Invertebrate Anatomy OnLine
Corbicula fluminea ©
Copyright 2001 by
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.
Eumollusca, Conchifera, Ganglioneura, Ancyropoda, Bivalvia C,
Veneroida O, Corbiculoidea SF,
Corbiculidae F (Fig
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
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
coelom is reduced to small spaces including the pericardial cavity containing
the heart and the gonocoel containing the gonad.
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.
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.
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.
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.
Recent molluscs are ganglioneurans, only the small taxa Aplacophora,
Polyplacophora, and Monoplacophora are excluded. Neuron cell bodies are
localized in ganglia.
mantle cavity, with its gills, is lateral. The calcareous portion of the shell
is bivalve, with the valves opening laterally and joined dorsally by a
derivative of the periostracum.
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.
gills are adapted for filter feeding. Water enters the mantle cavity
have gills with tissue interfilamentar connections.
is usually equivalve and without a nacreous layer.
fluminea is an Asian
species that was introduced to the west coast of North America around 1925. Since
that time it has spread across the continent and is present in streams, canals,
lakes, and reservoirs south of 40 ° North
latitude. Its range continues to expand and it can be collected locally for
laboratory use in many parts of the United States. It
is common in California, western Arizona, parts of Washington and Oregon,
throughout the southeast north through Kentucky and sporadically in more
northern states. It is absent from most of the Great Plains and Great Basin. Corbicula lives
in sand or gravel bottoms with the posterior third of shell exposed above the
has very short siphons and consequently must live at the sediment surface.
often abundant and population densities can reach 130,000/m 2 but
are usually much less, about 10-3000/m 2.
It is used as human food in Asia. It
is often common in reservoirs where its densities are greatest near the shore.
hermaphroditic, both simultaneous and protandric, has a benthic crawling larva
known as a pediveliger which
has made it possible for this species to spread rapidly both upstream and
downstream in any drainage to which it is introduced. Corbicula competes
with native mussel species and is thought to reduce their population densities
and may be responsible for the extinction of some species.
recommended as for the study of bivalve anatomy as an alternative to the widely
used freshwater mussels. It
is a typical eulamellibranch and in many areas it is abundant and readily
available, making live dissection feasible at low cost. Furthermore,
it is an undesirable alien species. It
is preferable to use introduced, overwhelmingly abundant exotic species for
study than to sacrifice increasingly scarce native freshwater mussels, several
of which are threatened or endangered. Conduct
the dissection under magnification in a small dissecting pan immersed in 7%
ethanol (if living) or tap water (if preserved).
Study the external features of an intact clam. The
soft parts are completely enclosed in the shell. The
shell consists of two valves (Figs
1, 2), right and left. The
two valves are held together along their dorsal margins by the hinge. The umbo is
a protuberance beside the dorsal margin
of each valve. The umbo is displaced slightly toward the anterior end
of the valve. Knowing dorsal and anterior find right,
left, ventral, and posterior.
Note the shiny brown organic periostracum covering
the outside of the valves. Bright
white calcareous layers of the shell may also be visible where the periostracum
has been eroded, especially near the umbos
" Living Corbicula,
like most bivalves, are difficult to anesthetize and open. At
their first detection of anything undesirable in their environment the animal
“clams up” and refuses to expose itself to an anesthetic. Corbicula is
much easier to open than are larger clams such as Mercenaria.
Use the following instructions to open the clam so that the
adductor muscles are cut, the right valve is removed, and the clam is left
cradled in its left valve with its right surface exposed for observation. Be
very careful that the scalpel does not slip and cut you instead of the clam. Refer
to Figure 2 or 3 to determine the location of the anterior and posterior
adductor muscles. These muscles must be cut before the clam can be opened.
Carefully slip the blade of a scalpel between the valves at
the anterior end of the clam. Do
not push toward your hand while you do this. Cut
through the anterior adductor muscle and then gently push on the scalpel handle
so that the tip of the blade is against the inside surface of the right valve. Carefully
work the blade around the ventral perimeter of the shell from anterior to
posterior so that the blade scrapes the soft tissue away from the right valve. Cut
the posterior adductor muscle. You
should not cut any of the soft tissue other than the adductor muscles. To
that end, be sure to keep the blade against the right valve.
the two adductor muscles are severed or scraped away from the right valve,
gently lift the right valve. Open
the shell slightly so you can see inside and use the scalpel to scrape gently
(not cut) all remaining soft tissue away from the right valve so it stays with
the rest of the animal in the left valve. When
all soft tissue is removed from the right valve, remove the valve and set it
complete clam is now present in its left valve with its right surface uppermost
and ready for study. The right mantle skirt may have been damaged by the scalpel
but the rest of the clam should be intact.
Place the clam in a small dissecting pan of 7% non-denatured
clam should be completely immersed in the anesthetic. The
right surface of the clam should be up. Once
in the anesthetic the clam will begin to relax. Until
that time its muscles, including those of the mantle, will be contracted. Set
the pan aside. While
you wait for the clam to relax, study the anatomy of an empty shell.
a cleaned shell. It
consists of two valves (Fig
1, 2). The umbo is
a protuberance beside the dorsal margin
of the valve. It
is often called the "beak" and is the oldest part of the valve. It
makes a good landmark for orienting the clam. It is dorsal and in most bivalves
is displaced toward the anterior end
of the valve and/or points toward the anterior end. The
plane of symmetry passes between the two valves, which are thus right and left. Place
the two valves together, orient the animal, find the plane
of symmetry and relocate the
major directions;dorsal/ventral, anterior/posterior, and right/left.
The two valves of your dried shell are probably no longer
connected to each other but in life they would be held together along their
dorsal margins by an articulation known as the hinge(Fig
2, 12-92B). The
umbos are situated beside the hinge and arch toward it and toward each other.
The hinge region possesses projections of the shell known as hinge
teeth and a pad of elastic
protein known as the hinge
ligament (Fig 2, 12-92B). The
teeth are readily visible on the inside of the hinge of each valve and the
ligament should also be visible unless it has been broken off by handling. It
is a dark brown mass of protein that becomes very brittle in dried specimens. In Corbicula it
is external and located immediately posterior to the umbo.
The typical bivalve shell consists of three layers; the outer
periostracum, middle prismatic layer (= ostracum), and the inner lamellar layer
(= hypostracum) (Fig 12-91). All three layers are secreted by the mantle
epidermis. Corbicula has
a well developed and conspicuous periostracum and lamellar layers but the
prismatic layer is reduced. Most
of the mass of the shell is the lamellar layer.
which in Corbicula is
dark olive brown or black, is the outermost layer. It
is composed of the protein conchiolin. Inside the periostracum is a chalky white prismatic
layer of calcium carbonate
crystals deposited over an organic collagenous matrix. The
periostracum, in the vicinity of the umbo especially, is often eroded in
freshwater clams. Consequently,
the underlying white calcareous layer is exposed and visible externally. In
areas where the periostracum is missing the underlying calcareous shell is
subject to erosion by acidic water so that the shell is often pitted. Innermost
is the thick, lamellar layer ,
which is also calcium carbonate and an organic matrix. It
can be seen covering the inside surface of the valves. It
is purple and white.
In some molluscs the structure of the lamellar layer is such
that its appearance is smooth and lustrous. This
type of shell is known as "mother of pearl" or nacre (pronounced NAKE ur). The
lamellar layer of freshwater “pearly” mussels (Unionida) is nacreous but that of Corbicula, and
other Veneroida, is not.
Figure 1. Exterior
of the left valve of the Asiatic clam, Corbicula
New shell material is deposited by the mantle epidermis along
the margins of the valve. Periods
of growth are marked by conspicuous concentric growth
ridges on the outer surface
of the valve (Fig 1).
Look at the inside of one of the valves and observe the
architecture of the hinge region. Corbicula is
a good species for demonstrating the basic pattern of bivalve hinge teeth. The
function of the hinge teeth is to keep the valves in alignment. The
dentition on the right and left valves differ (because they must mesh with each
other) but in Corbicula the
differences are slight so it makes no difference which valve you study.
In the center of the hinge, immediately ventral to the umbo,
are the cardinal teeth (Fig
2, 12-92B). (In
freshwater mussels the teeth in this position are known as pseudocardinal
teeth.) In Corbicula there
are three cardinal teeth in each valve and they are easily recognized because
they look like teeth.
Figure 2. Interior
of the right valve of the Asiatic clam, Corbicula
In addition to the cardinal teeth, two lateral
teeth are present in the
hinge of each valve but they don't look much like teeth. They
are low straight ridges paralleling the dorsal edge of the valve. The anterior
lateral teeth are anterior
to the cardinal teeth and the posterior
lateral teeth are posterior
to them. The
right valve has two of each, whereas the left valve has only one. Look
at the two valves and see if you understand the functional reason for this
Fit the two valves together to demonstrate how the teeth mesh
together to keep the valves in alignment when closed. Try
to shear the two valves past each other while the valves are tightly closed and
you will appreciate the effectiveness of the hinge teeth.
The teeth are located on a part of the hinge known as the dental
is typically a cavity under the dental shelf and inside the umbo known as the beak
cavity. Corbicula has
a deep beak cavity.
In life the two valves are pulled together by a pair of
adductor muscles, one anterior and one posterior. These
muscles extend transversely across the clam from one valve to the other. When
contracted they pull the valves together. This
action also stretches the hinge ligament, which is elastic. When
the adductor muscles relax the hinge ligament returns to its original shorter
length and this pulls the umbos closer together and the ventral margins of the
valves move apart. The
shell thus opens slightly along the ventral border. The
small gap thus created between the edges of the two valves is the gape. It
is just wide enough to allow the foot to slip out.
Look again at the inside surface of one of the valves. You
will see two smooth elliptical areas, one anterior and one posterior. These
are the anterior and posterior
adductor muscle scars ,
respectively, and they are the
sites of attachment of the adductor muscles.
Associated with the adductor muscle scars are scars of the
pedal retractor muscles that withdraw the foot into the shell before the valves
are adducted. These
small scars are located on the margins of the adductor scars and often coalesced
pedal retractor muscle scar is
located at about 1:00 on the circumference of the anterior adductor scar. It
is hidden by the overhang of the anterior lateral tooth. Theposterior
pedal retractor muscle scar is
located at about 11:00 on the outline of the posterior adductor scar and is
hidden by the posterior lateral tooth.
line extends from one
adductor scar to the other and parallels the ventral border of the valve. This
line marks the site of attachment of the mantle and its pallial muscles. The
mantle will be considered in more detail later.
your attention to the opened, live clam you set aside earlier. It
should be anesthetized now and unable to contract. Place the dissecting pan on
the stage of the dissecting microscope and examine the animal with low power. If
the soft anatomy is intact, you will be looking at the outside surface of the right
mantle skirt (Fig 3, 12-90). The
right mantle skirt (= mantle lobe) is penetrated by the two adductor muscles. You
cut these muscles in order to open the shell but they will still be in place
attached to the left valve. Find
the anterior adductor muscle ventral
to the anterior lateral tooth and the posterior
adductor muscle ventral to
the posterior lateral tooth (Fig 3, 12-89A).
Mantle and Mantle Cavity
In life the periphery of the right mantle skirt would be
attached to the right valve by a sheet of transparent, slightly yellowish periostracum but,
since you have removed the valve, that connection has been broken. With
magnification, look at the edge of the right mantle skirt to see the remnants of
this sheet of periostracum. It
looks like plastic wrap or cellophane. The
periostracum is secreted by the margin of the mantle and the sheet you see was,
before dissection, continuous with the periostracum covering the right valve. Lift
the right mantle skirt and find the margin of the left
mantle skirt. The
periostracum of this skirt should still be intact and connected with the shell. Examine
it with the dissecting microscope.
space between the right and left mantle skirts is the mantle
space is outside the body and is not a body cavity, even though it is largely
enclosed by the shell and mantle. In life it is filled with the water that is
the animal's environment. The
mantle cavity consists of two parts. The part you see now is its inhalant
chamber (= branchial chamber).
at the posterior edges of the right and left mantle skirts and see that they are
joined with each on the midline to form a pair of openings, the siphons (Fig
3, 8, 12-89). The
mantle tissue is thickened in the vicinity of the siphons and is pigmented. The
ventral siphon is the inhalant
siphon and it is continuous
with the inhalant chamber. It
is the larger of the two and its external opening is guarded by tentacles of
various sizes whose purpose is sensory and mechanical (to exclude large
a needle or probe to demonstrate the connection between the inhalant siphon and
the inhalant chamber.
Figure 3. The
right side of Corbicula with
the right valve and mantle skirt removed. Mussel85La.gif
dorsal opening is the exhalant
siphon and it is the smaller
of the two siphons (Fig 3, 8, 12-89). It
does not possess the array of sensory tentacles found on the inhalant siphon. It
is the outlet from the exhalant chamber of the mantle cavity dorsal to the
cannot demonstrate the continuity between the exhalant siphon and the exhalant
chamber at this time.
The mantle cavity is divided by the gills into a ventral inhalant
chamber (which you have
seen) and a dorsal exhalant chamber, or suprabranchial chamber, (which you
cannot see yet). Water
passes through the inhalant siphon to enter the inhalant chamber, flows across
the gills and then into the exhalant chamber. As
the water crosses the gills food particles are filtered from it and oxygen is
the exhalant chamber the water flows out the exhalant siphon.
fine scissors to remove the right mantle skirt. Leave
the siphons intact but cut around them to remove the rest of the right mantle.
The margin of a typical bivalve mantle skirt has three longitudinal folds, each
with a specific function (Fig 4, 12-91). The outer
fold secretes two of the
layers of the shell (periostracum and prismatic layer), the middle
fold is sensory, and theinner
fold is muscular. The
lamellar layer of the shell is secreted by the entire outer surface of the
mantle skirt, not by the folds.
Figure 4. Section
and oblique view of the shell and mantle margin of Corbicula.
Examine the margin of the left mantle skirt with the
dissecting microscope at about 12X. You
may also find it instructive to refer back to the edge of the right mantle skirt
the light and focus carefully. Find
the periostracum emerging
from near the edge of the skirt (Fig 4, 12-91). Note
that it is attached to the mantle and then extends to the inner edge of the
valve and wraps over this edge to continue over the outer surface of the valve. The
periostracum is secreted by the inner surface of the outer fold of the mantle
arises in theperiostracal groove between
the outer and middle folds.
Removal of the right mantle exposed the inhalant chamber of
the mantle cavity to view and gave you access to the structures in it. You
can now see the right gill, foot, visceral mass, and left mantle skirt (Fig 3,
left gill is hidden by the foot and visceral mass. The right
gill is a double sheet of
corrugated tissue lying on top of the foot and visceral mass (Fig 3). It
extends obliquely across the mantle cavity.
The foot is
a large semicircular mass of muscle occupying most of the ventral region of the
mantle cavity (Fig 3). The
size of the foot varies depending on its state of contraction. It
is smaller in preserved specimens.
mass is the thick globular
mass of tissue dorsal to the foot and ventral to the hinge (Fig 3, 12-89B). The
foot is attached to the ventral border of the visceral mass. The
left mantle skirt has already been identified. In
dissected specimens contractions of its muscles may pull it away from the margin
of the valve.
at the right gill. It
is a single gill, or holobranch,
even though it may appear to be two. It
is a typical eulamellibranch gill whose dual purposes are filter feeding and gas
exchange. In addition,
as is typical of many other freshwater bivalves including the freshwater
mussels, part of it serves as a brood chamber for the incubation of eggs.
The gill consists of two sheets of coalesced filaments folded
into a "W" shape (in cross section) (Fig 12-90, 12-96C,D). It
is attached to the dorsal wall of the mantle cavity by a longitudinal central
axis coinciding with the
middle point of the "W". This sheet divides the mantle cavity into the ventral
inhalant chamber and the dorsal exhalant chamber. To
get from the ventral chamber to the dorsal, water must pass through ostia
(pores) in the gills.
or whole gill, is composed of two half gills, or demibranchs.
Each demibranch is a sheet of fused filaments. Each
demibranch corresponds to one of the two "Vs" of which our "W" model is
to you is the lateral
demibranch of the right gill
and under it is the larger medial
demibranch (Fig 3). The
two are attached to each other and to the mantle along the longitudinal central
axis of the gill. They
are also attached to the body (either mantle or foot) along their borders. The
demibranchs are hollow and the space inside them is the exhalant chamber.
The surface of each side of a demibranch is a lamella. Each
demibranch has lateral and medial surfaces, and thus two lamellae. Each
holobranch thus has four lamellae. Each
demibranch is connected to the central axis by its descending
lamella and to the mantle
skirt or foot by its ascending
lamella (Fig 12-90).
The lamellae are covered by a ciliated epithelium and some of
these cilia (the lateral cilia) generate the feeding current that brings water
in through the inhalant siphon and then through the ostia into the exhalant
chamber and then out the exhalant siphon.
Bivalves feed on suspended particles too large to pass
through the ostia and thus are retained on the inhalant side of the gill. Other
cilia (the frontal cilia) are responsible for moving these particles, both
organic and mineral, over the surfaces of the lamellae and eventually to the
labial palps and mouth.
1a. If your specimen is
alive, remove it from its pan and looking at the gills, with magnification,
while they are covered by a thin film of water. Focus
on areas where light is reflected from the surface and you will see it shimmer
from the activity of the cilia. <
Bivalve gills are composed of numerous long, fused filaments. Look
as the surface of a demibranch with about 25X magnification. At
this magnification you can easily see the parallelfilaments (Fig
5, 12-96D) of which the lamella, demibranch, and holobranch are composed.
Look at the ventral edge of one of the demibranchs with high
you see the filaments of one lamella bend 180 º and
become the filaments of the opposite lamella (Fig 12-90). Along
the bend, which is the free edge of the demibranch, there is a distinct ciliated food
groove (Fig 5, 12-96B,D).
The cilia in this groove create a stream of mucus and food particles that moves
anteriorly to the labial palps and ultimately to the mouth.
you have a living specimen, place it in a dish of seawater and arrange it in the
dish so the flat surface of the exposed lamella is horizontal, or nearly so. Look
at the surface of the gill with magnification. Place
a little chalk dust or a drop of carmine-seawater suspension on the surface of
the gill while watching it with the dissecting microscope. You
should be able to see the particles moving rapidly over the gill to the ventral
food groove. The
particles, and the mucus surrounding them, are moved anteriorly by the ciliary
transport mechanism of the food groove. <
Figure 5. A
small portion of the food groove on the ventral margin of the medial demibranch
of Corbicula. Mussel87La.gif
about 2-3 mm of the edge of a demibranch, place it on a slide, tease the
filaments apart, affix a coverslip, and examine it with the compound microscope. If
your specimen is alive, the beating cilia of the filament will be easy to see. Look
for the ventral food groove at
the edge of the gill. Note
that it is a deep groove with a narrowed opening formed by the ends of the
filaments of the descending and ascending filaments. In
living specimens, the beating cilia of the filaments are easily seen. <
every 16 th filament
of the descending lamella is connected by interlamellar junctions to the same
filament on the opposite ascending lamella. The
other 15 intervening filaments are not connected in this fashion and are free to
bulge away from each other. Consequently,
the surface of the gill appears corrugated, or plicate. Each
ridge is a group of 16 filaments held together only by the 16 th filaments.
fine scissors cut along the central axis of the right gill at its posterior end,
in the vicinity of the posterior adductor muscle. This
will reveal the exhalant chamber inside
the gill. Note the
vertical water tubes extending into the demibranchs (Fig 12-98C,D). Water
from the inhalant chamber passes through ostia in the lamella to enter the water
the water tubes it moves to the exhalant chamber and exhalant siphon. With a
probe, demonstrate the connection between the exhalant chamber and the exhalant
of food and mucus are moved anteriorly along the ventral food grooves of the
demibranchs to the labial
palps at the anterior end of
the mantle cavity on either side of the mouth (Fig 3). There
is a labial palp on the right of the mouth and another pair on the left. Each
pair consists of two triangular sheets of tissue, known as palp lamellae that
resemble small gills (Fig 12-100). The
outermost is the lateral
lamella and hidden under it
is the medial lamella.
One surface of each lamella is a sorting
field of ciliated ridges and
grooves. The other surface is smooth. The ciliated surface of each lamella faces
the opposing ciliated surface of the other lamella. More
specifically, the medial surface of the lateral lamella faces the lateral
surface of the medial lamella. Food and mucus from the food groove of the gill
move onto the sorting fields where organic food particles are separated from
mineral particles. The
food moves along a ciliated oral groove to the mouth, again in a mucus string
powered by cilia. The
mineral particles, also mixed with mucus, are discarded into the inhalant
chamber as pseudofeces. Occasional
contractions of the adductor muscles compress the chamber and expel the
pseudofeces through the inhalant
siphon. Sorting on the gills and labial palps is imperfect and final sorting
occurs in the stomach.
labial lamellae of each side are connected with their counterparts on the
opposite side of the anterior visceral mass by a pair of transverse lips. The
right and left lateral lamellae are connected by the upper
lip and the right and left
medial palps are connected by the lower
lip (Fig 12-100). The
upper lip passes dorsal to the mouth and the lower lip passes ventral to it. The
food string travels in the oral groove between the upper and lower lips to reach
the clam with one hand so you can see the anterior surface of the visceral mass
and the lips with the dissecting microscope. Use
a minuten nadel to
lift the upper lip so you can see the mouth. The
mouth is a tiny, inconspicuous opening located on the midline between the two
can be difficult to demonstrate.
the two lamellae of the left labial palp as if opening a book. Place
a little carmine-seawater on the ridged surfaces of the lamellae and watch it as
it is transported by their cilia. Try
to trace currents and watch for the development of a stream of particles in the
oral groove leading into the mouth. This
is probably the easiest way to find the mouth. The
mouth is a small and inconspicuous pore but is easy to see if it has a string of
red carmine particles entering it. <
a moment or two on a superficial preliminary examination of the visceral mass
before considering it in more detail. Relocate
the visceral mass (Fig
is the large, thick mass of tissue situated immediately ventral to the hinge and
occupying the dorsal half of the valve.
The laterally compressed, muscular, cream-colored foot is
attached to the ventral edge of the visceral mass. While
its appearance is variable, it is likely to be more or less tongue-shaped with
the tongue probably pointing anteriorly.
The visceral mass is thickest dorsally and here you may see
dark lobes of the digestive
ceca through its walls. The
color of the digestive ceca depends on the color of the food and may be
yellowish, brownish, green, etc.
You may also see parts of the gray nephridium, or
cavity and the heart are
located on the dorsal median edge of the visceral mass immediately ventral to
the middle of the posterior lateral tooth (Fig 3, 8). A
lobe of the nephridium is present just posterior to the pericardial cavity. The
heart is located inside the pericardium.
Although you cannot see it now, the rectum passes through the
pericardial cavity, extends posteriorly just ventral to the posterior lateral
tooth, between the tooth and the posterior adductor muscle, and then ends at the
anus, in the exhalant chamber, on the posterior side of the posterior adductor
Most of the interior of the visceral mass, dorsal mantle
skirts, and foot are filled with the gonads. Corbicula is
hermaphroditic and there may be testis, ovary, or both present in your specimen.
These organs reach the surface of the visceral mass and parts of them are
visible without dissection (Fig 3).
Figure 6. The
eggs of Corbicula with
sperm drawn at the same scale. Mussel88La.gif
The testis is bright white and the ovary is dark gray. The ovary consists
of innumerable fairly large digitiform follicles which contain female sex cells
in various stages of development. The
follicles are easily seen on the surface of the visceral mass and in the dorsal
part of the mantle skirts. Don't
confuse the ovary with the nephridium, which is also gray and composed of
digitiform processes. The
processes of the nephridium are much smaller than those of the ovary.
The testis consists
of microscopic seminiferous tubules which contain male sex cells in various
stages of development. You
cannot see individual tubules with the dissecting microscope.
two wet mounts using small bits of ovary for one and testis for the other. Squash
the tissue in a drop of water and add a coverslip. Examine
the preparation with the compound microscope and look for gametes. Eggs
are large nucleate spheres (Fig 6). Sperm
are tiny, elongate and biflagellate (Fig 7). <
Sperm are released to the environment in large balls
(morulae), each consisting of thousands of sperm, via the exhalant siphon. These
enter the inhalant siphon of another clam where the morulae break up into
individual sperm which penetrate the gills and enter the exhalant chamber where
they fertilize eggs present there. The
eggs are then brooded in the interior water tubes of the medial demibranchs.
larvae (pediveligers) are crawlers, not swimmers, and thus are well adapted for
life in flowing water where they can move upstream or downstream along the
bottom and avoid being swept downstream as would planktonic larvae. Nor
are they dependent on fishes for dispersal as are the parasitic glochidia larvae
of freshwater mussels. Larvae are released twice yearly, once in spring and in
Figure 7. The
biflagellate sperm of Corbicula. Mussel89La.gif
Pericardial Cavity and Heart
heart and pericardial cavity are enclosed in a thin membranous pericardium ventral
to the posterior lateral tooth (Fig 3, 8). Use your fine scissors to make a
longitudinal incision through the pericardium to open the pericardial cavity. Extend
this incision posteriorly around the dorsal edge of the posterior adductor
muscle and then to the siphons. Cut
away the right side of both siphons. This
will expose the rectum, heart, and exhalant chamber (Fig 8).
the heart in
the pericardial cavity. It
consists of a muscular ventricle to which are connected two thin-walled atria,
one from each gill. The ventricle has
thick, opaque walls and is wrapped around the tubular rectum (Figs
3, 4). The
triangular, thin-walled, transparent right
atrium extends from the
dorsal edge of the right gill to the right side of the ventricle. The
atrium may have been damaged or destroyed when you opened the pericardium.
The rectum enters
the pericardial cavity anteriorly, passes through the ventricle, and then exits
the pericardial cavity posteriorly (Fig 8). It
lies immediately ventral to the posterior lateral tooth. It
crosses the posterior pedal
retractor muscle and then
passes over the dorsal surface of the posterior adductor muscle, between the
muscle and the lateral tooth, and then curves around the posterior side of the
muscle to end at the anus located
at about 7:00 on the circumference of the muscle. The
anus opens into the exhalant chamber.
Figure 8. Side
view from the right of the pericardial cavity, heart, kidney, and exhalant
chamber of Corbicula.
JC, Morton B. 1982 . A
dissection guide, field and laboratory manual for the introduced bivalve Corbicula
Rev. Supp. 3:1-82.
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.
RF. 2001. Mollusca:
Bivalvia, pp.331-430 in Thorp JH, Covich AP (eds.), Ecology and classification
of North American freshwater invertebrates, 2 nd ed.
Academic Press, San Diego. 1056pp.
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.
Small dissecting pan
Empty cleaned shell
Living or preserved Corbicula
7% non-denatured ethanol
be collected, sometimes in enormous numbers, along the shallow littoral zone of
lakes and reservoirs. They
are especially easy to collect when exposed by deliberate water level drawdown
in the winter. At
other times they can be collected by wading with a shovel and sieve or from a
boat or dock using an Ekman dredge or similar sampler. Once
collected, specimens can be held in finger bowls, but out of water, in a
refrigerator for several weeks. They
should be moistened occasionally.
be maintained indefinitely in laboratory aquaria on strained (babyfood) spinach
but they will not grow or reproduce on this diet (Britton & Morton, 1982).