea that separates the epidermis from the gastrodermis.
Their shape is not hydrodynamic, which makes them slow swimmers. But speed and low water resistance are not important as they are drifters that feed on plankton. It is more important for them that their movements create a current where the water (which contains their food) is being forced within reach of their tentacles. They accomplish this by having a body shaped like a bell which is rhythmically opened and closed.
Since jellyfish do not biologically qualify as actual "fish", the term "jellyfish" is considered a misnomer by some, who instead employ the names "jellies" or "sea jellies". The name "jellyfish" is also often used to denote either Hydrozoa or the box jellyfish, Cubozoa. The class name Scyphozoa comes from the Greek word skyphos, denoting a kind of drinking cup and alluding to the cup shape of the animal.


Life cycle and reproduction


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The developmental stages of jellyfish.
Most jellyfish pass through two different body forms during their life cycle. The first is the polyp stage; in this phase, the jellyfish takes the form of either a sessile stalk which catches passing food, or a similar free-floating configuration. The polyp's mouth and tentacles are located anteriorly, facing upwards.
In the second stage, the jellyfish is known as a medusa. Medusae have a radially symmetric, umbrella-shaped body called a bell. The medusa's tentacles hang from the border of the bell. This is the form most people know jellyfish as.
Jellyfish are dioecious (that is, they are either male or female). In most cases, to reproduce, a male releases his sperm into the surrounding water. The sperm then swims into the mouth of the female jelly, allowing the fertilization of the ova process to begin. Moon jellies, however, use a different process: their eggs become lodged in pits on the oral arms, which form a temporary brood chamber to accommodate fertilization.
After fertilization and initial growth, a larval form, called the planula, develops from the egg. The planula larva is small and covered with cilia. It settles onto a firm surface and develops into a polyp. The polyp is cup-shaped with tentacles surrounding a single orifice, perhaps resembling a tiny sea anemone. Once the polyp begins reproducing asexually by budding, it's called a segmenting polyp, or a scyphistoma. New scyphistomae may be produced by budding or new, immature jellies called ephyra may be formed. Many jellyfish can bud off new medusae directly from the medusan stage.

詳情 : http://en.wikipedia.org/wiki/Jellyfish

Turtles feed on jelly fish. Some turtles are killed when they mistaken white plastic bags disposed by people for their food.

Man eats jellyfish. Some species of jelly fish are seen as a delicacy by the Chinese.