11.0.3 11.5: Jellyfishes and other cnidarians are among the most poisonous animals in the world.

If you feel a sting when you are swimming in the ocean, you have met a jellyfish. If you are lucky, it will be one of the thousands of species that feed on tiny floating organisms called plankton, because these species have only mild stings. If it is a species that feeds on larger prey such as fish or shrimp, however, you are definitely unlucky. These jellyfishes have powerful stings that can cause extreme pain or even death.

The jellyfishes, along with sea anemones and corals, belong to the phylum Cnidaria (pronounced nigh-dare-ee-ah). As with all animals except the sponges, the 11,000 species in this phylum have defined tissues. The cnidarians also all have a simple body plan, with radial symmetry (FIGURE 11-7).

There are two types of cnidarian bodies: a sessile polyp and a free-floating medusa, which is the form most people picture when they think of a jellyfish. In some species, individuals spend part of their life cycle as a polyp and part as a medusa. Other species exist only as medusas or, as in corals and sea anemones, only as polyps.

Cnidarians are carnivores and use their tentacles to capture and feed on a wide variety of marine organisms, from protists to fish and shellfish. Their method of capturing prey relies on a sort of weapon that is unique to the cnidarians—a stinging cell called a cnidocyte. All cnidarians have tentacles, located near their mouth, that are armed with rows of stinging cnidocytes. Each cnidocyte has a coiled thread with barbs inside and a “trigger” on the outside. When something comes in contact with the trigger, perhaps prey (or perhaps your leg as you swim in the ocean), the coiled thread is ejected and, like a harpoon, can penetrate the prey, often injecting a toxin (FIGURE 11-8). The Portuguese man-o’-war is one of the dangerously poisonous species of cnidarians. Being stung by a Portuguese man-o’-war is painful (extremely painful if the unlucky swimmer becomes entangled in the tentacles and receives hundreds of stings), but these encounters are rarely fatal.

We explore here some of the great diversity among three familiar groups of cnidarians.

Corals The corals live as small (just a few millimeters long) polyps in large colonial groups, familiar as the beautiful structures found in coral reefs. Corals sting prey and use their tentacles to catch plankton (and even, on occasion, small fish), which are directed into the mouth by the tentacles, then digested in the stomach. Corals most commonly reproduce sexually, with both external fertilization (releasing sperm and eggs into the water, where they can fuse and form larvae) and internal fertilization (in which only sperm are released and can fertilize eggs within female corals). Corals can also reproduce asexually.

Corals grow in a wide variety of shapes—from free-standing spherical or branched forms to crusts growing on rocks or other corals. All of this is possible because corals secrete calcium carbonate to create hard shells on which a multitude of individual coral polyps can live as a colony. Such assemblies of giant calcium carbonate skeletons and corals are called coral reefs. Coral reefs provide an environment that is home to a greater diversity of species than any other marine habitat. The Great Barrier Reef, located in the Coral Sea, off the coast of Queensland, Australia, extends more than 1,500 miles (about 2,600 km) from north to south. A truly awesome display of biological productivity, this reef is the largest biological structure in the world and is easily visible from the International Space Station.

Coral reefs are among the first casualties of global warming. Although coral polyps can catch prey using their cnidocytes, they obtain most of their nutrition from algae, called zooxanthellae, that live symbiotically within the polyps. Each partner contributes and receives something of value in the relationship. The algae use the carbon dioxide produced by the polyps to conduct photosynthesis, and the polyps gain oxygen and nutrients produced by the algae. Strangely, when coral polyps get too hot they expel their zooxanthellae. Because the zooxanthellae are responsible for the colors of many corals, as well as much of their nutrition, when coral polyps expel their zooxanthellae, the coral appears white—a phenomenon called coral bleaching. The zooxanthellae may return to the polyps when the water cools, but repeated bleaching events often lead to the death of the coral polyps. The world’s oceans are already warming, and coral bleaching events are becoming more and more frequent (FIGURE 11-9).

Sea Anemones Sea anemones resemble flowers, and some of the colorful species are popular additions to saltwater aquaria. The polyp form of a sea anemone looks a bit like an upside-down medusa form of a jellyfish—the tentacles with their stinging cells are at the top of the anemone’s body, surrounding its mouth. Sea anemones have a larval stage that swims freely, then settles on a rock and metamorphoses into the adult form. Even as adults, many sea anemones can craphelan a few inches a day, and wandering sea anemones are often found in seaweed.

Jellyfishes The jellyfishes (which are not fish!) range tremendously in size. The Asian giant jellyfish is more than 6 feet (about 2 m) across and weighs nearly 500 pounds (more than 200 kg). When swarms of this giant jellyfish appear, they clog the nets of fishing boats with a sticky mass of toxic stingers. At the opposite end of the size scale is the Irukandji jellyfish, about the size of a hen’s egg. This species is so deadly that, in 2007, the sighting of just five Irukandji in Hervey Bay, Queensland, completely halted the filming of a major Hollywood movie (Fool’s Gold), which had to be completed in the safety of a studio.