Phytoplankton, A Necessity For Clams

By Ronald L. Shimek, Ph. D.

Animals that filter phytoplankton out of sea water are common; in fact, they probably are the most abundant animals on Earth. Most such critters are planktonic, and in terms of biomass, most of them are small crustaceans such as the calanoid copepod, Calanus finmarkiensis. Somebody with far too much time on their hands once calculated that species produced 100,000,000,000 tons of shed exoskeletons from its molting each year; given that these animals are about a quarter of an inch long, that estimate of productivity implies a LOT of tiny sea-going bugs... However, even amongst bottom-dwelling or benthic animals, this feeding mode is a predominate one.

Examples of animals totally dependent upon phytoplankton are found in virtually every major animal group, but relatively few animal groups are as wholly committed to this food source as are the bivalved, or two-shelled, mollusks, otherwise known as "clams." Reef aquarists often consider that there is only one type of "clam," that being a Tridacnid, and they largely ignore the remaining approximately 10,000 species of other clams. Nonetheless, however interesting and beautiful Tridacnids are, they are only very specialized clams and can be understood only in the context of other, more normal, clams. Consequently, to discuss the necessity of feeding in Tridacna, I first will have to discuss feeding in more typical clams. Have no fear, however, I will close this discussion with a return to feeding in Tridacna and Hippopus, but their cases are easier to understand if one knows about the methodology of clam feeding in general.

Some Necessary Anatomy:

clam diagram

Figure 1. This is a diagram of an animal that may have looked something like the ancestral mollusk that gave rise to both bivalves and snails. Note the mantle cavity or gill chamber under the shell in the rear of the animal. The incurrent, or inhalant, and excurrent, or exhalant, water current paths are shown. Note the shape of the gill with water flowing between the filaments.

The ancestral mollusk was probably a small creeping limpet-like creature called a Monoplacophoran. They had a single shell on the top of the animal, a broad foot that they crept upon, and gills along each side of, or behind the foot. This is a condition seen in modern Monoplacophorans such as are found in species of Neopilina, and Vema. This condition is also similar to what occurs in chitons, except that the primitive mollusk had only one shell as opposed to the eight shells found in the chitons.

These primitive mollusks were small, and breathed by pushing water through the gills with hairlike, microscopic, cellular processes called cilia. Seen from the top, a molluscan gill looks somewhat like a feather, with a central axis with flaps or filaments off each side. It functions rather like the respiratory equivalent of a radiator or heat exchanger; water is pumped between the flaps or filaments due to the covering of tiny cilia on the gill’s surface. Gases are exchanged as the blood is pumped through the thin gills; carbon dioxide leaves the blood and oxygen enters.

Breathing water is a messy process; along with the water being pushed through the gills, come all sorts of trouble in the form of small particles. These particles consist of bacteria, sediments, detritus, particulate organic material and, of course, various types of plankton. Particulate material fouls the gills, and if allowed to remain on them could cause diseases. Gill tissues are thin and delicate and, of course, absolutely necessary for the survival of the animal. Consequently, in all aquatic animals, they are continually cleansed. In mollusks cleaning consists of bathing the gills in a continual flood of mucus. As the mucus flows over the gills any small particles impacting in it are carried down to the bottom of the gills, or through specialized rejection grooves, and dropped off. Gills similar to this basic gill described above are still found in most major molluscan groups.

Figure 2. The basic mollusk gill structures are indicated on this composite diagram. In both parts of the diagram water flow is shown to the left and particle paths to the right. In the real gills, both of these processes occur on both sides of the gill. The left diagram shows a portion of a basic gill as it might appear if all surrounding tissue were removed. It is suspended in the mantle cavity by tissue connecting to the central axis from above. There are flaps or filaments on either side and water flows around them as shown by the yellow arrows. Particles are intercepted by the cilia covering the gills and moved off the gill as shown in red (and please note, that not all the rejection paths are shown; each filament will have them). In the right drawing, the gill has been cut across and internal structures are shown. Blood flow is indicated on the left with oxygenated blood shown as blue (the respiratory pigment of mollusks is hemocyanin, not hemoglobin, and is bright blue when oxygenated). A couple of particle paths are indicated as well.

It is not hard to see how such a gill might change from being a respiratory organ, to an organ that functions both in respiration and food capture. Really, all that would be necessary to turn the basic molluscan gill into a feeding organ would be a way to sort the food particles captured on the gill into useful food and rejected particles, and a means of transferring the food to the mouth. Such sorting mechanisms, based on food density and particle size have developed several times in the mollusks, and while there are several different groups of snails that feed using their gills, the best examples of such feeding are seen in the bivalves.

Bivalves are, by-and-large, animals dedicated to the filtering of large volumes of water. The small dorsal shell of primitive mollusks has been replaced by a pair of large shells, one on each side of the animal connected by a small hinge located in the center of the animal’s upper surface. These shells grow down around both sides of the bivalve and enclose a hydrodynamic volume where the animal directs the water flow for feeding. The shells are lined internally with a tissue called the mantle, and the internal cavity is, therefore, called the mantle cavity. The mantles, and sometimes the shells, are often elaborated out into long tubular processes called siphons, which can extend some distance into the water. Not all clams have long siphons, but some of those that do can live deeply buried in sediments and still possess a pipeline to some feeding water above the sediment surface.

Tridacna

Figure 3. Basic clam anatomy is indicated by the small diagram to the lower left. The relatively large gills are evident. The water flow paths are indicated by blue and the paths of captured phytoplankton are shown in tan. The larger diagram of Tridacna derasa, on the half shell, shows the large and evident food grooves found on the gills. Note how the anatomy of Tridacna relates to that of normal clam; the body has rotated almost to the point where the animal is inverted relative to the standard clam.

A normal clam’s body is generally suspended from the upper part of the shells, directly under below the hinge, and the large gill flaps hang down into the mantle cavity much in the manner of pleated curtains, and function, much as fishing nets or seines. Water is sucked into the mantle cavity from the rear and passes forward under, and then up through the gills, generally passing from the inside toward the outside and exiting the gills up near the body near the anus. From there the water is passed out of the body via the excurrent siphon.

In most bivalves the gill filaments have become very large, and although they still function in respiration, they are far larger than is necessary for gas exchange given the mass of respiring tissue. Additionally, in many clams, these filaments actually fuse to form sheets of tissue that appear pleated and are regularly perforated so that they function as a net or sieve. The ciliary tracts on their surfaces have expanded and become specialized, depending on their locations. Some of these cilia move sheets of mucus over the surface of the gills. Other, often longer, cilia move particles toward and into various "food or rejection grooves."

These food grooves sort the particles by density and size. Acceptable foods, such as phytoplankton, are small and light. The food is transferred to grooves going into the mouth, while rejected particles, often sediment particles which are heavy or larger, are shunted into grooves leading to the bottom edge of the gills. Here they are wrapped up in mucus to form a small mucoid blob and dropped off the edge of the gill as a piece of "pseudofeces." Pseudo- or "false" - feces look much like the real thing and accumulate with actual feces in the bottom of the clam’s mantle cavity. Periodically the clam claps its shells together blowing both the feces and the pseudofeces out of the excurrent siphon.

IT IS VERY IMPORTANT TO REALIZE THAT THE SORTING OF PARTICULATE MATERIAL INTO EATEN FOOD AND REJECTED PSEUDOFECES HAS ABSOLUTELY NOTHING TO DO WITH THE QUALITY OF THE PARTICLE AS FOOD. RATHER IT HAS EVERYTHING TO DO WITH THE SIZE AND DENSITY OF THE PARTICLE.

Perfectly nutritious material will be rejected if it is the wrong size, density or, in few cases, shape. Bivalves have been feeding on phytoplankton for the proverbial aeons and natural selection has worked to optimize their passive sorting mechanism to keep as much of the appropriate sized particles as is possible. In the real ocean, planktonic food is not in short supply, so losing a few odd sized food particles is a small price to pay to optimize the processing of the rest of the food.

AS FAR AS CLAM FOOD IS CONCERNED, SIZE AND DENSITY ARE EVERYTHING. THE RIGHT SIZE AND DENSITY ARE NATURALLY FOUND ONLY IN PHYTOPLANKTON.

Capturing the food particle is just the first step in the feeding process. The food then has to be digested and the nutrients assimilated. This means that not only must the eaten food be of the right size and consistency, it means that the animal must possess the appropriate structures and enzymes to digest it. This latter capability is very important; plant and algal material has a much different chemical composition from animal flesh. For an organism to be an effective phytoplankton feeder it must have enzymes to break down starches, polysaccharides, and plant lipids. Additionally it needs the intracellular metabolic pathways to use such materials in building its own tissues.

The ancestral and primitive mollusks probably were, to use a bit of invertebrate zoologist jargon, microphagous feeders. In other words, they fed on tiny particles, much as do present day grazing snails such as species of Trochus or Turbo. These snails feed on small one-celled algae such as diatoms and dinoflagellates. Such little algae are different from the phytoplankton filtered out of the water only in their choice of substrate. In essence, animals feeding on such surface-living algae are "pre-designed" to feed on plankton. Trochid snails, as did the ancestral mollusks before them, dislodge and sweep such little particles into the mouth with their radulae, where the particles are wrapped in a mucous strand and moved to the stomach.

Bivalve Stomach

Figure 4. This is a diagram of the basic bivalve stomach. The food enters from the upper left and indigestible material exits on the lower right. The rotating style rod is shown in purple. Most digestion occurs in the digestive glands located on either side of the stomach. The sorting region on the floor of the stomach separates denser materials such as sand grains, from digestible particles. The former enter the intestine and the latter are sent to the digestive glands.

The basic molluscan stomach design is shared by several groups of animals such as grazing snails, Strombid conchs, and bivalves. This stomach and digestive apparatus is very different from the stomach of vertebrates, and is absolutely perfectly designed for the processing of phytoplankton.

In clams, the phytoplankton collected from the gills is moved through the mouth and into the esophagus. Unlike the snail grazers, clams lack a head or any head-like structures to move the food into the esophagus. Instead the food simply moves into the mouth in mucous stream moved by cilia. As the food enters the mouth and passes into the esophagus more mucus is added and the strand become quite ropy.

The mucous rope containing the phytoplankton passes through the short esophagus and into the stomach. The clam stomach is ovoid, "foot-ball shaped," with several pouches or sacs found in it. One of these pouches, located more-or-less opposite the esophageal opening secretes a clear stiff rod of hardened mucus called the "crystalline style." This mucous rod contains digestive enzymes such as crystalline amylases, which break down sugars. The crystalline style may be quite large, sometimes several inches long, and looks like clear hard gelatin.

The rod lies in a groove and rotates, at speeds exceeding 500 rpm in some clams, driven by the cilia lining the groove. The mucous rope from the mouth is wound around the rod and the spinning action of the rod helps pull the food rope into the gut. As the rod spins, the free end is pressed against an abrasive patch called the "gastric shield." The pressure forcing the rod on to the shield, combined with the rotational velocity of the rod, causes the rod to be abraded away, releasing the enzymes, and grinding the particles, mucus, enzymes and style fragments into a sort of mush. In this region of the stomach, the contents are acidic and this causes the mucus to dissolve and activates the enzymes.

The phytoplankton particles, now partially digested by the amylases, are moved by ciliary pathways, on the stomach floor, to the digestive gland openings on either side of the stomach. Once inside the glands, which occupy much of the foot mass in bivalves, the food particles are engulfed and digested inside of by individual cells. The backs of these cells are bathed in blood, and nutrients are released into the blood stream to be transferred to the rest of the animal. Indigestible residues are returned by ciliary tracts to the stomach and passed down to the intestine, bound again into mucus, and eliminated through the anus.

In addition to the amazing and complex crystalline style apparatus, the bivalve stomach differs from the vertebrate stomach by having very little digestion occurring in the stomach proper. The gut further differs from the vertebrate on in that the bivalves have almost no musculature surrounding the gut. Virtually all the movement of the stomach contents is generated by the action of cilia.

The Strange Case of Tridacna.

As reef aquarists are aware, Tridacna and Hippopus clams have symbiotic zooxanthellae located in their blood, as well as in mantle and associated with their digestive glands. Probably as a result of natural selection to maximize mantle volume, and hence increase the number of zooxanthellae carried in it, tridacnids have undergone quite an extensive modification of the basic clam structures.

In essence, while the foot and byssal glands remain ventral and in contact with the substrate, most of the other organs have been displaced by differential growth, so that the shell and mantle edges, which normally face downward in clams, are positioned upward in Tridacna and Hippopus species allowing the mantle to be facing upward so that its zooxanthellae may be well illuminated.

At this point, two statements need to be made.

The production of feeding structures, such as gills and ciliary sorting tracts is metabolically costly, it uses energy that might otherwise go into reproduction or growth.

Likewise, the mucus produced for feeding is also expensive; after all, mucous is a protein/sugar combination, and these materials could be otherwise used in building more tissues, gametes, or in basic metabolism.

From these observations, some conclusions should be obvious.

All bivalves, including those containing photosynthetic algae, need a lot of energy and materials to stay alive. Clams appear to be passive animals, but actually they are burning food at a pretty good rate. Pumping water, even by ciliary action, is expensive and they pump ALOT of water. Consequently, they need a lot of energy to survive.

Natural selection acts to minimize unnecessary costs. If clams from Tridacna or Hippopus species didn’t need to feed, the feeding structures would be eliminated. There are a number of clams that live totally on the byproducts of symbiotic bacteria living on their gills. These clams are totally gutless. The fact that every Tridacna and Hippopus individual has a good and functional feeding apparatus ABSOLUTELY PROVES that they need to feed.

Indeed this is the case, researchers have found (Klumpp and Lucas, 1994; Griffiths & Klumpp, 1996) that small Tridacna, those about 10 cm (4 inches) in shell length or shorter, simply do not have enough mantle volume to hold sufficient zooxanthellae to support the metabolic needs of the clam. Only as the clams grow larger can the zooxanthellae produce enough respiratory energy, or sugars, to keep the clams alive.

Even after this period in their lives, these clams are dependant upon captured phytoplankton for much of their needs. All animal tissue is mostly protein, and to make proteins the animals must, absolutely must, have a nitrogen source. Researchers (Ambariyanto & Hoegh-Guldberg. 1999) have found the clams depend upon their feeding to provide a nitrogen source for their own protein metabolism. Although the zooxanthellae can help with protein synthesis, the clams need a nitrogen source and that source is their phytoplankton food.

How can you tell if your clams are getting enough food?

First off, they will be actively growing. Tridacna grow rapidly if well fed, and if they have enough calcium in the water. Second, you will see them actively defecating fecal mucus rope segments. If you feed DT’s Phytoplankton or some other live plankton, you may be able to look into the clams mantle cavity, through a siphon, and see the mucus covering the gills actually lightly colored with the color of the culture. Dead, off the shelf, chemically preserved phytoplankton cultures contain mostly cellular debris and the clams will reject most of this material as it is both the wrong shape and the wrong size.

Even small clams require a lot of algal food, without which their chances of survival are marginal. Many aquarists have had the experience of keeping Tridacna alive for a few months after which they mysteriously die, after seemingly "doing well." Well... they haven’t done well, they have slowly starved to death using up all their energy reserves and finally dying. All of these deaths - ALL OF THEM - could have been prevented by adequate feeding with good phytoplankton.


Cited and Useful References:

Ambariyanto and O. Hoegh-Guldberg. 1999. Net Uptake of dissolved free amino acids by the giant clam Tridacna maximua: alternative sources of energy and nitrogen. Coral Reefs. 18:91-96.

Belda, B. C. A., M. Sison, V. Silvestre, K. Villamor, V. Monje, E. D. Gomez and B. K. Baillie. 1999. Evidence for changing symbiotic algae in juvenile tridacnids. Journal of Experimental Marine Biology and Ecology. 241:207-221.

Griffiths, C. L. and D. W. Klumpp. 1996. Relationships between size, mantle area and zooxanthellae numbers in five species of giant clam (Tridacnidae). Marine Ecology Progress Series. 137:139-147.

Hoegh-Guldberg, O. 1996. Nutrient enrichment and the ultrastructure of zooxanthellae from the giant clam Tridacna maxima. Marine Biology. 125:359-363.

Kozloff, E. N. 1990. Invertebrates. Saunders College Publishing. Philadelphia. 866 pp.

Klumpp, D. W. and J. S. Lucas. 1994. Nutritional ecology of the giant clams Tridacna tevoroa and T. derasa from Tonga: influence of light on filter-feeding and photosynthesis. Marine Ecology Progress Series. 107:147-156.

Ohno, T., T. Katoh and T. Yamasu. 1995. The origin of algal-bivalve photo-symbiosis. Palaeontology (Leeds). 38:1-21.

Ralph, P. J., R. Gademann, A. W. D. Larkum and U. Schreiber. 1999. In situ underwater measurements of photosynthetic activity of coral zooxanthellae and other reef-dwelling dinoflagellate endosymbionts. Marine Ecology Progress Series. 180:139-147.