Understanding Hong Kong's Mysterious Ocean Ghosts

There are definitely jellyfish present in Hong Kong's waters, though much is still unknown about these mysterious creatures.

From locations as easily accessible as Victoria Harbour to the far-flung reaches of Sai Kung, it's possible to spot jellyfish pretty much wherever you are in the city.

In the summer months, when people begin to flock to the ocean to partake in activities like snorkeling, kayaking, swimming or even chilling on a junk boat, the chances of encountering jellyfish can be increased.

Here's what you need to know about Hong Kong's resident jellyfish and what you should do if you spot or are stung by one.

What Are Jellyfish?

What is a jellyfish?

Let's get into the (sea)weeds and discuss what exactly a jellyfish is.

Jellyfish belong to the phylum Cnidaria (which includes anemones and coral) and must fulfil several specific criteria to actually be classified as 'true' jellyfish. These criteria include:

• Having umbrella-shaped bells and trailing tentacles (adult medusa form)
• Usually being free-swimming, although some species can be anchored to the seabed
• Possessing stinging cells called nematocysts for defense and capturing prey
• Exhibiting radial symmetry
• Having a nerve net but lack a centralized brain

Getting even more specific, jellyfish belong to the subphylum Medusozoa, which is in reference to the adult life stage of a jellyfish called the Medusa stage.

The 4 classes of Medusozoa are: Scyphozoa (true jellyfish), Cubozoa (box jellyfish), Hydrozoa (Hydroids and Siphonophores) and Staurozoa (stalked jellyfish).

5 Amazing Facts About Jellyfish

Mysterious cannonball jellyfish courtesy of Wikimedia Commons

1. Jellyfish are the oldest multi-organ animal in the world (500-700 million years old), surviving all five of Earth’s mass extinction events
2. Jellyfish are 95% water, which is why they look so different in the water than out of it
3. Jellyfish have no brain, heart, or lungs, but they do have a nerve net that allows them to sense their environment
4. Jellyfish come in all shapes and sizes. The smallest jellyfish is the size of a pinhead, the largest jellyfish can have a diameter of up to 2 meters and the tentacles of a jellyfish can reach up to 200 feet (two blue whales)
5. There is a species of jellyfish, aptly named the immortal jellyfish, that is biologically immortal

John Terenzini, Founder of the Hong Kong Jellyfish Project, is trying to get a better picture of what species of jellyfish live in Hong Kong waters, when you're most likely to see them and how many people get stung by jellyfish each year.

Speaking exclusively to Hong Kong Hike, Mr Terenzini explains that based on data collected by the Hong Kong Jellyfish Project "there are about 15 Scyphozoa (“true” jellyfish), three Cubozoans (box jellyfish), three Ctenophora (comb jellies), and a small number of Hydrozoa" that can be found in Hong Kong.

Mr Terenzini further states that the "AFCD records only six jellyfish as present in Hong Kong, however they mean only the Scyphozoa recorded in previous literature."

This discrepancy highlights the need for further study into Hong Kong's jellyfish population.

Hong Kong's Scyphozoa (True Jellyfish) Species

Barrell jellyfish courtesy of Wikimedia Commons

Much of the information below came from the Hong Kong Jellyfish Project's page on local Hong Kong jellyfish species. They also have great pictures of each local species taken by locals.

Cyanea nozakii (Ghost Jellyfish)

• Color: White or light brown in color
• Max Size: 120 cm wide
• Rarity: Most common species found in Hong Kong
• Sting: Responsible for many stings in Hong Kong

Mastigias papua (M. albipunctatus) (Papuan spotted jellyfish)

• Color: blueish green to brownish green with white spots
• Max Size: 8 cm wide
• Rarity: Uncommon in Hong Kong
• Sting: Uncommon

Aurelia aurita (Moon jellyfish)

• Color: white, rose, blue, transparent
• Max Size: 40 cm wide
• Rarity: Common in July
• Sting: Practically painless

Nemopilema nomurai (Nomura's jellyfish)

• Color: brown to transparent
• Max Size: 200 cm wide
• Rarity: Very rare
• Sting: Quite potent

Pelagia panopyra (noctiluca) - (Mauve stinger)

• Color: Mauve
• Max Size: 10 cm wide
• Rarity: Exceedingly rare in Hong Kong
• Sting: Painful, but very rare in Hong Kong

Acromitus flagellatus (River jellyfish)

• Color: mostly clear with blueish spots
• Max Size: 12 cm wide
• Rarity: Regularly seen in Hong Kong, sometimes in large blooms
• Sting: Little to no effect on humans

Rhopilema hispidum (Sand jellyfish or Flower jellyfish)

• Color: Milky color with brownish spots
• Max Size: 80 cm wide
• Rarity: Very common in Hong Kong
• Sting: Painful sting. Best avoided.

Lobonema smithii (Hairy jellyfish)

• Color: Milky color
• Max Size: 50 cm wide
• Rarity: Very rare in Hong Kong
• Sting: Very painful. Even has stingers on its bell!

Anomalorhiza shawi 

• Color: Clear bell, pinkish internals and purple/brown tentacles
• Max Size: 50 cm wide
• Rarity: Rare in Hong Kong
• Sting: Not much known

Cephea cephea  (Purple crown jellyfish)

• Color: Purple
• Max Size: 60 cm wide
• Rarity: Only one reported sighting
• Sting: stings and venom not harmful to humans

Netrostoma setouchianum 

• Color: White
• Max Size: 20 cm wide
• Rarity: Infrequent in Hong Kong waters
• Sting: Unknown

Thysanostoma loriferum (purple jellyfish)

• Color: Green...nah, it's purple
• Max Size: 20 cm wide
• Rarity: Only 2 reports in Hong Kong so far
• Sting: Unknown

Cyanea purpurea

• Color: purple
• Max Size: 36 cm wide
• Rarity: Spotted in large numbers in 2022
• Sting: sharp sting

Chrysaora chinensis (Malaysian sea nettle)

• Color: milky white/pink
• Max Size: 15 cm wide
• Rarity: Infrequent in Hong Kong
• Sting: Severe sting

Hong Kong's Cubozoa (Box Jellyfish) Species

Box jellyfish courtesy of Wikimedia Commons

Tripedalia maipoensis (Mai Po box jellyfish)

• Color: Transparent
• Max Size: 1.5 cm wide
• Rarity: Newly discovered, so uncertain
• Sting: Uncertain

Malo filipina 

• Color: Transparent
• Max Size: 3-4 cm wide
• Rarity: Only seen once in Hong Kong
• Sting: Severe

Morbakka (Giant boxjelly)

• Color: Transparent
• Max Size: 20 cm wide
• Rarity: First reports in 2022 and 2023
• Sting: Severe

Hong Kong's Hydrozoa Species

Portuguese Man o War

There are 60 species of Hydrozoa reported in Hong Kong. Here are just a few that are seen in Hong Kong.

Hydroids

Aequorea spp. (Crystal jellyfish)

• Color: Bioluminescent
• Max Size: 10 cm wide
• Rarity: Common in Hong Kong
• Sting: Harmless. May cause slight itching

Porpita porpita (Blue button)

• Color: Blue
• Max Size: 3 cm wide
• Rarity: Common in Hong Kong
• Sting: Mild sting

Siphonophores

Physalia physalis (Portuguese Man o' War)

• Color: Blue and purple
• Max Size: 30 cm long
• Rarity: Uncommon in Hong Kong
• Sting: Painful and causes welts

Hong Kong's Ctenophora (Comb Jelly) Species

Comb jelly courtesy of Wikimedia Commons

Ctenophora are not considered to be 'true' jellyfish by some scientists, and are given a pass by others.

Mr. Terenzini weighs in by saying, "I personally like the definition I saw in a journal article by Brotz et al. 2012" which states:

"Here, the word ‘‘jellyfish’’ refers to gelatinous zooplankton that include medusae of the phylum Cnidaria (scyphomedusae, hydromedusae, cubomedusae, and siphonophores) and planktonic members of the phylum Ctenophora."

These are some Ctenophora that can be found in Hong Kong

Beroe sp. (Comb jellyfish)

• Color: Clear (bioluminescent)
• Max Size: 10 cm wide
• Rarity: Fairly common in Hong Kong, especially in spring
• Sting: No sting

Pleurobrachia globosa (Sea Gooseberry)

• Color: Clear
• Max Size: 2 cm wide
• Rarity: Unknown numbers in Hong Kong
• Sting: No sting

Ocyropsis crystallina

• Color: Clear
• Max Size: 5 cm wide
• Rarity: Unknown numbers in Hong Kong
• Sting: No sting

What Jellyfish Are You Most Likely to See in Hong Kong?

Ghost jellyfish spotted from high up on Cape D'Aguilar cliffs

More research needs to be done by documenting jellyfish when you see them. This will help to build a bigger and better picture of what types of jellyfish live in Hong Kong and how their numbers fluctuate throughout the seasons. If you see a jellyfish in Hong Kong, contribute to jellyfish science by reporting your sightings!

With the current available data, the two most common jellyfish you are likely to see in Hong Kong according to The Jellyfish Project include:

1. Ghost jellyfish
2. Sand/Flower jellyfish

In addition to these two species of true jellyfish, "huge numbers of comb jellies (not true jellies but included in “gelatinous plankton”) have been reported in large groups less frequently."

On occasion large groups of jellyfish will bloom in certain areas of Hong Kong. Mr Terenzini shares a few such experiences:

1. "In June [2023], people were reporting 100s of Acromitus flagellatus (river jelly) in the bays near Discovery Bay."
2. "I came across a mass stranding of over 200+ Rhopilema hispidum (Sand/flower jellyfish) on Lower Cheung Sha beach on Lantau in April 2021"
3. "Paddleboarders and surfers tell me that large aggregations of Rhopilema can frequently be seen off of Lantau in springtime."

Which of Hong Kong's Jellyfish Cause the Most Stings?

Sand jellyfish stranded on Tai Long Beach, Sai Kung

Knowing which jellyfish sting people the most in Hong Kong is very difficult. Mr Terenzini explains that "The Hong Kong Hospital Authority does not keep records of stings and most are likely taken care of on-site, so it is hard to tell exactly how many stings occur."

But thanks to the work of The Jellyfish Project, people can report a jellyfish sting and contribute valuable data that not only helps understand more about jellyfish behavior in Hong Kong's waters, but can also help to keep watersports enthusiasts safe!

With the data available, the two jellyfish responsible for the most stings according to the Hong Kong Jellyfish Project are:

1. The Ghost jellyfish, "likely due to its long, trailing tentacles that can sting even when detached."
2. Sand/Flower jellyfish "especially in the Lantau area because many of them get swept into the beaches on the east side by the prevailing currents."

Here's what to do if you get stung by a jellyfish.

As with so much of the natural world, new species are being discovered all the time!

Mr Terenzini and the Jellyfish Project were the first to publish peer reviewed sightings of Thysanostoma loriferum in Hong Kong waters!

In addition to this, researchers at Hong Kong's Baptist University also discovered a new species of Box jellyfish in 2023!

This only highlights the importance of reporting any jellyfish you might spot in the wild. Who knows if it's a brand new species or a jellyfish that people never would have expected to exist in Hong Kong!

Here are five ways jellyfish help create balance in Hong Kong (and the world)'s marine ecosystem according to Mr. Terenzini:

1. They predate on plankton and small fish, keeping their numbers in check
2. Jellyfish are "a major prey item for over 160 species of fish, some economically important"
3. They provide shelter and transport for several symbiotic marine species
4. "They can also provide carbon cycling benefits as they swim through the water column"
5. Jellyfish also "act as carbon sinks when blooms die off and drop to the bottom"

Share the Love for our Gelatinous Friends

We understand so little about jellyfish that scientists aren't even certain if they need protecting or not! Mr. Terenzini explains, "we don’t know enough about them [jellyfish] to determine whether any are endangered or what the impacts of certain environmental changes may be."

So, whether you're partying on a junk boat, going snorkeling, scuba diving, wake boarding, surfing or hiking, if you see a jellyfish, or are stung by one, try to snap a picture of it and share your exciting discovery/painful suffering with the Hong Kong Jellyfish Project.

There's nothing more satisfying than feeling like you're contributing to science by helping Hong Kong understand more about its wonderful gelatinous inhabitants.

Who knows, you may end up discovering a new species and having a jellyfish named after you!