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Jellyfish

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 Manes 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

The term jellyfish is an imprecise term that refers to an enormous variety of creatures from those animal

(which includes corals and sea anemones) and the

jellies). The jellyfish are those members of these two groups (which are sometimes collectively called the

) 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 jellyfishs 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

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,

, the Moon Jellyfish, and see if you

can use this diagram to identify some of the labelled structures on our 3D model!

Medusa: underneath (subumbrella or oral) view.

, 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

or umbrella. The domed surface of the bell, which is topmost, is called the

surface, which ofter curves inwards, is called the

. Contractions of the bell, cause it to pulse

from the cavity beneath the umbrella (the

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

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

at its terminus. The mouth is often surrounded by four

sometimes number a multiple of four, such as 8, depending upon species. Hanging from the edge of the

. 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

How do jellyfish feed, grow and reproduce? How do jellyfish know which way is up? Where is the jelly?

. Note that the tentacles have been cut away from 4 of the 8 sectors for clarity. There

is the lion-manes jellyfish. There are 16 tentacles per cluster in

this specimen, but there may be as many as 150 per cluster. The lions-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

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.

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.

is whitish-blue in colour (the specimen that I

saw was a striking translucent sea-blue colour).

The bell of the jellyfish is essentially two layers of cells, one on the outside surface, called the

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

. This inner cell layer, or gastrodermis, continues to line the stomach. The stomach in

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

body. Thus, the mature animal (especially in the larger jellyfish) contains more than simply two layers of cells!

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.

(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

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

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

which connects to the rhopalia and little

(dense balls of nerve cells) each associated with one rhopalium, and they also have a nerve net. The

, 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

medusas needs. Note: true highly integrated nerve rings are found in

wasp)though not in most scyphomedusae (true jellyfish like

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.

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

The tentacles, and sometimes other surfaces of the jellyfish, are armed with stinging cells called

. 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

radiate away from the stomach and then connect to the

(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

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

medusa can clear the plankton from 700 ml of water in less than one hour and it doesnt

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

. 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

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

) 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

Manes 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 Manes 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!

, 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

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

. 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

. 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

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.

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 thats 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 dont get the chance to travel and see these wonders or maybe you cant 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

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

A 3D computer (Pov-Ray) model of the Lions Mane jellyfish,

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

Above: a model of the moon jellyfish,

Above: a scyphistome feeding. This polyp-stage larva developed from a planula which attached to the rock.

. 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,

, the funnels disappear during the course of development to be replaced by shallower

. These are visible in the diagram as a small circle in the subumbrella

The most familiar jellyfish, especially to those living in temperate regions, belong to the order

The cnidarians (a type of colenterate) are diverse organisms and include

corals and the true jellyfish. The true jellyfish, like

, are so-called because the bulk of their bodies

are composed of gelatinous or cartilaginous

(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

hanging down from the edge. The upper surface is the

hangs down from the subumbrella, on a

. The four corners of the mouth are typically drawn-out into

The model below is an older Pov-Ray jellyfish computer model, which illustrates the main

The Importance of Sleep: Sleep and Vision in Jellyfish

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

, 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

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

Kavanau, J.L., 2006. Is sleeps supreme mystery unraveling? An evolutionary analysis of sleep encounters no

mystery; nor does lifes earliest sleep, recently discovered in jellyfish.

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