It is best to take care entering the water if you see jellyfish, and you should not touch them even if you find them washed up on a beach. Their tentacles can still release venom when they are dead. However, if you do take part in water sports, a full body wetsuit provides very good protection. There are also commercially available lotions, developed by studying the protective mucus of clownfish, that can somewhat reduce the severity of stings. 

The Hong Kong Hospital Authority does not collect statistics on jellyfish stings and most stings are treated on-site and do not get reported. But if you are stung, Terenzini says that the most up-to-date advice is to get out of the water, carefully remove any tentacles with tweezers if available, then soak the wound in the warmest water you can tolerate for 20 minutes. Hot water denatures the proteins in the venom, rendering it ineffective. He says not to scrub, apply pressure, or try to flush the area with fresh water as this might activate the nematocysts (stinging cells) to release more venom. Some older advice suggests soaking the wound in vinegar, but that only works on certain types such as box jellyfish. Terenzini also debunks the persistent myth that urine neutralizes jellyfish stings. It is completely ineffective, not to mention unhygienic. 

The ecological value of jellyfish

The second thing that John Terenzini would like you to know is that jellyfish play a very valuable role in the ecosystem. Scientists used to think that jellyfish, whose bodies are made up almost entirely of water and proteins, were a “trophic dead end”, i.e. so low in nutritional value that nothing else ate them. This turned out to be incorrect. There are in fact many sea creatures that eat jellyfish – sea turtles are a major example. Leatherback turtles in particular live almost entirely on jellyfish. Jellyfish are also eaten by at least 167 species of fish, some of which are economically valued like tuna, chum salmon, sablefish, and rockfish. Jellyfish also play a complex role as predators. “They eat both up and down the food web”, says Terenzini, meaning that they eat both smaller creatures like plankton and tiny shrimp, and the eggs and young of bigger creatures, like crabs and fish. They also support the ecosystem in other ways. Their daily migrations up and down the water column, sinking into the depths during the day to hide from predators, and rising to the surface at night to feed, help to cycle nutrients through the ocean’s layers. Juvenile fish are known to swim inside their bells to hide from predators while picking up bits of food dropped by the jellyfish. 

However, sudden increases in the jellyfish population (called jellyfish blooms) can be an environmental warning sign. Terenzini explains that blooms are a natural part of the jellyfish’s life cycle, but a long-term increase in the size or frequency of blooms might be a sign that something is wrong.

Jellyfish are opportunistic animals that can tolerate more polluted waters than many other sea creatures. When the water is polluted with excess nutrients (e.g. from agricultural run-off), which causes algae to multiply and use up the oxygen, jellyfish can move into these dead zones, though after a certain point, even jellyfish cannot tolerate the conditions.  Overfishing also removes too many predators from the environment, allowing jellyfish to reproduce unchecked. Once they become dominant, it is difficult for fish populations to reestablish themselves because the jellyfish eat their eggs and larvae. Climate change may also bring about conditions that enable jellyfish to reproduce more quickly and expand their geographic range. 

In Hong Kong, jellyfish blooms usually occur in spring and summer when the warm water helps them to reproduce quickly. A lot of jellyfish appearing in one location during the warm season is probably normal as they may have just been pushed into a bay by the current. Or they could be part of a broader pattern. That is one of the reasons why Terenzini established the Hong Kong Jellyfish Project, a citizen science project where members of the public can report jellyfish sightings with photos and location data so that researchers can use the information to track changes in the abundance and distribution of jellyfish in Hong Kong over time. This can help them to study whether and how human activities affect jellyfish populations, how jellyfish impact the ecosystem and people, and help develop safety advice for watersports groups.  

Since 2021 when the Hong Kong Jellyfish Project started, citizen scientists have already identified 45 species of jellyfish, 25 of which had never been seen in Hong Kong before. (This does not mean that they were previously absent, only that no-one had documented them.) There are altogether at least 105 known species of jellyfish in Hong Kong waters, although there are probably more in reality as jellyfish are very understudied. In 2023, researchers at WWF Mai Po, Ocean Park, Hong Kong Baptist University and the University of Manchester discovered a completely new-to-science jellyfish at Mai Po marshes, naming it Tripedalia maipoensis, the Mai Po box jellyfish. They are tiny, translucent white creatures only 15mm across, with four sets of threadlike tentacles. There are only 50 box jellyfish species in the world, and as far as anyone knows, this one only exists at Mai Po. For a city so densely populated and geographically small, Hong Kong has an incredible level of biodiversity, with 8,900 recorded species, and now we know that one of them is a unique jellyfish. 

Fascinating creatures

The third thing that John Terenzini would like to tell you is that jellyfish are fascinating animals in their own right. A longtime outdoor educator, he first became interested in jellyfish after encountering them while kayaking in Alaska and found himself intrigued by how strange and alien they are. They are an ancient lifeform, about as different from humans as any animal that evolved on earth can be. Only sea sponges and comb jellies are more genetically distant from us. Jellyfish are much older than the dinosaurs, older even than bilateral symmetry. The vast majority of animals from frogs to worms are bilaterally symmetrical, meaning that their body plan is divided into left and right along a central axis. Jellyfish do not have left and right sides, they have four-part radial symmetry around a central point. 

 “They’re not like a bear or a dog that has a head and four limbs,” says Terenzini. “Jellyfish don’t have a face. There’s nothing in them that we can relate to.” Jellyfish do not have heads, or hearts, or intestines. Some species have a single opening that serves as both their mouth and anus. Others eat through tiny minimouths located along their arms. They do not have brains either, at least not a centralized one. Instead they have a neural net spread over their entire body, with several independent nerve centres located on the edge of their bells. These centres, called rhopalia, which also contain their sensory receptors, communicate with one another to coordinate the jellyfish’s swimming, feeding and reproduction. Terenzini says that when he explains jellyfish biology, people’s reaction is often, “How is this even an animal?” 

Terenzini hopes that through his citizen science project, other people in Hong Kong will become curious about jellyfish instead of simply afraid of them, and that they will want to learn more about them in their variety and wonder. While the community of jellyfish spotters is still much smaller than the community of birdwatchers, Terenzini does say that he has observed some of his regular contributors learning over time. The first time, they will upload a picture and simply label it “unknown jellyfish”. The second time, they will try to identify the species, which he will either confirm or correct. The third time, they will get it right.

You can participate in the Hong Kong Jellyfish Project by reporting a jellyfish sighting. Every observation contributes to our understanding of Hong Kong’s marine biodiversity. 

Sources:

Gaskill, “No brain? For jellyfish, no problem,” Nature, PBS, 20 November 2018.

Hong Kong Jellyfish Project, “Biology and Ecology”, 2020. 

Hong Kong Jellyfish Project, “Stings and Treatment”, 2020.  

Joshua Rapp Learn, “The upside of rising jellyfish numbers? Many animals eat them”, National Geographic, 18 January 2019.

Yanan Sun et al., “A New Species of Box Jellyfish (Cnidaria: Tripedaliidae: Tripedalia) from Hong Kong, China”, Zoology Studies, vol. 62 i17, March 20 2023. 

John Terenzini, “Citizen science is expanding our knowledge of Hong Kong’s jellyfish”, Croucher Foundation, 18 January 2022.