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The "Golden Clam"
Corbicula fulminea

The "Golden Clam"
Corbicula fulminea

By Justin Pierce

There has recently been a surge in both interest and availability in the aquarium trade of a small freshwater clam, Corbicula fulminea.  There seems to be much misunderstanding concerning this little bivalve and its proposed “benefits” to the aquarium.  I thought I would write this article to help paint a clearer picture about how it functions, how to care for it, and its overlooked threat to the natural environment through intentional or unintentional release from captivity. 

This clam goes by a number of common names.  The most popular name is "Golden Clam", but it is also referred to as "Asian Clam", "Asiatic Clam", and "Prosperity Clam", among others.  The name "Asian Clam" is obvious in light of its origin, namely southeastern Asia, including  Southeast China, Korea, and southeast Russia.  In its native land, it is a prized food source to many people.  Indeed, it is this popularity that has enabled it to cross overseas into new countries, notably the United States, and become established as an invasive species.

A typical Corbicula sp. clam.  The golden color is particularly attractive against the dark substrate.

History of Corbicula in North America

Sometime prior to 1938, Corbicula was introduced to the United States by Asian immigrants.  Many experts believe this time to be as early as the middle 1800’s, although the exact method of introduction, intentional or unintentional, is not known.  Once in the North American environment, Corbicula made one of the fastest range expansions ever recorded for a non-native species.  From California, Corbicula spread eastward across the U.S., making its way to the east coast and down into Florida in about five decades.  This occurred because of a combination of human assistance and the general reproductive capability of the animal itself.  Transport barges that operate through out inland waterways are a prime source of dispersal.  Ballast water that is taken into barges or live-well water taken into boats also takes up free-swimming larvae from Corbicula.  When emptied at a different location ,these animals are released into a potentially new habitat and may become established.  Also, gravel from riverbeds is transported in a similar way on barges and can contain the adult clams, which can be introduced to new locations. 

The clams are also popular bait with fishermen.  The most common way that the clams can become established upstream in a flowing waterway is for fishermen to carry some with them in their pocket or bait bucket and once finished fishing to toss the remainder of the bait into the new water.  The clams are even sold as bait and can be transported to entirely new bodies of water, even secluded “secret” fishing spots.  Another controversial, although mostly discounted, means of dispersal is for the larvae to attach to the feet of wading birds and hop off when the bird flies to a new area.  Unfortunately, another method of human- aided dispersal involves aquarium-keepers releasing their live specimens, whether intentional or not.  This is a major black eye on the aquarium industry, not just for releasing Corbicula, but many other species as well.  This is a "hot button" issue for aquarists, since in some cases the actual role of aquarists in the introduction of non-native species is small or exaggerated when compared to other sources.  This is the situation with Corbicula, in particular, with the shipping and fishing industry responsible for the vast majority of introductions. 

It is also difficult to deny that there is also a fair amount of media hype/sensationalism that surrounds stories of escaped “pets”.  In some cases the blame is justly affixed to the aquarium industry, and in other cases we seem to have become a "punching bag" for various groups that want to avoid laying the blame on other, more powerful groups.  But whether our actual or perceived role is sole, major, or minor, the fact that irresponsible aquarists are a cause of the spread in non-native organisms in any capacity should be unacceptable.  The bottom line here is that we need to have an awareness of how these introductions can happen and act in responsible ways to ensure that they do not.  We must take a closer look at Corbicula, especially because of its proven adaptability and ability to invade new environments.

The characteristic golden color of young clams (above) darkens with age.  Eventually the burrowing activity of the clam will abrade the dark color to reveal the white calcium carbonate underneath (below).

Identification of Corbicula to the species level requires careful examination of the shell (below).

Meet The Species

Corbicula fulminea (and possibly C. manilensis) can be found in many bodies of water through most of the United States, except for a few of the most northern states.  They have been recorded as growing as large as 50-65mm, but it is uncommon to find them larger than 25mm (about the size of a U.S. half-dollar coin).  Their color can change with age/size but is typically a golden yellow color, especially when small, then changes to a dark brown or black color as they grow.  Different areas produce clams of different color, which may be due to genetics or environmental factors.  As with many other bivalves, there is an organic layer that covers the shell and protects it from erosion or dissolution.  This layer, called the periostracum, is what gives the otherwise white shell its color.  As the clam ages, the periostracum can become worn, especially around the oldest part of the shell, thereby exposing the white calcium carbonate.  The combinations of white, gold, brown, and black make this clam quite attractive. 

Corbicula are usually found in great abundance in areas that they occur, so catching many of them in a small amount of time is rather easy.  Their attractive appearance and ease of collecting has led some enterprising individuals to begin marketing them to aquarium keepers.  Previously, there were only a small number of people involved in keeping them.  They would go to their local stream or lake and pick up a few to put into their tanks at home.  Recently however, with the advent of the internet, the Golden Clam has begun appearing in online auctions, jobbers’ web pages, online aquarium outlets, and local aquarium stores.  Due to simple misunderstanding- or the quest for the almighty dollar- the marketing involved with these little -known clams is often exaggerated or completely misleading.  Let’s examine some of these claims and see if we can make some sense of them.

#1.  Golden Clams: "The Ultimate Bio-Filters".

This is true if you are referring to “bio-filter” in the broadest sense.  Clams are bio-filters, or “living filters”, in that they are alive and they do filter the water.  The difficulty comes with the definition that “bio-filter” has come to represent in the aquarium industry.  In actuality you could properly describe anything that is alive and filters water either mechanically, like bivalves, sponges, tunicates, fan worms, bryozoans, etc. or chemically, like bacteria or algae, as a bio-filter.  Unfortunately, the aquarium genre has put so much emphasis on the bacterial (nitrification) aspect of “biological filter” that nitrification (the process of oxidizing ammonia to nitrite and then nitrate) and biological filtration have become synonymous.  I am afraid that referring to anything aquarium-related as a "biological filter" will give the impression that this animal will "process" ammonia and nitrite.  In the least it will cause confusion, especially with beginning hobbyists.  Some websites imply this function by Golden Clams, while others are explicit.  One claims that “there is no need for expensive filters” if you have Golden Clams.  I must make it clear that simply having a bunch of Golden Clams in your tank does not mean that you can get by without an undergravel filter, trickle filter, or some other type of filter that uses nitrifying bacteria (or live plants).  In actuality, these clams will add ammonia to the tank.  I’ll explain this further below.

#2.  Golden Clams Reduce Nitrate.

This one is simply not true, no matter what angle you would like to look at it.  Assuming that this is a misunderstanding and not just a lie for marketing’s sake, the only way I can imagine that this confusion originated is from comparing them to Tridacnid clams.  The popular Tridacnid clams, or Giant Clams, do remove nitrate from the water.  This is becuase of the presence of symbiotic algae in their tissue.  It is actually the algae that utilize the nitrate, not the clam itself.  Even so, Tridacnid clams have developed a reputation for being "nitrate reducers" and are sometimes even recommended for this purpose.  Golden Clams though do not have symbiotic algae and therefore do not remove nitrate.  In fact, they help to create nitrate.  Similar to many aquatic animals, when they digest proteins the waste product that is generated is ammonia.  This is then converted to nitrite and then to nitrate by the biological filter – oops, I’ll be specific: the nitrifying bacteria.  So Golden Clams can be considered as part of the bioload on a system, adding ammonia, and therefore nitrate to an aquarium.

A clam with it's foot extended.

#3.  Golden Clams are Hardy and Can Live Without any Special Care.

This is half true.  Golden Clams are very hardy.  They can tolerate a wide range of environmental conditions, which I will expand on momentarily.  This makes them very adaptable to aquarium life and also enables them to easily invade new ecosystems.  The fact that they will live for months without doing anything special is also true.  Unfortunately, Golden Clams suffer from the same situation that many other bivalves in aquaria suffer from, usually without the aquarist even realizing it.  The condition is starvation, and it is difficult to notice because of the hard shell.  Too often, bivalves do not get the right type or amount of food in captivity.  This leads to slow starvation which happens in a similar way to most animals.  The body uses up its own mass for energy.  If this was say, a fish for example, you could notice that it was appearing to get skinny.  Perhaps the bones would start to become visible through the skin, or the gut area would become sunken in, etc.  Bivalves lose weight in a similar way, except for one key difference.  You cannot notice it because the hard shell, which stays the same thickness does not reveal the fact that the inner tissue it withering away.  There is no advanced warning.  You simply find the clam one day with its shell gaping open having died. 

Clams do not have as high of a metabolic rate as fish do, and take a lot longer to starve to death.  Aquarists should not take comfort in the fact the clams “live” for months without food.  Some assume that the clam is "getting its food from the water" by eating bacteria and other plankton produced naturally in the aquarium.  In reality, the tank is highly unlikely to produce the type and quantity of food needed by the clam.  Without adding food specifically for the clam, it is simply taking a long time to die.  If one assumes that the clam has a short lifespan, then this problem may go unrecognized.  But learning that the clams may live up to seven years really puts the situation into perspective.

There are other misconceptions about Golden Clams, but those covered above are the most troubling and frequently used in advertising the animals for sale within the hobby.  We will continue on, though with the topic of feeding.  Hopefully, by now it is clear that supplemental food must be added to sustain the clams.  But what do they eat?  Their major food source is phytoplankton, tiny free swimming single celled algae, and small protozoans.  There are a few types of phytoplankton that are available to hobbyists now that are worth trying.  Also, Spirulina and Chlorella, found at health food stores are also worth experimenting with.  Additionally, aquarists can make “green water” cultures as a food source.  The information on how to do this can be found in many other places on the internet, so I won’t go into it here.

No matter what form of phytoplankton you use though, it is best to use a drip system.  This will deliver the food in small amounts as a continuous supply.  If you don’t use a drip system and add the food in batches at a time, just use enough to slightly cloud the water.  Adding too much food can foul the water as well as clog up the filtering system of the clam.  There will have to be some experimentation on the part of the aquarist to figure out the correct amount to feed.  The best indicator of the clam receiving adequate quality and quantity of food is growth.  Golden Clams have high growth rates compared to other freshwater bivalves.  Starving clams will not grow at all.  If you observe any growth over a period of say, one month then they are receiving adequate food.

Another factor for growth is the presence of calcium and carbonate dissolved in the water.  If you happen to live in areas that have “hard” water, you probably won’t have to do anything other than routine water changes to ensure that the clams get the minerals they need to build their shell and grow.  If you have “soft” water, you will need to supplement with calcium and alkalinity boosters, both of which can be purchased at your local aquarium store.  Another method is to use some calcareous substrate in the tank to achieve the same goal.  It will slowly dissolve, releasing calcium and carbonate.   Make sure that the other animals in the tank can tolerate a pH in the upper 7’s since the addition of these two substances can elevate the pH.  If you have animals that require soft water with a low pH, it is best not to add clams to the system.

The total of all the dissolved minerals in water is referred to as "salinity".  Golden Clams are tolerant of a range of salinities, usually less than 13ppt, but may be able to be acclimated up to 24ppt.  In nature, they are only infrequently found in brackish water though with most of them occurring in what would be considered fresh water.

An additonal environmental factor which Golden Clams have a wide tolerance for is substrate type.  Many other freshwater bivalves can be quite specific about the type of substrate they need to live in, requiring a certain size: sand, pebbles, stones, boulders, etc.  Golden Clams, however, can live comfortably in just about any type of substrate.  Their numbers seem to be reduced in areas that have superfine silt although they are more tolerant of silt than most other native fauna.  The accumulation of silt, or siltation, is one the biggest environmental problems affecting freshwater systems.  Altering the flow of waterways, damming, channelization, agricultural runoff, and other human activities results in silt that is capable of smothering entire waterways.  Sadly, it is becoming more and more common to find little other than Golden Clams living in what used to be areas of high diversity including native bivalves.  The preferred substrate of the Golden Clam is clay, fine sand, and coarse sand.  Sand would make an ideal substrate for them in aquaria.  But they can be found in almost any substrate, amongst boulders or even in cavities inside solid rock.  It is even possible to keep them in a bare bottom aquarium with no substrate at all.  If kept with sand, they will burrow down so that only the very edge is visible.  If you want them to be more noticeable in your aquarium, only use a thin layer of sand or gravel so that they cannot hide completely.

One environmental parameter that is often overlooked in freshwater systems is water flow.  Many freshwater bivalves are particular as to the velocity of water flow which they prefer.  The Golden Clam however, can adapt to a wide range of water velocities ranging from churning streams to still lake environments.  The same goes for aquarium care.  They do prefer swifter moving water, and it is advisable to provide at least moderate circulation in the aquarium.

To Keep or Not to Keep Corbicula

Now that we have discussed how to keep them, we must ask ourselves should we keep them?  I will leave that answer up to the individual as long as there is a clear understanding of the challenges and responsibilities of doing so.  The hardiness of these clams enables them to adjust easily to captivity and also allows them to invade a large portion of North America with their range still increasing at a fast pace.  But simply being hardy, able to survive, is only part of the equation for their invasion.  It also has to do with reproduction.  One website selling these clams claims that “they have no interest in reproduction”.  I hate to break the news, but reproduction is a trait of every living organism and there is indeed a (putting it lightly) great “interest” in reproduction by all living organisms, including the Golden Clam.

In North America, these clams breed from spring to fall with the highest fecundity occurring in the fall.  However, they are capable of breeding year-round in captivity.  They are synchronous hermaphrodites, meaning that they have both male and female reproductive organs, producing both sperm and eggs at one time.  They may also be capable of self fertilization, meaning that it only takes one to reproduce.  Also, they reach maturity very quickly, being able to reproduce in as little as a few months of age.  Clams filter the water by drawing in water through an incurrent siphon and expelling it through an excurrent siphon.  During reproduction, sperm is released through the excurrent siphon in the water column.  It is taken in through the incurrent siphon of (usually) other clams as they filter the water, collecting both food particles and sperm.  The eggs are then fertilized internally and form veliger larvae in a specialized pouch.  The larvae are then ejected into the water where they swim for about a week or so.  They then settle to the bottom and begin to grow into the familiar “clam” form.  The number of larvae that are released is staggering.  Each clam (remember they are hermaphrodites) can release up to 400 larvae per day!  The water becomes loaded with larvae, although they are very tiny and are usually not noticeable to the naked eye.  The same thing happens in aquaria.  Herein lays the danger in keeping them.

Most, not all, aquarists are aware that you should NEVER release any organism from your aquarium into the wild, whether it is native or not.  Even if it is native, the organism may have been exposed to diseases; bacteria, fungi, viruses, or larvae or microscopic stages of other organisms that are not native.  It is not sufficient to simply avoid releasing the clams, which you should do of course.  The prodigious amounts of larvae released by Golden Clams can infest any water that they are in contact with as well.  Transport water, aquarium water, bait buckets, etc. have the potential to increase the range expansion of these clams if it should be disposed of outdoors.  Something as simple as washing an aquarium filter in the driveway of your house could potentially result in Golden Clams finding their way into the watershed.  Similarly, keeping them in outdoor ponds, as some sellers have recommended doing, is also very risky.  One major introduction event happened due to Auburn University holding these clams in outdoor experimental ponds.

For those of you that simply brought some Golden Clams home to put in your aquarium from the stream near your house (many of you can do this already) there is probably little risk in doing so.  The damage is already done.  Those of you sending and receiving Golden Clams in the mail or transporting them otherwise, including aquarium stores, are dealing with a different situation, one that needs to be addressed.  It is the responsibility of the buyer to educate themselves on the animals they keep, but also the responsibility of the sellers to make sure that the buyers understand this issue.

Once in a new area, some of the clams do become prey for some crayfish and a few species of fish.  So what’s the problem?  Everyone has heard of Zebra mussels in the Great Lakes being a problem.  Why haven’t you heard about Golden Clams?  I would guess that it is because Zebra Mussels are much more obvious in their presence than Golden Clams are.  Zebra Mussels grow on top of things; smothering other bivalves, clogging water pipes, attaching to boats, etc.  Perhaps the media is more inclined to report on them because they directly affect human activities and business.  Golden Clams are not visible to the casual passer-by except for some empty shells lying on the shoreline.  But if you look for them, they are everywhere.  They are typically buried in the sand or underneath rocks.  Every handful of sand you scoop up or every rock you turn over will usually yield at least a few.  While they do not directly smother the native fauna like Zebra Mussels, they affect the native fauna in a less obvious indirect way.  Most notably, they compete with native mussels, which are in a crisis right now.  Firstly, they compete for space.  Many native mussels are very particular about the type of substrate they reside in.  Golden Clams are much more general and are able to occupy a number of habitats that species of native mussels need.  Coupled with this is the fact that these needed substrates are becoming less available or non existent due to siltation or other human activities.  Golden Clams also compete for food with native bivalves.  This can be a problem to a greater or lesser extent depending on the habitat, but many native bivalves live in clear streams where food is less abundant.

Feeding by Golden Clams though is not only a problem for other bivalves but can be trouble for the entire aquatic ecosystem.  Bivalves filter the water and trap various particles, with some being very specific as to the type and size of the particles they will eat.  This “feeding selectivity” can cause a shift in the balance of ratios of predators and prey starting at the microscopic level but echoing throughout the entire food web.  Since Golden Clams occur in high numbers where they are found, they can be very influential in upsetting this trophic balance.

Golden Clams are able to out compete native bivalves in a number of ways.  They grow fast, mature early, produce lots of offspring, can tolerate a wide range of environmental factors including temperature, salinity, dissolved oxygen, water flow and substrate type, and do not need an intermediate host to carry the larvae like native mussels do.  This spells trouble for our native mussels who are already facing other threats.  The changes that Golden Clams create in the food web spell trouble for many other species as well.  While they do make interesting and hardy aquarium specimens, Golden Clams must be dealt with according to the risks they pose to the environment.  If you choose to keep these clams in aquariums do not ever release them into the wild, educate others about them, and sterilize the aquarium water with a little bleach before disposing of it. Even though the problem of these clams is already widespread, we should still do our part to ensure that we as aquarists are not the cause of further introductions.  Be careful with them not only to avoid having the industry look bad, but because it is the right thing to do.

Captions for pictures;

2302- Corbicula lying on its side.  The white area is the older section of the clam where the protective covering is wearing away.

2303- Another view of the same clam.

The interior of the shell of a Golden Clam without the tissue.  Notice the shiny “mother of pearl” layer, also known as nacre.

2307- An assortment of clams in various colors and stages of growth.

2310- A clam extending its muscular “foot” into the substrate.  It uses this muscle to dig and for locomotion.

2311-  A young golden clam exhibiting the coloration for which they are known.

2312-  The top view of a clam in its correct orientation in the substate.

Author Bio:

Justin Pierce has been an aquarist since early childhood.  He has a degree in Marine Science and is most passionate about aquatic invertebrates.  His life-long commitment to aquaria has led him to work at many aquarium stores, public aquariums, zoos and research labs.  Over the years Justin has been published in many hobbyist and peer reviewed journals and continues to enjoy contributing to advancements in this field.

Copyright  7/3/08
<Hi there>
Would it be possible earn permission to use a picture from the following URL in an educational presentation?
Asiatic clam:
<Mmm, unfortunately this piece is not by me... but a non-universal purchase property of Justin Pierce... whom I have no extant contact info.. The short answer is I cannot grant use of other people's property. Bob Fenner>
Caroline R. Nguyen
Coastal Rivers Water Planning & Policy Center
Georgia Southern University

Re: Copyright  7/3/08
Okay thank you so much for your reply.
<Welcome. If I had good images of this... I'd send them along. Good hunting! BobF>

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