learn only 3 things about them ...
shape of the shell reveals the snail's way of life.
are molluscs without shells. They are not worms.
Living snails are often MORE beautiful and interesting
than their empty shells.
knows what a snail looks like. The familiar land snails that we see,
however, are the tip of the snail iceberg. Most snails are marine!
Sea slugs are sea snails that have lost their
shells as adults. There are some land slugs too, but again, most slugs
are marine. And marine slugs and snails can be quite beautiful.
Where seen? You are almost certain
to see a snail on almost every shore even when the tide is not very
low. Snails and slugs are found on moist rocks and boulders, on mangrove
trees and other hard surfaces near the sea. They also creep among
the seagrasses, while small ones creep ON seagrasses and seaweeds.
Others plough through the sand. Yet more specialise on plants and
animals of the coral rubble area or reefs.
What are gastropods? Gastropods belong to Phylum
Mollusca. Gastropods include shelled snails as well as slugs without
shells.There are about 30,000 known species of gastropods. They are
thus the largest group of molluscs.
Features: The snails that most
of us are familiar with typically have a large muscular foot supporting
a visceral mass (the rest of the body and internal organs). In most
snails, the entire body can be retracted into a protective shell.
'Gatropoda' means 'stomach foot'. Gastropods indeed have a large obvious
foot. Most creep along on their foot, but Conch
snails can hop!
Shell facts: Many molluscs produce
a shell. A snail makes its own shell and stays in the same shell all
its life. It does not moult its shell like
a crab does. You can only remove a snail from its shell by killing
it. Shells sold as souvenirs are mostly obtained by harvesting living
snails and killing them.
A shell is made mostly of calcium carbonate and shell material is
added to both the outer edge as well as existing shell so that a shell
gets both bigger and thicker with age.The snail's shell is secreted
by a thin, specialised tissue called the mantle. The outer surface
of a shell is usually covered with a tough protein layer. Some snails
may have a layer of fine brown hair (called the periostracum) on their
shells. Pigment cells in the mantle create the beautiful colours and
patterns of the shell.
Mobile home: The shell protects
a snail from drying out as well as from predators. Shells come in
a a wide range of shapes, textures and sizes. These tell us a lot
about the way of life of the owner. Some like Nerite
snails have nearly globular shells which may help them stay on
wave-washed rocks and make it difficult for crabs to grip them. Other
rounded snails like Moon
snails burrow just beneath the sand. Some have spikes to keep
off predators or large lips to protect them as they forage for food.
Others have pointed tips to protect the siphon (long tube-like body
part). In some shells, the opening may be narrow and thickened so
crabs can't stick their fat pincers in.
Snail door: There's one problem
with the shell, there's a big hole in it! This hole is where the snail
comes out of its shell. Most snails close the shell opening with an
operculum (a hard 'trapdoor') attached to the foot. Here
is a series of diagrams showing how a snail uses its operculum to
seal off the shell opening.
The operculum may be thick and tough to prevent crabs from getting
a grip of the edge of the trapdoor and digging out the snail. In others,
the operculum is thin and flexible so that the snail can withdraw
deep within the coiling shell out of reach of crab pincers. The operculum
may also be used for more than just shutting the opening. In Conch
snails, the operculum is shaped like a dagger and used like a
pole-vault to hop along.
Slugs are gastropods that have lost their shells.
Instead of shells, these animals have developed thickened skin or
chemical and other defences.
Other gastropod features: Most
gatropods have a head with a pair of tentacles. An eye is usually
located at the base of each tentacle. Some slugs, however, have eyes
on the ends of long stalks.
Snails also usually have a siphon, a tube created out of an extension
of the mantle. The snail can stick the siphon out of the shell to
suck water in and sample the water for chemicals, for example, to
find food. In burrowing snails, the siphon is used to get water water
to breathe with. In some snails, the tip of the shell forms a notch
through which the siphon emerges. This is called the siphonal canal.
In addition to a siphon, carnivorous snails usually also have a proboscis.
This is an extendible tube which contains the radula, mouth and gullet.
The Class Gastropoda may be divided into these three subgroups
Most prosobranchs are snails with well developed
shells and breathe through gills.
There are about 18,000 known species of prosobranch,
making them the largest of the gastropod subclasses.
Most opistobranchs are slug-like with a reduced
shell or no shell at all. They include nudibranchs,
slugs. Opistobranchs have an additional pair
of tentacles called rhinophores, usually behind
the first pair of tentacles.
There are about 2,000 known species of opistobranchs.
Pulmonates can breathe atmospheric air. The gills
are reduced or lost and the mantle cavity works
like a lung. Few pulmonates, however, are marine.
Most are found in freshwater or on land and include
slugs and land snails. Marine pulmonates include
and the onch
There are about 16,000 known species of pulmonates.
Most snails (prosobranchs) have separate genders and generally practice
internal fertilisation. Most slugs are simultaneous hermaphrodites
although they may act as a male or female at any one time. When two
slugs meet, they typically exchange sperm. Most gastropods lay eggs
in a case, capsule or in gelatinous strings and masses.
Most marine gastropods undergo metamorphosis and their larvae look
nothing like their adults. In some, free-swimming larvae with a tiny
shell hatch out. Eventually, they settle down and develop into miniatures
of their parents. In others, tiny crawling snails hatch out. Here
is a fascinating photo
of a gastropod larva on Image
Quest 3-D Marine Library
Doing the Twist: In their larval
stages, gastropods undergo a process called torsion in their development
from to an adult. This process twists the body so that the anus moves
directly over the head. Other body modifications usually ensure the
gastropod doesn't dump wastes over its own head.
Scientists don't really agree on whether there is in fact any advantage
gained from torsion. Torsion is believed to be advantageous because
it may help a snail carry its shell and retract headfirst into its
shell. The rearrangement also moves the mantle cavity and gills to
the front and thus avoid these being clogged up by sediments stirred
by movement. It also locates the osphradium (the sensory patch) to
the front where it can be more effective in sensing the gastropod's
All gastropods undergo some degree of torsion during development,
and gastropods are the only creatures to do this. Some gastropods
reverse the torsion later on in their development, e.g., slugs, nudibranchs.
Here is a nice diagram explaining torsion
from the Archerd
Shell Collection website.
Role in the habitat: Snails and
slugs play their role in maintaining the natural balance as predators
or as grazers on seaweeds. They are in turn eaten by many other animals,
including land creatures such as birds. Empty shells are vital to
crabs. We should not take home empty shells from the shore as
we might be depriving a hermit crab of a home. Also, these shells
eventually break down into calcium that baby snails need to make their
Human Uses: Gastropods are among
our favourite seafood. These include the Gong
gong and other Conch
snails, Chut Chut
and other Creeper snails. Abalone is a gastropod and not a bivalve!
Wild populations of this snail is under severe pressure from over
collection. Many snails are also haplessly killed merely for their
shells, for collectors or to be made into cheap souvenirs. Cowries
are among the most sought after shells. Often, living snails are harvested
and killed for the shell trade. The living snail, however, is often
more beautiful than its empty shell.
The study of marine snails, however, has contributed a lot to our
understanding of life including human biology, as well as to developing
medical and other applications. The study
of cone snail toxins, for example, is leading to more effective
pain relief among others. Marine snails can also be bio-indicators
of marine pollution. The study of snail shells have also led
to tough ceramics and other products.
Status and threats: Sadly, many
of our beautiful and fascinating gastropods are listed among the threatened
animals of Singapore (here is a list of
threatened molluscs of Singapore). Like other marine creatures, they
are vulnerable to habitat loss due to reclamation or human activities
along the coast that pollute the water. They are also vulnerable to
trampling by careless visitors and over-collection for food and for
their shells can affect local populations.
Spider conch: a magnificent snail that hops!
Labrador, Mar 05
Large foot of Top shell
Pulau Sekudu, Jan 05
The large operculum of a Turban snail.
Sentosa, Jul 05
Some snails lay eggs in capsules
Changi, May 05
The egg capsule of the Moon snail.
Pulau Sekudu, Jul 03
The body of a moon snail can expand to be
MUCH bigger than the shell
Changi, Jun 06
live on the sea fans that they eat.
East Coast, Jun 06
these worm snails are snails!
Changi, Jul 02
beautiful Rare spined murex
our threatened gastropods.
Changi, Aug 08
Answers to some
interesting questions about gastropods:
- Tan Siong
Kiat and Henrietta P. M. Woo, 2010 Preliminary
Checklist of The Molluscs of Singapore (pdf), Raffles
Museum of Biodiversity Research, National University of Singapore.
- Tan, K. S.
& L. M. Chou, 2000. A
Guide to the Common Seashells of Singapore. Singapore
Science Centre. 160 pp.
- Edward E.
Ruppert, Richard S. Fox, Robert D. Barnes. 2004.Invertebrate
Brooks/Cole of Thomson Learning Inc., 7th Edition. pp. 963
Jan A., 2005. Biology
of the Invertebrates.
5th edition. McGraw-Hill Book Co., Singapore. 578 pp.
- Abbott, R.
Tucker, 1991. Seashells
of South East Asia.
Graham Brash, Singapore. 145 pp.
Pauline, Mike Severns and Ruth Dyerly, 2000. Periplus
Nature Guides: Tropical Seashells.
Periplus Editions. 64pp.
Neville. 2003. 2002
Sea Shells: Catalogue of Indo-Pacific Mollusca.
Neville Colemanís Underwater Geographic Pty Ltd, Australia.144pp.