Presented by Charles & Linda Raabe
Mactan Island, The Philippines
© 2008 All Rights Reserved

  As great looking as some clams can be, they also seem to be a difficult species for new hobbyists to maintain. I believe this is greatly due to the poor water quality that most new tanks encounter. Such water quality issues are also compounded by the fact that many hobbyists believe that the clams must be fed small particulate foods. That may be the case for many clam species, but for those clams that have zooxanthellae, such feeding is not required regardless of how small or large the clam is. They will remove what amounts of dissolved nutrients (nitrates/phosphates) they require as food from the aquarium's water.
  However,  I still dose my aquarium system with phytoplankton but do not do so with the intent of feeding a zooxanthellate clam. Such dosing is meant to feed the numerous zooplankton species (pods and such) that in turn, through their breeding feed my corals.  I suspect that a lot of hobbyist who dose phytoplankton thinking that they are feeding the clams are not aware that they are indirectly creating a better habitat for the entire aquarium system while the clams ignore such feeding.  Below are some great links concerning the identification and care of specific species of clams as well as some clam biology.


Photo by Charles Raabe


   
VARIETY OF CLAM ARTICLES  -  A variety of article authored by Daniel Knop.

A Brief Guide to the Selection and Placement of Tridacnid Clams  -  " Do you want to dress up your reef tank by adding a Tridacnid clam?  If you're like many reef hobbyists, the answer is a resounding "Yes!"

Identifying the Tridacnid Clams  -  "
It is the shell of each specimen which is typically the key to identification, not just the flesh. "

Clams  -  " Tridacnids, however, are very unusual clams; they have symbiotic zooxanthellae and they also have an internal anatomy that is oddly oriented relative to their shells.  "

Clam Data Base  -  Species specific information, a very good presentation.

The Flame Scallops  -  "A detailed article about the care of these mobile bivalves. "

   When placing your new clam or having to move one already in the tank, I have found that for the majority of species, they do better when placed up into the rock landscaping, which is usualy a hard thing to accomplish since the clams have a habit of slamming their shells shut and tipping themselves over or having something else, such as a snail, push them over and out of the rocks. Since clams can and do excrete anchoring filaments when given the time to do so, usually a matter of a day or two at most, I will use super glue gel to glue one side of the shell to a rock holding the clam in place long enough for it to anchor itself. Keep in mind, only glue one half of the shell to the rock(s) leaving the other half with enough room between any rocks for the clam to able to open. Do not glue to bottom of the clam as this is where it is open and excretes its anchoring filaments.
   For those with sufficient light intensity (metal halides), placing the clams on the sandbed seems to do them no long term harm that I am aware of. Although I have never seen a clam in the wild that was sticking up out of the sand and are always found in amongst the rocks and corals, its shell being embedded down into the rock/coral substrate affording the clam protection from below and its sides.


Also see:
The Giant Clam: an anatomical and histological atlas
 ( A 7-part PDF series )

   Circulatory - The heart of a clam lies just below the hump of the shell within the pericardial cavity, the only remains of the coelom. Therefore, the coelom of the clam is said to reduce. The heart pumps blue blood, containing the pigment hemocyanin instead of red hemoglobin, into vessels that lead to the various organs of the body. Within the organs, however, blood flows through spaces, or sinuses, rather than through vessels. Such a circulatory system is called an open circulatory system because the blood is not contained within blood vessels all the time. This type of circulatory system can be associated only with an inactive animal because it is an inefficient means of transporting oxygen and nutrients throughout the body.

   Digestive - The clam is a filter feeder. Food particles and water enter the mantle cavity by way of the incurrent siphon, a posterior opening between the two valves. Mucous secretions cause smaller particles to adhere to the gills, and cilia action sweeps them toward the mouth. The digestive system includes a mouth, a stomach, and an intestine, which coils about in the visceral mass and then goes right through the heart before ending in an anus. The anus empties at an excurrent siphon, which lies just above the incurrent siphon.


   Exocrine - There is an accessory organ of digestion called a digestive gland. The digestive gland surrounds the stomach and is composed of numerous tubules, which are formed at the distal ends of branching ciliated ducts. Nutrients from the stomach enter the tubules through these ciliated ducts and are absorbed by the glandular cells lining the tubules and digested intracellularly. The crystalline style (stalk) extends from the style sac into the stomach. The style contains amylase, which helps digest the starches present in the food. Thus, the digestive gland is both secretory and absorptive. In addition, the style usually (in other bivalves) harbours many spirochete bacteria that are thought to secrete additional digestive enzymes. Wastes from the digestive gland tubules are returned to the stomach and are eventually swept into the intestine.

   Excretory - There are two excretory kidneys in the clam, which lie just below the heart and remove waste from the pericardial cavity for excretion into the mantle cavity. The clam excretes ammonia, a poisonous substance that requires the concomitant excretion of water, Land-dwelling animals tend to excrete a less toxic substance in a more concentrated form.

   Immune - Internal defence in bivalve molluscs, like all invertebrate species, is based on an innate, non-lymphoid immune system which consists of a variety of cell types and effector molecules interacting to maintain efficient elimination of foreign bodies. Phagocytic blood cells (cell that removes unwanted substances) or haemocytes, are the principal effector cell for immune defence. The phagocytic response of the haemocytes is complemented by an array of killing mechanisms which may include, release of degradative enzymes and the generation of reactive oxygen metabolites. Antioxidant enzymes may also be present to minimise the potential damage for adjacent tissues and cells from these reactive oxygen metabolites. Haemocytes may also release other soluble compounds as part of their defence strategies. Recent research has identified the composition of a number of antibacterial peptides in bivalve molluscs.

   Musculo-skeletal - In a clam, the shell is secreted by the mantle and is composed of calcium carbonate with an inner layer of mother-of-pearl. If a foreign body is placed between the mantle and the shell, pearls form as concentric layers of shell are deposited about the particle. The adductor muscles are used to open and close the shells.

   Nervous and Sensory - The clam nervous system is composed of three pairs of ganglia, which all are connected by nerves. Clam lack cephalization. The foot projects anteriorly from the shell, and by expanding the tip of the foot and pulling the body after it, the clam moves forward.

   Reproductive - The male or female gonad of a clam can be found about the coils of the intestine. Reproduction takes place when male and female clams release sperm and eggs into the water. Fertilized eggs develop into free-swimming larvae before settling to the bottom. While all clams have some type of larval stage, only marine clams have a trochohore larva.

   Respiratory - Within the mantle cavity, the gills hang down on either side of the visceral mass, which lies above the foot. Gills are composed of vascularized, highly convoluted, thin-walled tissue specialized for gas exchange.


Clams as filtration ?  -  Having seen this being promoted and asked about, I thought it a good idea to address such a "method".  If by chance you read the above anatomy descriptions you would have noted that clams are filter feeders, of plankton. Which can only produce the same waste materials, most notably, ammonia, that any animal that eats would produce. I fail to see how loading an aquarium system with clams could ever have an effect on nitrate production as being suggested by others. Quite the opposite would happen. That and being plankton filter feeders, I would see them as a big competition for food that other tank inhabitants could and would utilize, such as my corals and other invertebrates.
  Having a reef system, and trying to provide for the animals food requirments is hard enough without having a number of clams being in direct competition for the same resources. Any area, such as a sump or refugium compartment would much better serve a reef aquarium if used to promote the production of plankton, not its removal.
  For nitrate reduction, its not going to happen through the use of clams. For water clarity, sure, if you want to deny your corals the chance at what little plankton our systems can produce.  All around, I feel this is just an idea that has not been thought out through to its biological conclussions. As happens quite frequently within this hobby.