What Is the Biggest Slug in the World? Giant Land Slugs

The record holder: the ash-black slug

The ash-black slug, Limax cinereoniger, belongs to the keeled slugs, the same family as the leopard slug. It lives across much of Europe, from Britain, Ireland and Scandinavia east to the Ural Mountains, but is rare or absent in the far south.

Adults vary in color from jet black to dark or pale gray and brown, with a white to pale tan stripe running along a long, prominent keel on the back. The sole of the foot is pale in the middle with dark edges, which helps distinguish it from the leopard slug. Young ash-black slugs are a toffee-brown color and look quite different from the adults.

Most adults measure 10–20 cm (4–8 in) long when crawling, but large individuals can exceed 30 cm (12 in). That is the figure usually cited for the world’s largest land slug. The ash-black slug lives mainly in old, undisturbed woodland, where it hides under logs and dead leaves and feeds mainly on fungi and algae. It is assessed as Least Concern by the IUCN and is not considered a garden pest.

The runner-up: banana slugs

Banana slugs (genus Ariolimax) live in the moist coastal forests of western North America. The best known, the Pacific banana slug (Ariolimax columbianus), ranges from southeastern Alaska to central California. It can reach about 25 cm (10 in) long when fully extended and weigh up to about 115 g (4 oz), which makes it the largest slug in North America and the second-largest land slug in the world.

Banana slugs are often bright yellow, but they can also be greenish, brown, tan or white, and some have so many black spots that they look almost black. They are important decomposers on the forest floor, eating dead leaves, fungi, animal droppings and fallen fruit. The banana slug is the mascot of the University of California, Santa Cruz, and in 2024 it was named California’s official state slug.

How big are leopard slugs and other common slugs?

Many familiar garden and woodland slugs are smaller than the giants above. Here is how the largest well-known land slugs compare:

The leopard slug’s scientific name, Limax maximus, means “biggest slug”, but it is only the second-largest member of its own genus. It is common in gardens and cellars, so it is the giant slug most people actually see. It is also famous for its unusual courtship, described in how do leopard slugs mate?

Why is it hard to measure a slug?

A slug’s body has no skeleton and no rigid shell, so its length changes all the time. A crawling slug stretches out long and thin; a resting or disturbed slug contracts into a short, thick lump that may be half its crawling length. Body size also depends on how much water the slug holds, which changes with weather and humidity.

For this reason, slug sizes are normally given for animals that are fully extended while crawling. Records based on a single exceptional animal are hard to check, and different books give slightly different maximum figures. The safest way to state the record is that the ash-black slug can exceed 30 cm (12 in), while the banana slug reaches about 25 cm.

The biggest sea slugs

If sea slugs are included, land slugs are easily beaten. The term sea slug covers many marine snails with small, internal or no shells, including sea hares and the colorful nudibranchs.

The largest is the black sea hare (Aplysia vaccaria), which lives off California and Mexico’s Baja California. The biggest recorded specimen measured about 99 cm (39 in) long when crawling and weighed nearly 14 kg (31 lb). Black sea hares eat brown seaweeds and kelp, which gives them their dark color, and they grow remarkably fast, living only about a year. They are rarely attacked by predators, partly because they are simply too large for most of them.

Sea hares are not closely related to land slugs. Shell loss has happened many times in snail evolution, both on land and in the sea, which is why “slug” describes a body shape rather than a single family. You can read more in slug vs snail.

Why do giant slugs live in cool, damp places?

The biggest land slugs all live in cool, humid forests: European woodland for the ash-black slug and the coastal rainforests of the Pacific Northwest for the banana slug. Slugs have no shell to retreat into, so they lose water through their skin and in their slime. A large body has less surface area for its volume than a small one, which slows drying out, but a big slug still needs a constant supply of moisture and shelter to stay active.

Mild, wet climates also provide what these large slugs eat: fungi, rotting wood, dead leaves and other decaying matter that is abundant on damp forest floors.

Myths and facts

Myth: The leopard slug is the biggest slug because its name means “biggest slug”. Fact: The ash-black slug can grow longer, and so can the Pacific banana slug.

Myth: Giant slugs are only found in the tropics. Fact: The largest land slugs live in temperate Europe and North America.

Myth: Sea slugs are just slugs that live in water. Fact: Sea slugs are marine snails that lost their shells independently of land slugs, and most breathe with gills rather than a lung.