Cnidarians

Cnidaria is a large phylum composed of some of the most beautiful of all the salt and freshwater organisms: the true jellyfish, box jellyfish, coral and sea anemones, and hydra. Although Cnidaria is an incredibly diverse group of animals, there are several traits that link them together.

Most cnidarians are dipoblastic, which means that they are composed of only two layers of cells. The outer layer is known as the ectoderm or epidermis, and the inner layer is known as the endoderm or gastrodermis. These layers contain the nerve nets that control the muscular and sensory functions of the animal. Between these layers is a jelly-like noncellular substance known as mesoglea, which in true jellyfish constitute the vast bulk of the animal (hence their common name). In other species, the mesoglea may be nearly absent. All cnidarians have a single opening into the body which acts as both the mouth and anus, taking in food and expelling waste. In most species the mouth is lined with tentacles which act to capture food. The mouth leads to a body cavity known as the coelenteron, where the food is digested. This body cavity has given this phylum its other, less commonly used, name of Coelenterata.

Cnidarians have a complex life cycle that, depending on the species, may alternate between two forms. The first form is known as a polyp, which is sessile (anchored to one spot). The polyps are tubular in shape, with the mouth, often lined with tentacles, facing upwards. The bodies often contain a type of skeleton that may surround the tissues (exoskeleton) or be surrounded by the tissues (endoskeleton). These skeletons may be composed of minerals like calcium carbonate, and/or may consist of organic material such as chitin. Polyps also have a hydrostatic skeleton, where the muscles in the endoderm work against the fluid contained in the coelenteron, thus extending the polyps. Hydrostatic skeletons are also present in the tentacles, allowing them to be extended to capture food. Polyps often form large colonies, where a trait known as polymorphism may occur: various polyps in the colony may take on specialized roles. For example, one polyp may only be used for defence, while another is used for reproduction and another for capturing food. Not all polyps do this, however, and may live solitary lives. Some cnidarians, such as true coral and sea anemones, live their entire lives in the polyp stage and do not metamorphose into the second form, which is known as the medusa.

In true jellyfish and in box jellyfish, the medusa is the most prominent form. They are free-floating or free-swimming, with the mesoglea giving them buoyancy. Medusae generally have only hydrostatic skeletons, which allow the muscles to work against the fluids in the coelenteron to enable the medusae to swim. The life cycle of cnidarians that contain both the polyp and medusa forms goes generally as follows: adult medusae reproduce sexually, creating a small, ciliated (cilia are small hairs that beat back and forth, allowing for locomotion) larva known as a planula. The planula eventually settles on the sea floor and changes into a polyp. The polyp can then reproduce asexually, commonly using one of two ways: it can split in two, creating two clones of the original, or it can form a colony, where the new polyps do not split from the original but rather seem to grow off its sides. Then, depending on the species, medusae can be formed asexually from the polyps, or, as occurs with the box jellyfish, the polyp itself can metamorphose into a medusa, and the cycle begins again. As mentioned above, this cycle is common for the jellyfish and box jellyfish. The coral and sea anemones remain as polyps. The hydrozoans are the most diverse when it comes to life cycles: some species may consist of both polyps and medusae, while others are polyp-free, being only medusae, and yet others are medusa-free, being only polyps.

Cnidarians are generally carnivorous in nature, but some species, such as coral, get some of their food from special symbionts (organisms that are benefited from and benefit the organisms they are with) living within them. There are two main types of symbionts: zooxanthellae, which are photosynthetic protists (single-celled organisms) known as dinoflagellates, and zoochlorellae, which are photosynthetic algae. These symbionts capture the energy from the sun to produce sugars which are then passed on to their host as a source of food. Not all cnidarians possess these creatures however, and thus must capture their own food. Because most cnidarians lack such sensory organs as eyes, it is thought they hunt passively: namely, they just wave their tentacles and hope something brushes near. When prey does come in contact with the tentacles, special structures known as nematocysts fire like harpoon guns into the flesh of the organism, either injecting a toxin that paralyses and/or kills the prey, or entangling the prey.

Nematocysts are common in all cnidarians, and are the one major trait that separates this phylum from the others. There are approximately 20-30 types of nematocysts know to date that help distinguish between the various classes. Nematocysts are just one of three types of structures located within the cells of the tentacles and/or mouth lining that are known as cnidae (which is how this phylum is named). The other two types are known as spirocysts and ptychocysts. They all work in basically the same way: when the cnida receives the appropriate physical or chemical signal, a cover known as the operculum is moved back, allowing a hollow structure known as a tubule to fire. This tubule may contain toxins (as is the case with nematocysts), adhesives that stick to the prey (spirocysts), or may just entangle bits of mud that then form a type of mud “home” around the organism (ptychocysts - found only in one order of the anemone and coral class). Once a cnida has fired, it can no longer be used.

Most cnidarians are marine in nature, found from the shallow water to the depths of the abyss. Some species of hydrozoans can be found in freshwater ponds and lakes.

Some specialists recognize only three classes of cnidarians, but today it is generally accepted that there are four, with the box jellyfish, previously being grouped with the true jellyfish, as the newest addition. There are approx 9200 species in 4 classes:

Anthozoa (true coral, sea anemones, sea pens) 6000 spp
Cubozoa (box jellyfish) 20 spp
Hydrozoa (freshwater hydra, fire coral) 3000 spp
Scyphozoa (true jellyfish) 200 spp

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