Open Journal of Cell Biology, 2012, 2, ***-***
Published Online September 2012 (http://www.SciRP.org/journal/ojcb)
Micromorphology and Ultrastructure of the Foot of the
Equilateral Venus Gomphina Veneriformis
(Bivalvia: Veneridae)
Jung Jun Park1, Jung Sick Lee2, Yeon Gyu Lee3 , Jae Won Kim4*
1Pathology Division, NFRDI, Busan 619-902, Korea
2Department of Aqualife Medicine, Chonnam National University, Yeosu 550-749, Korea
3Faculty of Marine Technology, Chonnam National University, Yeosu 550-749, Korea
4*Department of Marine Life-Science, Gangwon Provincial College, Gangneung 210-804, Korea
Received ********* 2012
ABSTRACT
The shape and microscopic structure of the foot of the equilateral venus, Gomphina Veneriformis are described by light
and electron microscopy along with the substrate conditions of their habitat. The habitat sediment of G. veneriformis is
composed of sand (2 - 0.063 mm in diameter), mainly. The foot is wedge-shaped with multiple vertical furrows on the
surface. Although the foot is composed of an epithelial layer, a connective tissue layer and a muscular layer, the boun-
dary between the connective tissue and muscular layer is not clear. The epithelial layer was composed mostly of ciliated
columnar epithelia and secretory cells. Epithelial cells forming the apical region of the fold were long columnar, while
cells of the interfold were mostly short columnar. The cilia and microvilli were commonly observed on the free surface
of epithelial cells, while tight junctions of apico-lateral aspect and membrane interdigitations were found between the
epithelial cells. Secretory cells were found to contain acidic mucopolysaccharide, and were classified into two types in
accordance with the shapes and ultrastructures of secretory granules. The muscle fibers were composed of thin and
thick microfilaments, the proportions of which were 81.3% and 18.7%, respectively. It was determined that such mor-
phology and structural characteristics of the foot of G. veneriformis would present advantageous conditions for borrow-
ing into substrate and mobility.
Keywords: Foot; Gomphina Veneriformis; Microscopic Structure
1. Introduction
Bivalves are classified into attached and locomotive
types in accordance with their mobility, and into epifauna
and infauna in accordance with borrowing into sediment
[1]. Foot of attached bivalves such as mussel has the ca-
pability to attach to substrate by forming byssus thread,
while foot of locomotive bivalves such as Mercenaria
mercenaria has capabilities including locomotion and
burrowing into soft substrate [1,2].
In most infaunal bivalves, the foot is large and wedge-
shaped, being adapted for burrowing in soft substrate. It
is also laterally flattened, highly muscular and extends
nearly the entire ventral surface of the visceral mass [2].
The epithelial cells of bivalve foot expediently respond
to various environmental stimuli along with the epithelial
cells of mantle and gills [3-6].
As such, information on the structure of these cells
could be used in assessing the physiological conditions
of organisms. In addition, it is also thought that these
structural characteristics would differ in accordance with
environmental conditions, habitat and taxon [2,7-9].
Gomphina Veneriformis is an infaunal bivalve that
lives in the sandy subtidal zone within depth range of 1 -
20 m, a dominant species in the eastern coastal waters of
Korea, and important to the local commercial clam in-
dustry [10,11].
However, until now no study has examined the foot
ultrastructure of G. Veneriformis. This study reports the
morphology and ultrastructure of the foot of G. Veneri-
formis along with the substrate conditions of their habitat
and basic information provided for future studies on foot
structural changes induced by environmental factors.
2. Materials and Methods
2.1. Specimens
Twenty adult G. Veneriformis of shell length 35.0 - 40.0
mm were used in this study. The clams were collected by
*Corresponding author.

J. J. PARK ET AL.
2
diving from the shallow (2 - 3 m) subtidal zone of Ju-
moonjin (East Sea of Korea; N 37˚54′20.23″, E 128˚49′
38.95″). Specimens of the foot were prepared by subdi-
viding them into three groups, light microscopy (0.5 cm3),
scanning electron microscopy (0.5 cm3), and transmis-
sion electron microscopy (2 mm3).
2.2. Habitat Sediment Analysis
Grain size analysis of habitat sediments was performed
using 10 g samples taken from experimental sites. Prior
to grain size analysis, organic materials and carbonates
from the sample were eliminated by adding 10% hydro-
gen peroxide (H2O2) and 0.1N hydrochloric acid (HCl),
sequentially. Subsequently, the samples were put through
automatic particle size analyzer (Sedigraph 5100, Mi-
cromeritics, USA) and sieve analysis. The weight of each
coarse and fine sample is shown by percentage weight at
each section [12].
2.3. Histological analysis
2.3.1. Light Microscopy
Specimen preparation for light microscopy was per-
formed according to the methodology of Drury and Wal-
lington [13]. Specimens were fixed in aqueous Bouin’s
solution and rinsed in running water and then dehydrated
through a graded ethanol series (70% - 100%). The spec-
imens were then embedded in paraplast (McCormick,
USA), frozen and subsequently sectioned at 4 - 6 μm
thickness using a microtome (RM2235, Leica, Germany).
Specimens were stained with Mayer’s hematoxylin-0.5%
eosin (H-E), Masson’s trichrome stain, periodic acid-
Schiff solution and alcian blue (AB-PAS, pH 2.5), and
aldehyde fuchsin-alcian blue (AF-AB, pH 2.5) reaction.
Stain affinity of mucous cells were determined using the
Pantone® formula guide coated first edition 2002 (Pan-
tone Inc. USA) as standard, and its unique code was in-
dicated in parenthesis [6,14].
2.3.2. Electron Microscopy
Specimen preparation for electron microscopy was per-
formed according to the methodology of Cormack [15].
Specimens were fixed in 2.5% glutaraldehyde solution
(pH 7.2, buffered 0.1 M phosphate buffer) for 2 - 4 hrs at
4˚C and rinsed in 0.1 M phosphate buffer and then post-
fixed in 1% osmium tetroxide (OsO4) solution for 2 hrs at
4˚C. After fixation, the specimens were washed with 0.1
M phosphate buffer 4 times for 2 hrs and dehydrated
with ascending grades of ethanol. Specimens for scan-
ning electron microscope (SEM) were dried with a criti-
cal point dryer and the outer surface was coated with
gold ion particles (10 nm in thickness) on an ion sputter
(E-1010, Hitachi, Japan). Viewing of samples occurred
on the SEM (JSM-7500F, Hitachi, Japan). Specimens for
transmission electron microscope (TEM) were embedded
in Epon 812, cut at ultrathin sections (70 nm in thickness)
and placed on copper grids (200 mesh) in order to
double-stain with uranylacetate and lead citrate. Speci-
mens were examined using a TEM (JEM-1200EXII,
JEOL, Japan).
2.3.3. Image Analysis
An image analyzer (IMT, Visus, USA) was used to
quantify the proportion and thickness of thin and thick
microfilaments in the muscle fibers. Ten muscle fibers
were analyzed. The total sarcoplasm area under the TEM
image, and each of the sums of the areas of thin and thick
filaments within the sarcoplasm were measured. After
the area of, and ratio between the thin and thick filaments
in the cross-section of the sarcoplasm, was calculated,
the thickness of thin and thick filaments was computed
for the cross-section of ten muscular fibers.
3. Results
3.1. Sediment Composition of Habitat
The habitat sediment of Gomphina Veneriformis is com-
posed of sand (2 - 0.063 mm in diameter), mainly (Table 1).
3.2. Foot Morphology and Light Microscopical
Structure
The foot of G. Veneriformis is wedge-shaped with mul-
tiple vertical furrows on the surface and extends nearly
the entire ventral surface of the visceral mass (Figure 1).
Table 1. Habitat sediment composition of Gomphina Vene-
riformis.
Sediment
(Size) Gravel (≥ 2 mm)Sand (2 mm - 63 μm) Mud (63 μm >)
Composition
(%) 1.13 98.87 0.00
Figure 1. External (A) and internal morphology (B) of
Gomphina Veneriformis. Es, excurrent siphon; Is, incurrent
siphon; G, gill; F, foot; L, ligament; Lp, labial palp, M,
mantle; S, shell; T, tooth.

J. J. PARK ET AL.
3
Histological analysis revealed that it is composed of an
epithelial layer, connective tissue layer and a muscular
layer. While the epithelial layer has definitive boundary,
the boundary between the connective tissue layer and the
muscular layer is not clear (Figures 2(A) and (B)).
Foot surface was folded multiple times, while the epi-
thelial layer was mostly composed of ciliated columnar
epithelia and mucous cells. The epithelial layer thick-
ness was approximately 15 μm on the ventral side, while
a well-developed striated border was formed on the free
surface. The epithelium shape differed depending on its
position. Epithelial cells forming the apical region of the
fold were long columnar, while cells of the interfold
were mostly short columnar (Figure 2(C)).
Mucous cells were observed in both epithelial and
muscular layers, with vacuolar form in H-E and Mas-
son`s triple stains (Figures 2(A)-(C)). Distribution of
mucous cells was found to be higher in the anterior and
posterior tip than those in the ventral region (Figures
2(D) and (E)). AB-PAS (pH 2.5) reaction revealed two
types of mucous cells; where one type reacted with a red
color (496C) and the other responded to a blue color
(542C). Higher numbers of blue-colored mucous cells
were observed in the ventral region (Figure 2(D)). In the
anterior tip, mucous cells displayed a blue color due to
reaction with alcian blue (Figure 2(E)). Mucous cells
also displayed a blue color (7464C) in the results of the
AF-AB (pH 2.5) reaction (Figure 2(F)).
The connective tissue layer appeared very thin (Figure
2(F)), was loose, and composed of mainly collagen fibers
(Figure 2(A)). The muscular layer was composed of
collagen fibers and muscular fiber bundles, with presence
of hemolymph sinus. The muscular fiber bundle was dis-
tributed regularly in both horizontal and vertical direc-
tions (Figures 2(A)-(F)).
3.3. Electron Microscopical Structure
SEM observation showed that the foot surface was cov-
ered with ciliary tufts (Figure 3(A)). However, the den-
sity of cilia was not uniform across the fold area. Cilia
density was higher in the apical region of the fold com-
pared to the interfold region (Figure 3(B)).
TEM observation revealed that the epithelial layer is
composed of ciliated columnar epithelia and secretory
cells.
Ciliated columnar epithelial cells in apical region of the
fold have well-developed cilia and microvilli on the free
surface. Length of these cells was approximately 16 μm,
while the cilia length was approximately 8 μm. Tight
junctions of the apico-lateral aspect and membrane inter-
digitations were found between epithelial cells. The nuc-
leus was oval shaped and located in the middle or basal
portion of the cell (Figure 4(A)). Also, epithelial cells in
lateral and basal regions of the fold have well-developed
cilia and microvilli on the free surface. These cells have
irregular oval shaped nucleus in the basal cytoplasm and
heterochromatins with high electron density are distri-
buted near the nuclear membrane (Figures 4(B) and
(C)).
In the basal epithelial cells of the fold, tubular mito-
chondria appear clustered in the apical cytoplasm (Fig-
ure 4(C)) and have connected ciliary rootlets (Figure
4(D)). Cross section of cilia showed "9+2" microtubular
structure (Figure 4(E)).
Figure 2. Light microscopical feature of the foot of Gomphina Veneriformis. (A): Sagittal section showing the epithelial layer
(El), connective tissue layer (Ctl) and muscle layer (Ml). H-E stain. (B): Section of anterior tip. Masson’s trichrome stain. (C):
Epithelial layer, showing the simple ciliated columnar epithelium (Ec). Masson’s trichrome stain. (D): Epithelial layer of
ventral region, showing the mucous cell (Mc) of alcian blue positive. AB-PAS (pH 2.5) reaction. (E): Anterior tip, showing the
numerous mucous cell of alcian blue positive in the connective tissue layer. AB-PAS (pH 2.5). (F): Epithelial layer of ventral

J. J. PARK ET AL.
4
region, showing the mucous cell (Mc) of alcian blue positive. AF-AB (pH 2.5) reaction. C, cilia; Cfb, collagen fiber bundles;
Cml, circular muscle layer; Hc, hemocyte; Hs, hemolymph sinus; Lml, longitudinal muscle layer; Sb, striated border.
Figure 3. Scanning electron micrographs of the foot surface
of Gomphina Veneriformis. (A): Frontal view showing the
numerous folds and ciliary tuft (Ctf). (B): Cross section sho-
wing the developed ciliary tuft on epithelial layer (El). M,
mucous.
Figure 4. Transmission electron micrographs of the foot
epithelial layer of Gomphina Veneriformis. (A): Section of
long ciliated columnar epithelial cell in apical region of the
fold showing the cilia and zonular occuludens (Zo) and zo-
nular adherens (Za) in the apico-lateral cytoplasm. (B):
Ciliated columnar epithelial cell in lateral region of the fold.
(C): Section of short ciliated epithelial cell of basal region of
the fold showing the numerous mitochondria (Mt) in the
apical cytoplasm. D: Longitudinal section of cilia (C) on the
free surface of the epithelial cell. Note the basal body (Bb)
and rootlet complex connected with mitochondria. E: Cross
section of cilia showing the 9 + 2 arrangement. Cmt, central
microtubule; Mv, microvilli; N, nucleus; Pmt, peripheral
microtubule.
Secretory cells are unicellular glands and can be di-
vided into two types (A and B) depending on cell shape
and characteristics of secretory granules. Type A secre-
tory cells are circular and develop from the epithelial
layer to the connective tissue layer. The cytoplasm was
filled with secretory granules of low electron density
(Figure 5(A)). The distribution of these cells was lower
than the type B secretory cells. Type B secretory cells
exist mainly in the connective tissue and muscular layer,
exhibit a typical goblet form and have a length of ap-
proximately 8 μm. These cells have secretory granules
with granular materials, the electron density of which
was higher than those of type B secretory cells. Further-
more, numerous rough endoplasmic reticula and Golgi
complex were found in the basal cytoplasm of these cells
(Figure 5(B)).
Muscle fibers and some collagen fibers were observed
in the muscular layer. The type of muscle fiber was
mostly smooth, while tubular mitochondria and small
number of sarcoplasmic reticula were observed in the
cortical sarcoplasm (Figure 5(C)). The muscle fibers
were composed of thin and thick microfilaments. The
diameter of thin filament was approximately 5 nm (2.5 -
7.8 nm) while that of thick filament was 52 nm (34 - 81
nm) (Figures 5(C), (D)). The proportions of thin and
thick microfilaments within muscle fibers were 81.3%
and 18.7%, respectively. In the longitudinal section, some
transverse striations of high electron density were identi-
fied in the collagen fibers (Figure 5(D)).
4. Discussion
The shape of bivalve foot differ greatly depending on the
habit conditions. Foot of attached bivalves is simple and
degenerated [1]. However, the foot shape of Mercenaria
Mercenaria (Veneridae), which is a borrowing bivalve,
Figure 5. Transmission electron micrographs of the foot of
Gomphina Veneriformis. (A): Type A secretory cell. Note
the secretory granules (Sg) of low electron density with
fibrous materials. (B): Type B secretory cell. Note the se-

J. J. PARK ET AL.
5
cretory granules of high electron density with granular ma-
terials. (C): Cross section of muscle fibers showing the thin
(Tnf) and thick filaments (Tkf). (D): Longitudinal section of
collagen fibers (Cf). Mf, muscle fiber; Mt, mitochondrion;
N, nucleus; Sr, sarcoplasmic reticulum.
is in wedge shaped, has multiple vertical furrows on the
surface and contains mucous cells in the epithelial layer
[2]. In this study, the foot of Gomphina Veneriformis also
displayed similar shapes and structural characteristics. It
was determined that such characteristics would present
advantageous conditions for borrowing into substrate.
Histological analysis of bivalve foot have revealed that
it is composed of an epithelial layer, which is simple and
composed of columnar epithelial cells and secretory cells,
a connective tissue layer, which is relatively thin, and a
muscular layer, which is composed mainly of collagen
fibers and smooth muscle fibers [2]. Analysis of M. mer-
cenaria foot revealed that it is covered with simple co-
lumnar epithelia that are ciliated near the tip of the ante-
rior extremity, while the crests of folds are tall columnar
and troughs are low columnar [2].
This general structure was confirmed here, where the
foot of G. Veneriformis was shown to possess an epi-
thelial layer, connective tissue layer, and a muscular
layer. Our analysis also revealed that the foot epithelial
layer of G. Veneriformis was similar to M. Mercenaria
[2].
Ciliated columnar epithelium in bivalves has also been
reported in Solemya Reidi [16] and M. Mercenaria [2].
These ciliated cells contained numerous tubular mito-
chondria in their apical cytoplasm. The function of cilia
on free surface of the epidermis is related to discharging
of foreign materials entering the mantle cavity, in addi-
tion to moving mucous substances outside of the mantle
cavity [3,17]. In this study foot ciliated cell of G. Vene-
riformis contained numerous mitochondria in the apical
cytoplasm.
Functions of foot gland have been determined to be
habitat-specific. In the case of burrowing bivalves, it is
mainly used for borrowing into sediments, while in the
case of attached bivalves it appears to be associated with
formation of attaching apparatus. Mya Arenaria, a bur-
rowing bivalve, has two types of glands in its pedal
aperture; bacillary mucous cells and mucous goblet cells.
Bacillary mucous cells secrete glycoprotein as the prin-
cipal type, and mucous goblet cells secrete sulfated and
nonsulfated mucosubstances. Their main functions are
associated with pseudofeces formation and burrow into
sediments [8].
The characteristics of mucous secreted by the gland
cells have been reported to be species-specific [18]. Foot
mucous cells of M. Mercenaria were reported to contain
acid glycosaminoglycans rich in sulfate and carboxylate
groups [2].
AB-PAS (pH 2.5) reaction in this study confirmed that
mucous cells contained mainly acidic mucopolysaccha-
rides. This study confirmed that mucous cells contained
acidic material abundant in carboxylate group from the
results of AF-AB (pH 2.5) reactions.
G. Veneriformis, like M. Arenaria and M. Mercenaria,
is a burrowing bivalve. Therefore, mucous secreted from
their foot are deemed to function in formation of pseudo-
feces, purification of mantle cavity [6], and burrowing
into sediments along with mucous secreted from the
mantle.
In general, exocrine glands are classified into unicel-
lular and multicellular glands according to the number of
composition cells, and can be divided further into holocr-
ine glands and merocrine glands depending on their pat-
terns of secretion [19]. Based on these standards, all se-
cretory cells reported in S. Reidi [14], M. Mercenaria
(Eble, 2001) and Scapharca Broughtonii [4] were un-
icellular glands.
It was presumed that these were merocrine glands, as
cell death and cellular components were not observed in
the lumen. This study also revealed two types of secre-
tory cells in the foot of G. Veneriformis, both of which
are unicellular and merocrine glands.
The muscular cells of invertebrates can be divided into
three major classes on the basis of their striation pattern;
transversely striated, obliquely striated, or smooth mus-
cle. Invertebrate smooth muscle differs from that of ver-
tebrates, principally in the higher proportion and larger
diameter of thick myofilaments [20].
Smooth muscle cells can be categorized into four types
(A, B, C and D) in accordance with characteristics of 1)
the diameter of the thick myofilament, 2) density of an
arrangement of dense bodies, 3) the size of the cell, and 4)
other characteristics (structure of sarcoplasmic reticulum
system, mitochondria, etc.) [21].
Among theses, the diameter of thick myofilament in
C-type is 60 - 120 nm with small number of large sized
dense bodies. The size of sarcoplasmic reticulum is small
and distributed at periphery. This type of muscle cells in
bivalves can be found in the adductor of Atrina, Astarte
and Meretrix [21].
In this study, the foot muscle fiber of G. Veneriformis
is determined to be the C-type in accordance with distri-
bution of thin and thick filaments, thickness of thick fi-
laments, and size and location of sarcoplasmic reticulum
and mitochondria.
Thin and thick filaments are irregularly mixed in foot
muscle fibers of G. Veneriformis. The diameter of thin
filament was approximately 5 nm (2.5 - 7.8 nm) while
that of thick filament was 52 nm (34 - 81 nm).
Muscle fibers in the translucent part of the adductor of
Crassostrea Angulata contained thick and thinner fila-
ments [22]. The opaque portion in the adductor of
Chlamys Nobilis was composed of smooth muscle cells

J. J. PARK ET AL.
6
that contained thin and thick filaments. The thick fila-
ments were classified into two kinds; thinner and shorter
filaments, and thicker and longer ones. The thinner and
shorter filaments were about 26.5 nm in diameter and 7.5
μm in length, and the thicker and longer ones were about
42.0 nm in diameter and 13.0 μm in length, respectively
[23].
5. Acknowledgement
This research was supported by Basic Science Research
Program through the National Research Foundation of
Korea(NRF) funded by the Ministry of Education,
Science and Technology(2012-0004670).
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