Jellyfish

Jellyfish

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Jellyfish

Jellyfish are free-swimming members of the phylum Cnidaria. They have several different basic morphologies that represent several different cnidarian classes including the Scyphozoa (about 200 species), Staurozoa (about 50 species), Cubozoa (about 20 species), and Hydrozoa (about 1000-1500 species that make jellyfish and many more that do not)^[1]^[2]. The jellyfish in these groups are also called, respectively, scyphomedusae, stauromedusae, cubomedusae, and hydromedusae; "medusa" (plural "medusae") is another word for jellyfish. Jellyfish are found in every ocean, from the surface to the deep sea. Some hydrozoan jellyfish, or hydromedusae, are also found in fresh water. Most of the information about jellyfish that follows in this article is about scyphozoan jellyfish, or scyphomedusae. These are the big, often colorful, jellyfish that are common in coastal zones worldwide.

In its broadest sense, the term jellyfish is sometimes used also to refer to members of the phylum Ctenophora. Although not closely related to cnidarian jellyfish, ctenophores are also free-swimming planktonic carnivores, are also generally transparent or translucent, and occur in shallow to deep portions of all the world's oceans. Ctenophores move using eight rows of fused cilia that beat in metachronal waves that diffract light, so that they sparkle with all of the colors of the rainbow. The rest of this article deals only with jellyfish in the phylum Cnidaria.

Anatomy and morphology
Body systems
Jellyfish blooms
Life history

Etymology and taxonomic history
Importance to humans

Culinary uses
In biotechnology
In captivity
Toxicity to humans

Systematics
See also
References
External links

Anatomy and morphology

A typical scyphomedusan jellyfish.

Jellyfish are simple animals. Most, but not all of them, swim through the water (a few spend their lives attached to the bottom). Although there are many variations on this form, the body is composed of a bowl-like bell, also called the umbrella, which rhythmically opens and closes, by contracting muscles and then releasing the contraction, in order to swim. They may swim in any direction, but the closed end of the bell always leads if they are actively swimming. In different species, the bell can be hemispherical, even more fully closed, or quite open and flat. The outside of this bowl-shaped body is technically known as the exumbrella and the inside or underside is called the subumbrella.

This body usually, but not always, has tentacles attached to the bell margin - there can be many hundreds of tentacles, or as few as one (or zero). In most species, arrangement of tentacles around the bell is in some sort of four- or eight-part symmetry, or multiple of four or eight. In a few species, the tentacles are attached either on the outside of the bell above the margin, or on the inside, above the margin, but attachment at the margin is most common. In the center of the underside of the bell (the subumbrella) is some sort of feeding structure that includes a central mouth opening and often some other highly ornamented structures, called oral arms, that are used to help grasp (and sometimes digest) prey. The oral arms may be many cm long, spiraled and frilly, or they may be short and highly divided, sometimes with perforations, and sometimes with accessory clublike structures hanging down around the central mouth.

The tentacles, edges of the mouth (usually called lips), and the oral arms are covered with thousands of microscopic stinging structures called cnidocysts or nematocysts (produced by cells called cnidocytes), that are used to catch prey or for defense. (These structures, also known as cnidae are common to all animals in the phylum Cnidaria.) Cnidocysts are tiny capsules filled with a combination of toxins and enzymes that either are triggered, largely by mechanical pressure, to evert a long, spined tube that either penetrates or entangles the prey, making it easier to capture and move to the mouth. In some species of jellyfish, the outer surface of the bell is also sprinkled with cnidocysts, which may have either protective or prey-capturing roles.

The digestive system of most jellyfish includes a short stomach or gut (called the manubrium) above the mouth, which then opens into a system of thin canals that carry nutrients around the bell. These radial canals are located on the underside of the bell. They sometimes terminate in a ring canal that runs around the bell margin, but that isn't present in some species. In many species, the tentacles are hollow and open to the canal system for further circulation of nutrients. Waste materials are voided back out through the mouth, usually in a loosely compacted bolus.

Different species of jellyfish have different innate buoyancies, so when they are not swimming, some hang neutrally in the water, while others slowly sink when passive; a few float. Although a few of the largest jellyfish are strong swimmers, jellyfish are subject to the oceans currents, tides and waves for their large-scale movements. Animals in the sea whose movements are determined by these oceans currents are called plankton. In contrast, fish, marine mammals, squid, and a few other strong swimmers who can move beyond where natural ocean forces carry them are called nekton.

Jellyfish are carnivores, feeding on a wide variety of small ocean organisms, including zooplankton, small fish, and often, other jellyfish. Most jellyfish are passive drifters and/or slow swimmers. They move so as to create a local current forcing nearby prey within reach of their tentacles or oral arms, which then ensnare and paralyze the prey with their venomous cnidocysts, and bring the prey to the mouth. Some species digest prey within folds of the long trailing oral arms.

Jellyfish lack basic sensory organs and a brain, but their nervous systems and rhopalia allow them to perceive stimuli, such as light and odour, and respond quickly. The body of an adult is made up of 94–98% water. The bell consists of a layer of epidermis, gastrodermis, and a thick, intervening layer called mesoglea that produces most of the jelly. Some jellyfish, including the edible group known as the Rhizostomeae, have very thick, stiff, almost cartilaginous jelly, whereas other species are very soft and floppy. Most species are approximately the consistency of cold Jell-O (clear dessert gelatin).

Body systems

A jellyfish detects the touch of other animals using a nervous system called a "nerve net", located in its epidermis. Touch stimuli are conducted by nerve rings, through the rhopalial lappet, located around the animal's body, to the nerve cells. Some jellyfish also have ocelli: light-sensitive organs that do not form images but are used to determine up from down, responding to sunlight shining on the water's surface. They also sting when another organism touches their tentacles.

Jellyfish don't have specialized digestive, osmoregulatory, central nervous, respiratory, or circulatory systems. They digest using the gastrodermal lining of the gastrovascular cavity, where nutrients are absorbed. They do not need a respiratory system since their skin is thin enough that the body is oxygenated by diffusion. They have limited control over movement and mostly free-float, but can use the hydrostatic skeleton of the water pouch to accomplish vertical movement through pulsations of the disc-like body.

Jellyfish blooms

Flower hat jelly, Olindias sp.

Jellyfish are, by the nature of their life cycles, "bloomy". Their presence in the ocean is usually seasonal, responding to the availability of prey, which is seasonal in most places, increasing with temperature and sunshine in the spring and summer. Ocean currents tend to congregate jellyfish into large swarms or "blooms", consisting of hundreds or thousands of individuals. In addition to sometimes being concentrated by ocean currents, blooms can furthermore be the result of unusually high populations in some years. The formation of these blooms is a complex process that depends on ocean currents, nutrients, temperature and ambient oxygen concentrations. The news media recently has been full of stories about increases in jellyfish blooms ^[3]^[4]^[5]^[6]^[7]. It is important to realize, however, that there is very little data about changes in global jellyfish populations over time, besides "impressions" in the public memory. In most places in the world, scientists have no quantitative data about what jellyfish populations used to be like, or in fact, quantitative data about what is happening in the present^[8]. Recent speculations about increases in jellyfish populations often are based on no "before" data.

According to Claudia Mills of the University of Washington, increasing frequency of jellyfish blooms globally might be attributed to humans' impact on marine systems. She says that in some locations jellyfish may be filling ecological niches formerly occupied by overfished creatures, but notes that we lack data to show that is indeed true^[9]. Jellyfish researcher Marsh Youngbluth further clarifies that "jellyfish feed on the same kinds of prey as adult and young fish, so if fish are removed from the equation, jellyfish are likely to move in."

Some jellyfish populations that have shown clear increases in the past few decades are "invasive" species, newly arrived from other parts of the world: examples of regions with troublesome non-native jellyfish include the Black Sea and the Caspian Sea, the Baltic Sea, the eastern Mediterranean coasts of Egypt and Israel, and the American coast of the Gulf of Mexico. Populations of some invasive species expand rapidly because there are no natural predators in the ecosystem to check their growth - such blooms would not necessarily reflect overfishing or other environmental problems.

Aurelia sp., commonly known as the moon jellyfish, occurs in very high numbers in nearshore waters many places in the world. Several sibling species are difficult to casually distinguish.

Increased nutrients in the water, ascribed to agricultural runoff, have also been cited as an antecedent to the proliferation of jellyfish. Monty Graham, of the Dauphin Island Sea Lab in Alabama, says that "ecosystems in which there are high levels of nutrients ... provide nourishment for the small organisms on which jellyfish feed. In waters where there is eutrophication, low oxygen levels often result, favoring jellyfish as they thrive in less oxygen-rich water than fish can tolerate. The fact that jellyfish are increasing is a symptom of something happening in the ecosystem."^[10]

By sampling sea life in a heavily fished region off the coast of Namibia, researchers found that jellyfish have overtaken fish in terms of biomass. The findings represent a careful, quantitative analysis of what has been called a "jellyfish explosion" following intense fishing in the area in the last few decades. The findings were reported by Andrew Brierley of the University of St. Andrews and his colleagues in the July 11, 2006 issue of the journal Current Biology^[11].

Areas which have been seriously affected by jellyfish blooms include the northern Gulf of Mexico. In that case, Graham states, "Moon jellies have formed a kind of gelatinous net that stretches from end to end across the gulf."^[10]

Life history

The developmental stages of scyphozoan jellyfish.

Most jellyfish pass through two distinct life history phases (body forms) during their life cycle. The first is the polypoid stage, when the animal takes the form of a small stalk with feeding tentacles; this polyp may be sessile, living on the bottom or on similar substrata such as floats or boat-bottoms, or it may be free-floating or attached to tiny bits of free-living plankton or even (rarely) fish. Polyps generally have a mouth surrounded by tentacles that face upwards, like miniatures of the closely-related anthozoan polyps (sea anemones and corals), also of the phylum Cnidaria. Jellyfish polyps may be solitary or colonial, and some bud asexually by various means, making more polyps. Most are very small, measured in millimeters or a fraction of an inch tall.

In the second stage, the tiny polyps asexually produce jellyfish, each of which is also known as a medusa. Tiny jellyfish (usually only a millimeter or two across) pull away from the polyps by swimming, and then grow and feed in the plankton. Medusae have a radially symmetric, umbrella-shaped body called a bell, which is usually supplied with marginal tentacles - fringe-like protrusions from the border of the bell that are used to capture prey. (Medusa is also the word for jellyfish in Finnish, Portuguese, Romanian, Hebrew, Serbian, Croatian, Spanish, French, Italian, Hungarian, Czech, Slovak, Russian and Bulgarian.) A few species of jellyfish do not have the polyp portion of the life cycle, but go from jellyfish to the next generation of jellyfish through direct development of the fertilized eggs.

Jellyfish are dioecious; that is, they are either male or female. In most cases, to reproduce, both males and females release sperm or eggs into the surrounding water, where the (unprotected) eggs are fertilized and mature into new organisms. In a few species, the sperm swim into the mouth of the female, allowing the fertilization of the ova within the female's body. Moon jellies use a different process, in which the eggs become lodged in pits on the oral arms, which form a temporary brood chamber to accommodate fertilization and early development.

After fertilization and initial growth, a larval form, called the planula, develops from the egg. The planula is a small larva covered with cilia. It settles onto a firm surface and develops into a polyp. The polyp is cup-shaped with tentacles surrounding a single orifice, resembling a tiny sea anemone. After an interval of growth, the polyp begins reproducing asexually by budding and, in the Scyphozoa, is called a segmenting polyp, or a scyphistoma. New scyphistomae may be produced by budding or new, immature jellies called ephyrae may be formed. A few jellyfish species are also capable of producing new medusae by budding directly from the medusan stage; such budding has been described from the tentacle bulbs, the manubrium (above the mouth), or the gonads of hydromedusae (each species buds only from one location). Fission of medusae (splitting in half) has been described for a few of species of hydromedusae.

Predators of jellyfish include tuna, shark, and swordfish, but other jellyfishes are some of the most important jellyfish predators.

Jellyfish lifespans typically range from a few hours (in the case of some very small hydromedusae) to several months. The life span and maximum size of each species is unique. One unusual species is reported to live as long as 30 years and another species, Turritopsis dohrnii as T. nutricula, is said to be effectively immortal because of its ability to transform between medusa and polyp, thereby escaping death^[12]. Most of the large coastal jellyfish live about 2 to 6 months, during which they grow from a millimeter or two to many centimeters in diameter. They feed continuously and grow to adult size fairly rapidly. After reaching adult size (which varies by species), jellyfish spawn daily if there is enough food in the ecosystem. In most jellyfish species, spawning is controlled by light, so the entire population spawns at about the same time of day, often at either dusk or dawn.

Etymology and taxonomic history

Since jellyfish are not fish, some people consider the term "jellyfish" a misnomer, and American public aquariums have popularized use of the terms "jellies" or "sea jellies" instead. Others find the word "jellyfish" to be equally useful and picturesque. The word "jellyfish" is used to denote several different kinds of cnidarians including scyphozoans, staurozoans (stalked jellyfish), hydrozoans, and cubozoans (box jellyfish). In its broadest usage, some people also include members of the phylum Ctenophora when they are referring to jellyfish.

The class name, Scyphozoa, comes from the Greek word skyphos (??????), denoting a kind of drinking cup and alluding to the cup shape of the organism.

A group of jellyfish is sometimes fancifully called a "smack". ^[13]

Importance to humans

Culinary uses

Cannonball jellyfish, Stomolophus meleagris, are harvested for culinary purposes.

Jellyfish are an important source of food to the Chinese community and in many Asian countries.^[14] Only scyphozoan jellyfish belonging to the order Rhizostomeae are harvested for food; about 12 of the approximately 85 known species of Rhizostomeae are being harvested and sold on international markets. Most of the harvest takes place in southeast Asia^[15]. Rhizostomes, especially Rhopilema esculentum in China (Chinese name: h?izh?, meaning "sea sting") and Stomolophus meleagris (cannonball jellyfish) in the United States, are favoured because they are typically larger and have more rigid bodies than other scyphozoans. Furthermore, their toxins are innocuous to humans.^[14]

Traditional processing methods, carried out by a Jellyfish Master, involve a 20 to 40 day multi-phase procedure in which the umbrella and oral arms are treated with a mixture of table salt and alum, and compressed.^[14] The gonads and mucous membranes are removed prior to salting. Processing reduces liquidation, off-odors and the growth of spoilage organisms, and makes the jellyfish drier and more acidic, producing a "crunchy and crispy texture."^[14] Jellyfish prepared this way retain 7-10% of their original, raw weight, and the processed product contains approximately 95% water and 4-5% protein, making it a relatively low calorie food.^[14] Freshly processed jellyfish has a white, creamy color and turns yellow or brown during prolonged storage.

In China, processed jellyfish are desalted by soaking in water overnight and eaten cooked or raw. The dish is often served shredded with a dressing of oil, soy sauce, vinegar and sugar, or as a salad with vegetables.^[14] In Japan, cured jellyfish are rinsed, cut into strips and served with vinegar as an appetizer.^[14]^[16] Desalted, ready-to-eat products are also available.^[14]

Fisheries have begun harvesting cannonball jellyfish along the southern Atlantic coast of the United States and in the Gulf of Mexico for export to Asian nations.^[14]

In biotechnology

The hydromedusa Aequorea victoria

In 1961, green fluorescent protein (GFP) was discovered in the jellyfish Aequorea victoria by scientists studying bioluminescence. This protein has since become a quite useful tool in biology. Its use is mainly for scientists studying in which tissues genes are expressed. The technique, using genetic engineering, fuses the gene of interest to the gene of GFP. The fused DNA is then put into a cell, to generate either a cell line or (via IVF techniques) an entire animal bearing the gene. In the cell or animal, the artificial gene gets turned on in the same tissues and the same time as the normal gene. But instead of making the normal protein, the gene makes GFP. One can then find out what tissues express that protein -- or at what stage of development -- by shining light on the animal or cell, and looking for the green fluorescence. The fluorescence shows where the gene of interest is expressed.^[17] Jellyfish are also harvested for their collagen, which can be used for a variety of scientific applications including the treatment of rheumatoid arthritis.

In captivity

A group of Pacific sea nettle jellyfish, Chrysaora fuscescens, in an aquarium exhibit.

Jellyfish are commonly displayed in aquaria in many countries. Often the tank's background is blue and the animals are illuminated by side light to produce a high contrast effect. In natural conditions, many jellies are so transparent that they are almost impossible to see.

Holding jellyfish in captivity presents other problems. For one, they are not adapted to closed spaces. They depend on currents to transport them from place to place. To compensate for this, professional exhibits feature precise water flows, typically in circular tanks to prevent specimens from becoming trapped in corners. The Monterey Bay Aquarium uses a modified version of the kreisel (German for "spinning top") for this purpose.

Toxicity to humans

The Lion's mane jellyfish, Cyanea capillata, is known for its painful, but rarely fatal, sting.

When stung by a jellyfish, first aid may be needed immediately. The stings of Scyphozoan jellyfish are not generally deadly, though some species of the completely separate class Cubozoa (box jellyfish), such as the famous and especially toxic Irukandji, can be fatal. However, even nonfatal jellyfish stings are known to be extremely painful. Serious stings may cause anaphylaxis and may result in death. Hence, people stung by jellyfish must get out of the water to avoid drowning. In serious cases, advanced professional care must be sought. This care may include administration of an antivenin and other supportive care such as required to treat the symptoms of anaphylactic shock.

There are three goals of first aid for uncomplicated jellyfish stings: prevent injury to rescuers, inactivate the nematocysts, and remove any tentacles stuck on the patient. To prevent injury to rescuers, barrier clothing should be worn. This protection may include anything from panty hose to wet suits to full-body sting-proof suits. Inactivating the nematocysts, or stinging cells, prevents further injection of venom into the patient.

The sting of some species of Mastigias have no discernible effect on humans.

Vinegar (3 to 10% aqueous acetic acid) should be applied for box jellyfish stings.^[18]^[19] Vinegar, however, is not recommended for Portuguese Man o' War stings.^[18] In the case of stings on or around the eyes, vinegar may be placed on a towel and dabbed around the eyes, but not in them. Salt water may also be used in case vinegar is not readily available.^[18]^[20] Fresh water should not be used if the sting occurred in salt water, as a change in tonicity^[21]can cause the release of additional venom. Rubbing the wound, or using alcohol, spirits, ammonia, or urine will encourage the release of venom and should be avoided.^[22] A strange but effective method of treatment of stings is meat tenderizer which efficiently removes the nematocysts. Though often not available, a shower or bath as hot as can be tolerated can neutralize stings. However, if hypothermia is suspected this method may cause other serious complications.

A species of Mediterranean jellyfish, Cotylorhiza tuberculata, on display at the Monterey Bay Aquarium.

Once deactivated, the stinging cells must be removed. This can be accomplished by picking off tentacles left on the body.^[22] First aid providers should be careful to use gloves or another readily available barrier device to prevent personal injury, and to follow standard universal precautions. After large pieces of the jellyfish are removed, shaving cream may be applied to the area and a knife edge, safety razor, or credit card may be used to take away any remaining nematocysts.^[23]

Beyond initial first aid, antihistamines such as diphenhydramine (Benadryl) may be used to control skin irritation (pruritus).^[23] To remove the venom in the skin, apply a paste of baking soda and water and apply a cloth covering on the sting. If possible, reapply paste every 15-20 minutes. Ice can be applied to stop the spread of venom until either of these is available.

Systematics

Systematics within the Cnidaria, as with all organisms, are always in flux. Many scientists who work on relationships between these groups are reluctant to assign ranks, although there is general agreement on the different groups, regardless of their absolute rank. Presented here is one scheme, which includes all groups that produce medusae (jellyfish), derived from several expert sources:

Phylum Cnidaria

Subphylum Medusozoa

Class Hydrozoa ^[24]^[25]

Subclass Hydroidolina

Order Anthomedusae (= Anthoathecata or Athecata)

Suborder Filifera - see ^[24] for families

Suborder Capitata - see ^[24] for families

Order Lepmedusae (= Leptothecata or Thecata)

Suborder Conica - see ^[24] for families

Suborder Proboscoida - see ^[24] for families

Order Siphonophorae

Suborder Physonectae