You might be suprised
to know that
all these shapes
are forms of the
common moon jelly, Aurelia aurita.
For your convenience, this information sheet is also available as part of the full Amazing Jellies teachers guide pdf.
What do you think of when you hear the word “jellyfish”? Most people think of a squishy, clear blob that stings. What shape comes to mind when you hear the word “jellyfish”? Most people think of an umbrella or bell shape.
In fact, not all jellies sting and they come in many shapes and sizes. Even the jellies with familiar bell-shaped medusae have complex life cycles that exhibit forms as different as a butterfly is from a caterpillar.
General characteristics
Jellies are classified into two groups (phyla), cnidarians (nˆ-dareq-e-ahns) and ctenophores (teenq-o-forq-ah). The common forms of cnidarians are the familiar, bell-shaped medusa and the cylindrical polyp. Ctenophores are usually spherical and smaller than a golf ball.
Jellies consist of two or three layers of cells formed around a central opening that is fringed by tentacles. This arrangement of similar parts around a central axis is called radial symmetry. With this structure, no part of a jelly is ever far from the necessities: oxygen, nutrients and water.
Jellies are 95% water by weight, which makes them fragile and transparent. Most move by pulsing. Despite the lack of a heart, brain and real eyes, they are effective predators. Many are planktonic, moving with the currents and capturing small prey such as crustaceans. Large jellies can even capture fish. Others stay put and use their tentacles to sting and engulf passing prey. The ctenophores don’t sting, but instead capture prey with sticky tentacles. Most jellies can reproduce asexually through budding, or sexually.
See our jellies fact sheet for more highlights of jellies.
Cnidarians
The term “jellyfi sh” is most
appropriately
used to describe
members of the class
scyphozoa.
However, it is not uncommon
to hear the term used for any medusa in
the phylum cnidaria.
Cnidarians are best known for their nematocysts, specialized structures in
certain cells that shoot out stinging threads for defense or to kill prey.
Other features that help them function are simple nerve nets, contractile cells
(instead of muscle cells), simple eyespots and receptors that respond to touch
and chemicals. Cnidarians are generally divided into four classes: Hydrozoa,
Scyphozoa, Anthozoa and Cubozoa.
Hydrozoans
Hydrozoans include the Portuguese man o’ war, freshwater hydra, fire coral (hydrocorals) and hydroids such as “snail fur,” which is commonly found on shells occupied by hermit crabs.
Most Hydrozoans are small and colonial, but many do have a distinctive medusa form as part of their life cycle. The Hydrozoan medusa is distinguished from that of Syphozoans by its smaller size and by a shelf-like structure called a vellum along the inside edge of the bell. In colonial forms, the polyps bud, but stay attached and share continuous body layers. However, individual members of the colony can perform different tasks, such as capturing prey, digestion and reproduction. Hydroid colonies are frequently mistaken for seaweed because the small budding polyps form soft, branching structures. The Portuguese man o’ war, on the other hand, is mistaken for a “jellyfish” because of its size and form. It is not a medusa, but a colony of specialized polyps, and some of those stinging polyps can be very painful.
Scyphozoans
Scyphozoans are commonly known as “jellyfish” because the medusa is the dominant form in the life cycle. They include moon jellies, lion’s manes and sea nettles. Most Scyphozoans are marine animals that live in shallow water. Typically, the central layer, or mesoglea, is thick and large, giving them the characteristic jelly texture. They have long, frilly oral arms and circular gonads that are visible through the bell.
Anthozoa
Anthozoa, the largest class of cnidarians, includes sea anemones, corals and seapens. Anthozoans do not have a medusa stage and can live as solitary individuals or in colonies. Though similar in overall shape to Hydrozoan polyps, they are usually much larger and have a signifi cantly different internal structure. Some sea anemones can reach one meter across.
Cubozoans
Cubozoans, or “box jellies,” resemble Scyphozoans at fi rst glance. However, the umbrella of the Cubozoan medusa is a distinct cube shape and has four rubbery appendages to aid in swimming. This class is frequently represented by the infamous “sea wasp.” It can reach two meters in length, but is difficult to see because it is transparent. More than 70 people in Australian waters have received fatal stings from this jelly since the 1980s.
Ctenophores
Ctenophores are commonly known as “comb jellies.” The name ctenophore is derived from the Greek work for “comb” and refers to the eight rows of comb plates that move in sequence along its body. Even when abundant, they are difficult to see due to their small size and transparency. Sea gooseberries and sea walnuts are representatives of this phylum. Like cnidarians, ctenophores are drifters that are made of thin tissues surrounding a gastric cavity. Unlike cnidarians, they are harmless. Instead of stinging, they capture food with specialized sticky cells.
Luminescence is a spectacular characteristic exhibited by comb jellies. The light produced in the body wall emanates into the comb rows, creating the effect of neon lights. Cnidarians possess true muscle cells for movement. For the most part, they are hermaphrodites and can reproduce only sexually. They cannot replicate themselves by budding. Unlike cnidarians, they do not have a polyp stage.
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