Blue blubber JellyfishCatostylus mosaicus

Blue Blubber Jellyfish- Facts and Photo’s

Blue blubber Jellyfish Catostylus mosaicus

An unusually colored blue blubber jellyfish (Catostylus mosaicus), spotted off the west coast of Mafia Island in Tanzania. These jellyfish vary in colour, depending on which part of the world they are from. In some parts of the world they are known as the jelly blubber. They can vary from white to light blue and purple. Blue blubber jellyfish in Tanzania have always been known to have blue tentacles, however this one is unique with red-brown tentacles. Possibly the change of colouring is related to breeding behaviour, but this is speculation.


The bell of the blue blubber jellyfish grows to approximately 30-45 cm (12–18 in) in width although they are usually slightly smaller. Bell colors range from a creamy white to a brown and blue color. The bell pulsates and is used to propel the jelly forwards. They have eight arms with tentacles that can also vary in color. Each of the eight arms have several small mouths that catch and transport food to the jelly’s stomach.There is no mouth as such, rather there are numerous small openings on each arm. Food is passed through these openings to the stomach. The tentacles have stinging cells that can capture tiny crustaceans and small fish.

blue blubber jellyfish (Catostylus mosaicus)


The eight tentacles  have large numbers of nematocysts or stinging cells on them. These are similar to a tube that has been pulled inside out into its self with a sharp point and has a mechanism to pump poison through the end of the point. These nematocysts are extremely small, about 0.001 mm in diameter. There is a trigger on the exterior of the tentacle and when this trigger is activated either chemically or physically the tube or tubule as it is known inverts its self. Pushing outwards and pushing the point into the prey, it then pumps poison up the tubule into the prey. This process of firing takes place in 700 nano seconds which is extremely fast and occurs with substantial pressure. In some species the point is able to penetrate the shell of small crustaceans.
There are two types of stinging cells in jellies tentacles , nematocysts and spirocysts and these inject canhypnotoxin, thalaxin and congestin into the prey. Hypnotoxin has an anaesthetic effect and paralyzes the prey. Thalaxin has an allergenic action, generating an inflammatory reaction. Congestin paralyzes the circulatory and respiratory systems of the prey.

The effect of the sting of the blue blubber jellyfish on humans is mild compared to some jellyfish. Usually the sting results in a slight redness around the area where the tentacles have touched, however some individuals may be more allergic than others. The area may be itchy for some time.


In Tanzanian waters blue blubber jellyfish are often seen in spring and summer, usually solitary but large blooms can develop. See below for images of these blooms. They are slow swimmers and easy for a swimmer to avoid. Juveniles are often white in color and are sometimes shadowed by small fish.

White Jellyfish


Blue blubber jellyfish do not have what we would traditionally call a brain or centralized concentration of nerve cells. Instead they have a nerve net distributed throughout the body which has various simple sensors on it and operates in a decentralized fashion. Information is transmitted through the net and via a circular net that runs around the bell with rhopalia on it, small structures containing sensors to sense light and gravity.

The circular net around the bell also controls the contraction of the bell for propulsion. Jellies use their light sensing cells to orientate themselves against the light. They use pulsations of their bell to propel themselves forward and having no sight, have limited control over where they will end up, their directional logic being generally limited to either moving away from light or towards it. Recent research indicates that some jellies may orientate their direction on a current but this is not yet proven.


The Blue blubber jellyfish is found in coastal parts of the Indo-Pacific preferring the warmer waters.

blue blubber jellyfish (Catostylus mosaicus)


Blue blubber jellyfish obtain their nutrition in two ways. They have a symbiotic relationship with various types of algae (zooxanthellae) which provide them with some of their nutrition. During the day they spend their time closer to the surface for the algae to be exposed to more sunlight. At dusk they head lower in the water table and rely on their tentacles to catch small fishes as well as crustaceans and zooplankton. The stinging cells on the tentacles are used to capture prey.


Jellyfish or sea jellies can reproduce through both sexual and asexual reproduction. During the Medusa stage specialized organs known as gonads form either eggs or sperm. In  some species, female jellies release eggs into the water table. These are fertilized by sperm released by the males. In others the eggs are fertilized in the stomach of the female when the sperm released by the males swims into the females stomach and then fertilize them there.

Some jellies such as the moon jelly have special areas on the oral arms which function as brood chambers. The eggs hatch into larvae known as planula with cilia on them allowing them to swim and these attach themselves onto a firm substrate and develop into polyps called scyphistomae.  These polyps produce numbers of ephyra, that detach and swim off to grow into the medusa stage. The polyps can also bud asexually producing other polyps.

juvenile blue blubber jellyfish (Catostylus mosaicus)


Research has shown that there are commensal relationships between certain copepods and the blue blubber jellyfish.  Paramacrochiron maximum, a copepod are found in high numbers living on the oral arms of blue blubber jellyfish. Some species of fish use the blue blubber jellyfish (Catostylus mosaicus) for protection and swim inside the jelly when they feel threatened as can be seen in the images below.

blue blubber jellyfish (Catostylus mosaicus) blue blubber jellyfish (Catostylus mosaicus) blue blubber jellyfish (Catostylus mosaicus)


In the far east in countries such as China and Japan, blue blubber jellyfish are regarded as delicacies. They are sometimes dried and powdered or added to stews, but they are also eaten as sushi and sashimi. Being 90 %  water they have very little fat in them !

blue blubber jellyfish (Catostylus mosaicus)


Blue blubber jellyfish are kept in various public aquariums and some enthusiasts keep them in special tanks. Advances in technology now allow jellies to be kept in small tanks with currents in them that prevent the jellies from hitting the sides.

Because they are easy to feed blue blubber jellyfish are becoming more popular. Small relatively cheap tanks which can be purchased to house them and they can live for up to a year in captivity. Because jellies fluoresce under ultra violet light a jellyfish tank can make a very attractive addition to a room.


In some years the blue blubber jellyfish bloom in the channel between Mafia Island and the Tanzanian mainland, congregating in large numbers. This creates havoc for the local fishermen as they clog their nets. Because of these blooms, catches often consist of more jellyfish than fish as can be seen in the images below.

This deprives the local fishermen of their main food source and times are tough for them during these jellyfish blooms. As a result of having to remove large numbers of jellyfish from the nets the fishermen are often stung on multiple occasions.

Blue blubber jellyfish washed up onto the beach

It is not really known for sure as to what causes these blooms.  However in all probability, they relate to favorable water and feeding conditions.  Small fishes partially ingested can be seen in the images above.

The main predators of blue blubber jellyfish are leatherback turtles and some species of fish such as tuna and sunfish. Because populations of these predators are dropping drastically, this may be an additional contributory cause of the blooms.
Blue blubber jellyfish clogging fishermens net




Photographed with a Canon 7D and a 18-55mm Lense in an Ikelite eTTL Housing with Ikelite Flat Port and two Ikelite DS-161 Strobes.