Cnidarians are a major phylum of aquatic (and mostly marine) invertebrates that include many of the most recognizable shoreline creatures, including sea anemones, jellyfish, and corals. The name of the phylum derives from cnidos, meaning "stinging nettle" in Greek, and refers to the stinging nematocyst class of cnidae—the microscopic structures that define the phylum. (In addition to nematocysts, the other two classes of cnidae are spirocysts and ptychocysts.) The phylum first appears in the late Precam-brian or Vendian period, and the group is generally considered the sister group of all meta-
zoans besides sponges (Porifera). Cnidarians are divided into four extant classes—Anthozoa (containing sea anemones, stony corals, soft corals, gorgonians), Scyphozoa (true jellyfish), Cubozoa (box jellies or sea wasps), and Hydro-zoa (hydroids, fire corals, jellyfish, siphono-phores, Portuguese-man-o'-war). The phylum contains approximately 10,000 species.
The basic body organization of cnidarians is simple: a radially (or biradially) symmetrical animal composed of two cell layers (the ectoderm/epidermis and the endoderm/gas-trodermis), with a single body opening—the mouth. The mouth is surrounded by tentacles covered with the cnidae, and the body is either shaped into a tubelike polyp stage usually attached to the benthos or a mushroomlike medusa stage ("jellyfish") generally found swimming in the water column. A jelly layer with relatively few cells, called the mesoglea, is found between the ectoderm and endo-derm. In some groups, this layer is quite thin, while in others (for example, jellyfish), the mesoglea makes up most of the mass and the structural support of the animal. The saclike body cavity is called the coelenteron (hence the alternative name of the phylum, Coelen-terata). It functions in digestion, circulation, gas exchange, and as a hydrostatic skeleton for support and extension of polyps and tentacles, and in medusae for swimming. Many cnidarian polyps also secrete mineral or organic endoskeletons or exoskeletons. In the case of coral reef-building scleractinian (stony) corals, these skeletons can be quite massive.
The life cycle of an idealized cnidarian species alternates between the medusa and polyp phases, though the medusa stage is completely absent in anthozoans and missing from many hydrozoan life cycles. Other hydrozoans lack the polyp stage. Regarding the idealized life cycle, however, male and female medusae spawn sperm and eggs in the plankton. These gametes unite and develop into small, ciliated, pear-shaped larvae called planulae. A planula settles to the bottom and metamorphoses into an upright, feeding polyp. In many cnidarians, this polyp develops into a colony or clone as the polyp buds new polyps that either remain connected or divide, respectively. After this period of asexual growth, the polyp asexually buds new medusae into the water column, which upon sexual maturation repeat the cycle.
Although the evolutionary relationships among these classes are still controversial, most scientists consider Anthozoa to be the basal group, with the other groups diverging later. If correct, this suggests that cnidarians originally evolved with a single phase of sexual polyps (as in current anthozoans). Only later, with the evolutionary appearance of the other classes, did the sexual portion of the life cycle physically partition off into a planktonic medusa phase. Alternatively, the earliest cnidarians had both medusa and polyp phases, and the former were lost during the divergence of the Anthozoa. Such evolutionary modifications and losses of medusa are seen within Hydrozoa. Within certain hydro-zoan groups, the sexual medusa phase has gradually declined in importance, as medusae have become smaller, shorter lived, and weaker swimmers. In other groups, the medusa no longer detach from the polyp, while in others (for example, Hydra), they are so completely reduced that they are expressed as simple gonadal tissue in the polyps. In contrast, in other hydrozoans, scyphozoans, and cubo-zoans, the polyp stage has been reduced or lost, and most time and growth is devoted to the jellyfish stage, which can become quite large in the true scyphozoan jellyfish. The great variety of ways in which the polyp and medusa phases have evolved within Cnidaria clearly demonstrate the evolutionary versatility of this seemingly simple phylum.
Ecologically, all cnidarians are predators, using their tentacles and cnidae to capture and subdue prey, which then gets transferred into the mouth of the polyp or medusa. Individual polyps and colonies are either passive filter-feeders (also called suspension-feeders), in which colonies capture small prey and food particles that drift by in the water column, or sit-and-wait predators that catch prey that walks or falls into their tentacles. Some jellyfish are somewhat more active, catching prey that gets sucked into reach of their tentacles as they pulse and swim. Others float motionless and act as completely passive sit-and-wait (or ambush) predators. Some cubozoans are known to have particularly potent toxins in their nematocysts, causing the deaths of a small number of swimmers every year. Because of their stinging abilities, cnidarians have relatively few predators, though exceptions include animals that have evolved specialized abilities to deal with these defenses. Many cnidarians also contain diverse secondary metabolites that serve as additional chemical defenses against would-be predators, and many of these compounds are being investigated as potential sources of new drugs.
In addition to being predators, many antho-zoans and fewer hydrozoans and scyphozoans have evolved internal symbiotic relationships with microscopic algae (including dinoflagel-lates, called zooxanthellae, and green algae, called zoochlorellae), allowing the cnidarian-algal associations to photosynthesize and function as primary producers. Arguably, this primary producer ability reaches its height in tropical reef-building corals, and the productivity of coral reef ecosystems is largely driven by these symbiotic associations. In addition, the ability of many benthic cnidarians—such as hydroids, hydrocorals, black corals, gorgonians, and scleractinian (stony) corals—to contribute to and even define the three-dimensional structural complexity of their habitats makes these groups important ecosystem engineers within their communities.
Within the water column, some species of jellyfish have reportedly become more common in recent decades, with populations periodically experiencing explosions. These blooms have been interpreted as a sign of competitive release, with the overexploitation of many predatory fish species leaving an expanded role for jellyfish as higher-level pelagic predators.
—Daniel R. Brumbaugh
See also: Adaptive Radiation; Benthos; Coloniality; Communities; Coral Reefs; Plankton; Protoctists
Brusca, Richard C., and Gary J. Brusca. 1990. Invertebrates. Sunderland, MA: Sinauer; Ruppert, Edward E., and Robert D. Barnes. 1994. Invertebrate Zoology, 6th ed. Fort Worth, TX: Harcourt Brace; Tree of Life Web Project, Daphne G. Fautin and Sandra L. Romano. Cnidaria (Coelenterata). http://tolweb.org/tree (accessed April 28, 2002)
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