Are jellyfish found worldwide? Jellyfish are found all over the world, from the surface to the deepest parts of the ocean, and are known to have existed on earth for more than 650 million years. As many species are oceanic, these have wide, often global distributions.
Are jellyfish numbers increasing? Jellyfish numbers have been on the increase, usually in summer. The warmer water temperatures are probably a factor in this increase, as these are also known to boost reproductive rates. As these creatures tend to move with currents, seasonal changes in these also probably contribute to the appearance of jellyfish off our shores. In Gibraltar, the two main episodes we had were of Portuguese men-o-war (Physalia physalis) — strictly speaking not a jellyfish but a floating colonial organism of minute individuals called zooids —, and the purple stinger jellyfish (Pelagia noctiluca). Both of these species have been giving problems in the Med recently but the latter had also increased in numbers off places like the coasts of the British Isles, resulting, for example, in the loss of 100,000 salmon at a fish farm in Northern Ireland when a 10-mile square bloom of mauve stingers appeared in 2007, causing £1 million damage. Why are jellyfish numbers increasing? These and all the other organisms on the earth have evolved, like we have, over millions of years to exhibit a tremendous variety of forms and ways or surviving. Jellyfish have not changed much at all in over half a million years, which means that they must be doing something right. The truth is that numbers are probably so high because of what we (humans) are doing to their ecosystems such as reducing populations of their natural predators (things like sea turtles, tuna, sunfish), dumping high levels of fertilizers from agriculture into the sea which raises productivity levels, etc. Where do our jellyfish come from — do they migrate from area to area? The vast majority of jellyfish do not ‘migrate’ as such (although some do go up and down on a daily basis in the water column — known as diel vertical migration), but rather tend to move with ocean currents as they are weak swimmers, or in the case of the Portuguese man-o-war, using a ‘sail’ to also capitalise on wind power.
photo: Steve Warren
What do you think of the nets which will be in place to protect areas of each of our beaches this summer? I know of the proposed nets and it is understandable that measures are being taken to allow beaches to be more safely used, but I suspect that if swarms are large the nets will probably not stop them all so care will still need to be taken. Also the nets will need careful and constant monitoring and maintenance, especially as the last thing anyone wants is for any other animal, such as a sea turtle, to get caught up in these and perish as a consequence. Jellyfish really are very interesting creatures, who certainly have no evil design to spoil our summers. n
GIBRALTAR MAGAZINE • JULY 2013
Left: A swarm or bloom of purple stinger jellyfish in the Strait
photo: Steve Warren
Australian endurance swimmer Chloe McCardel’s attempt in June to become the first person to swim from Cuba to Florida without a protective shark cage was ended not by a shark, but by a jellyfish sting. This event came closely on the heels of our own Environment Minister, John Cortes, announcing that the Gibraltar Government will install purpose-built nets to protect Gibraltar’s beaches from jellyfish this summer. We thought it was a good time to speak to Dr Darren Fa, Deputy Director, Gibraltar Museum to find out more about these fascinating and spectacular sea creatures.
Jellyfish Facts Jellyfish first appeared about 650 million years ago and are found in every ocean. Some are found in fresh water. Jellyfish are not fish, so some people prefer the term sea jellies The collective term for jellyfish is a bloom or swarm Jellyfish don’t have a respiratory system — the body is oxygenated by diffusion — and they do not have a brain, instead they have a network of nerves located in the epidermis, called a ‘nerve net’ Jellyfish are dioecious; to reproduce, both males and females release sperm and eggs into the surrounding water, where the eggs are fertilized. Jellyfish larvae (planula) settle on the seabed and grow into something resembling a sea anemone or a coral, called polyps, which may be individual or colonial, and from which baby jellyfish bud off asexually. Box jellyfish venom is the most deadly in the animal kingdom and has caused over 5,500 recorded deaths since 1954. Each tentacle has about 500,000 sindasites which inject venom into the victim The lion’s mane jellyfish is the largest species of jellyfish — the largest recorded specimen had a bell (body) with a diameter of 2.3m and the tentacles reaching 36.5m.
Right: An individual purple stinger jellyfish
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