Help Out the Horseshoe Crabs At Shui Hau Wan, Lantau

You may have heard of Shui Hau Wan on Lantau Island, which is a popular location for clam digging and kite boarding. Shui Hau is actually an important wildlife habitat housing a wide range of precious species, including the horseshoe crab.
Horseshoe crabs are marine living fossils, probably dated back to 485 million years ago. There are two horseshoe crab species found in Hong Kong – Chinese horseshoe crab (Tachypleus tridentatus) and mangrove horseshoe crab (Carcinoscorpius rotundicauda). For more info on HK’s horseshoe crabs visit Billy Kwan’s webpage Horseshoe Crab in Hong Kong.

Trash from visitors as well as marine debris are major threats to the juvenile horseshoe crabs living in the mudflat. To learn more about the features of the ‘living fossil’ horseshoe crab, their importance to humans and how their habitat is being affected by human activities, you can join the mudflat cleanup organized by Friends of the Ocean Park Conservation Fund. Details in the graphic below.


For enquiries, please contact Miss Chan at +852 3923 2217.


Lion’s manes, invisible barnacles, suicidal fish, and photoshop hoaxes

It’s definitely summer and the sea has seasons, too. With summer comes the dread of Hong Kong’s beach-goers, the season of the lion’s mane jellyfish Cyanea nozakii.
This large, flat-topped jellyfish can grow to 50 cm in width with 8 bunches of 70 to 120 stinging tentacles that can trail up to 10 m. These tentacles can still sting even when they break off or are torn from the main body (though they lose potency over time).
It is cream to pale yellow with a darker centre and transparent on the outside edge.

A lion’s mane jellyfish (Cyanea nozakii) drifts close to the surface off Lantau Island.

You start to see these guys arrive from late March onwards and they reach their peak in mid-summer and decline in numbers after that.

Although it occurs all over the Indo-Pacific, seasonal outbreaks of the lion’s mane occur in the Yellow and East China seas and the Gulf of Bohai where fisherman regard them as a nuisance : first because they clog up nets and even damage them, second because their presence is linked to the decline of the edible jellyfish Rhopilema esculentum. Although it used to appear sporadically in those areas, it seems to have become more and more of a problem. One Chinese researcher even said that more harm and damage was caused to the coastal waters of China by this species than by red tides. Personally I have not noticed any change in Hong Kong over the years, but maybe I am wrong. When I was a kid (in the 80’s) I remember trying to count the number of jellyfish we passed off Lamma Island in our boat and lost count after 100 in 5 minutes…on average adults weigh about 1.2-1.3 kg so we passed literally a tonne of jellyfish in 5 minutes (and it was a slow boat).
So what about the barnacle and the suicidal fish?
Well, remember symbiosis from high school biology? The jellyfish are sometimes accompanies by other creatures taking advantage of the bad-ass protection offered by the stinging tentacles. One interesting guy is a transparent, soft barnacle called Alepas pacifica that hitches a ride by attaching itself to the outer rim of the jelly’s bell with a stalk and spends the rest of its life in care free existence filter feeding particles out of the water – not a care in the world. Until the jellyfish dies, then it will, too.

Three Jellyfish Barnacles (Alepas pacifica) attached to a jellyfish.

And the other two creatures are two commercial fish: the Razorbelly Scad (Alepes kleinii) and the Malabar Trevally (Caranx malabaricus). Both fish as juveniles like to seek out a lion’s mane soon after they hatch from eggs which – with perfect timing – seems to coincide with the arrival of the jellyfish. Hanging around the jellyfish gives them protection from predators. Think this is perfect adaptation? You would be wrong. The fish might gain an advantage from hiding by the jellyfish but they have not evolved any mechanism to avoid or deal with the stinging tentacles! Think of a child hiding from the bogeyman on top of a high voltage electricity pylon and you get a good idea of the risk to the fish. They have to swim very, very carefully! And on top of all that they are very loyal or attached to this dangerous fortress, too. When a jellyfish are caught and handled the fish stay with the jellyfish until inevitably they are forced into contact with the tentacles and are repeatedly stung until the die. Suicidal fish.

You can find more information on Hong Kong marine symbiosis in the book “Partnerships in the Sea: Hong Kong’s Marine Symbiosis” by Brian Morton.

An adult razorbelly scad (Alepes kleinii).
An adult Malabar Trevally (Caranx malabaricus). As juveniles they use the lion’s mane for protection from predators.

And now – sex:

Jellyfish can reproduce sexually and asexually. And when they chose sex it’s between male and female jellyfish – not that you can tell the difference unless you have a microscope and a lot of time. But like most invertebrates they simply try to time their spontaneous ejection of sperm and eggs into the water and hope that some meet, fertilise and grow into larvae. The microscopic larvae, called planula, are tiny, hairy, slipper-shaped things that use their tiny hairs (called cilia) to propel themselves. The planula larvae  of the lion’s mane only last a day and a half before they settle on the seabed and turn into small polyps like coral except solitary instead of in big colonies. After that they bud off little 2-3 mm wide jellyfish which then grow into the adult 20-30 cm wide jellyfish we all fear coming into contact with while swimming.

The life cycle of a jellyfish. Staring from planula larvae (1) to adult medusa (14). (From Schleiden M. J. "Die Entwicklung der Meduse". In: "Das Meer", A. Sacco Nachf., Berlin, 1869.)
The life cycle of a jellyfish. Staring from planula larvae (1) to adult medusa (14). (From Schleiden M. J. “Die Entwicklung der Meduse”. In: “Das Meer”, A. Sacco Nachf., Berlin, 1869.)

Photoshop hoax:

Some time ago the below image circulated online of a gigantic lions mane jellyfish dwarfing a diver.  This is actually a different species (Cyanea capillata) to our lions mane (Cyanea nozakii) even though it shares the same common name. Thanks to Google’s reverse image search the image was quickly exposed as a photoshopped fake as you can read in this Forbes article .

Photoshoped image of a lions mane jellyfish with a diver. L=Impressive sight, sadly a fake.
Photoshoped image of a lion’s mane jellyfish with a diver. L=Impressive sight, sadly a fake.
And finally some interesting recent research on the lion’s mane jellyfish:

– Researchers in Qingdao investigating the venom of the stinging tentacles found it was most stable for storage at -80 degrees Celsius and that it was more effective at killing human colon cancer cells compared with human liver cancer cells.

– Researchers in Lianyungang attempting to find some sort of use for the enormous unwanted by catch of lions manes in fisheries found that it is a good source of collagen and that compared to other sources of collagen it had lower levels of arsenic, mercury and lead.

– Researchers in Japan have tried to isolate the gene responsible for the lions manes production of an enzyme called endoglycoceramidase which is useful in synthesising some other molecules. Also in Japan researchers worked on the acoustic signature of different jellyfish species to develop an avoidance system for fishing boats. The acoustic signature is essentially the characteristics of the echo received from a pinger when the sound waves hit the jellyfish. Unfortunately they were not able to tell jellyfish species apart , which would have been a wonderful way to quickly measure their populations by just towing a pinger over a vast area and then estimating the population from the echo data.

Hong Kong Divers Damage Corals

A new study published this year has found that divers cause significant damage to corals on Hong Kong reefs. At most of the locations studied, the percentage of broken corals exceeded the recommended no-action threshold of 4%, which means management intervention is justified.

Scientists from Hong Kong’s Baptist University surveyed coral breakage from diving activities in Hong Kong using transects at seven different sites. The results show a total of 81 broken corals with 3–19 broken colonies per site. The team found a significant link between the number of broken coral colonies and the number of divers visiting the site.

Broken corals
Damaged Corals

The branching Acropora and the plate-like Montipora suffered more frequent damage than expected from their numbers. This means that some corals species are more vulnerable to damage from divers. The implication is that high numbers of divers may alter the species composition and ecology of the coral reefs as a whole.


Acropora coral
Acropora coral
Montipora coral

The study concludes that popular dive sites should be classed as “no-go’ areas for training divers.

The Horrors of the Black Sea Cucumber

Known to science as Holothuria leucospilota, the black sea cucumber is the most common sea cucumber in Hong Kong, but is considered endangered in mainland China because of overexploitation. Dive or snorkel anywhere on the eastern half of Hong Kong and you are certain to find a few!

The Basics:
While relaxed it can be up to 40 cm (16 in) long, but it can stretch to about a metre (yard). It is sausage-shaped, tapering towards the back end. At the head end, there are twenty oral tentacles with branched tips. These surround the mouth which is on the under side of the body and help shovel food into its mouth. The animal is soft and covered with small fleshy protrusions called papillae. The usual colour is charcoal grey or reddish-black with pale grey tube feet on the underside.

Holothuria leucospilota is normally found in shallow water from east Africa to the western Indo-Pacific region including the South China Sea and Hong Kong waters. It is also a common species on the north-east coast of Australia where it is found on reefs and rocky coasts, often partly concealed under a boulder. It is generally more common near boulders, corals and seaweed clumps than on the open seabed at depths from 0-15 m. A study from Singapore found that the Black Sea cucumber is relatively tolerant of changes in the salt content and temperature of seawater. During identical salinity and temperature changes, the Japanese sea cucumber (Apostichopus japonicus) shrank in size, eviscerated (spewed out its own guts!), and died within three days. The black sea cucumber, however was fine.

Holothuria leucospilota is a scavenger and when feeding it usually has its back end anchored underneath a rock or in a crevice so that it can contract back out of sight if disturbed. It feeds by using its tentacles to shovel organic debris lying on the seabed into its mouth. In the process it swallows a lot of sand, which passes through the gut. In doing this it can remove and feed on the thin layer of bacteria and micro-algae that coats the grains of sand – this layer is normally called ‘biofilm‘.

Here is a nice clip of a black sea cucumber feeding on sand:

And here is a lovely clip of one feeding on detritus floating its way in the water. You can also see its tiny tube feet on the glass:

A study by HKU found that the sea cucumber can detect the sand patches with the most biofilm, because the sand it eats has about twice the amount of organic matter than the average sand patch. It churns through just under half a kilo of sand a year and feeds 3-4 times faster in summer than in winter.


Black sea cucumbers are commercially harvested in Malaysia, China and Vietnam, processed by eviscerating and freeze drying. Although the black sea cucumber is not highly prized: wholesale prices are around USD 5 per kg compared with USD 42 per kg for some other species.

The 1st Horror
As a boy I learned that when threatened or handled (especially out of the water) the black sea cucumber can release a sticky white substance from its bottom called Cuvierian tubules. These can entangle potential predators and lets the sea cucumber escape. As the name implies the substance is actually made up of very fine tubes and these form inside the sea cucumber.

Warning! It gets a bit gross now: when stressed, the sea cucumber faces away from the attacker and contracts its body muscles sharply, causing the wall of the cloaca to tear, the anus to gape open and the free ends of some of the tubes to be ejected. I warned you…
The Cuverian tubules are presumably what give the black sea cucumber its other common name ‘white thread fish’ which is pretty silly as it isn’t a fish by any stretch of the imagination. Sea cucumbers , or Holothurians to scientists, are related to starfish and sea urchins and are invertebrates….but I digress.
The Cuverian tubules lengthen when they come into contact with seawater and become amazingly sticky when they meet objects or human hands! It takes fifteen to eighteen days to regenerate these tubules. Now that you know what it takes to project the sticky tubules (painful rectal tear) and that it takes 15-18 days to regenerate leaving the sea cucumber vulnerable, spare a thought for the sea cucumbers and don’t deliberately stress them just to see the effect. Especially not because the sticky white stuff actually has a toxin called holothurin (sea cucumbers are known in science as Holothurians from the Greek word meaning water polyps).

Instead you can watch someone in this YouTube clip “stressing” a black sea cucumber:

My personal experience with the Black Sea cucumber is that skin contact with the toxin does not cause any harm other than general unpleasant stickiness, but the general advice is to avoid contact with it completely and wash affected areas properly to avoid any contact between contaminated skin and the eyes. This is because people have reported holothurin to cause conjunctivitis and blindness! So whatever you do, do not let the tubules come into contact with your eyes (likely via your hands)!

The 2nd Horror
The black sea cucumber sometimes plays host to the worm pearlfish (Encheliophis vermicularis), a fish that lives as a parasite inside sea cucumber guts where it feeds on the sex organs (gonads). It can grow to 15 cm…nearly half the body length of its host sea cucumber! Oh yeah – and they like to live in male-female pairs inside the sea cucumber. They enter and exit the sea cucumber through its anus and seem to be attracted chemically to the Cuverian tubules – the sticky white substance mentioned before. They also somehow manage to not get entangled in the tubules because black sea cucumber (its favorite host) does not eject the tubules in response to the worm pearlfish’s “comings and goings”.

Here is a clip showing an example of different species of sea cucumber and pearlfish “interacting”…

Sex or cloning? Cloning, please
The black sea cucumber has two methods of reproducing: sexual and asexual. In sexual reproduction the sea cucumbers spawn releasing sperm and eggs into the water in the hope that some eggs will get fertilized. These then develop into larvae that float and feed in the plankton until they are big enough to settle and transform into juveniles. Asexual reproduction takes place by “transverse fission” – basically the sea cucumber splits itself in half down the middle an each half grows into a full sea cucumber. Cool, huh?

In Taiwan this species reproduces between June and September. Unfortunately, I couldn’t find any information the reproduction of the black sea cucumber in Hong Kong waters.

And finally, here’s is a short list of some of the interesting research being done on the black sea cucumber:
– evaluation of its polysaccharides in the treatment of tumours
– evaluation of its chemical compounds as antimicrobial or anti-fungal treatments
– using them to absorb the organic detritus falling to the seabed under fish farms while producing a secondary commercial crop of sea cucumbers

The Long-Spined Urchin (Diadema setosum)

When I was a boy snorkelling off beaches one of the sights I best remember and one of the potential hazards was the long-spined sea – Diadema setosum. Its quite a beautiful creature with black or brown-banded spines that are extremely long and narrow. They are also hollow and contain a mild venom which -despite being painful – does not pose a serious threat to humans. And if you have ever stepped on one you’ll know the spines are fairly brittle so getting them out is tricky!

They have a black skeleton shaped like a squashed ball that houses all the organs which is called a ‘test’ and essentially that’s the body of the urchin. Get close enough and you will see a bright, orange ring around the urchin’s “anus” (which is on the top of the urchin as you look at the seabed) and some bluish spots surrounding the orange ring. Similar blue spots are arranged in linear fashion along its test.


Adult urchins weigh 35 – 80 grams and measure no more than 7 cm in test diameter and around 4 cm in height. But the spines make them look much bigger!

It is commonly found on coral reefs, but also on sand flats and in seagrass beds. Its range stretches throughout the Indo-Pacific from the Red Sea and then eastward to the Australian coast, then as far north as Japan and as far south as the southern tip of the African east coast. Interestingly it has been introduced into other localities not within its natural range. For example in 2006, two living specimens were found in waters off Turkey.
In Hong Kong it’s very common and lives all across the coast of Eastern Hong Kong as well as the southern side of Lamma and probably anywhere where there is coral or rocks with reef-type algae. Check out Hong Kong Reef Check for more info on its distribution in Hong Kong waters.

During the day they hide in a sheltered crevice and at night they leave it to graze on a variety of algal species in an area about 1 m around their crevice. Very hungry individuals may become carnivorous. They can also bite bits of the rocks and corals producing coral sand as debris.

In Hong Kong they probably spawn in summer when the water temperature reaches at least 25 degrees. But other cues, such as a full moon may affect the spawning, too. They need to live in high densities to reproduce because males and females stimulate the opposite sex to spawn by releasing chemicals into the water and that’s only possible when there lots of them in one place. Females release 10-20 million eggs at each spawning, hoping that at least a few of them will get fertilised by the sperm the males release into the water at the same time. The fertilised eggs develop into larvae that will live and drift for 40-50 days as plankton before settling on the seabed. When the larvae finally settle on a naked rock somewhere, they metamorphose – that is transform into real sea urchins. It’s a bit like caterpillars and butterflies or tadpoles and frogs. Incidentally, almost all marine invertebrates go through such a metamorphosis. Frequently they live below another urchins spines to protect themselves from enemies.


In Hong Kong the long-spines sea urchin has been suspected of causing damage to reefs (along with some other invertebrates), so that the government has at least once undertaken manual removal of these urchins from the Hoi Ha Wan Marine Park. There is ssome research into this topic going on at the Swire Institute of Marine Sciences (HKU).

My old friend the Arabian Cowrie

Mauritia arabica shell
Credit: Source= |Author=Richard Parker (

Meet an old friend of mine, the Arabian cowrie known by its scientific name as Mauritia arabica (Linnaeus, 1758). When I started my first website as a teenager this was the first animal to feature and it’s still one of my favorites.  I have always loved the fine lines and curves on the shell that resemble Arabic script and give the shell its scientific name “arabica”.

They are pretty wide-spread ranging from South Africa all the way up East Africa , the Red Sea and Persian Gulf, India, Bangladesh, Myanmar, Thailand, Malaysia, Vietnam all the way across the Indonesian and Philippine islands and southern coast of China, down to Papua Guinea and northern Australia. It lives in shallow water near corals at cavern entrances and overhanging boulders or even under rocks where it feeds on algae during the night.
In Hong Kong if you know where to look its fairly easy to find them, too. Last weekend I was walking along a small beach in Ha Mei Wan (western side Lamma Island) south of the power station and spotted several bunches of 10 or more of them. The tide was very low which  exposed many of them to the air and huddled together in shady and moist spots of overhanging boulders near the water line.

Arabic Writing image
The inspiration for the name Arabian cowrie: the markings on the shell look a bit like Arabic writing. Source= |Date=2008-2-9 |Author=Danieliness

Another good place I have found this species is snorkeling among coral covered boulders at Shum Wan on Lamma Island. These guys like to cling to the overhangs of boulders near the sandy bottom.

I have inserted a picture from WikiCommons for this post, but for some really excellent images have a look at this site where you can see all the varieties of this little beauty from multiple angles:

PLEASE, DO NOT COLLECT LIVE SPECIMENS OR BUY THEM FROM SHOPS OR ONLINE: There are many websites that sell seashells for profit and to get a perfect shell for selling, most collectors will take live specimens. They will put them in 100% alcohol to make them swell out of their shell and die so that the shell can be stored without smelling of rotting snails. But why kill such a beautiful animal  simply to keep in a box and show off? If we all do that there won’t be any left. Sometimes  empty shells  wash up on  beaches, especially after typhoons. They might not be in perfect shape, but you will be a better person for not killing these pretty little things.