Exploring Our Fluid Earth

The phylum Cnidaria (pronounced “nih DARE ee uh”) includes soft-bodied stinging animals such as corals, sea anemones, and jellyfish (Fig. 3.23 A). The phylum’s name is derived from the Greek root word cnid- meaning nettle, a stinging plant. Cnidarians are found in many aquatic environments. Sea anemones are widely distributed, from cold arctic waters to the equator, from shallow tide pools to the bottom of the deep ocean. Jellyfish float near the surface of the open oceans and in some tropical freshwater lakes. Corals are found primarily in shallow tropical waters, but a few grow in deep cold ocean waters. Small anemone-like cnidarians like Hydra sp. are also found in freshwater lakes and streams. Cnidarians range in size from tiny animals no bigger than a pinhead to graceful giants with trailing tentacles several meters long.

 

Fig. 3.23. (A) Moon jellies (Aurelia aurita) from the phylum Cnidaria

Image courtesy of Luc Viatour / www.Lucnix.be, Wikimedia Commons

Fig. 3.23. (B) Comb jelly from the phylum Ctenophora

Image courtesy of Marine Genomics, Flickr

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Some animals that look similar to cnidarians are actually not part of the same phylum. An example of this is a type of jelly called a ctenophore (Fig. 3.23 B). Ctenophores were removed from the phylum Cnidaria and placed in a new phylum called Ctenophora (pronounced ti-NOF-or-uh). Although both ctenophores and cnidarians have similar bodies with thin tissue layers enclosing a middle layer of jellylike material, scientists now group them separately. These comb rows, called ctenes (ctene meaning comb) is how the ctenophores get their common name of comb jellies.

 

 

In the phylum Porifera we saw a body formed of aggregated cells with no organization into tissue layers or organs. Cnidarians have a slightly more organized body plan, and have tissues, but no organs. Most cnidarians have two tissue layers. The outer layer, the ectoderm, has cells that aid in capturing food and cells that secrete mucus. The inner layer, the endoderm, has cells that produce digestive enzymes and break up food particles. The jellylike material between the two layers is called the mesoglea. All of these body layers surround a central cavity called the gastrovascular cavity, which extends into the hollow tentacles (Fig. 3.24). Figure 3.24 demonstrates the anatomy of the main cnidarian forms.

 

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Fig. 3.24. (A) Polyp life form

Image adapted by MarkusZi, Wikimedia Commons, in courtesy of National Oceanic and Atmospheric Administration (NOAA)

Fig. 3.24. (B) Medusa life form

Image adapted by Mariana Ruiz Villarreal, Wikimedia Commons

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Fig. 3.24. (C) Polyps from the orange cup coral, Tubastrea faulkneri

Image courtesy of Nick Hobgood, Wikimedia Commons

Fig. 3.24. (D) Medusa form of a moon jelly, Aurelia aurita

Image courtesy of Julian Fahrbach, Wikimedia Commons

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The body plans cnidarians generally have radial symmetry (Fig. 3.25 A). Because the tentacles of corals, jellyfish, and sea anemones have this radial structure, they can sting and capture food coming from any direction.

 

Many cnidarians take two main structural forms during their life cycles, a polyp form and a medusa form. The polyp form has a body shaped like a hollow cylinder or a bag that opens and closes at the top (Fig. 3.25 A). Tentacles form a ring around a small mouth at the top of the bag. The mouth leads to a central body cavity, the gastrovascular cavity (Fig. 3.24 B). Polyps attach to hard surfaces with their mouths up. Because they are sessile organisms, they can only capture food that touches their tentacles. Their mesoglea layer is very thin. Corals and sea anemones are polyps. Most of these animals are small, but a few sea anemones can grow as large as 1 meter in diameter. The second structural form that cnidarians have is called the medusa form. Medusa bodies are shaped like an umbrella with the mouth and tentacles hanging down in the water. The mouth leads upward into the gastrovascular cavity. Medusae (plural; the singular form is medusa) are not sessile, but rather are motile, meaning that they swim freely in the ocean (Fig. 3.25 C). Their mesoglea is thick and makes up most of their bulk. Jellyfish are medusae. Medusae come in many sizes ranging from small 2.5-centimeter-long box jellies to the lion’s mane jellyfish, which has an umbrella over 2 m across. In many ways polyps and medusae are really the same basic body plan, except each is upside down compared to the other. Some cnidarians go through both a polyp and medusa phase in their life cycle. However, one or the other is the dominant phase in different species. Figure 3.25 demonstrates some examples of body plans showing radial symmetry.

 

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Fig. 3.25. (A) Cylinder shaped anemone

Image courtesy of Ed Bierman, Flickr

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Fig. 3.25. (B) Leptastrea purpurea coral polyp

Image by Narrissa Spies

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Fig. 3.25. (C) Jellyfish

Image courtesy of Doug Letterman, Flickr

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Fig. 3.25. (D) Soft coral Anthomastus sp.

Image courtesy of National Oceanic and Atmospheric Administration (NOAA) Okeanos Explorer

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Fig. 3.25. (E) Porpita porpita, known as a Blue Button, a colony of hydroids surrounding a float.

Image courtesy of National Oceanic and Atmospheric Administration (NOAA) Ocean Explorer

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Fig. 3.26. Diagram of a cnidocyte ejecting a nematocyst

Image by Byron Inouye

Cnidarians have a unique feature: stinging cells called cnidocytes (NID-uh-sites). Each cnidocyte cell has a long, coiled, tubular harpoon-like structure, called a nematocyst (Greek root word nema meaning thread; Greek root word cyst meaning bag). The unfired nematocyst is inverted into itself, much like a sock bunched up and turned inside out. When the nematocyst senses food either through touch or chemoreception, it fires outward, injecting venom through its tube into the prey (Fig. 3.26). Each nematocyst can fire only once, but new cnidocytes grow to replace used ones. The structure of cnidocytes is specific to different species of cnidarians.

 

All cnidarians are carnivorous predators. Jellyfish capture small drifting animals with their stinging cnidocyte-filled tentacles. Even the sessile coral polyps and sea anemones are predators ready to sting prey, grasp it in their tentacles, and push it into their mouth. The potency of the stinging venom varies among species. Some cnidarian venoms have little effect on humans. Others are extremely toxic. The venom of the Portuguese man-of-war (Physalia physalis) is potent enough to inflict a painful sting, even after it is washed up on the beach.

 

 

 

Fig. 3.27. Hydrostatic skeleton of a sea anemone (A) Hydrostatic skeleton filled with water and extending anemone tentacles (B) Hydrostatic skeleton emptied with anemone tentacles contracted

Image by Byron Inouye

Unlike sponges, which have skeletal structures made of spongin or spicules, sea anemones and jellyfish have no skeletal structure to support their soft tissues. For support, they fill the gastrovascular cavity with water and close the mouth tight, putting the water under pressure as in a balloon filled with water. The water pressure supports the soft tissues. This feature is called a hydrostatic skeleton (Fig. 3.27). If the sea anemone opens its mouth or contracts its body wall hard, the water flows out and the body collapses. It takes several minutes to pump water back into the cavity. Coral polyps also have a hydrostatic skeleton, but they are frequently sitting in a hard skeleton made of the mineral limestone (calcium carbonate or CaCO₃). Coral reefs are the aggregated limestone skeletons of many coral polyps.

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Fig. 3.28. Anatomy of a sea anemone showing some internal structures. 1. Tentacle, 2. Pharnyx, 5. Septum, 8. Pedal disk, 9. Retractor muscle, 12. Collar, 13. Mouth, 14. Oral disk

Image courtesy of Hans Hillewaert, Wikimedia Commons

Cnidarians lack organs. This means that they do not have respiratory or circulatory systems. Like the cells in sponges, the cells in cnidarians get oxygen directly from the water surrounding them. Nutrients from digested food pass through the liquid between the cells to nourish all parts of the body, and wastes pass out by the same route. Cnidarians have a very simple nervous system consisting of cells with long, thin fibers that respond to mechanical or chemical stimuli. The fibers connect, forming a network called a nerve net (Fig. 3.28). The nerves send impulses to muscle cells, which respond by contracting. Despite its lack of complexity, the nerve net does allow cnidarians to respond to their environment.

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Cnidarians do have a more sophisticated sensory biology than sponges. The ability to respond to a stimulus of touch or pressure is called mechanoreception. When something touches the surface of the sea anemone, the nerve cells send impulses to the muscle cells in the body wall, the muscle cells contract, and the anemone moves. Chemoreception is the ability to respond to chemical stimuli. Chemoreception includes taste and smell, two ways to detect chemicals. Chemoreception is crucial to finding and testing foods, detecting harmful substances, and, in some organisms, selecting and attracting mates and finding suitable places to live. Cnidarians rely on chemoreception for these things, too. The ability to respond to changes in light intensity is called photoreception. Most cnidarians have the ability to sense changes in light and dark. Box jellies have eyes that are able to form images, making them the most derived cnidarians in terms of sensory biology. Finally, most jellyfish also have a sensory structure called a statocyst that is denser than water. The gravitational pull on the statocyst helps ocean going jellies tell which way is down.

 

To respond to stimuli, cnidarians use a rudimentary muscular system consisting of muscle cells lying in bands up and down the body wall and in a circle around the mouth cavity (Fig. 3.27). The body shortens when the vertical bands contract. If muscles on only one side contract, the body bends in that direction. The mouth closes when the circular muscle contracts.

 

 

Fig. 3.29. Generalized body plan and swimming movements of a medusa

Image by Byron Inouye

Many jellyfish are supported by an umbrella shaped structure that is composed of a modified layer of mesoglea. When a ring of muscles contracts, a jet of water is forced out from under the umbrella, moving the jellyfish forward. When the muscles relax, the stiff mesoglea springs back to its original shape, and the umbrella opens again (Fig. 3.29). Alternating muscle contraction and relaxation creates pulsating movements that propel the jellyfish through the water. Even so, jellyfish are such poor swimmers that they are considered plankton. Plankton are aquatic organisms that cannot swim against a current.

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Check out the video for an introduction to jellyfish movement and function.

Cnidarians reproduce both sexually and asexually. Some species can produce both eggs and sperm in the same organism. These organisms are called simultaneous hermaphrodites and release gametes into the ocean in egg-sperm bundles. Some species are also either male or female and produce either eggs or sperm. Fertilization (the uniting of egg and sperm) can happen externally in the water column, but can also happen internally. Many coral species reproduce externally in a process called broadcast spawning (Fig. 3.30 B). These species tend to have synchronous spawning events in which all individuals in the colony or area release their gametes at the same time. This is often triggered by environmental cues like full moons, temperature, or chemical signals from other individuals. Broadcast spawning increases the likelihood of sperm and egg from the same species meeting and for genetic mixing to take place. In other cnidarians the male releases sperm into the water, but fertilization happens inside the body when sperm from a male colony enters the female and fertilizes eggs internally. This type of sexual reproduction is called brooding, resulting in the release of a fully formed larva (Fig. 3.30 C).

 

 

Fig. 3.30. (A) Diagram of external sexual reproduction in sea anemones and corals

Image by Byron Inouye

Fig. 3.30. (B) Brain coral releasing egg-sperm bundles during a spawning event

Image courtesy of Emma Hickerson, National Oceanic and Atmospheric Administration (NOAA)

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Fig. 3.30. (C) Internally brooded larva in the tentacle of a coral polyp

Image by Narrissa Spies

Fig. 3.30. (D) Coral polyp in the process of budding into two new polyps

Image by Narrissa Spies

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Fig. 3.30. (E) Fragments of coral, called nubbins, in a coral grow-out experiment

Image by Narrissa Spies

Following fertilization in broadcast spawning cnidarians, the new organism grows into a larva that swims by means of cilia—small hair-like structures that move it along by beating back and forth. Because larvae cannot easily swim against currents, they are classified as plankton, organisms that drift. The larval stage is important in dispersing sessile species like coral. Larvae can stay afloat for a long time, drifting hundreds of miles from the parent, or they can settle within hours after fertilization. An anemone or coral larva remains in the water column until it can find a suitable habitat, attach to a hard surface, and grow into a sessile adult (Fig. 3.30).

 

Cnidarians can also reproduce asexually, by budding or fragmentation (Fig. 3.30 D, E). If many attached buds are produced, they can form a large colony. This is the mode of reproduction for which reef-building corals are famous. They can form such large colonies that they alter the structure of the ocean floor. Cnidarians can also replace lost or damaged parts by regeneration. Damaged or lost tentacles can often grow back. A small chunk of detached tissue may even regenerate into an entire new organism, as in the freshwater anemone Hydra sp. Sea anemones can also regenerate lost parts.

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