Molluscs

Molluscs are a huge group of soft-bodied invertebrates, including familiar creatures like snails, slugs, octopuses, squid, clams, oysters and mussels. There are around 85,000 known species, making it the second-largest animal phylum after the arthropods. Molluscs live almost everywhere: from the deepest ocean trenches to the tops of mountains, from polar ice to tropical jungles, even high in trees and underground. Their basic body plan (a soft body, often covered by a shell, with a special muscular "foot") has been remarkably successful for over 500 million years.

  • Known speciesapprox. 85,000Second-biggest animal phylum
  • LargestColossal squidUp to 14 m, eyes the size of footballs
  • SmallestAmmoniceraMicroscopic, under 1 mm
  • SmartestOctopusUse tools, solve puzzles, recognise faces
  • Four main groupsGastropods, bivalves, cephalopods, chitons
  • Most speciesGastropodsAbout 70,000 (snails and slugs)

What makes a mollusc?

Despite their huge variety, all molluscs share a common body plan with several features.

  • Soft body: no internal skeleton.
  • Muscular foot: used for crawling, swimming, anchoring or (in squid) jet propulsion.
  • Mantle: a special layer of tissue that often secretes a shell and houses the gills.
  • Shell (in most): made of calcium carbonate, secreted by the mantle. Some molluscs have lost their shells over time (slugs, octopuses).
  • Radula (in most): a unique rasping tongue covered in tiny teeth, used to scrape food.

The main groups

  • Gastropods ("stomach-foot"): snails and slugs. Around 70,000 species, by far the biggest group. Live on land, in fresh water and in the sea.
  • Bivalves ("two-shelled"): clams, oysters, mussels, scallops. Around 9,200 species. They have two hinged shells and are usually stationary filter-feeders.
  • Cephalopods ("head-foot"): octopuses, squid, cuttlefish, nautiluses. Around 800 species. The most intelligent and most active molluscs.
  • Chitons: oval-shaped molluscs with shells made of eight overlapping plates. About 900 species, all marine.

Snails and slugs: the masters of slow

Snails and slugs are the most familiar molluscs. Both have a single muscular foot they use to glide along a slime trail of mucus. Snails carry their shells with them; slugs have lost or greatly reduced their shells. Most are herbivores or scavengers, eating leaves, algae, fungi or rotting plant material.

Snails can be extraordinarily diverse. The giant African land snail can be over 20 cm long. The Hawaiian tree snails were once famous for being some of the most beautifully coloured animals in the world (most are now sadly extinct due to invasive species). Aquatic snails include the lovely sea slugs (nudibranchs), which can be more colourful than any tropical fish.

Cephalopods: the smartest molluscs

The most intelligent molluscs, and indeed some of the most intelligent invertebrates of any kind, are the cephalopods: octopuses, squid and cuttlefish. They have:

  • Large brains for their body size, plus extra "mini-brains" in each arm.
  • Excellent eyes: octopus eyes are remarkably similar in design to vertebrate eyes, despite evolving completely independently.
  • Camouflage: can change colour and texture in less than a second, the most advanced disguise in nature.
  • Jet propulsion: shoot water out through a siphon to swim quickly.
  • Ink defence: spray clouds of black ink to confuse predators.
  • Complex behaviour: many can solve puzzles, use tools and recognise individual humans.
Fact The largest mollusc in the world is the colossal squid, found in the deep waters around Antarctica. Adults can be up to 14 metres long and weigh almost 500 kg. Their eyes are the largest of any known animal, the size of footballs, the better to see in the deep ocean dark. Only a handful of complete specimens have ever been studied, mostly recovered from the stomachs of sperm whales (which hunt them).

Octopuses: the alien intelligence

The octopus is one of the most remarkable animals on Earth. With around 300 species, octopuses range from tiny (the pygmy octopus, 2.5 cm) to giant (the Pacific octopus, 9 m arm span). They have three hearts, blue blood (based on copper rather than iron), eight arms with around 280 suckers each, and they can squeeze through any hole bigger than their beak.

What is most extraordinary is their intelligence. Octopuses have been documented:

  • Unscrewing jar lids to get at food inside.
  • Using coconut shells as portable shelters.
  • Escaping from aquariums by squeezing through tiny pipes.
  • Recognising individual human caretakers and behaving differently towards different people.
  • Playing with floating toys for no obvious reason.
  • Solving multi-step puzzles to get food.

Because their last common ancestor with us was 600 million years ago, octopus intelligence evolved completely separately from any vertebrate. They are sometimes described as "the closest thing on Earth to an alien intelligence".

Did you know? Pearls are made by oysters and other bivalves. When a tiny irritant (often a grain of sand or a parasite) gets stuck inside an oyster's shell, the oyster coats it with the same material it uses to make the shiny inner lining of its shell (called nacre or mother-of-pearl). Layer after layer of nacre builds up over years, eventually forming a smooth, round pearl. Natural pearls are very rare; most pearls today are cultivated by carefully implanting a small bead into a farmed oyster and waiting several years.
Deeper dive: why an octopus is the closest thing to an alien

If you ever wonder what intelligent life on another planet might be like, the closest you can get without leaving Earth is to look at an octopus. The octopus brain evolved from a completely different ancestor than the brains of any vertebrate. Their behaviour, biology and even their sense of self are profoundly strange.

An octopus has roughly 500 million neurons, similar to a dog. But unlike a dog, more than two-thirds of those neurons are NOT in the central brain: they are distributed across the eight arms. Each arm has its own mini-brain that can process information and act independently. A severed octopus arm can keep moving and even avoid pain for a while after being cut off. The central brain coordinates the arms but does not micromanage them.

Octopus vision is equally strange. Their eyes are excellent, capable of high-resolution colour vision, but their actual photoreceptor cells (the bits that detect light) are colour-blind. Recent research suggests that octopuses may be able to "see" colour through chemical sensors all over their skin, including on their suckers. This means an octopus may literally taste colours with its arms.

Octopuses also live short, solitary lives. Most species live only 1 to 5 years. They typically die soon after reproducing, the female starving herself while guarding her eggs. There is no parental care after hatching, and almost no chance to learn from other octopuses. Yet they still display astonishing intelligence and behaviour, which all has to be learned fresh in each generation.

Many scientists who study octopuses come away convinced they are conscious in some meaningful way. The exact nature of what an octopus "experiences" of the world remains one of the most fascinating open questions in modern biology. Whatever it is, it is almost certainly very, very different from what you or I experience.

For other invertebrate groups, see insects, arachnids and crustaceans.