Introduction to Clams

Phylum:  Class: Order: Family: Subfamily Genus:
Mollusca Bivalvia Venerioda Cardiidae Tridacninae Tridacna
Gigas Clam

Large Gigas Clam



Tridacna clams are the most commonly kept clams in the reef aquarium.  This is the family of clams that include the largest clams in the ocean and most get moderately large   (6″) to very large (3′ or more).  Coloration varies from species to species, but most are very attractively colored.  They all survive off symbiotic algae called zooxanthellae that live within their mantle tissues and require no direct feeding except when very small before their zooxanthellae become established.  Related bivalves sometimes sold into the reef hobby include flame scallops and oysters of various types.  These clams are all filter feeders and are generally not very suitable for the reef tank unless the appropriate foods are offered.

The Tridacna species typically available for the reef aquarium are the T. maxima, T. crocea, T. derasa, T. squamosa and T. gigas with T. maxima and T.derasa being the most commonly available.

The common names for Tridacna clams are typically just the species name.  For instance the Tridacna maxima is typically referred to as the Maxima clam.

All tridacnid clams require at least moderate lighting and some require intense lighting.  Ensure that the tank has sufficient lighting to support the type of clam you are thinking about buying.  If lighting is insufficient, the clam may show very high mantle extension.  Although this may look like a happy clam, in fact it can indicate that the clam is trying to extend its mantle for maximum light gathering due to inadequate lighting conditions.  Clams in the wild do not show much mantle extension.  If the light is too bright, it can adjust for this by not extending the mantel as much or by keeping the shell partly closed.

At times you may notice your clams expelling a brownish-green stringy material from their exhalant siphon.  This is normal and is a sign that the clam is expelling excess zooxanthellae algae or other waste products and is not a sign for concern.  They may do this more while adjusting to their new environment.

You may notice a white border on the mantel edge of the shell.  This is an indication that the clam is growing quickly and is typically seen on Derasa and similar fast growing species.

Clam Expelling Waste

Expelling waste



Most of the more popular Tridacna clams such as Maxima and Derasa are now being captive breed in the locales that they occur in naturally.  Clams will sometimes spawn in the reef aquarium once they reach maturity, but it will not result in any viable baby clams being produced in the home tank.  The photo here is of a Derasa clam spawning.

Guidelines to buying tridacna clams:

  • Chose specimens that respond to a shadow or touch.  The clams should shut it’s shell with some amount of force.

  • Inspect the mantle.  It should extend past the edge of the shell and not have tears, holes or obvious damage to it.

  • Ensure you have the lighting to support the type of clam you are contemplating buying.  Maxima and Crocea typically require the brightest lighting while Derasa and Squamosa can do OK under moderate lighting.

  • Keep in mind that some types of Tridacna clams can get large fairly quickly, especially Gigas and Derasa which can grow from a length of 2″ to 8″ in just 12 to 18 months.  These types of clams are really only suitable for fairly large tanks where they have room to grow or be prepared to trade them back in for smaller clams when they outgrow their tank.

  • Look for gapping.  This is when the clam opens it’s shell but with little or no mantle extension.  The intake siphon opening is frequently widely distended as well and the clam has an overall flaccid appearance.  This is typically accompanied with a low response to stimuli such as a touch or shadow.  Gapping is a sign of a sick clam that should not be purchased.  Unlike some marine animals, sick clams never recover even when put in optimum conditions.  By the time they show signs of being unhealthy they are already on their way out, so resist the temptation to try to save a sick clam at your local LFS.
    Tridacna Clam Growth Ring

    Growth Ring



When placing the clam in the aquarium, there are some general guidelines to follow:

    • Position the clam where it cannot be stung by any of its neighbors.  Clams are not very tolerant of bothersome neighbors.  Clams can be placed next to each other as long as they have room to fully expand their mantles..

    • Clams vary by species in the amount of water current that they like.  The Crocea and Maxima clams come from the reef crest zones with a lot of water motion while Gigas and Derasa clams tend to be deeper water species with little water flow.  In aquariums, most clams do OK with a moderate water flow that keeps their mantles fluttering in the current and not be blown about.  Definitely keep them away from laminar water flows such as a power head.
    • Ensure you place the clam on a suitable substrate.   Some clams such as Derasa do best on the sand bed.  If it is positioned up on the live rock, it should be kept in place by rocks or similar or the clam may ‘jump’ from its position by extending it’s foot and using it to launch it off its perch you put it on and tumble down the live rock.  Typically clams will do this if they don’t like the light or water current in the location that you thought was perfect or they are being bothered by neighbors.  Consider finding a different spot to see if the clam is happier.

    • Clams like the Maxima will attach themselves to any firm substrate that they are resting on.  If you have them placed on the liverock, they will permanently attach themselves over time making it difficult or impossible to remove without causing them serious damage, so it is best to place them on smaller pieces of rock so that you can move both the clam and rock later should it become necessary to do so.
  • Clam Gapping

    Clam Gapping



    Watch the clam after introduction to ensure it is not being molested by any of the fish or invertebrate inhabitants.  A new clam will frequently have tasty stuff on its shell that is new to the tank and so it is not uncommon for fish to show an interest in it when first introduced and should not be a cause for concern as long as they pick at the shell and do not bite at and tear the mantle.  Once they eat all the good stuff off the shell, they’ll leave the clam alone.

Further reading about Tridacna clams