What are jellyfish?
Some drift along the currents aimlessly, while others glide through the sea to sting their prey.
Some glow in the dark, while others unleash deadly venom.
These sea jellies, which are made up of 95 per cent water, have evolved and roamed the seas for at least half a billion years.
These ancient creatures are boneless, bloodless and brainless, but they outlived dinosaurs.
They have simple sensory organs on their bells and tentacles to detect light, changes in the water and the touch of other animals.
Jellyfish are found all around the world, from deep oceans to shallow coastal areas.
If a jellyfish washes up on the beach, it would most likely evaporate and dry up due to its high water content.
But do not touch the dying blob as its stingers can still shoot venom.
Do all of them sting?
All jellyfish have stings, but not all of them are strong enough to harm the skin. Some stings barely feel like a tingle, while some are not felt at all.
For instance, the renowned Jellyfish Lake in Palau is popular with tourists, who swim alongside a swarm of harmless golden sea jellies.
On the other end of the spectrum of these harmless jelly friends is the box jellyfish - one of the most deadly animals on the planet.
All jellyfish have stings, but not all of them are strong enough to harm the skin. Some stings barely feel like a tingle, while some are not felt at all. For instance, the renowned Jellyfish Lake in Palau is popular with tourists, who swim alongside a swarm of harmless golden sea jellies.
They can grow up to 3m long and, once a person is stung, the venom can immediately attack the heart, nervous system and skin cells. It may cause the victim to go into shock within minutes.
The bells and tentacles of most jellyfish are dotted with tiny cells filled with a dart-like stinger, which releases venom when triggered. This means that the gelatinous bell may not be as innocuous as it looks.
Some jellyfish can also sting in two other unusual ways - without touching the victim, and through larvae.
Some upside-down jellyfish with no tentacles will release a gooey mucus filled with microscopic particles containing stinging cells. When divers swim near those jellies, they feel a stinging sensation similar to an irritating itch or burn.
The larvae of the thimble jellyfish, which are usually found off the coasts of Florida and the Caribbean, are barely visible, appearing like a speck of finely ground pepper, but they pack a painful punch.
Once stung, the skin can develop a bumpy rash, sometimes with blisters. This condition is called seabather's eruption. Other symptoms, which include headache, vomiting and urethritis, may occur. As the larvae are small enough to creep into swimwear and swim caps, covered skin will not be spared.
In 1987, more than 300 thimble jellyfish specimens were collected from the waters off the west coast of Singapore.
Sting incidents at the beaches of Sentosa were reported from 2010 to 2017. There was a spike in cases in 2010, when 101 sting incidents were reported, compared with the average of 30 cases annually in other years. During those years, more incidents were reported at Palawan Beach, compared with Siloso and Tanjong beaches.
In 2010, there was a bloom of unidentified Semaestomae jellyfish in the waters around Sentosa.
If you are stung by a jellyfish, seek medical attention immediately. Meanwhile, rinse the affected area with seawater or vinegar and do not try to remove the tentacles yourself.
How do they eat?
Jellyfish have a mouth underneath their bells. They use their tentacles and ribbon-like frills, called oral arms, to capture and stun their prey before eating them.
The carnivores eat plankton and other tiny marine creatures, such as shrimp and fish larvae.
Waste is also secreted from their mouths.
Why do some of them glow?
Some types, such as the comb jelly and the moon jelly, are bioluminescent and the light is produced through a chemical process.
Green fluorescent protein (GFP) from jellyfish is extracted and used in biomedical research.
For instance, the GFP, which emits a green glow, can be inserted into the DNA of cells to track the proteins that are being studied. GFP has been used to monitor the spread of cancer cells as well.
Are climate change and rising ocean temperatures impacting jellyfish?
Unlike other marine creatures, jellyfish can thrive in warmer and more acidic waters, giving rise to an explosion in their population.
Jellyfish usually reproduce more in warmer waters, which means blooms may persist in tropical waters.
A jellyfish explosion can disrupt the ecological balance, clog up power plants and affect tourism at beaches, as the risk of getting stung will be higher.
In 2016, a huge swarm of jellyfish thronged a power plant's cooling system in East Java, forcing the plant to close for 20 days. As the power plant was the main source of electricity for Java and Bali islands, electricity supply to parts of Jakarta and West Java had to be shut down.
The only animal known to have this ability, the Turritopsis dohrnii jellyfish can transform into its younger form, called a polyp, when it is damaged, starving or facing other crises.
Popularly known as the "immortal jellyfish", this tiny 4.5mm creature can restart its life multiple times until it gets eaten.
Jellyfish have also gone beyond the sea and into space. In 1991, the National Aeronautics and Space Administration launched close to 2,500 moon jellyfish polyps on a space mission. The creatures were contained in flasks and bags filled with artificial seawater.
The mission was to test how the small amount of gravity in space affects the animals as they develop into full-fledged jellyfish and reproduced. About 60,000 jellyfish returned to earth safe and sound when the experiment ended 20 years later.
• Sources: National Geographic, Nat Geo Wild, Ms Iffah Iesa, Mr Nicholas Yap, Harmful Jellyfish Country Report in Western Pacific, Smithsonian Ocean, Wild Singapore
Some jellyfish seen in local waters
Acromitus sp. This jellyfish can be spotted in the mangroves and estuarine seagrass meadows along the northern shores of Singapore, and a bloom of these little stingers was spotted in the Pasir Ris Park mangrove in 2017.
Its bell is about 6cm to 8cm wide, with thick, sausage-like oral arms extending down. The jellyfish contracts its bell energetically to propel itself through the water.
This species comes in a few colours, including white, brown, pink and purple. Sightings: Sungei Buloh Wetland Reserve, Pulau Ubin, Chek Jawa and Pasir Ris Park.
Chrysaora sp. This species is recognised by its long, ribbon-like oral arms that can extend up to 50cm, and large brown spots at the edge of the bell. Also known as the Indonesian sea nettle, the jellyfish carries powerful stings that can cause an intense burning sensation on skin. Swelling will occur, and it may take several weeks to heal.
Do not touch the jellyfish if it is stranded on the beach, as the stingers remain active even after the creature is dead.
However, small fish have been seen swimming alongside the jellyfish's arms and tentacles, unharmed. These fish could be seeking protection from predators, or feeding on morsels of undigested food that the sea jellies leave behind. Sightings: Raffles Marina, St John's Island, Pulau Hantu and Terumbu Semakau.
Cassiopea sp. This jellyfish is so named because it prefers to rest its bell against the sea bed, with its stout oral arms outstretched. It is sometimes mistaken for a sea anemone.
Local researchers suspect that the jellyfish can release mucus filled with venom into the water, but there is currently no data to prove this.
The jellyfish contains single-celled algae that undergoes photosynthesis to produce food. The food is shared with the jellyfish, which provides shelter and minerals for the algae in return.
Since it relies on photosynthesis, the animal is found in shallow waters, on seagrass meadows, reef flats or mangroves - usually in the Southern Islands. Sightings: Pulau Semakau and Kusu Island, among other Southern Islands.
Cubozoa jellyfish Known to be one of the most lethal animals in the world, the box jellyfish's sting can cause paralysis, cardiac arrest and even death. Named for its cube-shaped bell, the box jellyfish has tentacles that can extend to 3m.
It has sharp eyesight, thanks to the clusters of eyes on each side of its bell.
Unlike most other jellyfish that drift with the currents, the box jellyfish - which can swim 2m per second - moves intentionally to hunt small fish.
This jellyfish, usually found off the coasts of Australia, Malaysia and Thailand, has also been spotted in local waters. Sightings: Tanah Merah and Sentosa.
Aequorea sp. With its glassy, almost transparent bell and thin, thread-like tentacles, this jellyfish can be difficult to spot in the water. Washed up on shore, it resembles a thick, gel-like blob, and can still sting when triggered.
The bell, which is decorated with radiating white stripes, can be up to 8cm wide, with more than 20 tentacles attached.
Contact with this jellyfish may cause an itchy rash or intense swelling lasting several hours. Sightings: Chek Jawa, Pasir Ris Park, Lazarus Island, Pulau Semakau, Tuas and other parts of the coastline.
• Sources: Wild Singapore, A Survey of Jellyfish (Cnidaria) around St John's Island in the Singapore Straits, National Geographic, Mr Nicholas Yap.