Jellyfish
Stauromedusae
Coronatae
Semaeostomae - Disc jellyfish
Rhizostomae
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Jellyfishes (also called
jellies or
sea jellies as they are not true fish) are
animals that belong to
Phylum Cnidaria, included in the
class Scyphozoa. The name "jellyfish" is also sometimes used for the related classes of medusae (
Hydrozoa) and box jellyfish (
Cubozoa). Almost all jellyfish live in the seas and though they lack true
organ structures they feature specialized
tissues. The
adult forms of these creatures are composed of 95-99%
water. All species are found in each of the world's
oceans, with a few
species living in
fresh water. 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
Coelenterates which means "hollow
gut", and are made up of a
layer of
epidermis, gastrodermis, and a thin
jelly-like layer called mesoglea that separates the epidermis from the gastrodermis.
The jellyfish have two major body forms throughout their life.
The first form is called the
polyp stage and is characterized by a either a non-moving (sessile) stalk that catches food drifting by or a similar form that is free-floating. Their mouth and tentacles are located anteriorly, facing upwards. The second form looks like a saucer is called the
medusa stage and is characterized by a round (
radially symmetric) dome-shape body plan with food catching tentacles hanging down. It is this form which is most able to respond to and interact with its environment and is also the form most people are familiar with.
During the polyp stage, jellyfish do not have
males or
females, thus only
asexual reproduction occurs. This happens in two ways: (1) budding, to produce other polyps; and (2) strobilating, to produce medusae. During budding, the egg or planula of the jellyfish attaches itself to a hard surface where it grows into its polyp form called scyphistoma. The scyphistoma then asexually produce many ephyra that look like round jagged disks and becomes a strobila. Then the ephyras detach themselves from the strobila and become mature free living medusae. At this stage is when they can
reproduce sexually. The male will release their sperms into the water where the eggs will be fertilized.
Like all other cnidarians, jellyfish have stinging cells called
cnidocytes which contains the stinging
nematocysts on their tentacles. Whenever a prey comes in contact with a tentacle, hundreds to thousands of nematocysts fire one or another type of "hook and line" into the prey's direction. These stinging cells are thus able to latch onto the prey and the tentacles bring the prey item into their large "mouth" for digestion.These cells are activated by a simple but precise
nervous system called a
nerve net which is located in the
epidermis of the jellyfish. Impulses to these
nerve cells are sent from the nerve rings that have collected information from the environment of the jellyfish through the
rhopalial lappet, which is located around the animal's body. Jellyfish also have "eyes" or ocelli that cannot form images, but are sensitive to light. Jellyfish do not have a specialized
digestive system,
excretory system,
respiratory system, and
circulatory system. They are able to digest with the help of the gastrodermis that lines the
gastrovascular cavity, where nutrients from their food is absorbed. They do not need a respiratory system since their skin is thin enough that oxygen can easily diffuse in and out of their bodies. They do not have a
brain, a
heart, a central nervous system, a
skeletal system and also no bones and no blood. Jellyfish move using a
hydrostatic skeleton that controls the water pouch in their body to manipulate their movements.
Most jellyfish are not dangerous to
humans but a few are highly toxic, such as the
Cyanea capillata. Contrary to popular belief, the menacingly infamous
Portuguese Man O' War (
Physalia) isn't actually a jellyfish, but a colony of hydrozoan polyps.
Many aquaria, such as
Monterey Bay Aquarium, Vancouver Aquarium, and
Sea nettle
