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Invertebrates · 8 species guides

Molluscs: Octopuses, Squid, Snails & Clams

Molluscs are soft-bodied invertebrates that include octopuses, squid, cuttlefish, snails, slugs, clams and giant clams; many have a shell secreted by a mantle, while cephalopods are active marine predators with complex behaviour, ink defences and camera-like eyes.

Soft body and mantle
A muscular mantle covers the body and may secrete a shell. Insects and crustaceans use hard external cuticle instead.
Cephalopod arms
Octopuses typically have eight arms; squid and cuttlefish have eight arms plus two longer tentacles.
Shell presence
Snails and clams usually show obvious shells; octopuses lack an external shell; cuttlefish have an internal cuttlebone.
Eyes
Cephalopods have complex camera eyes; many other molluscs have simpler eyespots.

Molluscs range from garden snails to open-ocean squid. The mantle, radula in many lineages, and soft body are shared; shells may be external, internal or absent. Octopus versus squid versus cuttlefish is the cephalopod confusion set. Giant clams and overfished snails show that shellfish are not inexhaustible.

Cephalopods are short-lived and highly intelligent by invertebrate standards. Many shelled species face ocean acidification because calcium carbonate becomes harder to form as seawater pH falls. Garden snails belong to the same phylum as octopuses — a fact that reframes what “mollusc” means for readers who only know seafood. This type page separates the everyday groups; species guides carry harvest and Red List detail. WARN mollusc guides linked from this page cover octopuses, squid, cuttlefish, snails and giant clams with harvest and acidification context where relevant. Seafood labels rarely explain life history; short-lived cephalopods and long-lived bivalves need different management assumptions when stocks are discussed.

Key facts

  • Molluscs include cephalopods, gastropods and bivalves.
  • Octopuses have eight arms; squid add two hunting tentacles.
  • Giant clams are among the largest bivalves and need clear tropical water.
  • Many cephalopods are short-lived and highly intelligent by invertebrate standards.
  • Overfishing and ocean acidification threaten shelled species that build calcium carbonate.
  • Garden snails are molluscs — the same phylum as octopuses.

How to recognise them

The features that separate this group in the field, in the order they are usually most useful.

Soft body and mantle
A muscular mantle covers the body and may secrete a shell. Insects and crustaceans use hard external cuticle instead.
Cephalopod arms
Octopuses typically have eight arms; squid and cuttlefish have eight arms plus two longer tentacles.
Shell presence
Snails and clams usually show obvious shells; octopuses lack an external shell; cuttlefish have an internal cuttlebone.
Eyes
Cephalopods have complex camera eyes; many other molluscs have simpler eyespots.
The confusion pairs
Octopus versus squid: body shape and tentacles. Snail versus slug: shell present versus reduced or absent.

8 molluscs guides

Search or filter by conservation status. Every guide carries sourced facts, IUCN status and FAQs.

Showing 1-8 of 8 animals

Animal group
Cuttlefish hovering above the seabed with a patterned mantle and wavy fins
Least Concern

Fish

C

Cuttlefish

Sepia officinalis

The color-changing master of camouflage

Oceans

Open guide
Giant clam on a coral reef with brightly colored, wavy mantle tissue
Vulnerable

Other animals

G

Giant Clam

Tridacna gigas

The reef-dwelling giant of the mollusks

Oceans · Oceania

Open guide
Chambered nautilus with coiled striped shell and tentacles extended above a deep reef slope
Vulnerable

Other animals

N

Nautilus

Nautilus pompilius (Linnaeus, 1758) — family Nautilidae; genus Nautilus among six living nautiloid species

The only living cephalopod with an external shell — pinhole eyes, ancient lineage, modern trade pressure.

Indo-Pacific coral reefs and slopes · Philippines, Papua New Guinea, Australia and neighbouring waters

Open guide
A common octopus on the seafloor with eight suckered arms spread outward, its mottled skin blending into surrounding rock and coral.
Varies by species

Invertebrates

O

Octopus

Octopoda

Eight arms, three hearts, blue blood, and one of the sharpest minds in the sea.

Global oceans

Open guide
Colourful nudibranch sea slug with orange cerata and blue margin gliding over a temperate reef rock
Not Evaluated as a group

Other animals

S

Sea Slug

Clade Nudibranchia — over 3,000 described species in order Nudibranchia

A shell-less mollusc that can steal chloroplasts from algae — or weapons from jellyfish relatives.

World oceans · Shallow reefs and temperate coasts worldwide

Open guide
Leopard slug with mottled brown-grey body extended on damp garden foliage at night
Group page

Other animals

S

Slug

Shell-reduced gastropods — class Gastropoda; many families and independent origins

Snails that lost the shell — repeatedly, independently, and not always completely.

Worldwide in damp habitats · Greatest diversity in temperate and tropical forests

Open guide
Garden snail with spiral shell on damp moss
Group page

Other animals

S

Snail

Class Gastropoda (shelled members) — tens of thousands of land, freshwater and marine species

A single muscular foot, a tongue with thousands of teeth, and the group with more recorded extinctions than any other.

Worldwide — land, freshwater and marine · Greatest land-snail diversity on oceanic islands and limestone hills

Open guide
A squid swimming in open water, showing its streamlined mantle, paired fins, large eye, eight arms and two longer feeding tentacles
Varies; most Least Concern / Not Evaluated

Invertebrates

S

Squid

Order Teuthida

Jet-propelled cephalopods with colour-changing skin and the largest eyes on Earth.

Atlantic Ocean · Pacific Ocean

Open guide

Conservation: what this group faces

Molluscs with shells face ocean acidification and harvesting; cephalopods face fishing pressure and bycatch. Short lifespans can mask decline until catch rates fall suddenly.

Fund invertebrate & habitat protection Current appeals

Molluscs: frequently asked questions

What is a mollusc?
A soft-bodied invertebrate, often with a mantle and shell, including snails, clams, octopuses and squid.
What is the difference between octopus and squid?
Octopuses usually have eight arms and a rounded body; squid are more torpedo-shaped with eight arms plus two tentacles and often an internal gladius.
Is a squid a fish?
No. Squid are molluscs — invertebrates related to octopuses and cuttlefish.
Do octopuses have shells?
Most living octopuses lack an external shell. Ancestors had shells; a few deep lineages retain internal remnants.
Why do shells dissolve in acidifying oceans?
Many molluscs build shells from calcium carbonate, which becomes harder to form as seawater pH falls.
Are giant clams endangered?
Several giant clam species are threatened by harvesting and habitat loss; status varies by species on the IUCN Red List.

Other types of invertebrates

All invertebrate guides Animals A–Z