Seashells of all sorts from Hong Kong waters: gastropods, bivalves, barnacles, crustaceans, sand dollars, limpets, mussels, scallops, oysters. A look at the shells and the amazing animals that make them. Ecology and biology of indigenous Hong Kong seashells.
The venomous Conus textile is one of the most abundant and widespread cone shell in Hong Kong waters. Easily recognized by the tent-shaped markings on the orange-colored cone-shaped shell with wavy chocolates lines, you are advised to stay away from these beautiful but dangerous reef predators. They like to hide in sandy patches under rocks and also occurs widely throughout the Info-Pacific and grow to a maximum of 15cm shell length. The danger they pose comes from a tiny venom-laden harpoon they can fire from their proboscis. They normally use this to hunt other sea snails by injection them with conotoxin through the harpoon-like needle teeth they can fire out of their proboscis. They can reach around to any point on their shell with this proboscis, and several human death have resulted from handling.
To see exactly how subtle and fast the venom injection is, I recommend this clip from YouTube of a textile cone in a tank hunting down a prey snail. The prey snail in the clip has actually sealed itself into its shell and shut the opening with a special door called an operculum – but apparently to no avail!
The giant clam Tridacna gigas which can grow to 120 in width and 200 kg in weight is one of a group of clams called giant clams – the Tridacnids. Although T. gigas is the biggest of them, there are others which are still pretty huge by clam standards. One of these giant clam species, the Maxima clam Tridacna maxima, used to occur in Hong Kong. It grows to about 40 cm width, though typically is only around 20 cm in width. But consult the IUCN records for this clam species and you will see that although it occurs throughout the tropical and sub-topical waters of the Indian and western Pacific Oceans, the entry for Hong Kong sadly reads ‘regionally extinct’. When did that happen? Why did they vanish? These are the questions I wanted to find answers to.
When did it go ‘regionally extinct’?
The first entry I could find for the Maxima clam being regionally extinct in Hong Kong is 1983. Unfortunately that’s about all I could find. But at it has been gone from Hong Kong since at least 1983. Unless of course it has returned….its a slow-growing animal but has a very wide dispersal through free-floating larvae that live in the plankton – that’s why it occurs over such a wide geographical area spanning nearly a quarter of the planet! So theoretically, if conditions are right (see more below) and larvae are swept over Hong Kong or are purposely introduced by humans, the Maxima clam could reestablish itself in Hong Kong. So the next question is why did it go extinct in Hong Kong in the first place?
What happened to the Maxima clam in Hong Kong?
Apart from the fact that the Maxima clam lived on coral reefs in Hong Kong waters, I can not find any more details of how big the population was, which exact areas in inhabited (obviously coral reefs, so that narrows it to Eastern and Southern Hong Kong waters) or whether it was harvested locally. It is however clear that Hong Kong used to be a big regional market for giant clam species in Asia. Giant clams were and still are a delicacy in Asia (mostly the meat big abductor muscle) and the shell was used for decoration (though not extensively). Even the UN Food and Agriculture Organization has no useful fisheries statistics for Tridacnids. So there is no information on how many – if any – were harvested from Hong Kong waters before they disappeared. The maxima clam like all clams is a filter feeder which suck in water, filters out and swallows edible particles and then ejects the water out again. As such it is quite vulnerable to toxic substances in the water. In addition, it harbors symbiotic microscopic algae called zooxanthellae (zoo-oh-zan-the-lay) in its tissue. These absorb the clams waste products like CO2 and photosynthesize turning them into sugars in the presence of sunlight and giving off oxygen for the clam.
When open, the bright blue, green or brown mantle of the clam is exposed and obscures the edges of the shell which have prominent distinctive furrows. The attractive colours of the mantle are the result of pigment cells, with a crystalline structure inside. These are thought to protect the clam from the effects of intense sunlight, or to bundle light to enhance photosynthesis of the zooxanthellae.
This is essentially what corals do, too, which is why they share the same habitat – coral reefs. And like most reef corals the maxima clam also gets most of its nutrients from its zooxanthellae . Coral reefs suffer enormous damage from smothering by sediment that washes into the sea from rivers and rainfall and from clouding of the water and smothering by excessive algal blooms. Both of these were and still are to some extent big problems for Hong Kong waters, whereas in the past this was not the case. Algal blooms and sediment runoff increased a lot as a result of the increase in human population in Hong Kong and as a result of rapid industrialization and the associated water pollution. This combined with harvesting seems to be the most likely reason for the disappearance of the Maxima clam from Hong Kong waters before 1983.
Will there ever be giant clams in Hong Kong again?
I hope so. Like I said earlier, if conditions are right, any of the wide-ranging planktonic larvae of the clam that stray into Hong Kong waters could settle and grow to adulthood. Failing that humans could also try to establish them by attaching cultured juveniles to appropriate spots on reefs – but this is more complicated and costly, although Singapore has attempted this with initial success using another giant clam species Tridacna squamosa. But the main criteria is suitable conditions for a population to establish and grow – in other words we need clean seas again. Hong Kong has improved a lot on this front up until very recently, when the increased coastal development in southern China started to create a lot of water pollution which somewhat diminishes the results. There is still a long way to go. But I would say that divers should keep an eye out. In fact, the ReefCheck 2015 recorder forms even have a section for giant clams (Tridacna sp.), so its not just me that is hopeful! You never know. you could be diving some coral reef in Sai Kung, Tung Ping Chau, Hoi Ha Wan or the Ninepin Islands and come across a maxima clam. It might be an old dead one stuck in a reef with just the wavy outline of the two shell valves (probably) or it could have the fat, bright blue or green mantle of a live clam – in which case 1) hooray for Hong Kong and 2) please report your finding to the AFCD and Reef Check!
On the 21st of April a committee meeting was held in Legco (HK law making body) to discuss a new regulation on harmful anti-fouling systems. The legislation seeks to ban the use of organotin such as tributylin (TBT) compounds in paints used on ships which kill any organisms trying to colonize the hulls such as barnacles, worm, mussels, algae and others. Despite a lot of petty squabbling (and personal digs at the previous colonial administrations) it looks hopeful that regulation may eventually make it to a 2nd and 3rd reading in Legco and pass into law.
Tributylin (TBT) is a chemical that was used extensively in anti-fouling paints (bottom paints) on ships to improve efficiency by preventing invertebrates and plants clinging to the hulls. TBT is the most successful anti-fouling agent ever invented and was relatively cheap. It was used extensively for 40 years. But TBT slowly leaches out into the marine environment where it is highly toxic to a wide range of organisms beyond the organisms that it was intended to kill. By poisoning barnacles, algae, and other organisms at the bottom of the food chain, TBT levels are concentrated (biomagnified) up the marine food web and even up to us humans. It also causes developmental problems in marine organisms. One of the most studied organisms are marine snails, such as the Dog whelk (Nucella lapillus) in Europe and America. TBT leads a condition termed ‘imposex’ where female snails are ‘masculinized’ and grow penises. Since fewer fertile females are then available for mating, the population begins to decline, which disturbs the balance of the ecosystem.
Vertebrates such as fish and mammals can become affected by TBT through contact with waters contaminated with TBT and by eating already poisoned seafood. The Japanese rice fish (Oryzias latipes), has been used as a model to test the effects of TBT at different developmental stages of the embryo. Scientists found that as TBT concentration increased the developmental rate decreased and that tail abnormalities occurred as a result. Studies have shown that TBT is harmful to the immune system. Research also shows that TBT reduces resistance to infection in fish which live on the seabed and experience high levels of TBT. These areas tend to have silty sediment like harbours and estuaries (like Hong Kong). Mammals, exposed to TBT through their diet, also suffer. TBT can lead to immunosuppression in sea-otters and dolphins. High levels of TBT were found in the livers of sea otters (Enhydra lutris) and stranded bottlenose dolphins. TBT has also been blamed by hearing experts for causing hearing loss in mammalian top predators such as whales. Because hearing is important for mating and predation in these animals, long-term consequences could be drastic.
How TBT can move up the food chain was shown by one study that found most samples of skipjack tuna tested positive for TBT. Tuna from waters around developing Asian nations had particularly high levels of TBT most likely because the regulation of TBT is not as well enforced in Asia as it is in Europe or the US.
In addition TBT last for a long time in the marine environment. Its half-life in the marine environment is around 25 years. TBT sticks to seabed sediments. But that process is reversible and depends on the pH of the seawater. Studies have shown that 95% of TBT can be released from the sediments back into the aquatic environment. This release makes it difficult to quantify the amount of TBT in an environment, since its concentration in the water is not representative of its availability.
TBT Pollution in Hong Kong
In 1995 a study showed that 3 of the 4 species examined had sign of distorted sex organ development (imposex) and the authors inferred TBT as the cause. Another study in 2000 of 24 species found 5 species with imposex. Again the author inferred TBT as cause. In 2001 a study found the concentration required to cause imposex for one local species was as low as 0.000001 grams per liter.
In the 2004-2012 monitoring, TBT was generally not detected in marine water, river water, sewage effluent or storm water runoff samples by the Environmental Protection Department (EPD). According to the EPD the levels of TBT in Hong Kong marine sediments mostly met Australia‘s sediment quality guideline for the protection of benthic organisms and were generally within the range (falling on the low-side) reported in other Asian countries, such as Japan, Vietnam and Malaysia. The levels of TBT in the biota species were low and largely comparable with the levels for biota in the Pearl River Estuary area.
The Actions of the Hong Kong Government
The production, import and export of TBT paints was already banned in HK. In 1990, the Marine Environment Protection Committee recommended that the Government eliminate the use of TBT-containing antifouling paints on smaller vessels. This was intended to be a temporary restriction until the International Maritime Organization could implement a complete ban of TBT anti-fouling agents for ships, which it did in 2001. The use of TBT in antifouling paint was banned (deregistered) in Hong Kong in 1992. But this new HK regulation seeks to now fully implement the International Convention on the Control of Harmful Anti-Fouling Systems on Ships from 2001 which includes certification requirement for large ships in Hong Kong waters and also extends to Hong Kong registered ships anywhere in the world.
The Convention on the Control of Harmful Anti-Fouling Systems on Ships came about in 2001. Mainland China implemented this Convention in 2011, but Hong Kong (as a major port city) is only now discussing this.
Despite the international bans, TBT will most likely be present in the water column and sediment for up to twenty years because of its long half-life.
Violations of the Ban
Even though its banned by some international agencies, TBT anti-fouling paints are still being used in some countries with poor regulation enforcement, such as countries in the Caribbean.
Note: the new subsidiary legislation (title: Merchant Shipping (Prevention and Control of Pollution) Ordinance (Cap. 413) Merchant Shipping (Control of Harmful Anti-fouling Systems on Ships) Regulation) was published in the Gazette on the 20th of March 2015, introduced into the Legislative Council on 25 March 2015. It was refered for discussion to the subcommittee on Merchant Shipping on the 10th of April 2015. In their report published on the 6th of May 2015 the subcommittee supported the subsidiary legislation. Having cleared the first hurdle.
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.
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: http://www.cypraea.eu/species/cypraea_arabica.htm
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.