molluscs text index | photo index
Phylum Mollusca > Class Bivalvia | Class Gastropoda | Class Cephalopoda
Phylum Mollusca
updated Sep 2020
if you learn only 3 things about them ...
Besides shelled snails, molluscs include slugs, octopuses, squids and cuttlefishes.
They have a tongue that is specialised to eat their food or prey.
Molluscs include some of the smartest and largest invertebrates.

While most people will find other invertebrates boring, almost no one can resist a mollusc.

Delicious! For a start, molluscs are among our favourite seafood! Some snails and clams have also attractive shells. Octopus are among the smartest invertebrates, the Giant squid is believed to be the largest invertebrate, while nudibranchs are among the most beautiful sea creatures.

Generalisations are difficult to make for this large and diverse Phylum. Major Classes under the Phylum include: Gastropoda (snails and slugs), Bivalvia (clams), Cephalopoda (octopus, squids and cuttlefishes). Not surprisingly, there isn't really such a thing as a 'typical' mollusc.
Class Gastropoda Class Bivalvia

Snails with shells


Order Nudibranchia

Order Sacoglossa

Sea hares
Order Anaspidea

Giant clam
Class Cephalopoda Class Polyplacophora

Order Octopoda

Order Teuthoidea

Order Sepioidea

Order Sepioidea


Mind-boggling Molluscs: Molluscs range from immobile oysters stuck to a rock, slow moving snails with large extenal shells, to speedy squids with light internal shells. While many may be dull, some like nudibranchs are among the most colourful marine creatures. The star of the molluscs are possibly the octopuses: the smartest of invertebrates, with some studies showing that they can learn. On the shores, molluscs may be tiny and microscopic, to enormous giant clams that can reach 40cm or more. In the deep sea, some squids can grow to monstrous size!

The Giant clam is among
the largest bivalves on our shore.
Sisters Island, Jan 04

The startling white colour of this Polka-dot nudibranch
warns that it's not good to eat.
Sentosa, Mar 05

The body of a moon snail can expand to be
MUCH bigger than the shell
Changi, Jun 06
Soft and squishy: 'Mollis' means 'soft-bodied'. Indeed, among the few features shared by all molluscs is a soft body. To protect this soft body, some snails produce a shell, while clams produce a two-part shell. Slugs that lack shells often have toxins and other defences. Others like squids and cuttlefish rely on speed instead.

Big foot:
In many molluscs, a large foot makes up much of the body. The foot has cells that produce mucous to help them move over a surface. The foot is often extensively modified in some members such as octopuses and squids.

A moon snail with a bivalve enveloped in its foot.
Changi, Aug 12

Clam shell with hole neatly drilled,
possibly by a moon snail?
Changi, Oct 10

The Scintilla clam uses its foot to
creep around or even 'jump'.
Terumbu Semakau, Jun 12
Terrible tongue: Most molluscs also have a radula; a firm ribbon-like structure made of protein-chitin, covered with sharp teeth made of chitin. The radula is usually used for feeding and can modified in amazing ways to rasp, grate, grasp, cut, stab and inject poison. For example, drills are snails that use their radula to make a hole through their hard-shelled prey. Here are diagrams of how a grazing snail uses its radula to feed on algae.

Other features: Many molluscs also have tentacles, a pair of eyes and a pair of balance organs. Most also have an osphradia, a sensory patch inside the body that is believed to process chemicals in the water flowing into the body. Most molluscs have gills. These are often inside a chamber in the body called a mantle cavity. Most also have a circulatory system and a heart, as well as kidneys.

Sometimes confused with barnacles, which are crustaceans like crabs and shrimps. Here's more on how to tell apart animals with conical shells that are found on rocks.

Mollusc babies: Most molluscs have separate genders. Although most slugs
are simultaneous hermaphrodites, that is, each animal has both male and female reproductive organs at the same time. The details of the way each reproduces is covered in the fact sheets on them.

Nerites mating with their white egg capsules.
Chek Jawa, Feb 02

The Spider conch lays bright orange egg strings.
Terumbu Hantu, Apr 12

A pair of Polka-dot nudibranchs possibly mating.
Pulau Semakau, Jul 08
Human uses: Molluscs have been exploited by humans for millenia. Uses range from food to adornments (dyes, shells, pearls) to even money (cowrie shells). Molluscs continue to play some of these roles today. In addition, some molluscs are being studied for modern medical applications. For example, the toxins of the highly venomous cone shells (Family Conidae) are being studied for applications in pain control.

Status and threats: Sadly, many of our beautiful and fascinating molluscs are listed among the threatened animals of Singapore. Like other marine creatures, molluscs 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.

Marine molluscs on Singapore shores
gastropods: text index and photo index of shelled gastropods on this site
other molluscs: text index and photo index of other gatropods and other molluscs on this site

Threatened marine molluscs of Singapore
from Davison, G.W. H. and P. K. L. Ng and Ho Hua Chew, 2008. The Singapore Red Data Book: Threatened plants and animals of Singapore.

Gastropoda: Snails with shells
  Family Architectonicidae
  Architectonica perspectiva (Clear sundial) (EN: Endangered)

  Family Cassidae
  Phalium glaucum (Grey bonnet) (EN: Endangered)

  Family Cerithiidae (Creeper snails)
  Cerithium trailli (Traill's cerith) (EN: Endangered)

  Family Conidae
  Conus consors (Singed cone) (VU: Vulnerable)
Conus textile
(Textile cone) (VU: Vulnerable)

  Family Cypraeidae (Cowries)
  Cypraea annulus (Gold-ringed cowrie) (EN: Endangered)
Cypraea arabica
(Arabian cowrie) (VU: Vulnerable)
Cypraea tigris
(Tiger cowrie) (EN: Endangered)

  Family Ellobiidae (Belongkeng snails)
  Ellobium aurismalchi (Mangrove land snail) (CR: Critically endangered)

  Family Fissurellidae
  Scutus sp. (Hoof-shield limpet)
*Scutus unguis (EN: Endangered)

  Family Haliotidae
  Haliotis asinina (EN: Endangered)
Haliotis clathrata
(EN: Endangered)
Haliotis dohrniana
(EN: Endangered)
Haliotis varia
(EN: Endangered)
Haliotis ovina
(EN: Endangered)
Haliotis planata
(EN: Endangered)

  Family Harpidae
  Harpa major (Major Harp) (EN: Endangered)

  Family Muricidae (Drills)
  Chicoreus ramosus (Ramose murex) (EN: Endangered)
Murex trapa
(Rare-spined murex) (VU: Vulnerable)

  Family Neritidae (Nerite snails)
  Clithon oualeniensis (Dubious snails) (VU: Vulnerable)
Nerita planospira
(Flat-spire nerite) (VU: Vulnerable)

  Family Olividae (Olive snails)
  Oliva sericea (Orange-mouth olive) (VU: Vulnerable)

  Family Strombidae (Conch snails)
  Lambis lambis (Spider conch) (VU: Vulnerable)
Strombus aratrum
(Dark Diana conch) (CR: Critically endangered)
Strombus urceus (Black-lipped conch) (VU: Vulnerable)

  Family Triviidae
  Trivirostra oryza (EN: Endangered)

  Family Trochidae (Top shell snails)
  Trochus niloticus (Giant top shell snail) (VU: Vulnerable)
Umbonium vestiarium
(Button shell snail) (VU: Vulnerable)

  Family Turbinidae (Turban shell snails)
  Turbo petholatus (Tapestry turban) (VU: Vulnerable)

  Family Turritellidae
  Turritella sp. (Turritella snail)
Turritella terebra
(Screw turritella) (VU: Vulnerable)

  Family Volutidae (Volutes)
  Cymbiola nobilis (Noble volute) (VU: Vulnerable)
Melo melo
(Bailer shell snail) (EN: Endangered)

Bivalvia (Clams)

  Family Clavagellidae
  Verpa penis (Watering pot shell) (NE: Presumed extinct)

  Family Pinnidae (Fan shells)
  Atrina vexillum (VU: Vulnerable)
Pinna bicolor
(VU: Vulnerable)

  Family Tridacnidae (Giant Clams)
  Tridacna squamosa (Fluted giant clams) (EN: Endangered)

Gastropoda: Slugs and others

  Family Chitonidae (Chitons)
  Acanthopleura gemmata (EN: Endangered)

  Family Onchidiidae (Onch slugs)
  Peronina alta (DD: Data deficient)

(Octopus, squid, cuttlefish)

none listed

Land snails

  Family Amphidromidae
  Amphidromus atricallosus perakensis (Green tree snail) (EN: Endangered)
Amphidromus inversus (Brown tree snail) (CR: Critically endangered)

Links References
  • 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.
  • Davison, G.W. H. and P. K. L. Ng and Ho Hua Chew, 2008. The Singapore Red Data Book: Threatened plants and animals of Singapore. Nature Society (Singapore). 285 pp.
  • Abbott, R. Tucker, 1991. Seashells of South East Asia. Graham Brash, Singapore. 145 pp.
  • Debelius, Helmut, 2001. Nudibranchs and Sea Snails: Indo-Pacific Field Guide. IKAN-Unterwasserachiv, Frankfurt. 321 pp.
  • Coleman, Neville. 2003. 2002 Sea Shells: Catalogue of Indo-Pacific Mollusca. Neville Coleman's Underwater Geographic Pty Ltd, Australia.144pp.
  • Edward E. Ruppert, Richard S. Fox, Robert D. Barnes. 2004.Invertebrate Zoology Brooks/Cole of Thomson Learning Inc., 7th Edition. pp. 963
  • Pechenik, Jan A., 2005. Biology of the Invertebrates. 5th edition. McGraw-Hill Book Co., Singapore. 578 pp.
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