Invertebrate Anatomy OnLine

Sphaerium, Eupera, Pisidium, Musculium ©

Fingernail Clams


Copyright 2001 by

Richard Fox

Lander University


            This is one of many exercises available from Invertebrate Anatomy OnLine , an Internet laboratory manual for courses in Invertebrate Zoology.   Additional exercises, a glossary, and chapters on supplies and laboratory techniques are also available at this site. Terminology and phylogeny used in these exercises correspond to usage in the Invertebrate Zoology textbook by Ruppert, Fox, and Barnes (2004).   Hyphenated figure callouts refer to figures in the textbook.   Callouts that are not hyphenated refer to figures embedded in the exercise. The glossary includes terms from this textbook as well as the laboratory exercises.  


Mollusca P, Eumollusca, Conchifera, Ganglioneura, Ancyropoda, Bivalvia C, Metabranchia sC, Eulamellibranchia SO, Veneroida O, Corbiculoidea SF, Sphaeriidae F (Fig 12-125, 12-122)


Mollusca P

            Mollusca, the second largest metazoan taxon, consists of Aplacophora, Polyplacophora, Monoplacophora, Gastropoda, Cephalopoda, Bivalvia, and Scaphopoda.   The typical mollusc has a calcareous shell, muscular foot, head with mouth and sense organs, and a visceral mass containing most of the gut, the heart, gonads, and kidney.   Dorsally the body wall is the mantle and a fold of this body wall forms and encloses that all important molluscan chamber, the mantle cavity. The mantle cavity is filled with water or air and in it are located the gill(s), anus, nephridiopore(s) and gonopore(s).   The coelom is reduced to small spaces including the pericardial cavity containing the heart and the gonocoel containing the gonad.

            The well-developed hemal system consists of the heart and vessels leading to a spacious hemocoel in which most of the viscera are located.   The kidneys are large metanephridia.   The central nervous system is cephalized and tetraneurous.   There is a tendency to concentrate ganglia in the circumenteric nerve ring from which arise four major longitudinal nerve cords.

            Molluscs may be either gonochoric or hermaphroditic.   Spiral cleavage produces a veliger larva in many taxa unless it is suppressed in favor of direct development or another larva.   Molluscs arose in the sea and most remain there but molluscs have also colonized freshwater and terrestrial habitats.


            Eumollusca, the sister taxon of Aplacophora, includes all molluscs other than aplacophorans.   The eumolluscan gut has digestive ceca which are lacking in aplacophorans, the gut is coiled, and a complex radular musculature is present.


       Conchifera, the sister taxon of Polyplacophora, includes all Recent molluscs other than aplacophorans and chitons. The conchiferan shell consists of an outer proteinaceous periostracum underlain by calcareous layers and is a single piece (although in some it may appear to be divided into two valves). The mantle margins are divided into three folds.

Bivalvia C

            Bivalvia is a large, successful, and derived taxon.   The body is laterally compressed and enclosed in a bivalve shell.   The two valves are hinged dorsally.   The the foot is large and adapted for digging in the ancestral condition.   A crystalline style is usually present but never is there a radula.   The mantle cavity is lateral and in most bivalves the gills are large and function in respiration and filter-feeding.   The head is reduced and bears no special sense organs.   The nervous system is not cephalized.   The group includes scallops, clams, shipworms, coquinas, marine and freshwater mussels, oysters, cockles, zebra mussels, and many, many more.

Metabranchia sC

            Metabranch gills are adapted for filter feeding. Water enters the mantle cavity posteriorly.

Eulamellibranchia SO

            Eulamellibranchs have gills with tissue interfilamentar connections.

Veneroida O

Shell is usually equivalve and without a nacreous layer.

Laboratory Specimens

            Sphaeriidae (= Pisidiidae) are the fingernail, pea, or pill clams.   These are small freshwater bivalves less than 25 mm in length and usually closer to 10 mm or less.   They occur world-wide and most bodies of freshwater have at least one species.   Native freshwater bivalves in North America are either unionaceans (freshwater mussels) or sphaeriids.   Sphaeriid genera are Sphaerium, Musculium, Pisidium, and Eupera. Introduced exotic freshwater clams in North America include Corbicula, the widespread and abundant Asian clam, and Dreissena, the zebra mussel.

            Sphaeriids are always hermaphroditic and engage in self-fertilization.   Development is direct, with no larva. The eggs are large and yolky and the embryos are brooded in the exhalant chambers of the gills and released as fully formed clams that can be very large, sometimes almost half the size of the adult.  

            The exercise is written for specimens preserved in 40% isopropyl alcohol. Living specimens can be used if available.   Sphaeriids are not available from commercial suppliers.   Living material should be anesthetized in 7% non-denatured ethanol.   A dissecting microscope should be used.

When preserved all molluscs contract their soft tissues and assume unnatural, shrunken shapes that are difficult to study and sphaeriids are no exception.   Because of this and their small size this study is limited to some of the most conspicuous external features.   

External Anatomy

Place the specimen in a 6-cm culture dish and add enough 40% isopropanol (if preserved) or 5 % non-denatured ethanol (if alive) to completely cover the specimen with liquid.   Handle the valves very gently as they are delicate and easily broken.   Manipulate them with microneedles and do not attempt to lift them with forceps except when you are ready to return them to their vial.            

            The valves of your specimen are probably gaped widely, far more widely than they would ever gape in life, but this is convenient because it allows easy access to the soft anatomy.   In preserved specimens the body may drop out of the shell and float free of it.   If it has not done so, tug it gently with your fine forceps to remove it from the shell. This will make study of both shell and body easier.  


            Look first at the outside of the shell (Fig 1).   It is composed of two valves held together by the proteinaceous hinge ligament (Fig 2, 12-92C).   The ligament is probably pale yellow.   The region of the valves in the vicinity of the ligament is the hinge.   The hinge is dorsal and the gape is ventral, anterior and posterior.  

            Locate the umbo (= beak) on the dorsal margin of the outside of each valve. The umbo is a conspicuous bump that is the oldest part of the valve.   It may be anterior, posterior or in the middle of the valve. Note the fine concentric growth lines, or striae, on the outside of the valve and the delicacy of the valve.  The valve grows as new calcareous secretions are added to its outer margin.   Larger and less numerous rest lines are also present on the outside of the valves. These indicate periods of slow, or no, growth.

On the inside of each valve examine the hinge area.   Arrange the two valves so you can easily compare the two hinges using 30-40X of the dissecting microscope (Fig 2, 3).   The hinge bears three sets of hinge teeth.  The hinge teeth assure proper alignment of the valves and prevent shearing when the valves are closed.  Near the center of the hinge, located ventral to the umbo, are the cardinal teeth.   These are short, pointed, and toothlike.   The left valve has two cardinal teeth but the right valve has only one (although it may be bilobed).   You now have the information necessary to distinguish the right and left valves from each other and you should do so at this time.  Bivalve molluscs are bilaterally symmetrical and the plane of symmetry divides the clam and the shell into right and left sides, each side with a valve. You may have noticed that the right and left cardinal teeth are not bilaterally symmetrical.   Hinge teeth are asymmetrical because they must interdigitate with each other when the shell closes. The single cardinal tooth of the right valve fits between the two cardinal teeth of the left valve.  

Also in the hinge, but anterior and posterior to the cardinal teeth are the lateral teeth.   These are elongate ridges that do not look very much like teeth.   The anterior lateral teeth are anterior of the cardinal teeth and the posterior lateral teeth are posterior to them.    Like the cardinal teeth, the lateral teeth are also asymmetrical. On the right valve the lateral teeth are paired so there are two anterior lateral teeth and two posterior lateral teeth.   The two teeth of each pair lie side by side and parallel to each other.   In the left valve the lateral teeth are not paired so there is only one anterior lateral tooth and one posterior lateral tooth.   Find these teeth and think about how those of the right and left valves interact with each other when the valves are adducted (closed).   The single lateral teeth of the left valve fit into the valley between the paired lateral teeth of the right valve.  

Figure 1. Left side of the fingernail clam, Sphaerium cornuta. Mussel96L.gif

Soft Anatomy

            Examine the body of the clam. It should be separate from the shell.   If it does not come out of the shell easily ask for assistance.   Use the microneedle to manipulate the body.   Avoid the use of forceps.   First orient the body so you know which is dorsal and ventral, which anterior and posterior, and which is right and left.  

The body is enclosed by the mantle which would normally lie against the inside surface of the valves.   It is composed of two lobes, one the right mantle skirt and the other the left mantle skirt (Fig 12-90).   The mantle gapes along its anterior and antero-ventral margins but posteriorly the right and left lobes are fused with each other on the midline (Fig 12-89).   This is an easy way to distinguish anterior from posterior.  

Figure 2. Hinge region of the right valve of a sphaeriid clam. Redrawn from Burch (1972). Mussel94L.gif

Figure 3. Hinge region of the left valve of a sphaeriid clam. Redrawn from Burch (1972). ,mussel95L.gif

Look at the posterior end in the median groove between the fused right and left mantle lobes.   The siphons are located in this groove.   In life the siphons of sphaeriids are as long, or longer than, the body (Fig 1) but in the contracted preserved condition they are so short they are barely visible.   If present they will appear as low elongate ridges on the posterior end of the body, in the groove.   Find the dorsal exhalant siphon (= excurrent siphon) and the ventral inhalant siphon (= incurrent siphon).    All sphaeriids have an exhalant siphon but Pisidium has no, or a much reduced, inhalant siphon.   It has a tiny, inconspicuous opening but no contracted siphon around it.  

Orient the body so you are looking at its ventral surface.   Look into the large anterior gape between the right and left mantle lobes.   The space between the two lobes is the inhalant chamber of the mantle cavity and it contains the foot and gills.   The cavity may be filled with congealed mucus that obscures your view of these structures.   Try to use your microneedles and/or a jet of water from a plastic pipet or squirt bottle to remove most of the mucus without damaging the tissue.  

The contracted foot (Fig 1) lies on the midline and is large and more or less cylindrical in its contracted condition.   It dominates the space in the mantle cavity.  In life it is extended through the gape and used to burrow in soft sediments.  Its surface is transversely ridged and its free distal end points anteriorly.  

Clams have two gills, one on each side of the foot (Fig 12-89A, 12-90).   The gills are composed of fused, side by side filaments which are easily seen if you have been successful in removing the mucus.   The filaments appear as oblique ridges. Each gill, 0r holobranch, is divided into two demibranchs, or half gills.

Look at the outside of the body from the side.   Identify the large visceral mass situated dorsally and in the center and occupying most of the volume of the body (Fig 12-90. 12-89B).   The mantle forms a skirt along its ventral margin.   Anterior and posterior to the visceral mass are the ends of the conspicuous anterior and posterior adductor muscles.   These muscles extend transversely from one valve to the other and are responsible for adducting, or closing the valves. They are usually dark brown in this end-on view.   Their white appearance in side view is due to their wrapping of connective tissue.

Look at the outer surface of the mantle skirts. The epithelium of this large surface is responsible for secreting the lamellar layer of the shell (Fig 12-91). Veneroid clams do not have nacre. Unionacean mussels, on the other hand, have a nacreous layer (= mother of pearl) for which they are harvested and exploited.   The thin free edge of the mantle skirt secretes the periostracum and the prismatic layer of the typical bivalve three-layered shell.   

Examine the mantle margin from the outside and note the numerous small white bundles of pallial muscles.   These muscles extend from the inner fold of the mantle margin to attach the mantle to the valve at the pallial line.  

            Orient the clam so its ventral surface is down, against the glass of the culture dish, and its dorsal surface is up, facing you.   The clam will probably stay in this position without assistance.   Anteriorly find the anterior adductor muscle again.   You can see its fibers extending transversely across the anterior gape and you can also see then ends of its fibers where it formerly attached to the valves. Immediately posterior to the ends of the adductor muscles are the ends of the anterior pedal retractor muscles.   They appear as small, usually brownish, circles in the mantle lobe.              A similar situation exists at the posterior end.   The large posterior adductor muscle extends transversely from valve to valve and a pair of much smaller posterior pedal retractor muscles, situated just anterior to the adductors, extend from the valve to the foot. The pedal retractor muscles pull the foot back into the mantle cavity prior in preparation for adducting the valves.

            You can see, through the dorsal mantle, the visceral mass filling most of the center of the dorsal surface (Fig 12-90).   On the right and left of the visceral mass are the dorsal edges of the gills, one on each side.   From this viewpoint the two gills look like parentheses enclosing the visceral mass.

            The ventricle of the heart is visible inside the thin-walled pericardial cavity on the posterior midline just anterior to the posterior pedal retractor muscles.   It is white and can be seen through the mantle.   The two very thin-walled atria extend from the ventricle to the right and left gills but are not readily visible.   Try pressing gently on the mantle above the heart.   This may cause particles to move in the atria.   If you can see the particles move in the atria you can get an idea of their position.  


Britton JC, Fuller SLH.   1979.   The freshwater bivalve Mollusca (Unionidae, Sphaeriidae, Corbiculidae) of the Savannah River Plant, South Carolina.   Savannah River Plant publication SRO-NERP-3.  

            Burch JB.   1972.   Freshwater sphaeriacean clams (Mollusca: Pelecypoda) of North America. .   US Env. Protect. Agency, Biota of freshwater Ecosystems Identific. Manual # 3.

Heard WH. 1968.   Mollusca, G1-G26 in F. Parrish (ed), Keys to water quality indicative organisms (Southeastern United States).   Federal Water Pollution Control Administration, Washington.  

            McMahon RF. 2001. Mollusca: Bivalvia Pp.331-429 in Thorp, J.H. & A. P. Covich (eds).    Ecology and classification of North American freshwater invertebrates. 2 nd ed.   Academic Press, San Diego. 1056 pp.

Pennak RW. 1989. Fresh-water invertebrates of the United States, 3 rd ed. Wiley, New York.

Ruppert EE, Fox RS, Barnes RB.   2004. Invertebrate Zoology, A functional evolutionary approach, 7 th ed. Brooks Cole Thomson, Belmont CA. 963 pp.    


Dissecting microscope

Living or preserved sphaeriid clams

5% non-denatured ethanol in springwater

6-cm culture dish

Microdissecting forceps and minuten nadeln