Journal of Environment and Life Sciences
J Environ Life Sci. September 2016; vol. 1 (Issue 1): 71-75.
www.imedpharm.com/journals/index.php/jels
ISSN 2456-6179
Research Article
Radular ultrastructure of freshwater apple snail
Pila virens (Gastropoda: Ampullariidae)
M. Gayathri*, M. Ramasamy and N. Santhiya
P.G & Research Department of Zoology, A.V.V.M Sri Pushpam College (Autonomous), Poondi-602 023,
Thanjavur, India
*For correspondence
M.Gayathri,
P.G & Research Department of
Zoology
A.V.V.M Sri Pushpam College
(Autonomous), Poondi-602
023, Thanjavur, India.
Email: [email protected]
ABSTRACT
Objective: The knowledge of the feeding biology of animals is thus very
essential towards understanding all levels of biological organization, and
it is also an important key in understanding the evolution of animal
diversity. The basic pattern of most radula consists of several to
numerous transverse rows of radular teeth with bilateral symmetry
relative to the main axis, supported by a thin radular membrane.
Methods: Radula of all samples was studied by light and scanning
electron microscopy. Scanning Electron Microscopy is the most powerful
tool in radular study.
Results: The radular formula of the analyzed specimens of the
Ampullariidae species here studied is (Taenioglossate type) with straight
rows of teeth. Gastropods replace their radular teeth throughout their
entire life cycle. The radula is a ribbon shaped structure, located on the
floor of the buccal cavity, bearing the teeth that are arranged in a definite
pattern in 33 - 41 similar transverse rows.
Conclusions: Studies on the freshwater snail P.virens of lower anaicut
reservoir is scanty and the present study on P.virens is the first work on
Indian Ampullariidae. Gastropods replace their radular teeth throughout
their entire life cycle.
Received: 13 July 2016
Accepted: 31 July 2016
Keywords: Ampullariidae, Radula, SEM, Taenioglossate, Rachidian
of the molluscan buccal cavity is the radula, one
of the hallmarks of the phylum is a chitinous
ribbon bearing many transverse rows of firmly
fixed teeth. Each row of teeth on the radula
normally repeats precisely the number and shape
of the teeth in the rows in front of it and behind.
Introduction
The radula has been frequently investigated
because of its importance as a tool in supra
specific systematic to diagnose the species. The
radula has also been recognized as an important
morphological criterion for the taxonomic
allocation of species. Scanning Electron
Microscope (SEM) is the most powerful tool in
radular study. The most characteristic structure
© Journal of Environment and Life Sciences
The radular teeth of gastropods are adapted for a
variety of feeding methods in different species.
The recurved radular teeth are arranged on a
71
Gayathri M. et al. J Environ Life Sci. 2016; vol. 1 (1): 71-75.
membranous belt supported by the cartilaginous
odontophore. The length of the radula appears
also to be related to the quantum of work which
has to be done in feeding. The radula has been
frequently investigated because of its importance
as a tool in supraspecific systematic to diagnose
the species. The radula has also been recognized
as an important morphological criterion for the
taxonomic allocation of species. It shows
general similarities at family and generic levels
with consistent differences at the species level.
The respective similarities and differences have
been utilized to an increasing extent in the
classification of strombid gastropods as the
shape of the radula teeth is directly related to the
kind of food, which the gastropods eat, and the
way in which it is manipulated. The radular teeth
of gastropods are adapted for a variety of
feeding methods in different species.
over the dorsal surface of the buccal mass on the
floor of the buccal cavity. The ribbon bears teeth
placed regularly alongside one another
transverse rows and regularly behind one
another in longitudinal series and the number of
these and shape of the teeth differ from species
to species, though remaining fairly constant
within one species. As a consequence of this and
the fact that they are imperishable and may be
extracted from dries bodies, the radula is an
important organ from the taxonomic point of
view.6 The radula has also been recognized as an
important morphological criterion for the
taxonomic allocation of species. It shows
general similarities on family and generic levels
with consistent difference on the species level.
The radula has traditionally been used as an
important source of characters in gastropod
systematics, both for taxonomy and for
phylogenetic reconstruction. The scanning
electron microscope is now routinely used for
examining radula, and systematizes recognize
the need to assess variation in systematic
characters of this kind. As a result, there has
been a growing appreciation of the extent of
intraspecific variation in radular dentition and, in
some cases, an understanding of its causes.
The radula, odontophoral cartilage, and muscles
of the molluscan buccal mass work collectively
to provide a steady stream of finely divided food
material to the oesophagus.1 This material may
include plant tissue or a variety of animal prey
including
sessile
erect
or
encrusting
invertebrates. Although buccal architecture
varies among cephalate molluscs and may be
associated with specialized predatory habits,2-4
the fundamental elements of feeding are similar
throughout members of this group. While
grazing, the radula (a flexible, toothed
membrane) and its supporting cartilage is
protruded through the mouth, accelerated across
the substrate (or into the prey) and withdrawn.
During radular retraction, food particles carried
on the surface of the radula are passed to the
oesophagus and to the posterior alimentary
canal. Although malacologists generally agree
on most aspects of radula function, until recently
they had been divided on the precise mechanism
of this small, structurally complex, organ
system.5
Each row of teeth on the radula normally repeats
precisely the number and shape of the teeth in
the rows in front of it and behind. In it there are
usually an odd number of teeth due to the fact
that the row consists of a single tooth, centrally
placed, with a series of others on either side,
those on the right being the mirror image of
those on the left. The middle tooth is the
rachidian or central; those on either side of it are
broadly known as the laterals. These usually
diminish in size from the mid-line laterally,
some-times in an even gradation of similar 5
teeth, but more commonly, a group of teeth
nearer the rachidian(the lateral or intermediate
teeth) is distinguishable from a group lying
further away(the marginal teeth). The biggest
lateral tooth may be called the dominant.7
The most characteristic structure of the
molluscan buccal cavity is the radula, one of the
hallmarks of the phylum. It is a chitinous ribbon
continually added to at its inner end, which is
placed at the posterior end of the radular sac, a
blind diverticulum from the posterior end of the
buccal cavity, and stretching forward from there
© Journal of Environment and Life Sciences
Materials and Methods
The snails were collected from the lower
Anaicut Reservoir for the radular study.
72
Gayathri M. et al. J Environ Life Sci. 2016; vol. 1 (1): 71-75.
Specimens preserved in alcohol were used for
extraction of radulae. Shell height (maximum
dimension parallel to axis of coiling) was first
measured with calipers and the sex noted; the
radula was then dissected from the buccal mass,
uncoiled, and its length measured with an ocular
micrometer (to 1 mm). In a few cases the radular
ribbon was incomplete, so that length could not
be measured. Only adult specimens were used,
close to maximum size for the sample. Radulae
were cleaned by soaking in a bleaching solution
(1% w/v sodium hypochlorite, 8% w/v sodium
chloride) at room temperature for up to 5
minutes, followed by gentle cleaning with fine
needles and thorough rinsing in distilled water.
Each radula was mounted damp on a thin layer
of polyvinyl acetate glue on a glass coverslip,
and the outer marginal teeth quickly folded
outwards with needles before the radula had
dried completely. Specimens were coated with
gold and palladium before examination in a
scanning electron microscope. Only fully
formed and unworn teeth from the central part of
the radula (about 80% of total length) were
examined. Within this range, all teeth were
inspected to check for uniformity, but only a
small part was photographed. Photographs were
taken in three standard orientations: flat view
from vertically above (to show shapes of teeth),
at 45° from front end (to show shapes of tooth
cusps), and at 45° from side (to show relief).
The numbers of cusps on the outer marginal
teeth were counted on all visible teeth on both
left and right sides of each radula, and the modal
number noted for each side separately. Relative
radular length was calculated as actual length
divided by shell height.8
(a)
(c)
(d)
Figure 1: SEM view of P.virens radular
ribbon. (a) Scale bar=2 mm (b) Scale bar=500
µm (c) Scale bar=200 µm (d) Scale bar=200 µm
Results and Discussion
The radular formula of the analyzed specimens
of the Ampullariidae species here studied is
(Taenioglossate type) with straight rows of teeth.
The transmission electron microscope is the
ideal means to observe, with high resolution,
variations in the cusps of the central teeth and
the level of tooth wear caused by raping activity
during feeding. The radula is a ribbon shaped
structure, located on the floor of the buccal
cavity, bearing the teeth that are arranged in a
definite pattern in 33 - 41 similar transverse
rows. The radula is taenioglossate consisting of
seven teeth in the transverse row, one central
(rachidian) flanked on each side by one lateral
and two marginals. The general radular formula
is 2M+1L+1R+1L+2M. The marginals, laterals,
and to a certain extent the rhachidian overlap
and obscure from view their structures. During
preparation, individual teeth were deflected in
order to reveal and clearly observe their fine
Radulae of all samples were studied by light and
scanning electron microscopy. The extracted
radulae was cleaned in 10% KOH for 24 h,
washed with deionized water and then,
dehydrated through alcohol series. After airdrying, the radulae was mounted flat on
aluminum stubs and coated with goldpalladium.9 We examined the external
morphology of radular teeth by using a VEGA 3
LMV (TESCAN) Scanning Electron Microscope
(SEM) at Periyar Manniyammai University,
Vallam, Thanjavur.
© Journal of Environment and Life Sciences
(b)
73
Gayathri M. et al. J Environ Life Sci. 2016; vol. 1 (1): 71-75.
features. The rhachidian teeth have horizontally
elongated body from which 7 cusps project. The
anterior margin of the body is grooved. The
central cusp is the largest, triangular, spade-like,
or dagger-like in form. There are two lateral
teeth per row. Each has a base that is elongated
obliquely, with prominent ridges on the dorsal
surface. The body is basically shouldered and
typically bears five cusps: a large mesocone, a
small endocone and ectocone, a basal cusp at the
antero-medial part of the base and hidden from
view by the body, and another cusp lateral to the
ectocone. The mesocone is the largest cusp,
dagger-like in form, and has smoothly curved or
straight lateral edges terminating in a pointed
structure. There are four marginal teeth to a row.
They are smaller than the laterals. Immediately
outer to the laterals are inner marginal teeth with
narrow bases and longitudinal ridges. An inner
marginal tooth has two cusps, a small inner and
a large outer cusp. A basal lobe is present at the
base projecting posteriorly and firmly attached
to the redular membrane. The outer marginal is
essentially identical to the inner one but lacks a
basal lobe and is smaller.
In all cases, when the odontophore is at rest the
outer marginal tooth accommodates over the
inner marginal one and the latter shows a
concavity where the lateral rests projecting
towards the central or rachidian tooth. The
region that arises from these teeth is much
thinner and it always ends in two points, on the
other hand, the base is broader and is lobe or
cone-shaped, this structure could be used as an
anchorage in the radular ribbon allowing the
coordinated movement of the remaining teeth of
the same row and allowing the mechanism of
cutting and collecting food by the snail.12
Interspecific variation in the alimentary tract: In
P. ampullacea, each spade shaped rhachidian
tooth has a central cusp flanked by two pairs of
smaller lateral cusps. Aberrant rhachidian teeth
without lateral cusps were also observed in some
snails. The lateral teeth have relatively smaller
mesocones with smooth lateral edges. There are
35 - 37 rows of teeth in the radula. The
rhachidian teeth of P. mainitensis are similar to
that of P. ampullacea but the central cusps are
more triangular, resembling an arrow head. The
lateral teeth have larger dagger-like mesocone
with straight lateral edges. There are 35-36 rows
of teeth in the radula. The radular teeth of P.
vittata and P. quadrasi are similar to those of P.
conica.
The secretion activity observed around the tooth
is supposed for the addition of organic material
for tooth replacement as they wear off in
usage.10 Some previous research has indicated a
close relationship between radular form and diet
in littorinids.8,9 Examination of the teeth on the
early part of the developing radular membrane in
different species possessing wishbone teeth
shows that, despite great variability in the shape
of the fully formed teeth, the process of tooth
`maturation' is very similar.
With respect to the morphology of the stomach,
in P. ampullacea, the more anterior of the
stomach pouches (SP) is not markedly
demarcated, giving the stomach an appearance
of having only one pouch. It also has a relatively
shorter style sac. The stomach of P. mainitensis
is similar to that of P. ampullacea, although, the
two stomach pouches are distinct. The stomach
of P. vittata and P. quadrasi is similar to that of
P. conica. No difference in the morphology of
the intestine, rectum, and anal papilla was noted
between the species examined.5
The radulae of the species of the Ampullariidae
family studied under SEM have a common
pattern (taenioglossate) characterized by a
central radular tooth or rachidian tooth, a lateral
and two marginal ones (inner and outer marginal
teeth) on each side. The number of rows varies
depending on the species; they possess some
wing-like extensions and a ribbon to which rows
of teeth are attached. The central teeth, one per
row, are subsequently displayed, being
perpendicular to the sagittal plan of the
odontophore.
© Journal of Environment and Life Sciences
Conclusions
Although the available information on the
internal morphology and mainly on the radular
ultrastructure of species of this family is still
incomplete, the aim of this work focuses on the
74
Gayathri M. et al. J Environ Life Sci. 2016; vol. 1 (1): 71-75.
comparison of the radulae of P. virens is to
verify the validity of this character as a
classificatory tool in the study of the above
family taxonomy. The radula is one of the most
characteristic organs of the phylum Mollusca,
lacking only in the class Bivalvia. The radula
has been recognized as an important
morphological criterion for the taxonomic
allocation of species.
5. Paoulayan RC, Remigio EA. Notes on the
family
ampullariidae
(gastropoda:
prosobranchia) in the philippines: digestive,
circulatory,
and
excretory
systems.
Biotropia. 1993;6:1-32.
6. Fretter V, Graham A. British prosobranch
molluscs, their functional anatomy and
ecology. Ray Society, London; 1962.
7. Fretter V, Graham A. British Prosobranch
Molluscs. Scientific Medical and Technical
Publications Press, 1994: 900.
8. Reid DG, Mak Y-M. Indirect evidence for
ecophenotypic plasticityin radular dentition
of
littoraria
species
(Gastropoda:
Littorinidae). J Moll Stud. 1999;65:355–70.
9. Andrade SCS, Solferini VN. Transfer
experiment suggests environmental effects
on the radula of Littoraria flava
(Gastropoda: Littorinidae). J Molluscan
Stud. 2005;72:111-6.
10. Van der Wal P. Structural and material
design of mature mineralized radula teeth of
Patella vulgata (Gastropoda). J Ultrastruct
Mol Struct Res. 1989;102:147-61.
11. Martin SM, Negrete LHL. Radular
ultrastructure
of
South
American
Ampullariidae (Gastropoda: Prosobranchia).
Braz J Biol. 2007;67(4):721-6.
Funding:
No funding sources
Conflict of interest: None declared
References
1. Russell-Hunter WD. A Life of Invertebrates.
New York: Macmillan; 1979.
2. Carrjker MR. On the structure and function
of the proboscis in the common oyster drill,
Urosalpinx cinerea Say. J Morph.
1943;73:441-98.
3. Crampton DM. Functional anatomy of the
buccal apparatus of Onchidoris bilamellata
(Mollusca: Opisthobranchia). Trans zool
Soc Lond. 1977;34:45-86.
4. Shimek RL, Kohn AJ. Functional
morphology and evolution of the
toxoglossan
radula.
Malacologia.
1981;20:423-38.
© Journal of Environment and Life Sciences
75