|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 porojections 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 they do have is a marginal nerve ring (in most species) 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 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
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 impails 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!
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 that cover its surface. 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 creature called a scyphistoma. 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) but in winter or early spring, the scyphistomea 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. 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.
Graphics illustrating the life-cycle of a typical jellyfish are coming soon...
Above and below: a Pov-Ray model of an ephyra larva of a jellyfish like the moon jellyfish.
Click images to enlarge
Above: a strobila strobilating.