Introduction

The cnidarians are a phylum that includes jellyfish, sea anemones, and corals. Cnidarians evolved in the Precambrian, and are amongst the earliest multicellular animals to be found in the fossil record. They are simple metazoans, with a primitive grade of organization.

Although their bodies are organized into tissues, they lack organs. Their body plan is usually radial, although this has been modified in some groups. The cup-shaped body is composed of two layers of cells with a supportive filling. Food is digested within the cup, whose only aperture is usually surrounded by tentacles. Stinging cells characterize the tentacles and outer cell wall, and are used for defense or to catch prey.

Cnidarians have an unusual life cycle, with a floating and a sedentary phase (Fig. 4.1). A swimming larva settles on the sea bed and forms a polyp. This can bud to form asexual clones that may stay attached to the parent or break away. It can also form a sexual phase that floats in the plankton, aiding dispersal of the species. This sexual phase then produces larvae that settle. Jellyfish are the sexual phase of one such group, and sea anemones the benthic stage of another. The duration of each element of the life cycle varies, with some groups being long lived on the sea bed and others in the plankton. Corals have abandoned the planktic stage altogether and reproduce sexually from the sea floor.

The asexual reproduction of corals has important consequences for their lifespan. Each polyp is a clone, and shares a single genotype. This is true for all of the polyps living at one time, and also for all of the polyps that build the colony over time. In addition, one coral colony can live indefinitely, even though the individual polyps may die off. Corals in the Caribbean have been shown to be several thousands of years old and may have colonized the sea floor as it was flooded by the postglacial rise in sea level.

Geologically, the most important class of cnidarians is the Anthozoa, which includes the three orders of skeleton-building corals: order Tabulata, order Rugosa, and order Scleractinia.

Medusoid stage

Sexual reproduction

Swimming larva

Medusoid stage

Sexual reproduction

Swimming larva

Cnidarians Sexual Reproduction

Fig. 4.1 Alternation of generations in cnidarians. Most cnidarians alternate between a planktic sexual stage and a sessile asexual stage, which can reproduce by budding or cloning.

Asexual reproduction Polyp stage

Fig. 4.1 Alternation of generations in cnidarians. Most cnidarians alternate between a planktic sexual stage and a sessile asexual stage, which can reproduce by budding or cloning.

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