Everyone has heard of jellyfish, but what are they really? Since they aren’t truly a type of fish, in the JelliesZone we’ll usually refer to them as “jellies.” We’re not talking about the kind you eat with your peanut butter! Not all gelatinous zooplankton are true jellyfish, however. Only those classified in the Phylum Cnidaria, the group that also includes corals, anemones and sea pens, are really types of jellyfish. Members of the Cnidaria are blessed with microscopic structures known as nematocysts, which are responsible for the notorious reputation of this group. These are the handy units that inject toxins into prey, and also may sting if you carelessly pick one up that’s washed up on the beach. The primary nematocyst action is located on the long, thin tentacles found on most cnidarian jellies. Species vary in the number of tentacles, from a couple to several hundred. Most jellies use the tentacles to capture zooplankton prey. Following capture, the prey are transferred to the mouth and into a single stomach (Hydrozoan jellies) or multiple pouches (Scyphozoan jellies). Radial canals are used to distribute digested food products throughout the rest of the jelly.
A conspicuous feature of cnidarian jellies is the bell, also known as the umbrella. This is the globular structure that forms most of the bulk of the jellyfish. Covered by a very thin epidermal tissue, the rest is made up of a gelatinous material called the mesoglea. The lower, inside portion of the bell is the subumbrella. It’s here that muscle tissue contracts to produce the distinctive pulsing motion of cnidarian jellies. The result is a type of jet propulsion that can vary from slow and gentle to relatively rapid depending on the species. Cnidarian jellies get along just fine without a brain or heart. They can respond to environmental stimuli, however, using sensory structures positioned in many species around the bell margin. These specialized sensors can often detect light and determine bell orientation. A simple network of nerve cells in the bell epithelium connects these sensory structures and can relay signals to the muscles to alter swimming patterns. It should be emphasized that jellies (cnidarian and ctenophores) lack true organ systems that most other multicellular organisms possess (like kidneys, livers, hearts, lungs, etc.). They are thus considered to be at the tissue level of organization. There may not appear to be much to a jelly, but nevertheless, they thrive in the worlds oceans and have been successful for over 500 million years!
In addition to the true jellyfish, a splendid variety of additional types of gelatinous animals ply the seas. Comb jellies resemble the cnidarian jellyfish, but are actually members of an unrelated phylum (Ctenophora). They are distinguished by the 8 comb rows, which are used for propulsion. Ctenophores lack the stinging nematocysts and capture food using other methods (although several species can incorporate and use nematocysts from cnidarian jelly prey). Other gelatinous animals include a variety of swimming gastropods (relatives of snails, the pteropods and heteropods) and pelagic tunicates (salps, doliolids, larvaceans and pyrosomes). The common feature uniting all these disparate and unrelated groups is the characteristic delicate gelatinous tissue. Out of the water they appear to be nothing more than quivering blobs; when immersed in their natural habitat, they are among the most graceful and beautiful creatures of the sea.
How do gelatinous animals eat?
Imagine trying to eat your dinner without any teeth or jaws. In fact, what if you didn’t have any hard parts (like our skeleton) in your body. Capturing living prey and consuming it without getting torn apart could be a challenge! That’s why the cnidarian jellies use the stinging nematocysts, and why they can be such a pain (literally!). By using various types of toxins, the true jellies can immobilize zooplankton prey like krill, copepods, larval fish and even other gelatinous animals. The long, thin tentacles, often not visible to potential prey, are studded with batteries of nematocysts. By drifting with extended tentacles in areas of high prey density, food items that contact the tentacles are stunned and killed by the nematocysts. Other nematocysts function more to hold on to the prey. Once subdued, the prey can then be brought to the mouth, and into the stomach for digestion. With their transparent bodies, it’s often easy to see what a jelly has recently consumed.
Other gelatinous animals lack nematocysts and have devised other methods for acquiring food. Some comb jellies (the “sea gooseberries”) employ a pair of sticky tentacles to ensnare small zooplankton prey. Another group, the lobate comb jellies, utilize a pair of mucous-covered oral lobes to funnel zooplankton into the waiting mouth. Perhaps the most amazing comb jelly feeders are the various species of Beroe. Pity the poor comb jelly that encounters one of these voracious beasts. Gluttonous to the max, certain Beroe can engulf ctenophores up to their own size, or even larger! Despite what most of us think about mucus (yuck!), it plays a valuable role for many gelatinous snails and tunicates. Certain planktonic snails known as pteropods produce sticky mucous webs that effectively capture zooplankton prey. Other types of pteropods, and another gastropod group known as heteropods, are more active predators. With well developed swimming ability and relatively complex mouthparts they seek out gelatinous fare. Pelagic tunicates also take advantage of the stickiness of mucus, using it in a variety of ways to collect tiny zooplankton and phytoplankton (microscopic algae).
Does anything eat jellies?
Although it might appear that dining on gelatinous tissue could be either dangerous or not worth the effort, a number of marine creatures have developed a taste for jellies. There’s not much substance to something that’s 95% or more water, but at least it’s easy to chew and digest! Sea creatures that have a thing for jellies include blue rockfish (left, top photo), salmon, leatherback sea turtles, certain marine birds such as fulmars (left, 2nd and 3rd photos), and even other jellies such as Phacellophora and Beroe.
How do gelatinous animals swim?
Among the advantages of the gelatinous lifestyle is built-in buoyancy. With tissue that lacks bone and dense muscles, and is at least 95% water, jellies have about the same density as water. They lack accessory buoyancy structures like the swimbladders of fish, and don’t need to expend a lot of energy in swimming to keep from sinking. Jellies also can be carried relatively passively over long distances by oceanic currents. Nevertheless, jellies and other gelatinous animals do possess decent swimming capabilities using a variety of mechanisms. Cnidarian jellies utilize coronal (circular) muscle fibers in the subumbrella of the bell (the “bottom” of the bell). By contracting these muscles, water is pushed out, which propels the jelly forward. Jellies vary in the frequency and power of the bell pulsing, with some having very rapid, jerky pulsing and others possessing a more gentle, slow-motion style of swimming. The frequency of pulsing is under some nervous control. The margin of the bell is lined with concentrations of nerve cells known as the nerve ring (Hydrozoans) or marginal nerve centers (Scyphozoa). These nerve cells serve as a type of pacemaker controlling the rate of contraction of subumbrella bell muscles, and can also receive input from sensory cells that may then lead to a change in the rate of pulsing.
Comb jellies have adopted a totally different means for getting around. All possess 8 comb rows (hence the name for the group). Formed by multitudes of cilia fused into rows of plates, the comb rows serve to move the ctenophore with sometimes surprising speed. Look carefully at the comb rows and you’ll see shimmering waves of color as the plates beat in sequence to act like tiny oars.
Gelatinous snails typically use modified extensions of the gastropod foot. They include the heteropods which possess a single undulating ventral fin (photo at left shows typical swimming behavior), and the pteropods which have a pair of wings that flap. Gymnosome pteropods (some of which are known as “sea angels”) are very active swimmers. Of the pelagic tunicates, salps and doliolids utilize loop-like bands of muscles to squeeze water out of the body for a form of jet propulsion. Pyrosomes depend on cilia to move water through the body of each individual, and collectively use jet propulsion to move the colony at a gentle pace compared to the more frenetic pattern of some salps and doliolids.
How do gelatinous animals reproduce?
A survey of reproductive methods among gelatinous animals will reveal a bewildering array of methods for spreading the genetic word. Cnidarian jellies typically undergo an alternation of generations, with a sessile (attached) asexual polyp phase and a free-swimming sexual medusae phase. The medusa is the form most familiar to everyone. Among most scyphozoan jellies, males and females release eggs and sperm into the surrounding water. The fertilized eggs develop into tiny ciliated larvae known as planulae that eventually settle out on appropriate substrates. These develop into polyps (scyphistomae) that may divide asexually to colonize an area. Eventually the polyps undergo a process known as strobilation in which tiny medusae known as ephyrae are serially released. A strobilating polyp resembles a stack of dinner plates, with the largest, most developed ephyra at the end. Ephyrae quickly develop into the medusa form and eventually mature as separate male and female individuals. Many hydrozoan jellies also have an attached polyp phase (usually known as hydroids). Anthomedusae, Leptomedusae and Limnomedusae retain a hydroid phase in the lifecycle, while open ocean and deep-sea dwelling Trachymedusae and Narcomedusae lack a benthic phase.
Other gelatinous animals are holoplanktonic, which means they lack any benthic, attached phase. These drifters spend their lives entirely as members of the plankton. Comb jellies tend to favor the hermaphroditic route, typically with both sexes appearing simultaneously. Eggs and sperm are released from gonads beneath each comb row (eggs visible in this Beroe cucumis at left). Eggs are fertilized externally and develop into ciliated larvae. Cydippid (such as the sea gooseberry, Pleurobrachia), cestid (like the Venus girdle, Cestum) and lobate ctenophores all start out with paired tentacles and only 4 comb rows. The beroid comb jellies lack the cydippid stage with tentacles.
Many gelatinous molluscs use copulatory organs for internal fertilization. Male heteropods have a conspicuous penis to transfer spermatophores into females. Fertilized eggs are released in long strings. Pteropods are typically hermaphrodites. Thecosome pteropods (the “sea butterflies”) start off as males, and later convert over to the female side (known as protandrous hermaphrodites). This may sound pretty bizarre but is actually not uncommon in the marine realm. Fertilized eggs are released as floating masses. Predatory gymnosome pteropods (the “sea angels”) are simultaneous hermaphrodites with internal fertilization. Pelagic tunicates like salps play the alternation of generation game, although without the attached polyp stage of cnidarians. Solitary individuals are responsible for producing the long aggregate chains by asexual budding. Individuals in the chain (each known as a zooid) are protogynous hermaphrodites, which means they start as females and later convert to males. Eggs produced by the females are fertilized by older male zooids and develop directly into solitary individuals attached to the body wall of the parent. Doliolids have even more complex reproductive affairs involving sexual and asexual phases.
Where can you find jellies?
Gelatinous animals inhabit all marine habitats, and even some freshwater ones. Jellies live in estuaries, bays, harbors, nearshore habitats, open ocean (far offshore) and even in the deep sea. All the worlds oceans, from the cold Arctic to the warm tropical seas of the Pacific and Indian Oceans can harbor swarms of gelatinous creatures. If you live near the ocean, you’re pretty much guaranteed of seeing some jellies. A wide variety of hydromedusae, many very small and inconspicuous, reside in nearshore habitats like harbors, quiet bays and estuaries. If you have access to a boat dock in relatively clean water you often can see them gently bobbing. A good time to view them is at night – by using a flashlight you’ll easily see any near the surface. Some of the large conspicuous scyphozoan jellies periodically invade nearshore waters and are easily seen while boating or diving. Other gelatinous animals like many of the gelatinous snails and the pelagic tunicates are more characteristic of offshore oceanic waters. On occasion, they may be carried closer to shore by currents, where they can be found with only a short boat ride from the dock. Midwater and deep-sea habitats are endowed with a respectable share of gelatinous types. Siphonophores are among the most prevalent predators in midwater habitats. Occasionally they are carried into surface waters. Many midwater gelatinous animals are vertical migrators, spending days at depth and venturing to feed in shallower waters under cover of darkness. Deep-sea habitats below 1000 meters support a splendid array of cnidarian jellies and ctenophores that thrive in the cold, dark depths. You’ll never see these mysterious species unless you have access to a submersible vehicle or deep-water trawling equipment (not likely unless you’re a scientist!). Although the vast majority of gelatinous animals must live in seawater environments, a few hydromedusae inhabit freshwater lakes and ponds (most notably the peach blossom jelly, frequently found in ponds and lakes throughout the U.S.), so it’s possible you’ll see small jellies in your favorite swimming hole. Many types of gelatinous zooplankton have widespread distributions and can be found in more than one ocean (like moon and lion’s mane jellies). Good places to find jellies include Monterey Bay, Puget Sound in Washington, the Gulf of Mexico, and Chesapeake Bay on the East Coast.
Are all jellies and other gelatinous animals dangerous?
Among the gelatinous animals, only a few of the cnidarian jellies present any real danger to people. Without any ability to sting, comb jellies, gelatinous snails and pelagic tunicates are harmless (unless of course if you are a copepod or other small prey!). All cnidarian jellies possess nematocysts for use in acquiring food. Many species, particularly among the hydromedusae, lack nematocysts that are capable of penetrating human skin and thus are basically harmless. If handling a jelly that doesn’t appear to sting, it would still be prudent to avoid touching your eyes or other sensitive parts of the body. Relatives of hydromedusae, the siphonophores, are another matter. Endowed with hundreds or even thousands of tentacles, each studded with batteries of potent nematocysts, many of these long chains can give you a stinging jolt you won’t soon forget. Fortunately the sting of most is only painful and not a serious threat to your health. A notable exception is the Portuguese man-of-war (Physalia) which is capable of ruining your day or occasionally even ending your life. North of southern California, however, it’s unlikely that you’ll ever see a Physalia. Some of the large West Coast scyphozoan jellies are also capable of packing a fairly potent nematocyst punch, particularly sea nettles (Chrysaora fuscescens), purple-stripe jellies (Pelagia colorata) and lion’s mane jellies (Cyanea capillata). In all the strongest sting is concentrated in the sinuous tentacles so these are the structures to avoid. Usually the worst that happens is some pain, itching and a rash that may last for a few hours. Of course, some people may be sensitive to the sting and may have more serious allergic responses. Even jellies that have a reputation for lacking a sting (like the moon jelly, Aurelia) have been known to sometimes produce an irritating sting. Mention should be made of jellies known as sea wasps. These troublemakers, members of the box jellies (Class Cubozoa), inhabit warm tropical seas. The sea wasp of Australian waters (Chironex fleckeri) has a well-deserved deadly reputation. Swimmers are well advised to leave the water when this box jelly is sighted since the sting carries a strong possibility of the ultimate penalty.
If you see a gelatinous blob on the beach, it would be prudent to avoid contact. Even a jelly that’s long dead and beaten to bits can still sting since the nematocysts continue to remain active. When seeing a jelly from a boat or while diving, there’s no need to fear it – jellies won’t come after you! Watch carefully from a respectable distance and you’ll have the opportunity to observe some of the most beautiful and remarkable creatures of the sea.
What should you do if you are stung by a jellyfish?
If contact with a jelly results in pain, then you’ve been injected with toxins from hundreds or even thousands of nematocysts. It’s important to remove any tentacle pieces as soon as possible since the nematocyst function will continue even when detached. A seawater rinse is best since freshwater will only trigger more nematocyst firing. Other things that should be avoided are rubbing alcohol and human urine (yes, it actually is a folk remedy, albeit one that only makes things worse!). With a mild to moderate sting on sensitive areas of the body, you may develop a burn-like red rash with pain, itching and raised bumps on the skin. Symptoms will usually disappear within a few hours without any treatment. Many websites (including this one!) have advocated the use of ice packs to the affected area for several minutes to reduce pain and inflammation. Recent research, however (see Wilcox and Yanagihara, 2016), has shown that for most painful stings, immersion in hot water (~45 degrees C) is the most effective treatment, both for pain reduction and improved clinical outcomes. Presumably the jellyfish venoms and toxins are heat labile and thus rendered less effective in performing their unpleasant actions.
More potent stingers (such as from the Portuguese man-of-war) can have far more lasting effects. Pain may be severe and last for several hours (or longer), with welts and lesions lasting a number of days. After such a stinging episode, and following a seawater rinse, you can try vinegar or sodium bicarbonate to help disable any remaining nematocysts. If possible, immersion in hot water may the preferred course of action, but this can be difficult if the affected area covers a large area of the body. Hot water may also not be readily available at the site of injury. Ice pack therapy can then be used for the pain, followed by application of topical analgesic creams. With a severe reaction to the toxins that may include respiratory and cardiac distress, it’s very important to seek immediate medical attention. Fortunately this is a rare situation on the West Coast.
Why should you care about jellies?
It might appear at first glance that something made of up to 95% water could hardly make a difference to anything. Most people regard jellies as a nuisance at best and a source of fear and loathing. In reality, jellies and other gelatinous animals are among the most beautiful creatures of the sea and also serve influential roles in marine ecosystems. Many animals rely on tasty gelatinous fare as a source of food, including sea turtles, at least 50 species of fish such as the ocean sunfish (Mola)and blue rockfish (Sebastes mystinus), marine birds like the northern fulmar, and even other jellies. Juvenile crabs and fishes of a variety of species utilize gelatinous animals as protective havens and a means of distribution. As predators, jellies can cause profound ecological shifts in only a matter of days. Gelatinous animals are opportunists that can quickly overwhelm an area. When jellyfish blooms occur, zooplankton populations (copepods and larval fish) can be dramatically reduced, with resultant impacts on anything dependent on them. Among other types of gelatinous animals, rapidly growing and reproducing salps are superbly efficient grazers on phytoplankton. There is mounting evidence that human influences in coastal habitats may be creating conditions more favorable to jellyfish, leading to an increased frequency of blooms and reduced populations of larval fishes. They may seem insignificant when washed up on a beach, but gelatinous animals are certainly worthy of our attention and study.
Do jellyfish have nerves or some way of feeling someone touching them? Do they feel pain?
Jellyfish do have a very simple type of nervous system known as a nerve net. They have nerve cells in the their tissue, but these are not concentrated into anything like a brain. Although they can respond to contact by swimming faster or in a different direction, they have no sense of pain since there is no brain to create the sensation. When stranded on a beach the tissue probably can survive for no more than a few hours. Since they don’t have a hard skeleton, without the support of the water the tissue collapses into a gelatinous blob and cells won’t be able to get the oxygen they need. It’s important to know that the stinging structures in the tentacles (the nematocysts) can remain active for many days even if the jellyfish is dead. For that reason it’s best not to touch a dead jellyfish or even pieces of one on a beach.
How and why do jellyfish “light up”?
Many types of jellies can produce light, known as bioluminescence. This process is a chemical reaction that produces a blue-colored light when one type of chemical (known as luciferin) is oxidized by the action of another chemical (luciferase). In jellies these are combined to form a photoprotein. Some hydrozoan jellies also possess another protein known as green fluorescent protein, which takes the blue light and shifts it to a green color. Light production typically happens after a jelly is touched, perhaps by a predator seeking a meal. Not all parts of a bioluminescent jelly have these light producing substances.
There are lots of reasons for marine organisms to produce light. Since most jellies lack vision (some box jellies are a notable exception!) they don’t produce light as a way to recognize each other like some fish. Instead it probably helps to scare away predators that might try to eat a jellyfish. If a fish, for example, tries to eat a jellyfish which lights up when attacked, the fish may be startled and swim away. Also, since it is completely dark where deep-water jellyfish live, producing light when under attack by a fish or some other predator may result in the attacker being visible to other nearby potential predators. Just imagine trying to pick up something that lights up in the dark when you touch it. Your presence will be advertised to anyone else in the area, which is not a good thing if you want to hide.
How long do jellyfish live?
Most jellyfish probably live no more than a year, and many may only live a few months. In captivity, jellies like the purple-stripe jelly, moon jelly and sea nettle can live for 2 to 3 years. In their natural habitat it’s generally thought these and other larger types may live about a year, and maybe up to a year and a half. Small jellies, like crystal and umbrella jellies, probably live for 4 to 6 months at most. Generally the smaller jellies are pretty short-lived and may only be around for part of the year. It’s difficult to know for sure how old jellies are since they can’t be tagged and tracked like fish, and they don’t have any kind of growth rings that indicate age. For comb jellies, the sea gooseberry may live 4 to 6 months, and in captivity, lobate comb jellies can live for over a year (probably a lot less than this in natural habitats).