True jellyfish (or scyphomedusae) belong to a group of animals called ‘cnidarians’. This group includes relatives such as corals and sea anemones. There are more than two hundred species of true jellyfish globally but only six species commonly found in British waters.
Jellyfish consist of a gelatinous ‘umbrella’ above a circle of tentacles that surround a central mouth. The mouth opens into a simple stomach and is also used to get rid of any waste. The umbrella is surrounded by sensory organs, which detect light, gravity and chemicals in the water.
Jellyfish have stinging cells, called nematocysts. The ‘sting’ is coiled and fired like a harpoon when triggered. All species have nematocysts in their tentacles, some also have them on their umbrella.
The word ‘jellyfish’ is used to describe a range of species, many of these are related to true jellyfish, but comb jellies are not closely related at all and do not sting. Instead they use sticky tentacles to catch their prey. Close relatives of true jellyfish include the sometimes deadly box jellyfish and the Portuguese Man O’War. There are thirty six different species of Box jellyfish they have twenty four eyes, are able to detect colour and can swim as fast as an Olympic swimmer! The Portuguese Man O’War is commonly mistaken for a jellyfish but it is colony of animals called siphonophores each individual has a specific role to play in the colony, including one which acts as the large float.
- A group of jellyfish is called a ‘smack’ although it is more commonly referred to as a ‘bloom’.
- Jellyfish have no eyes but can sense light, they don’t have a sense of smell, a brain or a skeleton.
- Although jellyfish tentacles are covered in poisonous stinging cells, some young fish hide among them for protection
- Jellyfish are an important food for humans and catches are increasing. They are usually preserved by drying and are considered a delicacy in some parts of Asia
“How do they move?”
Muscles contract rhythmically causing the umbrella to pulse, allowing them to swim. Jellyfish are considered by many to be plankton, as they are relatively weak swimmers and drift long distances on ocean currents. This often leads to large numbers becoming stranded on beaches.
“What do they eat?”
Jellyfish are predatory animals and feed on fish, crustaceans and other zooplankton, fish eggs and larvae.
“How do they feed”
Jellyfish have a loose network of nerves called a ‘nerve net,’ around their tentacles which they use to detect prey. They can trap and sting their prey with their nematocysts and use their tentacles to move food into their mouth.
“What is the biggest?”
The Lion’s Mane jellyfish is the longest recorded jellyfish in the world. It can grow up over 30m in length (although this is not common). Arguably the largest , sometimes measuring two metres across and weighing 200 kg is Nomura’s jellyfish (Nemopilema nomurai), found off the coasts of Japan and China.
“Where do they live?”
Jellyfish are found around the world at all depths and from tropical to polar waters . Whilst most species are found in the sea, some can be found in fresh water.
“How long do they live?”
The lifespan of jellyfish ranges considerably between species. Some living only a few hours as free swimming adults and others several years. Although they generally have short adult phases and die following a period of reproduction.
The six species commonly found around the British Isles
Moon jellyfish (Aurelia aurita) – The most common jellyfish found around the British Isles and is easily identified by four white rings (its gonads), visible through its umbrella.
Compass jellyfish (Chrysaora hysoscella) – Easily identified by dark ‘compass’ markings on its umbrella. Juvenile fish hide among the tentacles, which provide protection from larger predators.
Lion’s mane jellyfish (Cyanea capillata) - the second longest recorded animal in the world. tentacles are often bright red or orange and arranged in eight bunches, each containing over a hundred tentacles.
Dustbin-lid jellyfish (Rhizostoma pulmo) – A large, fleshy jellyfish, which grows over 90 cm across and is often found washed up in large numbers in the British Isles.
Blue jellyfish (Cyanea lamarckii) – A close relative of the lions mane jellyfish, but bright blue with a smaller maximum size of only 30 cm across.
Mauve stinger (Pelagia noctiluca) – A fairly small species with a powerful sting that is capable of bioluminescence (it can create its own light). Large blooms can cause serious damage to fish farms.