The cnidarians, a varied group of animals that includes corals, sea anemones, sea pens, sea pansies, jellyfishes, and hydroids, are among the most beautiful of all marine invertebrates. Many species have a flower-like look and they are often considered delicate and soft. But beware. Their delicate looks belie their potent nature. Cnidarians possess an armament of well-concealed yet potent stinging cells.
In fact, the word Cnidaria, (pronounced with a silent “C,” ni-dare’-ee-uh) derived from the Greek word knide which means nettle, refers to the stinging cells found in the tentacles of the members of this phylum. These cells comprise the defining characteristic of this phylum; thus all animals that bear the stinging cells known as cnidocytes are cnidarians by definition.
Worldwide, there are slightly more than 9,000 living species of cnidarians. The vast majority occur in salt water, but there are a handful of freshwater species. Biologists say cnidarians represent the most basic forms of animals describable as complex organisms. The phylum consists of four classes. Hard corals, soft corals, sea anemones, sea pens and sea pansies are described in the class Anthozoa (an-thuh-z¯o -uh). Jellyfishes are described in the class Scyphozoa (si-fuh-z¯o -uh). All hydroids, fire corals, and the Portuguese man-of-war are members of the class Hydrozoa (hi-dra -z¯o -uh), while the class Cubozoa (coo-ba-z¯o -uh) includes box jellies and sea wasps. It is largely believed that all cnidarians evolved from hydroid-like ancestors. Unfortunately, many divers are all too familiar with the stinging cells of many cnidarians. Even slight contact with many species can produce painful stings that result in burning welts. They eventually blister and are slow to heal. Most cnidarians are attached to the bottom, and those that swim are rather slow moving, depending more upon wind, current and wave action than their own muscular contractions to take them on their life journey.
Having little mobility, but needing to eat and escape predators presents a potential problem for any animal. Cnidarians are incapable of pursuing prey or outdistancing predators. While some creatures such as sponges solve the dilemma of limited mobility by filtering the water for nutrients, cnidarians overcome the problem by deploying fast-acting neurotoxins through their stinging cells. These toxins can immobilize many prey and repel many predators upon contact.
Experiments show that some cnidarians can fire their explosive stinging cells within 3 milliseconds after contacting another animal. This lightening-fast reaction is one of the fastest cellular responses in nature, and it occurs with enough velocity to easily penetrate the hard shells of many crustaceans. Slight contact or even a pressure-wave from a nearby moving object can cause the stinging cells to fire due to mechanical stimulation. Specific chemical stimulation can also cause the stinging cells to fire. Despite the potency of their arsenal, many cnidarians are commonly preyed upon by a variety of predators ranging from nudibranchs to turtles.
As a rule, the stinging cells, known as cnidoblasts (nii’-doe-blasts), are concentrated in the tentacles of cnidarians. The stinging cells are equipped with nematocysts (nuh-mat’-o-sists) which look like a miniature harpoon. These structures are always coiled and ready to be unleashed. When stimulated, the stinging cells explode, releasing the coiled nematocysts. The stinging cells are highly concentrated and, when acting in near unison, collectively spear and ensnare the victim while the fast-acting neurotoxins are injected.
The harpoon-like design of the nematocysts enables the tentacles of the cnidarian to hold smaller prey. The enshrouding tentacles are then drawn in toward the mouth of the deadly cnidarian and the prey is consumed.
The Body Plan of Cnidarians
The bodies of all cnidarians display radial symmetry, meaning that the body parts radiate outward from the center. The animals have a top and bottom, but no front or back, or left or right side. A ring of tentacles surrounds a centrally located mouth. The body of living animals is gelatinous, and some animals such as the corals are surrounded by a hardened, limestone skeleton.
Cnidarians occur in one of two general body forms: the bottom-dwelling polyp and the free-floating medusa. As divers, we are most familiar with the easily recognizable polyp form found in adult corals and in sea anemones. In corals and anemones the polyps are attached to the sea floor via a foot-like disc, although in many instances the presence of a disc is not immediately obvious to the casual observer. In corals, each tiny polyp is an individual animal. The mouth is on the surface opposite the disc, and the mouth and surrounding tentacles face the surface or water column.
The body design of the medusa can be seen in jellyfishes. With this body, the animal is free-swimming and the mouth and tentacles usually point toward the sea floor.
Cnidarians are the most primitive animals that possess specialized body systems. They lack specialized organs and their nervous system consists only of a diffuse nerve net that coordinates nerve actions and reactions. Cnidarians have a very simple digestive system and lack specialized systems controlling basic bodily functions such as respiration, circulation, and the elimination of wastes.
Surprising to many divers is that corals are living animals. Each individual polyp is an organism. However, with very few exceptions individual polyps can not survive outside an associated colony.
The foundation of any coral reef is the skeletal remains of uncountable numbers of once-living corals, most of which were no larger than an American penny. Over time these polyps commonly create reef systems that extend for hundreds of square miles. In fact, Australia’s Great Barrier Reef, the world’s largest reef system, is more than 1,000 miles long and in some places as much as 150 miles wide.
Corals are typically categorized two ways: hard or stony corals and soft corals. Hard corals include all species known as reef builders. When stony corals die, deposits from their limestone-based skeletons contribute to the foundation and growth of the reef. Conversely, while soft corals add considerably to the beauty of coral reefs, they add little to the long-term development of a reef.
All reef-building corals are colonial animals. Living generations of coin-sized polyps reside atop limestone skeletons of countless generations of their ancestors. Because stony corals form the very foundation of all of the world’s tropical reefs, many biologists consider these organisms the single most important invertebrates in warm, shallow seas.
Despite their hard feel and rugged appearance, stony corals are fragile. Most require a minimum water temperature of 68º F (20º C), and that is why hard corals can not survive in temperate seas. The solitary coral which occurs in California waters is an exception.
Depending upon the species, some hard corals capture their planktonic prey by stinging it with their tentacles. Others use a layer of mucus to help ensnare their prey. Some water movement in the form of current or upwelling is needed to help supply nutrients for many corals.
Hard corals depend upon the symbiotic relationship with tiny algae known as zooxanthellae that live within the tissues of the hard corals. The corals use the byproducts of photosynthesis conducted by the algae for their own food. This means that hard corals can survive only in water that is shallow and clear enough for sufficient sunlight to penetrate.
The common names of many species of hard corals are derived from their general appearance. As examples, some of the more prominent Caribbean species include staghorn, elkhorn, finger, brain, pillar, star, plate, flower and lettuce coral. The various species’ preferences range from shallow waters where the surf breaks to considerably deeper, calmer portions of the reef.
Numerous species of small organisms such as flatworms, copepods, mollusks, echinoderms and crustaceans hide in the nooks and crannies formed by coral heads. Countless fishes use the corals as shelter, hiding and sleeping under ledges and within the structures of branched species. Some fishes such as sergeant majors use corals as spawning sites with the eggs being attached to suitable dead surfaces. And many worms, sponges and other boring organisms use the corals as a food source and a surface for attachment.
During the day, the polyps of many corals retract, but at night the polyps open and the stinging tentacles extend into the water column where they can capture their prey of plankton.
While soft corals do not contribute to the long-term growth of coral reefs because their skeletons lack the calcium carbonate found in hard corals, they do add considerable beauty to many coral reef communities. A list of soft corals includes the colonial animals known as sea fans, sea whips, sea rods, sea plumes, sea fingers, deep-water gorgonians, and wire corals. All are also referred to as gorgonians, a term derived from their order, Gorgonacea.
As is the case with hard corals, the various species of soft corals tend to be found in characteristic places along coral reefs. The Caribbean sea fingers, sea whips, sea plumes and sea rods are found in shallow water along the reef flat while sea fans grow in a plane that stretches across the path of the prevailing current. This orientation enables the individual polyps in the fan the optimal chance to catch food. A variety of animals such as brittle stars, hermit crabs and numerous species of cowries as well as a variety of cryptic fishes can be discovered on sea fans or on the skeletal base. Basket stars and crinoids commonly use sea fans at night to give them good vantage points from which they can reach into nutrient-rich currents and capture food.
Soft corals commonly provide homes for well-camouflaged fishes, mollusks, crustaceans and echinoderms. Some of these animals prey upon the soft corals, while others use the corals as a place to hide or deposit eggs. Finding small creatures hiding in soft corals can seem almost impossible at first, but with a little patience you are likely to discover a community of cryptic creatures going about their secretive lives.
Without question, sea anemones are among the most beautiful and delicate invertebrates. Sea anemones occur from tide pools to depths well below safe sport diving limits. Lacking a hard skeleton, their bodies often flow back and forth with surge and water movement. In some species, their long flowing tentacles give the animal a flower-like appearance. In fact, the name of their class, Anthozoa, is derived from the Greek word anthos which means flower, and zoa for animals. Anemones occur as solitary and colonial organisms. In many instances they appear scattered randomly about a reef. In other settings their high density obscures the reef.
Despite the fact that the tentacles of sea anemones are laced with potent stinging cells, several animals have adapted the ability to use the anemone as a place to live or hide. Numerous species of crabs, shrimps and some fishes can often be seen nestled among the tentacles. Who among us can resist spending hours watching colorful anemonefish wriggling away from the tentacles? Anemonefishes are protected from the anemone’s sting by a thick layer of mucus.
Sea Pens and Sea Pansies
Members of the class Anthozoa, sea pens are colonial organisms commonly found in the sand flats of temperate seas. These beautiful animals generally require quiet waters and rarely occur in surf zones. The colony of polyp-bearing lobes is organized around a vertically oriented axis made from calcareous secretions. The lobes are attached to a bulb buried in the sand.
During the day and in surgy conditions the lobes tend to retract and the stalk withdraws into the sand leaving a couple of inches of the sea pen exposed. As conditions improve the stalk extends upward and the lobes extend as the polyps reach out into water column where they can capture their prey.
Sea pansies are colonial animals closely related to sea pens. Sea pansies look like purple-to-blue heart-shaped discs embedded in the sandy bottoms of some temperate seas. The colony uses a stalk on the underside to anchor into the sand while specialized cells assist in the capture of food, respiration and defense.
All of the world’s approximately 200 species of jellyfishes are members of the class Scyphozoa. Jellyfishes display some control over their attitude and direction through the pulsating rhythmic contractions of their bell. They are relatively poor swimmers that end up going wherever the combination of wind, current, wave action and upwellings take them. As a result, they often occur in large concentrations due to the consistency in prevailing conditions.
The best advice from one perspective is “beware, when you see one jellyfish you are likely to encounter other members of the same species on the same day.” An opposing viewpoint states that many jellyfishes are gorgeous animals and we are quite lucky to see them in their natural habitat.
Some jellyfishes are richly colored hues of purple, burgundy and yellow, while many others are transparent to off-white coloration which can make them difficult to see in lightly hued surrounding water. But regardless, in most species the tentacles, which trail the bell, are nearly transparent and can be quite difficult to see. In a few species, the tentacles, where stinging cells are concentrated, trail as far as 40 feet behind the bell. It is also wise to keep in mind that size does not correspond to potency. Some of the smallest species of jellyfishes pack the most potent stings.
The prey of most jellyfishes consists of small fishes and myriad planktonic life forms. Some small fishes are known to hide among the deadly tentacles of some species of jellyfishes. For many years it was thought that these fishes were somehow immune to the stings of the jellyfishes, but studies have demonstrated that this is not the case. These fishes, too, are involved in a potentially deadly game and must be extremely wary to avoid the tentacles while using them as a means of protection from potential predators.
A variety of sea turtles actively prey on jellyfish. And, of course, just when you think you have jellyfish and cnidarians all figured out, you discover that a few species lack stinging cells. The dominant species of jellyfish that inhabit Palau’s famous Jellyfish Lake gain food as a byproduct of symbiotic algae that live in the tissues of the jellyfish. They do not need to sting prey to acquire food.
Box Jellies and Sea Wasps
A group of cnidarians known as box jellies and sea wasps represent the biggest “stinging” threat to divers and swimmers. While many divers think of these animals as types of jellyfish, there are enough differences between the two groups that scientists place the box jellies and sea wasps in a separate class.
Most box jellies occur in the waters of the South Pacific, especially Australia where concentrations commonly force beach closures. Box jellies get their common name from the fact that their bell is shaped like a box and their tentacles are attached to the bell at four “corner” points. In many specimens there appears to be more than four main tentacles as the tentacles commonly branch out, but a close look reveals four points of attachment. As a general rule of safety, any jellyfish that appears to have four main tentacles should be avoided.
Sea wasps occur in many parts of the Caribbean. While not as dangerous as box jellies to humans, sea wasps are, nonetheless, best avoided. Sea wasps are typically less than 10 inches (25 cm) long with a bell that is 1-3 inches (2.5-8 cm) wide. Despite their diminutive size, these little guys pack a powerful wallop. The bodies of sea wasps are nearly translucent and can be difficult to see.
Fortunately, sea wasps prefer to inhabit deeper waters during the day when most diving is done. At night, however, these small, semi-translucent animals tend to make their way to the surface where they prey upon plankton. Sea wasps commonly gather under the bright lights of docks and around boats that have bright anchor and deck lights, particularly on very calm nights.
Worldwide there are about 2,700 species described in the class Hydrozoa, a group of animals that include solitary and colonial hydroids, fire corals and the Portuguese man-of-war. Despite their jellyfish-like appearance, Portuguese men-of-war are colonial hydroids. Divers and boaters most often sight a man-of-war by noting its purple-to-blue float bobbing on the surface. Tentacles laced with potent stinging cells trail underneath the float. In some specimens the tentacles can reach 20 feet. The virulent stinging cells are easily capable of paralyzing any pelagic fish and in humans can cause extreme pain requiring medical attention.
Colonial hydroids are often seen on the blades of giant kelp plants in southern California waters, while larger, more prominent colonial species referred to as branching hydroids, feather hydroids and bushy hydroids are seen in temperate, semi-tropical and tropical regions.
Many divers who have explored the Caribbean and other tropical waters are quick to see the colonial hydroids that resemble small, budding twigs or groups of bushy feathers, but they fail to recognize a solitary hydroid. A typical solitary hydroid is only the size of a small coin. Observant divers exploring the waters of the Cayman Islands and the reefs off Belize find solitary hydroids on the ends of the branches of purple sea plumes. From this vantage point, the animal is able to reach into the water column and ensnare its prey of small, planktonic creatures. The location helps the hydroid, but is harmful to the sea plume. The branch of the sea plume near the hydroid often dies, and the sea plume will not bear polyps near the hydroid.
Though California waters are too cold to support reef building corals, the state’s waters are blessed with a beautiful species known as purple coral. Also known as hydrocoral, purple coral is actually a type of colonial hydroid. No less beautiful are the red, pink, and purple sheets of a species known as encrusting hydrocoral. Occurring from the low intertidal zone to depths of about 100 feet (30 m), solid sheets of this species commonly extend as wide as 3 feet (1 m).
Despite a hard, stone-like appearance similar to that displayed by reef-building corals, the species known as fire corals are not true corals. They are hydroids. While the use of the term “coral” is incorrect in a strictly scientific sense, the term “fire” is certainly apropos. Contact with any of a number of species of fire corals is likely to cause immediate, intense, burning pain.
Fire corals occur in a variety of forms, all of which should be considered as “look, but don’t touch” species. If you take a close look at fire corals you will see a number of fine, hair-like structures which contain the stinging nematocysts. The extremely virulent stinging cells not only enable fire corals to capture prey and avoid potential predators, but also enable them to invade and overtake territory already occupied by some hard corals.
Clearly, the cnidarians are a varied group of animals. For such relatively simple organisms they add considerable beauty and fascination to the underwater world. To enjoy, all you need to do is pay heed to the mantra, “look, but don’t touch.”
Not So Simple…
The lives of cnidarians are as simple as one might suspect in such relatively simplistic organisms. Many species reproduce both asexually and sexually. In some species, the polyps are separately sexed meaning some animals are male and others are female. However, other species are simultaneous hermaphrodites — male and female.
Asexual reproduction involves budding, a form of natural cloning, in which a young animal forms on and then splits off of an adult. Budding can be seen in some corals and sea anemones. Sexual reproduction entails genetic recombination via sperm and eggs of genetically different adults.
Both methods of reproduction offer their advantages and disadvantages. The primary advantage of budding is that no partner is needed. Thus, there is no need to spend energy looking for and selecting a suitable mate. The disadvantage is that a single predator, or a single defect or disease, can quickly wipe out an entire population because there is no genetic diversity.
Sexual reproduction results in populations of animals that are genetically dissimilar despite great similarities in appearance and design. At a genetic level, the concept of diversity plays a crucial role in the ability of a species to survive over the long haul in a world of constantly changing environmental factors. While the cross-fertilization accomplished by sexual union of sperm and egg contributed by different adults offers this significant advantage, it also has its disadvantages. In many species considerable energy must be spent in finding and wooing a mate.
In many cnidarians, a lack of mobility and immediate access to a willing partner capable of successfully mating pose significant problems. Communication via special hormones known as pheromones is the key to overcoming this dilemma. Pheromones are released into the water in order to advertise a state of readiness, and once this communication occurs, individual animals release sperm and eggs into the water.
What About Getting Stung?
There is good news and bad news when it comes to humans getting stung by cnidarians. The good news is twofold. One, human skin is too thick to be penetrated by the nematocysts of many cnidarians. And two, in some people the sting is almost unnoticeable, causing little if any pain and no long-lasting effect.
The bad news is that stings from several species can be very painful, and in some cases, fatal. A word to the wise: avoid getting too cavalier about the potential problems from a cnidarian sting, even one from species that look flower-like and harmless.
Similar to stings from bees and wasps, reactions in humans vary individually. This is not, however, to suggest that if you do or do not react severely to bee and wasp stings that you will or will not react in similar fashion to the sting of a cnidarian. Severe burning and itching, swelling and painful blistering rashes are common. In some cases of stings from sea wasps, jellyfishes and Portuguese-men-of-war, intense, paralyzing pain and respiratory difficulties occur almost immediately.
But before allowing yourself to get too alarmed, be aware that there is some more good news. A thin wet suit, hood, and gloves will go a long way toward preventing many stings.