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www.thevillagenews.co.za
4 March 2020
MY ENVIRONMENT
The lives of jellyfish By Dr Anina Lee
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trollers on Hermanus beaches recently noticed a large number of little jellyfish washed up on the sand. Regular swimmers in The Marine pool thought better of it and did not venture into the water. What were these jellyfish and what caused their sudden appearance? My motto is: If in doubt, do the research. And in the case of things marine-related, your best bet is the Two Oceans Aquarium in Cape Town. So by great coincidence a team from the Aquarium came to Hermanus last week on their turtle rescue road trip. One of the team members is Krish Lewis, the ‘jelly guy’ and I was able to learn a bit about jellies from him. These jellyfish that are washing up in their thousands, Krish explained, are called ‘nightlight’ jellyfish (Pelagia noctiluca). They commonly occur in large blooms in deeper waters off the Western Cape coast. Strong winds and swells carry them in-shore and this is not a cause for concern. However, the would-be swimmers in The Marine pool were wise not to venture into the water. Night-light jellies can sting humans, and although it is not likely to be serious, they are best avoided. Contact with their long tentacles triggers stinging cells with spiny filaments wound up like springs. These filaments inject toxins into the skin, causing nasty reactions. If you should get stung by a jellyfish, bluebottle or insect, rub the juice of a sour fig leaf (Carpobrotus sp.) on it for instant relief. Pelagia noctiluca are pinkish-purple jellyfish, also known as ‘mauve stingers’. They are about
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10cm in diameter and glow at night in the waves. Their name comes from the Greek pelagos, for open sea; nocti, for night; and luca, for light. If you are in an area where they are washing up, keep an eye on the waterline at night. Some populations are able to bioluminesce when struck by waves and you may be lucky enough to see this phenomenon. Life cycle of a jellyfish Jellyfish have a fascinating life cycle. When you think of a jellyfish, you are probably picturing a translucent, bell-shaped animal with long flowing tentacles. While iconic, that's only a short stage of a jelly's life – the final ‘medusa’ phase before it dies. Jellyfish live far longer, more interesting lives than you may have imagined! The life story of a jellyfish begins at dusk or dawn, when adult jellies, known as ‘medusae’, gather in large numbers to spawn. They release huge quantities of sperm and eggs into the ocean around them. These spawning events are triggered by their proximity to other jellies, abundant light and enough food. The fertilised eggs usually drift freely in the ocean currents. Some eggs survive to enter the next phase of their lives. The egg grows into a small larva called a ‘planula’. Each planula can swim freely and resembles a microscopic flatworm, covered in tiny hairs called cilia. The cilia beat rhythmically and allow the planula to swim about. When it has grown sufficiently large it finds a suitable hard surface and attaches itself to it. The planula then develops into its next life stage – the polyp. This jellyfish polyp very closely resembles other marine animals like anemones and corals. The difference is that a jellyfish will develop into new stages beyond the polyp, while an animal like an anemone will essentially become an ‘adult polyp’. When conditions are ideal, the jellyfish polyp begins to reproduce asexually, by cloning itself. The polyp elongates and forms segments which will eventually bud off to form independent
animals. The tiny, newly-budded segments of polyp become free-living organisms known as ‘ephyra’. As the ephyra grows, its bell takes on the characteristic shape and its tentacles and oral arms will grow into the adult forms unique to its species.
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The twist in the tale But there is always a twist in the tale – the exception to the rule. The night-light jellies that we now see on our beaches do not go through this elaborate life cycle. When a large number of them are swept or blown together and bump into each other, they are stimulated to spawn. Hundreds of thousands of eggs and sperm are released and the eggs, once fertilised, hatch out within three days into tiny forms of the adult jellies. The resultant ‘bloom’ of jellies was blown out onto our beaches by the on-shore winds. They will disappear again in a few days, feasted on by birds, plough snails and crabs.
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For a brilliant visualisation of the jellyfish lifecycle go to YouTube and search Plankton Chronicles- Pelagia (by Sharif Mirshak). 1. Some night-light jellyfish are able to bioluminesce in the waves at night. PHOTO: Hakai Magazine 2. Night-light jellyfish (Pelagia noctiluca) are pinkish-purple jellyfish, also known as ‘mauve stingers’. PHOTO: Shaun Matthew Yeo 3. Night-light jellyfish have been washing up on the shores of Hermanus beaches. PHOTO: Gerda Marais Bücker
4. The life cycle of a night-light jelly from Te Ara: The Encyclopedia of New Zealand.
