Jellyfish and Comb Jellies
Cnidaria and Ctenophora
Jellyfish and comb jellies are gelatinous animals that drift through the ocean's water column around the world. They are both beautiful—the jellyfish with their pulsating bells and long, trailing tentacles, and the comb jellies with their paddling combs generating rainbow-like colors. Yet though they look similar in some ways, jellyfish and comb jellies are not very close relatives (being in different phyla—Cnidaria and Ctenophora, respectively) and have very different life histories.
Both groups are ancient animals, having roamed the seas for at least 500 million years. And, in the modern age, they are having similar effects on ecosystems. As seawater temperature rises, predators of jellies are removed by fishing, more structures are built in seawater, and more nutrients flow into the ocean, some types of jellyfish and comb jellies may be finding it easier to grow and survive. Whatever the reason, huge explosions in jelly numbers (a jelly bloom) can disrupt fisheries, make for unpleasant swimming, or foul up the works of power plants that use seawater for cooling. Invasive jellies have also wreaked havoc in some parts of the world.
A Simple Body Plan
While jellyfish and comb jellies have several anatomical differences, the basics are the same. Both have two major cell layers: the external epidermis and the internal gastrodermis. (Ctenophores also have musculature in their in-between layer, the mesoderm, but it likely evolved separately from the mesoderm found in bilaterians like people.)
The gastrodermis lines the all-purpose gut and an opening where food enters and reproductive cells are released and taken in. Jellies have no need for a stomach, intestine, or lungs: nutrients and oxygen slip in and out of their cell walls through the gastrodermis or even their bodies' outer cells. The outer cells that make up the epidermis contain a loose network of nerves called the "nerve net." This is the most basic nervous system known in a multicellular animal. (See Brains of Jelly? for more.)
Between these layers is a gelatinous material called mesoglea, which makes up most of their bodies. (Although some small species have very thin mesoglea.) Jellyfish and comb jellies are 95 percent water and so, rightly, mesoglea is mostly water! It also contains some structural proteins, muscle cells, and nerve cells, forming a kind of internal skeleton.
Comb Jellies' Unique Features
Comb jellies are named for their unique feature: plates of giant fused cilia, known as combs, which run in eight rows up and down their bodies. The combs act like tiny oars, propelling the comb jelly through the water. Many microscopic organisms, such as bacteria, also use cilia to swim—but comb jellies are the largest known animals to do so. The comb-rows often produce a rainbow effect. This is not bioluminescence, but occurs when light is scattered in different directions by the moving cilia.
Until 2015 scientists believed that comb jellies removed their waste via their "mouth," or what was believed to be the one hole in their body plan. A new study showed that comb jellies in fact release indigestible particles through pores on the rear end of the animal. This discovery adds another piece to the evolutionary puzzle of when animals evolved to have anuses.
Many comb jellies have a single pair of tentacles (often each tentacle is branched, giving the illusion of many tentacles) that they use like fishing lines to catch prey. They are armed with sticky cells (colloblasts) and unlike jellyfish, the tentacles of comb jellies don’t sting. (See The Stings: Nematocysts and Colloblasts for more.)
Jellyfishes' Unique Features
Jellyfish transition between two different body forms throughout their lives. The familiar body plan that looks like an upside down bell with tentacles hanging down from the inside is called the medusa. The polyp, the other cnidarian body plan, is the opposite, with the mouth and tentacles above, like a sea anemone. (See more in Reproduction & Lifecycle.)
Jellyfish also have a stinging adaptation that is unique to them and their close relatives (including sea anemones and hydras): nematocysts, or stinging cells. (See The Stings: Nematocysts and Colloblasts for more.)
Jellyfish and comb jellies vary greatly in size depending on the species. Most jellies range from less than half an inch (1 cm) wide to about 16 inches (40 cm), though the smallest are just one millimeter wide! The largest jellies are the Lion’s Mane Jellyfish (Cyanea capillata), which can be almost 6 feet wide (1.8 m) with tentacles over 49 feet (15 m) long. Larger individuals have been seen, but they are not typical. Venus’s girdle (Cestum veneris), a belt shaped comb jelly, can be 40 inches (1 meter) long.
The Nervous System
Brains of Jelly?
Jellies don't have brains as we typically think of them: rather, they have a network of neurons ("nerve net") that allows jellies to sense their environments, such as changes in water chemistry indicating food or the touch of another animal. The nerve net has some specialized structures such as statocysts, which are balance sensors that help jellies know whether they are facing up or down, and light-sensing organs called ocelli, which can sense the presence and absence of light.
Additionally, some jellyfish have sensory structures called rhopalia, which contain receptors to detect light, chemicals and movement. One group of jellyfish, the cubozoan jellyfish, have complex eyes with lenses, corneas and retinas in their rhopalia. Although they respond to visual stimuli, scientists don’t know how the jellyfish interpret the images created by their eyes since they don’t have a brain with which to process them. Their nerve ring, a ring-shaped concentration of nerves found in jellyfish, seems to be involved, however.
A 2017 study of the upside-down jellyfish, Cassiopea, found that a brain is not required to experience sleep. At night Cassiopea enters a sleep-like state where it pulses less frequently than during the day and is slow to respond to disturbances. When kept awake throughout the night, the next day the jellyfish appear to be tired—their pulsing was noticeably slower than if they had a solid night of sleep. It is the first time an animal without a brain was observed sleeping. The discovery suggests sleep among all animals is an ancient characteristic with a shared evolutionary beginning, considering the neural network of jellyfish evolved before centralized nervous systems like a brain.
Diversity & Evolution
Types of Jellyfish
Jellies are found in oceans worldwide, in shallow and deep water, and a few can even be found living in freshwater. They have been found floating in the deepest parts of the ocean, like at the bottom of the Mariana trench.
All jellyfish are Cnidaria, an animal phylum that contains jellies, sea anemones, and corals, among others. There are more than 10,000 species of Cnidaria, and less than 4,000 of these are Medusazoa—those animals we think of as jellyfish. Those 4,000 jellyfish can be divided into four different groups.
SCYPHOZOA are the most familiar jellyfish, including most of the bigger and more colorful jellies that interact with humans, and are sometimes called "true jellyfish" for this reason. Scyphozoa spend most of their lives in the medusa body form, and there are at least 200 species.
HYDROZOA are jellyfish look-alikes but not in the same group as the “true jellyfish.” The swimming medusa stages of this group are often small and inconspicuous, whereas the bottom-dwelling polyps, or hydroids, usually take the form of large colonies. (See Reproduction & Lifecycle.) In the water column, the colonial siphonophores may be quite spectacular. These include the notorious Portuguese Man-o-Wars and many deep-sea forms, some of which stretch out up to 50 meters in length like giant fishing nets. Colonial siphonophores are composed of many specialized individuals called zooids that are genetically identical because they all come from a single fertilized egg. In 2016, researchers discovered what they believe to be a new hydrozoan species of Crossota, 12,140 feet (3,700 meters) deep within the Mariana Trench. Floating in the water column like a glowing spaceship, this Crossota jellyfish is an exception to most hydrozoans and will spend the majority of its life as a large medusa. There are around 3,700 species of Hydrozoa.
CUBOZOA are the box jellyfish, named for their box-like bells. Some cubozoans, such as the sea wasp (Chironex fleckeri), produce some of the most potent venom known. Cubozoan jellyfish also have a more developed nervous system than other jellyfish, including complex eyes with lenses, corneas and retinas. Some even engage in elaborate (for a jellyfish) courtship behavior! There are at least 36 species. In 2011, Allen Collins, a jellyfish expert at the Smithsonian, discovered a new species, which was named Tamoya ohboya in a public naming contest. (Listen to a podcast about box jellies.)
STAUROZOA are the stalked jellyfishes, which don't float through the water like other jellies, but rather live attached to rocks or seaweed. They are trumpet-shaped, and mostly live in cold water. There are around 50 staurozoan species, many notable for their unique combination of beauty and camouflage.
Types of Comb Jellies
Compared to jellyfish, there are far fewer species of ctenophores: only 100-150 species have been found, but quite a few are out there yet to be discovered and fully documented. The best-known comb jellies are those found close to shore because, there, they are most likely to run into people. Those can be roughly divided into three groups.
CYDIPPIDS all have rounded bodies—some spherical, some oval—with branched tentacles. (This means that their tentacles are fringed with smaller tentacles.) These tentacles can be withdrawn into the jelly's body into special sheaths or pouches on either side of their mouths.
LOBATES are defined by two flattened lobes that extend from the typical rounded ctenophore body down below their mouths. They also have short tentacles and tend to grow larger than cydippids.
Open ocean ctenophores are much less known. They tend to be very fragile because they don't have to endure rough coastal waves; many of them are so fragile that they cannot be collected by submersibles and are known only by photographs. They come in a great diversity of forms. Some are shaped like belts (Cestida), while others don't float in the water column at all, but live on the seafloor! (These are known as benthic ctenophores.)
Comb jellies live throughout the world's ocean, although most species prefer warmer water.
How Closely Related?
Jellyfish and comb jellies are in different phyla, but scientists have long argued over whether they have an especially close relationship apart from the rest of the animal kingdom. To distinguish them, all Cnidaria and Ctenophora were once described as Coelenterata—but that term is no longer commonly used.
To this day, some researchers believe they are sister groups, while others think they are not closely related. Some argue that a new fossil from the Chengjiang fossil site in China and a peculiar fossil in the Burgess Shale are similar enough to jellyfish that they might be ancient comb jellies that had recently split from the jellyfish lineage. Either way, there are still plenty of other questions to argue about, such as how long ago the two groups diverged, and even whether ctenophores might be the most ancient group of animals, diverging even earlier than sponges in the animal tree of life. These arguments continue because, as some of the simplest animals alive today, understanding their place in the tree of life helps people understand how all other animals—including people—evolved.
Whichever came first, comb jellies and jellyfish (and other Cnidarians) made an important step in evolutionary history: they are the earliest known animals to have organized tissues—their epidermis and gastrodermis—and a nervous system. They're also the first animals known to swim using muscles instead of drifting with the whims of the waves.
The oldest ancestors of modern day jellies lived at least 500 million years ago, and maybe as long as 700 million years ago. That makes jellyfish three-times as old as the first dinosaurs!
Because jellies have no bones or other hard parts, finding jellyfish fossils is rare. But in 2007, a group of scientists including Allen Collins from the Smithsonian National Museum of Natural History, discovered some beautifully-preserved jellyfish fossils buried in Utah from 505 million years ago. From around the same period, scientists have also found well-preserved comb jelly fossils in the Burgess Shale.
Fossils described in 2019 from the Quingjiang site in southern China are some of the best preserved Cambrian fossils ever found, with the cilia and plates easily visible. Scientists are optimistic this discovery will help tease out the relationship between jellyfish and comb jellies.
In the Food Web
Predators and Prey
Jellyfish and ctenophores are carnivorous, and will eat just about anything they run into! Most jellies primarily eat plankton, tiny organisms that drift along in the water, although larger ones may also eat crustaceans, fish and even other jellyfish and comb jellies. Some jellyfish sit upside down on the bottom and have symbiotic algae (zooxanthellae) in their tissues, which photosynthesize, and so get much of their energy the way plants do.
While their nematocysts and colloblasts do help them defend themselves, plenty of animals manage to catch and eat jellies: more than 150 animal species are known to eat jellies, including fish, sea turtles, crustaceans, and even other jellyfish. Jellies are the favorite food of the ocean sunfish (Mola mola) and endangered leatherback turtle (Dermochelys coriacea), which will migrate thousands of miles for the gelatinous delicacy. Young jellyfish are small enough to be part of the general zooplankton population and are eaten by many animals.
Humans also eat jellyfish: people have fished for jellies for at least 1700 years off the coast of China. Some 425,000 tons (more than 900 million pounds) of jellyfish are caught each year by fisheries in 15 countries, and most are consumed in Southeast Asia. Eating jellyfish may become more common around the world as we overfish more preferable fish species.
The Stings: Nematocysts and Colloblasts
Jellyfish and ctenophores both have tentacles with specialized cells to capture prey: nematocysts and colloblasts, respectively. Jellyfishes' nematocysts are organelles within special cells (cnidocytes) that contain venom-bearing harpoons. The cell is activated upon touch or chemical cue, causing the harpoon to shoot out of the cell and spear the prey or enemy, releasing toxin—a process that takes only 700 nanoseconds. A small number of jellyfish are very toxic to humans, such as the box jellyfish (Chironex fleckeri) and Irukandji jellyfish (Carukia barnesi), which can cause severe reactions and even death in some people.
Many comb jellies have colloblasts lining their tentacles, which work like nematocysts but release glue instead of venom. Upon touch, a spiral filament automatically bursts out of colloblast cells that releases the sticky glue. Once an item is stuck, the comb jelly reels in its tentacle and brings the food into its mouth. One species of ctenophore (Haeckelia rubra) recycles nematocysts from hydrozoan jellyfish it consumes and uses these to stun and kill prey.
Many Ctenophores, Many Ways to Feed
Comb jellies come in many shapes and sizes, and so within the group there are many ways to feed. The rounded and tentacled cydippids have branched tentacles lined with colloblasts that they use, in the traditional jelly style, like a fishing line to trap food and bring it to their mouths.
The lobate ctenophores have two flattened lobes that reach below their mouths. Special cilia waving between the lobes generate a current to pull planktonic food between the lobes and into the jelly's mouth, allowing them to feed on plankton continuously. They also use colloblast-lined tentacles to catch food.
The tentacle-less beroids depend on their large mouths. Instead of catching food with colloblasts, they swallow their prey (often other ctenophores!) whole and then clamp their mouths shut, giving them no escape route. Inside their mouths they have small cilia that act as teeth, pulling food apart, which also direct the food into the comb jelly's gut.
Color and Bioluminescence
Many jellyfish and comb jellies are able to produce light—an ability known as bioluminescence. They have proteins in some tissues that undergo a chemical reaction to produce blue or green light in response to stimuli such as touch. No one's quite sure why jellies bioluminesce, but it seems to be mainly a defense tactic. A bright enough flash could be enough to startle a predator—or to attract an even bigger predator to make the jelly's predator into prey.
Jellies have also adapted their body color to camouflage in the darkness. Most are nearly colorless and transparent, so they can be difficult for predators to see. However, some deep sea jellyfish and comb jellies are a bright red or orange color. Why would they be red instead of black to blend in with the dark water? Red cannot be seen in dark water (deeper than 200 meters), so there's no greater protection from black than red. But red is preferred to black because pigment is easier for animals to produce. Some deep sea jellies just have dark red guts, possibly serving to mask luminescent prey from other larger predators with eyes.
Reproduction & Lifecycle
Jellyfish: Medusa and Polyp Switch-Off
Jellyfish have a complex life cycle: a single jellyfish reproduces both sexually and asexually during its lifetime, and takes on two different body forms.
An adult jellyfish is called a medusa, which is the familiar umbrella-shaped form that we see in the water. Medusa jellyfish reproduce sexually by spawning—the mass release of eggs and sperm into the open ocean—with entire populations sometimes spawning all together. Male and female jellyfish (there aren't many hermaphrodites) release the sperm and eggs from their mouths. In most species, fertilization takes place in the water; in others, the sperm swim up into the female's mouth and fertilize the eggs within.
The fertilized eggs then develop into planulae (singular: planula), which are ciliated free-swimming larvae shaped a bit like a miniature flattened pear. After several days of development, the planulae attach to a firm surface and transform into flower-like polyps. The polyps have a mouth and tentacles that are used to feed on zooplankton.
Polyps reproduce asexually by budding—when a polyp divides roughly in half to produce a new genetically identical polyp—or they can produce or transform into medusae, depending on the type of jellyfish. Hydrozoan polyps bud medusae from their sides; cubozoan polyps each transform into a medusa.
In schyphozoans, a process called strobilation takes place (shown in video and in diagram). During strobilation, a polyp splits into 10-15 plate-like segments stacked atop one another in a tower called a strobila. After a segment separates from the strobila, it is called an ephyra, a juvenile jellyfish. Ephyrae mature into the medusa form.
Most jellyfish are short lived. Medusa or adult jellyfish typically live for a few months, depending on the species, although some species can live for 2-3 years in captivity. Polyps can live and reproduce asexually for several years, or even decades.
One jellyfish species is almost immortal. Turritopsis nutricula, a small hydrozoan, can revert back to the polyp stage after reaching adult medusa stage through a process called transdifferentiation. This is the only animal known to do so.
In comparison to the jellyfish, comb jellies have a very simple lifecycle. Most species are hermaphroditic and able to release both eggs and sperm into the water, which drift with the waves until they find other gametes. Because most species have both male and female gametes, it's thought that they can self-fertilize as well.
This method may not seem very efficient, since it's likely that most of the gametes never find a match. But ctenophores make up for this by releasing them every day. If they run out of food while producing so many eggs and sperm, they can shrink and hunker down until they run into more food and can start reproducing again.
Once eggs and sperm find each other, the embryo develops into a larva that looks just like a small adult ctenophore—and, from there, all it has to do is grow up.
One species (Mertensia ovum) can reproduce even when it is still larva, and scientists think other species are also able to reproduce at a young age. This means that comb jelly populations can grow very fast under certain conditions.
Around the world, vast aggregations of jellyfish and comb jellies seem to be more common. These aggregations are known as "jellyfish blooms" or "jellyfish outbreaks," which can cause a wide array of problems. Too many jellies in the water can be a danger to swimmers, forcing towns to close their beaches. Jellies have clogged up machinery at coastal power plants, causing power outages. They can interfere with fisheries by eating fish larvae, and fisherman catch jellies instead of the fish they want. Where they occur, blooms of jellyfish even change seawater chemistry. Scientists hope to address this problem through the discovery of a practical application for jellyfish, like substituting jellyfish for the fish used in aquaculture feed. Jellyfish mucus, which has been shown to bind to microplastics, may even one day be used in water treatment facilities to help combat the world’s growing plastic problem.
Why are jellies becoming more common around the world? It seems likely that their spread is human-caused, although some scientists have argued that the blooms are part of a natural cycle. If the blooms are human-caused, there are several probable culprits.
OVERFISHING Over the past two decades, between 100 and 120 million tons of marine life have been removed from the ocean by fisheries each year on average. A lot of these marine species, including fish and invertebrates such as squid, eat some of the same food that jellies do: mainly, zooplankton. As these other predators of plankton are fished from the sea, jellies have less competition for food, and are able to grow and reproduce with fewer limits.
NUTRIENTS When fertilizers runoff from the rivers to the seas, they can create dead zones: areas of ocean where little life survives. The nitrogen and phosphorus in fertilizer helps phytoplankton grow very quickly, and there can be so many of these single-celled plant-like animals that they deplete oxygen from the water. Most animals can't survive in these conditions, but many jellies can better tolerate low-oxygen environments.
CLIMATE CHANGE The ocean is warming, and this might give some jellies a boost. The warmer water could help jelly embryos and larvae develop more quickly, allowing their populations to grow more quickly. And jellies that prefer warmer water will have more area to live in. However, this could also hurt some species as cold-water jelly species see their habitat shrink.
SUBMARINE SPRAWL Many industries, such as shipping, drilling and aquaculture, build docks, oil platforms and other structures in the water—sometimes referred to as “ocean sprawl"—which can serve as nurseries for jellyfish. To undergo their polyp stage, jellyfish need solid surfaces to settle upon. It’s much easier for jellyfish polyps to attach to man-made structures made of wood, brick and concrete than sand. Ocean sprawl provides more and better habitat for jellyfish to reproduce and complete their lifecycles.
Invasive Species & Fisheries
Jellies are very good at surviving: they have broad diets, reproduce quickly, can shrink down if food runs out and then revive, and tolerate low-oxygen water. So, as you can imagine, they are also very good at thriving in new ecosystems once they arrive.
In the 1980s, the sea walnut (Mnemiopsis leidyi), a type of comb jelly, was brought to the Black Sea in ship ballast water. It reproduced and spread quickly, gobbling up zooplankton and leaving little behind for the larvae of commercial fish species, including anchovy, scad and sprat. Within a decade, the comb jellies took over the Black Sea and many of the fish populations collapsed, bringing local fisheries down with them. In a stroke of accidental luck, a different species of comb jelly (Beroe ovum)—a predator of the sea walnut—was brought over in a ship, and it's helping to bring down the population. A similar story of fishery collapse coinciding with jellyfish blooms is playing out off the coast of Japan.
However, the collapse of a fishery doesn't always end in jellyfish. A crash in the pollock and walleye fishery in the Bering Sea left an opening for jellyfish but, after reigning for a few years, the jellies gave up their crown as the fish returned. And when the Peruvian anchovy fishery collapsed in the 1970s, no jellyfish swarmed in to take their place.
Spineless: The Science of Jellyfish and the Art of Growing a Backbone by Juli Berwald
Jellyfish: A Natural History by Lisa-ann Gershwin
Stung!: On Jellyfish Blooms and the Future of the Ocean by Lisa-ann Gershwin