Phylum Cnidaria: Biology of Jellyfish, Corals, and Sea Anemones

Phylum Cnidaria includes animals that show radial or biradial symmetry and are diploblastic. Nearly all (about 99 percent) cnidarians are marine species. Cnidarians contain specialized cells known as **cnidocytes** ("stinging cells") containing organelles called **nematocysts** (stingers). These cel...

Phylum Cnidaria includes animals that show radial or biradial symmetry and are diploblastic. Nearly all (about 99 percent) cnidarians are marine species. Cnidarians contain specialized cells known as **cnidocytes** ("stinging cells") containing organelles called **nematocysts** (stingers). These cells are present around the mouth and tentacles, and serve to immobilize prey with toxins contained within the cells. Nematocysts contain coiled threads that may bear barbs. The outer wall of the cell has hairlike projections called cnidocils, which are sensitive to touch. When touched, the cells are known to fire coiled threads that can either penetrate the flesh of the prey or predators of cnidarians or ensnare it. These coiled threads release toxins into the target and can often immobilize prey or scare away predators. The illustration shows a nematocyst before (a) and after (b) firing. The nematocyst is a large, oval organelle inside a rectangular cnidocyte cell. The nematocyst is flush with the plasma membrane, and a touch-sensitive hairlike projection extends from the nematocyst to the cell's exterior. Inside the nematocyst, a thread is coiled around an inverted barb. Upon firing, a lid on the nematocyst opens. The barb pops out of the cell and the thread uncoils. Cnidarians display two distinct body plans: **polyp** and **medusa**. Examples of the polyp form are freshwater species of the genus *Hydra*; perhaps the best-known medusoid animals are the jellies (jellyfish). Polyps are sessile as adults, with a single opening to the digestive system (the mouth) facing up with tentacles surrounding it. Medusae are motile, with the mouth and tentacles hanging from the bell-shaped body. In other cnidarians, both a polyp and medusa form exist, and the life cycle alternates between these forms. The phylum Cnidaria contains four classes, Anthozoa, Scyphozoa, Cubozoa, and Hydrozoa. **[Class Anthozoa]** The class Anthozoa includes all cnidarians that exhibit a polyp body plan only; in other words, there is no medusa stage within their life cycle. Examples include sea anemones, sea pens, and corals, with an estimated number of 6,100 described species. These animals are usually cylindrical in shape and are attached to a substrate. A mouth opening is surrounded by tentacles bearing cnidocytes. **[Class Scyphozoa]** Class Scyphozoa includes all the jellies and is exclusively a marine class of animals with about 200 known species. The defining characteristic of this class is that the medusa is the prominent stage in the life cycle, although there is a polyp stage present. Scyphozoans display a characteristic bell-like morphology. Scyphozoans are dioecious animals, that is, the sexes are separate. The gonads are formed from the gastrodermis and gametes are expelled through the mouth. Planula larvae are formed by external fertilization; they settle on a substratum in a polypoid form known as scyphistoma. These forms may produce additional polyps by budding or may transform into the medusoid form. The life cycle of these animals can be described as polymorphic, because they exhibit both a medusal and polypoid body plan at some point in their life cycle.  Part a shows a photo of a bright red jellyfish with a dome-shaped body. Long tentacles drift from the bottom edge of the dome, and ribbon-like appendages trail from the middle of the body. Part b shows a cross-section of a jellyfish, which has nematocyst-bearing tentacles hanging from the bottom of the dome. Underneath the middle of the dome is an opening that serves as both a mouth and an anus. The opening leads to a gastrovascular cavity that is lined with a gastrodermis. The outer surface of the body is covered with an epidermis. Between the epidermis and gastrodermis is the mesoglea. **[Class Cubozoa]** This class includes jellies that have a box-shaped medusa, or a bell that is square in cross-section; hence, are colloquially known as "box jellyfish." These species may achieve sizes of 15--25 cm. Cubozoans display overall morphological and anatomical characteristics that are similar to those of the scyphozoans. A prominent difference between the two classes is the arrangement of tentacles. This is the most venomous group of all the cnidarians. **[Class Hydrozoa]** Hydrozoa includes nearly 3,200 species; most are marine, although some freshwater species are known. Animals in this class are polymorphs, and most exhibit both polypoid and medusoid forms in their lifecycle, although this is variable. The polyp form in these animals often shows a cylindrical morphology with a central gastrovascular cavity lined by the gastrodermis. The gastrodermis and epidermis have a simple layer of mesoglea sandwiched between them. A mouth opening, surrounded by tentacles, is present at the oral end of the animal. The true characteristic shared by all of these diverse species is that their gonads for sexual reproduction are derived from epidermal tissue, whereas in all other cnidarians they are derived from gastrodermal tissue.