Select a topic and discover the wonderful world of jellyfish
Author: Prof. Alan Deidun



How do jellyfish swarms come about?

Jellyfish swarms or blooms consist of large numbers of jellyfish which aggregate occasionally, mainly as a result of the direction of prevailing water currents. Why do such increases occur? Several theories have been proposed by different scientists, including the fact that nutrient concentrations in the sea have increased, that jellyfish predators, such as turtles, have decreased over recent years and that the Mediterranean Sea is warming up due to climate change.

It may well be that the recent observed increases in jellyfish numbers are simply part of natural life cycles.


Pelagia noctiluca blooms are the most frequently encountered in the Mediterranean during the summer months



Most jellyfish species have only limited control over their vertical position and so are swayed about by currents.

Can jellyfish swim?

Jellyfish are members of the plankton family – i.e. they cannot oppose the prevailing water currents but rather sway along with them.

For this reason, aquaria featuring jellyfish normally have continuous water flows, known as kreisels (from German for “spinning top”). Some jellyfish species are capable of making pulsating movements which allow them to control their vertical position in the water column.





Is the presence of jellyfish a sign of a polluted sea?

Jellyfish populations are normally highest in overfished seas and the Mediterranean Sea is a case in point, where large increases in jellyfish populations have been observed recently, at the same time that the fishing intensity in the same basin has been increasing in recent years.

An increase in dissolved nutrients in the sea (namely phosphates and nitrates) leads to an increase in phytoplankton abundances which normally leads to an increase in small zooplankton numbers which are then grazed upon by gelatinous zooplankton like jellyfish. Hence, increased pollution levels can lead to higher jellyfish numbers but research is needed to establish such a relationship.


Overfishing might be one of the causes of the recent upsurge in jellyfish blooms



The upside-down jellyfish is one of the alien species recorded recently from the Maltese Islands

Are there any jellyfish species that only live in the Mediterranean Sea?

There are no jellyfish which are restricted (i.e. endemic) to the Mediterranean but there are jellyfish species which are very common in the Mediterranean Sea. These include the mauve stinger (Pelagia noctiluca) and the fried egg jellyfish (Cotylorhiza tuberculata).

The Mediterranean is additionally being colonized by alien jellyfish species coming in through the Suez Canal, mostly from the warmer waters of the Indian and the Pacific Ocean. Examples include the upside-down jellyfish (Cassiopea andromeda), the nomadic jellyfish (Rhopilema nomadica) and the Australian spotted jellyfish (Phyllorhiza punctata).





Do all jellyfish sting?

No. In fact, many species are harmless. Those jellyfish which do sting (known as stingers) only make up a small proportion of the 200 known jellyfish species.

The jellyfish sting is actually a mild venom/poison contained in stinging cells called nematocysts, arranged in rows on jellyfish tentacles.


Some jellyfish species can inflict painful stings through their numerous stinging cells called nematocysts



Jellyfish are popular for human consumption in some parts of the world, including China, Japan and Indonesia

Who eats jellyfish?

A number of medusivorous (jellyfish-eating fish) exist, including the ocean sunfish Mola mola, besides marine turtles, like the common loggerhead turtle, which are known to consume jellyfish as well.

Numerous seabird species are known to scavenge on dead jellyfish once these are stranded on the beach. Jellyfish are also consumed by humans in a number of countries, namely in countries like Japan, China and Indonesia, after being soaked overnight in water to desalt them.





Which are the most dangerous and largest jellyfish?

The largest ever recorded jellyfish specimen belonged to the lion's mane jellyfish species washed up on an American beach in 1870, having tentacles which were almost 40m long (longer than a blue whale). Don’t worry – such a species is confined to the cold waters of the Arctic and northern Atlantic Oceans.

The most toxic jellyfish venoms belong to a group of jellyfish known as the box jellyfish (Cubozoa), most notably the Irukandji jellyfish and Chironex fleckeri. The latter’s venom suffices to kill 60 individuals and non-lethal symptoms include an increase in blood pressure, nausea and cramps.  


The box jellyfish Chironex fleckeri is reputed as the most dangerous jellyfish since its venom can presumably kill up to 60 people



Fried egg jellyfish blooms occur in late summer and early autumn, coinciding with the dolphin fish (lampuki) fishing season

Which are the most common jellyfish in the Maltese Islands?

The mauve stinger (Pelagia noctiluca) is by far the most common jellyfish species throughout the year, with swarms in local waters being recorded as far back as the 1950’s.

The fried egg jellyfish blooms at the end of summer (September-October) and for this reason this species is known by the monicker of “tal-lampuki” (dolphin fish jellyfish) since this period coincides with the “lampuki” fishing season.



  Visit OCEANA online for more interesting jellyfish facts!

NEW: click HERE for a recent article on the citizen science approach
adopted by the Spot the Jellyfish Initiative