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Feature: About Jellyfish

What causes jellyfish blooms? 

Scientists do not know the ultimate causes. The increase in the temperature of the water due to climatic change, the reduction in the number of predators due to over-fishing and the increase in nutrients due to contamination of the coasts may be some of the reasons.

What are jellyfish?

Jellyfish are invertebrates, which, together with corals, gorgonians and anemones belong to a group called the cnidarians (knidé = nettle, from the Greek). This animal group has stinging cells which they use both to capture their prey and also as a form of defence. These cells contain a capsule in the interior of which there is a rolled up filament and a poison. A prey animal makes contact with the surface of the jellyfish, the capsule opens and the filaments are ejected and stick into the prey, injecting their poison.

Why are they transparent?

Because their body is 95% water, which means they are perfectly camouflaged. The body of a jellyfish is divided into three main parts, the umbrella, the oral arms (around the mouth) and the stinging tentacles. They are animals with radial symmetry. They have an internal cavity, in which the digestion is carried out, denominated the gastro-vascular cavity and which has a single aperture which carries out the functions of both the mouth and the anus.

They show two different types of morphology: the polyp form, which lives fixed to the substrate, with a tubular body and with tentacles and its mouth directed upwards, and the jellyfish form, free-living and with the tentacles and the mouth downwards.

How do they reproduce?

The jellyfish have separate sexes, that is to say, there are male and female jellyfish. In order to reproduce, males and females release sperm and eggs into the water (sexual reproduction). After fertilisation, they develop larvae which give rise to new jellyfish or which settle on the sea bottom as polyps. From these polyps, by means of asexual reproduction, new free-living jellyfish may develop.

What kinds of jellyfish are there?

Within the classification of the Animal Kingdom, and within the sub-Kingdom of Metazoans (organisms with tissues, organs and systems of organs with radial symmetry such as the jellyfish, anemones, hydra and corals) there is the Phylum of the Cnidarians, which is the group to which the jellyfish belong.

Within the Phylum of Cnidarians, it is possible to differentiate four large groups, each with its own characteristics. They are:

• The class of Hydrozoa.

They show the phases of polyp and jellyfish alternately. They are generally small in size and they can be colonial or solitary. The siphonophores are included in this class. They are floating colonies of polyp individuals and jellyfish with great and abundant poisonous cells for self-defence which, in certain cases, can be lethal for people. The siphonophores form complex colonies of individuals specialising in different functions; some serve as the flotation organ, others for nutrition, defence or for feeling. Among the best-known species of siphonophores, are the by-the-wind sailor (Velella spirans) or the Portuguese man of war (Physalia physalis), which can produce painful burns for bathers, and even heart failure.

• The class of Scyphozoa.

This is the group of those known as true jellyfish. They are the great marine jellyfish, normally with a very short or even non-existent polyp phase. The following belong to this class: the moon jelly (Aurelia aurita) which is very common in the Mediterranean, the Rhizostoma pulmo which inhabits the Mediterranean and the Atlantic or the fried egg jellyfish (Cotylorhiza tuberculata). Some species are luminescent, such as the purple jellyfish or mauve stinger (Pelagia noctiluca), which can be really striking on a night-time dive. Its eight stinging tentacles can reach a diameter of ten metres when spread out.

• The class of Cubozoa.

A class with few representatives that some authors group together with the esciphozoa. They inhabit the waters of tropical and sub-tropical seas. These are the so-called box jellyfish or sea wasps. They have their umbrella in the form of a cube, with four sides. They have a powerful sting and they may cause death to a person in just a few minutes if he is not treated with an antidote.

• The class of Anthozoa.

All the representatives of this class are polyps, which never adopt the jellyfish stage. It includes corals, madreporas, actinias and sea anemones. They are generally species of a beautiful colour. Some individuals live in isolation, such as the beadlet anemone (Actinia equina), some anemones and the colour tube anemone, (Cerianthus membranaceus), and others form colonies such as the corals or the red gorgonians (Paramuricea sp.)

How do they arise and where do they live?

Jellyfish are inhabitants of the tropical seas and of the cold waters of the Arctic. They have been there for over 650 million years.

Jellyfish are pelagic animals, that is to say that they live in the open seas, and although they can propel themselves with rhythmic motions of their umbrella, they move basically at the mercy of the currents of the sea.

Why do we have these periodic invasions?

The superabundance of jellyfish does not happen by chance but rather it is a symptom of the fact that the characteristics of the water have changed due to variations in the oceanographic parameters (temperature, salinity).

The causes of the existence of great masses of jellyfish are not local, as has been verified by researchers from the Mediterranean Institute of Advanced Studies (IMEDEA, CSIC-UIB), but rather it is a result of effects of the currents. The seasonal conditions have not been identified as determining the appearance of these species in coastal waters as, in campaigns carried out in winter, the presence of jellyfish has also been identified both in oceanic and in coastal waters.

Jellyfish normally live at a distance of between 20 and 40 miles from the coast, where the water is more salty and hotter than by the coast. Coastal waters, which are colder and less saline, act as a barrier to jellyfish. However, when the water supplied by rivers (from rainwater) to the coast is at its most limited, because of the drought less water is contributed to the sea, the salinity of the coastal waters becomes equal to that of the waters further from the coast.

The most decisive factor is the effect of marine currents.

Another aspect to bear in mind is the influence of over-fishing as certain types of fishing incidentally catch the predators of jellyfish: like the loggerhead sea turtle.

Is it true that the jellyfish have proliferated because there are fewer turtles, which feed on them?

Among the predators of the jellyfish, the following have been identified: ocean sunfish, grey triggerfish, turtles (especially the leatherback sea turtle), some seabirds (such as the fulmars), the whale shark, some crabs (such as the arrow and hermit crabs), some whales (such as the humpbacks).

Some other cnidarians also feed on jellyfish such as anemones, certain nudibranches (small molluscs without shells) which may even take over their stinging cells to use in their own defence.

What do jellyfish feed on?

Jellyfish are carnivores and can increase in size rapidly and create a large number of individuals when food is abundant. However, if food is scarce, they can become smaller. These animals, of a gelatinous consistency, have a very unsophisticated anatomy which is nevertheless very effective. They feed mainly on zooplankton, small crustaceans, although some small fish and other jellyfish also form part of their diet. It is a strange sight to see the jellyfish’ latest prey inside its body before it is digested.

How does the defence system of the jellyfish work?

The tentacles, with their stinging cells, serve as defence and as a powerful weapon for capturing prey. When they come into contact with their victims, the nematocysts (cells loaded with poison) present in the tentacles, release their harpoons or filaments which they have inside them and, through these, a toxic substance is released which paralyses the prey. The oral arms help in the capture and ingestion of the captured animal.

What preventive measures should be taken against jellyfish?

It may be dangerous for human beings to swim too close to a jellyfish but a number of organisms have found a good refuge in them. The fry of some fish such as the bogue or the amberjack, hide within the protection of their tentacles.

A number of recommendations have been made about preventive actions on affected beaches, which are above all for health professionals, fishermen and bathers and are supplied by the researchers of the Mediterranean Centre for Marine and Environmental Research (CMIMA-CSIC), Josep-María Gili and Francesc Pages.

• If there are a large number of jellyfish in coastal waters, the beach should be closed for at least 24 hours, taking precautions even if the jellyfish are abundant at some distance from the coastline.

• If the jellyfish are close to the beach, the best thing is to stay out of the water and keep one’s distance from the breaking waves.

• If a jellyfish is seen in the water, it is better not to take any risks even if it is some distance off as, with the action of the waves, its tentacles can break and the cells in the floating fragments will remain active. What is more, it is necessary to advise bathers who are not familiar with these organisms that they should not touch them even if they appear to be dead.

• Jellyfish should not be touched in the sand, even though they appear to be dead, the stinging cells remain active for a period of time; even walking along the water’s edge can be dangerous as there may be remains of tentacles in the sand. It is necessary for a period of a day’s sun to de-activate the stinging cells located in the fragments.