|Building Bodies of Jelly - Jellyfish
The images of the jellyfish above were created in Pov-Ray and represent a 'typical' jellyfish. Of course there
is really no such thing, jellyfish come in an incredible range of varieties and range in size from one centimetre
or less in diameter, to over two metres in diameter and some, like the Lion Mane's jellyfish, may reach half a
tonne in weight. Despite their beautiful and enchanting appearance, jellyfish are highly efficient predators. In
the early oceans on the primordial Earth the jellyfish were the top predators, and even today they are vastly
abundant and often swarm in thousands. To produce so much mass at such a prolific rate jellyfish must
clearly be very efficient predators.
The term 'jellyfish' is an imprecise term that refers to an enormous variety of creatures from those animal
groups called the cnidarians (which includes corals and sea anemones) and the ctenophores (comb
jellies). The jellyfish are those members of these two groups (which are sometimes collectively called the
coelenterates) which swim or float freely. These include the medusas, like the one above, named after the
Greek myth of a beautiful woman who was cursed by a jealous goddess and left with a hideous appearance
and writhing serpents for hair - these serpents have been compared to the jellyfish's tentacles. They also
include forms like the Portuguese Man O' War which consists of a colony of many individuals fused together
into a single organism. This page deals mostly with the medusae, and specifically those that are most
commonly found washed up on seashores - the scyphozoan medusae.
Look at the image above and the other viewpoints of the same model shown below (click on thumbnails to
enlarge) and then look at the labelled diagram of a similar jellyfish, Aurelia, the Moon Jellyfish, and see if you
can use this diagram to identify some of the labelled structures on our 3D model!
Medusa: plan (exumbrella) view.
Medusa: underneath (subumbrella or oral) view.
A labelled diagram of Aurelia, the Moon Jelly, probably so-named both for its whitish disc-like body and its
nocturnal habit of swimming near the surface of the sea. Identify as many of these labels as you can on
the 3D jellyfish medusa model before we look at what these structures are and what they do.
The main part of the medusa body is the bowl, dome, lantern, cuboidal, goblet, trumpet or disc-shaped bell
or umbrella. The domed surface of the bell, which is topmost, is called the exumbrella, whilst the lower
surface, which ofter curves inwards, is called the subumbrella. Contractions of the bell, cause it to pulse
and expel jets of water from the cavity beneath the umbrella (the subumbrellar cavity) and these jets
propel the jellyfish along. It is often said that jellyfish are weak swimmers and at the mercy of the tides and
even that they can only swim upwards and sink downwards. However, much film footage clearly shows jellies
swimming horizontally as well as vertically and many are strong swimmers, but not as strong as large fish and
so they do sometimes get caught up in strong tidal currents, but they are better and more precise swimmers
than most give them credit for. The bell contains a thick ring of strong muscle, called the coronal muscle,
that generates most of the power. Other more complexly arranged muscles assist the coronal muscle.
Hanging from the centre of the subumbrellar is a projection, called the manubrium, which bears the mouth
at its terminus. The mouth is often surrounded by four oral arms, though these may be absent and
sometimes number a multiple of four, such as 8, depending upon species. Hanging from the edge of the
underside of the bell, are the tentacles. Some species lack the tentacles, some have hundreds of tentacles,
others only four tentacles, some species have very short tentacles (like Aurelia) others have tentacles many
metres in length.
How do jellyfish feed, grow and reproduce? How do jellyfish know which way is up? Where is the jelly?
Above: the jellyfish Cyanea. Note that the tentacles have been cut away from 4 of the 8 sectors for clarity. There
are several species of Cyanea, Cyanea capillata is the lion-mane's jellyfish. There are 16 tentacles per cluster in
this specimen, but there may be as many as 150 per cluster. The lion's-mane has 8 lobes and 8 rhopalia and the
bell diameter may reach 2.5 metres! The colour varies from yellowish to deep red or reddish-purple. The tentacles
of Cyanea can be up to 10 to 30 metres or more in length and are very sticky, and they can be fanned-out to form
a massive fishing net that the jellyfish trawls through the water. Click the image to enlarge. Cyanea lamarckii is up
to 15 cm or more in diameter (I once saw what was almost certainly a specimen of this jellyfish some two to three
feet across) and in this species the 8 primary lobes are divided into pairs of secondary lobes which divide at the
edge into pairs again, making a total of 32 lobes. Cyanea lamarckii is whitish-blue in colour (the specimen that I
saw was a striking translucent sea-blue colour).
Where is the jelly?
The bell of the jellyfish is essentially two layers of cells, one on the outside surface, called the epidermis,
and another which follows the lining of the subumbrella as it extends down the pendulous protuberance and
enters the mouth, at which point the cell lining takes on different characteristics and is called the
gastrodermis. This inner cell layer, or gastrodermis, continues to line the stomach. The stomach in Aurelia,
and in our model jellyfish, is divided into a central chamber and four pouches coming off the sides. These
pouches contain the gonads (reproductive organs that produce sperm and/or egg cells). The gonads are
visible in our model as the four pinkish horseshoe shaped structures in the centre of the bell.
Beneath these two layers of cells, the epidermis and the gastrodermis, the main bulk of the jellyfish is made
up of a jelly-like substance called mesogloea. In some tiny jellyfish, the mesogloea may be little more than a
thin sheet, but in large jellyfish it becomes a thick mass. Cells that develop from the epidermis and/or
gastrodermis of the developing baby jellyfish, migrate into the jelly (especially in the larger types) and form
muscle and nerve cells as well as wandering amoeboid cells, that resemble amoebae and wander around the
body. Thus, the mature animal (especially in the larger jellyfish) contains more than simply two layers of cells!
Radial Symmetry and the number 4
Jellyfish of the medusa type we are considering here, have what we call radial symmetry - meaning they are
essentially circular (or spherical). A human, on the other hand, has bilateral symmetry - meaning that your
body is in two mirror halves and has a definite front end and back end. Jellyfish are also built on around the
number 4, with most of their structures occuring either in 4s or in multiples of 4, such as 8 or 16 etc. Thus,
tentacles may number 4, 8, 16, ..., to 8 x 40 = 320 or more. Our model has 4 gonads, 32 (8 x 4) lappets (the
crinkly projections along the bell margin), 4 oral arms, etc.
Knowing which way is up and where things are
The rhopalia (singular rhopalium) are the small pink structures, 8 in number in our model, which can be
seen located around the bell margin at regular intervals, between lappets. These are sensors. Each
rhopalium contains a gravity sensor, which allows the jellyfish to tell which way is up and which way down,
and to know how much its body is tilted. These organs may also contain what look like olfactory (smell)
sensors and in some species each rhopalium has a tiny eye. These eyes may be simple light sensors, or
they may be complex eyes equipped with a lens. Some jellyfish do not have eyes, but even these can detect
light by other means.
Scyphozoan jellyfish avoid bright sunlight, descend deeper into the water at midday and in darkness, but
surface in the morning or late afternoon and during cloudy days. Thus, most jellyfish medusae prefer twilight
or diffuse light, though some do prefer sunlight. Medusae also descend into the water during rough and
What no brain?
Jellyfish have no obvious brain, as a large mass of nerve cells, but they clearly possess sophisticated
computers. What some do have is a marginal nerve ring which connects to the rhopalia and little ganglia
(dense balls of nerve cells) each associated with one rhopalium, and they also have a nerve net. The nerve
net, or plexus, is a network of neurones (not true nerves) that cover the subumbrella (and sometimes a
nerve net or plexus that covers the exumbrella) just beneath the surface. These structures function as a
sophisticated computer, not as complex or as sophisticated as the mammalian brain, but sufficient for the
medusa's needs. Note: true highly integrated nerve rings are found in hydromedusae and cubozoa (e.g. sea
wasp)though not in most scyphomedusae (true jellyfish like Aurelia and Cyanea) although the nerve net
tends to form circular bands overlying the coronal swimming muscle in scyphomedusae also.
Nerve rings occur in cubomedusae (cube jellyfish, e.g. the Sea Wasp) and also in the related hydromedusae
(not considered true jellyfish by zoologists, due to their much thinner layer of mesogloea giving them a more
glassy appearance, but colloquially also often referred to as 'jellyfish'). In jellyfish like Aurelia and Cyanea
there is no compact nerve ring, but neurones do form circular networks around the bell margin, what we may
call a neuronal ring. This, along with the rhopalia, is as close as jellyfish get to a 'brain' but is not a true brain
since it does not consist of centralised ganglia (though each rhopalium may be innervated by a ganglion).
Each rhopalium acts as a pacemaker to synchronise swimming muscles to produce coordinated and
appropriately-timed bell pulsations when swimming. Mathematical models demonstrate that having several
pacemakers connected in series, in this fashion, improves the degree of synchrony and the precision of the
pacemaker system. At any one moment, one rhopalium dominates the others, but this changes randomly in
the absence of stimulation. Otherwise, the most strongly stimulated rhopalium becomes dominant.
Common myth - jellyfish have no skeleton
Whilst it is true that jellyfish are soft to the touch and have no hard bony parts, they do have the mesogloea.
Stiffening fibres traverse the jelly and in some jellyfish, the mesogloea can form hardened plates, rather like
cartilage, that hinge together. These plates provide support for the animal and the muscles may attach to
these plates, so they function as a skeleton. Obviously, the jellyfish skeleton of jelly, of more or less
firmness, is not as hard as the mammalian bony skeleton, nor as hard as the cartilagenous skeleton that
sharks have, but it is still a skeleton, albeit more or less soft, and is sufficient for the jellyfish which does not
move the bulk of its body quickly in complex ways and can rely on the surrounding sea water to buoy up and
support its body.
Making a living - the Jellyfish's Sting
The tentacles, and sometimes other surfaces of the jellyfish, are armed with stinging cells called
nematocysts. These nematocysts are grouped into stinging batteries. Each cell, when triggered by the
touch of potential prey (or a predator), discharges a tiny thread which is a miniature harpoon that impales
the victim and injects venom. A prey item, such as a fish, will be injected with dozens of these harpoons.
Other nematocysts discharge sticky threads to trap the prey. There are many different types of nematocyst
found in the coelenterates, and which type or types an individual has depends upon species.
Each tentacle can be moved by its own muscles, as can the oral arms. The tentacles and/or oral arms pass
the captured food to the mouth. Once in the stomach, the food is digested into a broth within about six hours.
This liquid is transported around the animal by the circulatory system. This consists of radial canals that
radiate away from the stomach and then connect to the ring canal (if present) shown as a pink ring in the
model, and then back to the stomach, with remaining waste being carried out through the mouth (jellyfish
have no separate anus!). These canals together with the stomach (gastric cavity) form the
gastroendodermal system. In some jellyfish the stomach gives off complex branching canals, in others just
four straight radial canals are apparent. The gastrodermis lines these canals and each cell possesses a
flagella, a long (but microscopic) whip-like structure that stirs up the water, creating specific currents that flow
in the desired direction, transporting the broth around the body, along with sea water that enters through the
stomach. This circulatory system probably also transports dissolved oxygen around the body and removes
waste, including carbon dioxide.
Aurelia feeds in a different way. Tiny planktonic creatures, including molluscs, crustaceans, eggs, minute
worms and larvae, collect on the exumbrella surface, where they become trapped in mucus. Tiny beating
hairs (cilia or flagella) carry the food-laden mucus to the edge of the bell, where it collects in eight masses (in
the centre of the lappets) where it is licked off by the oral arms and carried by tiny hairs along a groove that
runs along the inside of each arm, through the mouth and into the stomach. The food is partly digested by
the stomach and then carried along 8 straight (ad)radial canals, along the ring canal, and back to the
stomach along the branched radial canals. Outward currents generated by tiny beating hairs on the oral
arms, carry the waste out through the mouth, as inward currents bring more food in. This is very efficient, for
a single Aurelia medusa can clear the plankton from 700 ml of water in less than one hour and it doesn't
have to do very much, just wait for the food to stick to its body as it swims past!
The stomachs of jellyfish generally consist of four pouches or less-distinct lobes (the coronal stomach)
radiating from a central stomach. The 4 muscular septa which divide the stomach into its 4 chambers are
pierced by a circular opening (septal ostium), forming a ring sinus which connects the 4 chambers together.
The inner edge of each septum is equipped with 2 rows of gastrodermal tentacles, called gastric filaments,
which consists of a mesogloea core lined by gastrodermis equipped with nematocysts and gland cells. The 8
rows of gastric filaments project into the central stomach. This gut pattern occurs in the adults of some
species, whilst in others (including Aurelia and Cyanea) it occurs only in the scyphistoma larval stage and is
modified in the adult: the septa disappear, leaving a central stomach which may be slightly scalloped into
four chambers. The gastric filaments now spring from the stomach floor, in rows or groups in an interradial
position (in-between the four main radii).
Most jellyfish, however, are fierce hunters, trapping and eating animals as large as fish. The huge Lion
Mane's jellyfish has a vast tangle of tentacles that sweep the oceans like fishing nets, spanning an area the
size of a tennis court. No wonder these Lion Mane's jellies often reach half a tonne in weight! In some
jellyfish, the oral arms are highly branched feathery structures, whilst in others these arms fuse to form a
conical structure, which may be truly massive in some jellyfish, and which contains hundreds of frilly mouths!
Reproduction - churning out new jellyfish
Most jellyfish are dioecious, meaning that individuals are either male or female, but some species are
hermaphrodite (having both male and female gonads). Jellyfish typically ripen in spring and summer. The
eggs develop either in the gonads, or in pockets on the oral arms (after being released from the gonads
through the mouth) depending on species. Each egg produces a tiny larval creature, called a planula, which
escapes and swims away with the help of tiny beating hairs (cilia) that cover its surface. The planula is either
hollow or solid. After a short planktonic existence, during which the planula may travel great distances, the
planula attaches to a solid surface, such as a submerged rock, and develops into a small trumpet-shaped
organism called a scyphistoma. In some species the planula puts out stolons (shoots) which bud new
scyphistomes at intervals and then detach. If the scyphistoma moves about, stolons may detach and each
fragment can regenerate into a new scyphistoma. In the related Stauromedusae, in which the adult is sessile
and attached to the substrate by a stalk, the planula is vermiform (worm-like) and may put out 1 to 4 stolons
may detach as vermiform creeping larvae which eventually develop into stalked sessile larvae.
The scyphistoma deveops tentacles around the mouth which is on its top (apical) surface. These tentacles
catch tiny food items, with the help of nematocysts, and so the scyphistome eats and feeds, rather like an
upside-down jellyfish stuck to the rock, but no more than a few centimetres long. The scyphistoma may
bud-off new scyphistoma (asexual reproduction) or grow stolons which bud off new scyphistomae. In winter
or early spring, the scyphistoma starts to split up into a stack of discs, rather like a stack of plates, a process
called strobilation. This stack of disks is called a strobila. One by one each disc detaches from the end of
the strobila and become a tiny jellyfish, slightly different from the mature form, and called an ephyra.
Depending on species, the scyphistoma may bud off one ephyra and then regenerate its tentacles before
later budding off another ephyra, and so on. This is called monodisk strobilation. Others undergo polydisk
strobilation, in which the scyphistoma fragments into a stack of plate-like structures, the most distal (tipmost)
detaching first ad the more basal develop. Ephyra detachment involves muscular constriction.
Each ephyra is only a few millimetres in diameter, but will feed and grow, and if it survives then it will become
a mature jellyfish, possibly weighing as much as half a tonne. Scyphistomae may live for several years,
strobilating each winter, and feeding each summer. In this way, each scyphistoma is like a jellyfish factory,
churning out dozens of jellyfish! Note that the life-cycle of some jellyfish is very different from that just
described, and indeed is unknown for many.
Where to see more jellyfish
It is impossible to do justice to the diversity, complexity and beauty of jellyfish in a couple of pages! However,
a search on Google will reveal dozens of stunning photographs. One of the best accounts ever written about
jellyfish, including many beautiful diagrams, is that given by Libbie Henrietta Hyman in her 1940 volume 1 of
The Invertebrates (unfortunately not in print at the moment!). Libbie Hyman was one of the greatest
zoologists of all time and motivated by the sheer appreciation of the beauty of living things to produce one of
the best series of zoology books ever written. The standard of this work is an example to all scientists and is
one of the best scientific works ever produced. It is unfortunate that she never lived long enough to complete
her review of the invertebrates, but then that's hardly surprising when one considers how many different
types of invertebrate there are! There are more living wonders on Earth than any individual can ever live
long enough to see, study and appreciate, but just to see some of these creatures is well worth the while! If
you don't get the chance to travel and see these wonders or maybe you can't travel to see these wonders,
there are a lot of ways to still see these creatures. Visit you local library or rent textbooks, or search the Web
where there are lots of resources. One other way could be to check out your local aquarium.
The life-cycle of a scyphozoan such as Aurelia. Click images to enlarge.
Above and below: a Pov-Ray model of an ephyra larva of a jellyfish like the moon jellyfish.
Above: a strobila strobilating. This is an example of polydisk
strobilation, in which the scyphistoma has constricted itself into a
series of developing plate-like structures which are released at the
tip as young ephyrae.
A 3D computer (Pov-Ray) model of the Lion's Mane jellyfish, Cyanea capillata. These organisms are phenomenal
fishing machines, trawling the seas for fish and other prey when they extend and spread their vast net of tentacles.
The model illustrates the 8 primary lappets of the bell margin, the 4 frilly oral arms and 8 V-shaped clusters of
tentacles (16 tentacles per cluster in this case, though this number is highly variable in life). The fishing tentacles
are being deployed.
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3 Feb 2018
Above: a model of the moon jellyfish, Aurelia.
Label your own jellyfish diagram!
Above: a scyphistome feeding. This polyp-stage larva developed from a planula which attached to the rock.
Above: subumbrella view of Aurelia. Each of the 4 gonads occur in the stomach floor and hang down into
the subumbrella cavity. Many jellyfish have subumbrellar funnels, 4 deep pits or invaginations in the
subumbrella (which occur on the interradii) of unknown function, though water flows in and out of them as
the bell pulses (so they may be respiratory, excretory or chemo/thermosensory). In other jellyfish,
including Aurelia, the funnels disappear during the course of development to be replaced by shallower
depressions, called subgenital pits. These are visible in the diagram as a small circle in the subumbrella
inside each arc-shaped gonad.
The most familiar jellyfish, especially to those living in temperate regions, belong to the order
Semaeostomeae. This includes Aurelia (e.g. Moon Jellyfish) and Cyanea (Lion's-mane Jellyfish).
The cnidarians (a type of colenterate) are diverse organisms and include hydra, sea anemones, true
corals and the true jellyfish. The true jellyfish, like Aurelia, are so-called because the bulk of their bodies
are composed of gelatinous or cartilaginous mesogloea (shown in blue in the above diagram). Jellyfish
alternate between attached and often stalked larvae (see below) which resemble hydra (the polyp stage)
and the sexually-reproducing and free-swimming medusa (named after the woman of Greek legend who
had her hair turned into snakes by goddesses envious of her beauty, in reference to the 'tentacled-head'
appearance of jellyfish). The medusae of the more familiar jellyfish are dome, saucer or bell-shaped with a
number of tentacles hanging down from the edge. The upper surface is the exumbrella, whilst the
undersurface is referred to as the subumbrella. The mouth hangs down from the subumbrella, on a
pendulous structure called the manubrium. The four corners of the mouth are typically drawn-out into
frilly oral arms.
The model below is an older Pov-Ray jellyfish computer model, which illustrates the main
The Importance of Sleep: Sleep and Vision in Jellyfish
Cubomedusae, such as Chironex fleckeri, the Sea Wasp, are particularly active swimmers, capable of swimming at up
to 0.5 m/s and changing direction with considerable agility. The box-shaped body of the cubomedusae, up to 24 cm in
diameter in C. fleckeri, has four pedalia, or muscular fleshy pads, one at each corner of the bell and each bearing
one or more tentacles. Midway between each pedalium is a rhopalium (sensory stalk) hanging down inside an
indentation in the bell margin. Each rhopalium bears a gravity-sensing statocyst and a cluster of 6 eyes (making 24
eyes in total). The eyes are of four different types: two pit-eyes, two slit-eyes and two lens-bearing camera-type eyes
of different sizes. The camera type eyes are complex, each equipped with a cornea, pupil, lens, a vertebrate-like
retina and pigment layer. This suggests that they are capable of image formation. Kavanau (2006) suggests that
these eyes are used in hunting for prey. The more hunting these jellyfish do, the more sleep they need, whereas
those which are 'hand-fed' in aquariums may need no sleep at all (for as long as 9 months in one case). During sleep
they may lie motionless on the sea floor, with their tentacles touching the bottom. They may also rest and perhaps
sleep upside-down, with their tentacles and captured food in the cavity of the bell. Individuals tracked in nature may
sleep for 2 to 15 hours each day. The hypothesis is that jellyfish rest upside-down whilst they remove food from their
tentacles and digest their food.
This rest-and-digest behaviour may have evolved into sleep in the Cubomedusae because they need to rest their
nervous systems. Their central nervous system consists of a nerve ring connecting the rhopalia and pedalia. The
notion is that the vast computational demands placed upon the visual systems of actively hunting cubomedusae
prevents their nervous systems from carrying out general housekeeping tasks, including memory formation (which
requires changes to the synapses). Visual processing is certainly a very demanding computational task (which is why
our computers have dedicated video cards) and processing images would probably saturate much of the capacity of
jellyfish nerve nets. The mammalian brain faces similar problems: it was discovered only a few years ago that during
sleep glymphatics open-up more to flush the brain, washing it and removing toxic waste products and leaked
neurotransmitters that accumulate during the day. (Glymphatics are channels around the smaller blood vessels of the
brain which carry circulating cerebrospinal fluid, CSF). Sleep is also evidently important for memory formation in
mammals. Combined with these recent and important studies in jellyfish, it would seem that the 'mysteries' of sleeep
are indeed unravelling!
Kavanau, J.L., 2006. Is sleep’s ‘supreme mystery’ unraveling? An evolutionary analysis of sleep encounters no
mystery; nor does life’s earliest sleep, recently discovered in jellyfish. Medical Hypotheses. 66(1): 3-9.