Can Woodlice Swim?

The short answer: no, woodlice cannot swim. They're land-dwelling crustaceans that evolved from marine ancestors but have lost the adaptations needed for swimming. They need moisture to survive, but none of their breathing structures is built for breathing underwater, and one that cannot climb out of water will in time drown.

This article covers why woodlice can't swim, the amphibious relatives that have re-adapted to wet environments, and what this means for keepers.

Why Woodlice Can't Swim

Despite being crustaceans (the same biological group as crabs, lobsters, and shrimp), woodlice have lost most aquatic adaptations during their evolutionary journey to terrestrial life. Specifically they lack:

• Streamlined bodies - woodlouse bodies are flattened for life in tight spaces under stones and bark, not for moving through water
• Swimming appendages - their legs are designed for walking on solid surfaces, not paddling or propulsion through water
• Functional gills - they breathe through their pleopods, the flat limbs under the tail that their marine ancestors used as gills, and many species also carry pleopodal lungs, but none of these is built for breathing underwater
• Water-tight exoskeleton - woodlice lack the waxy cuticle that would let them resist water absorption
• Salinity tolerance - most species can't handle saltwater immersion at all

The result: a woodlouse in water flounders, and one that cannot find a foothold will in time exhaust itself and drown. They're built for moist air, not for water itself.

The Moisture Distinction

Worth being precise about the moisture question. Woodlice need:

• Humid air - their pleopods only work while they stay moist
• Moist substrate - for hydration through cuticle absorption
• Damp microhabitats - for ongoing survival

What they CANNOT survive:

• Full submersion - under water they cannot take up enough oxygen to survive for long
• Waterlogged substrate - leaves them nowhere damp and airy to breathe
• Sustained immersion - one that cannot climb out of water will in time drown

The "needs moisture" requirement is often confused with "needs water". They're different - woodlice need humid air to breathe, not bodies of water to swim in.

Woodlouse Anatomy: Why They're Built for Land

The woodlouse body is optimised for terrestrial life:

Flattened Body

Suited to life in tight spaces under stones, logs, and leaf litter. Helps minimise water loss by reducing the surface area exposed to dry air, and helps them hide from predators.

Segmented Exoskeleton

Seven plates on the thorax (pereon) plus six smaller segments on the abdomen (pleon). Provides flexibility for movement and protection from predators. The exoskeleton's primary defence is against vertebrate predators (birds, mammals, amphibians) and larger invertebrates - NOT against bacteria or insects as sometimes wrongly stated.

Pleopodal Lungs

Located on the UNDERSIDE of the abdomen (on the pleopods, not on legs), in the many species that have them. These are the woodlouse equivalent of lungs - evolved from gills in their marine ancestors but adapted to extract oxygen from humid air. They appear as small white patches on the underside if you carefully turn over a live woodlouse.

Worth correcting a common AI-generated error: woodlice do NOT have "gills on their hind legs". The respiratory structures are the pleopods on the underside of the abdomen, and many species carry pleopodal lungs in them. The woodlouse anatomy guide has more on how woodlice breathe, including how the number of lung pairs varies between species.

Seven Pairs of Legs

Each attached to thoracic segments. Designed for walking, not swimming. They give woodlice their characteristic gait and let them navigate uneven terrain effectively.

Evolutionary History: From Sea to Land

Woodlice evolved from marine isopod ancestors, and their move to land is estimated to have begun roughly 300 million years ago, though the oldest woodlouse fossils are about 105 million years old. This transition involved several major adaptations:

• Breathing air through the pleopods, with pleopodal lungs evolving later in many species
• The brood pouch their marine relatives already had, turned into a closed chamber of fluid
• Improved water-conservation mechanisms
• Loss of fully aquatic locomotion
• Behavioural adaptations to humid microhabitats

Woodlice are one of the few crustacean lineages that have fully adapted to terrestrial life. The transition allowed them to exploit ecological niches unavailable to their marine relatives. For more on this transition see our article on isopods evolution from water to land.

Amphibious Relatives: The Exceptions

While most woodlice are decidedly terrestrial, some related species have re-adapted to amphibious or semi-aquatic lifestyles:

Sea Slater (Ligia oceanica)

The UK's largest native woodlouse and the most familiar amphibious species. Lives in the splash zone of rocky shores, where they're regularly wetted by sea spray but not actually submerged most of the time. They can survive brief immersion in seawater but can't survive prolonged underwater periods. See our dedicated sea slaters article for more.

Haloniscus Species

Australian semi-aquatic genus that lives in salt lakes and saline environments. Demonstrates that some isopod lineages can re-adapt to wet environments after their ancestors became terrestrial.

Cave-Dwelling Species

Species like Typhlotricholigioides aquaticus and Cantabroniscus primitivus are cave-dwelling species that have re-adapted to aquatic or semi-aquatic environments in stable cave conditions.

These amphibious species are exceptions, not the rule. The vast majority of woodlouse species (around 3,700 described globally in the suborder Oniscidea) are fully terrestrial.

How Woodlice Stay Hydrated Without Water

Woodlice need no standing water: they drink through the mouth from droplets and damp surfaces, and they also obtain moisture through:

• Humid air absorption - in very humid air some species take up water vapour at the pleopods under the tail
• Cuticle absorption - some moisture absorbed directly through the exoskeleton in humid conditions
• Food moisture - leaf litter, decaying wood, and other food provides incidental moisture
• Substrate contact - direct contact with moist substrate transfers some moisture

This is why a humid environment is critical for captive woodlice. For broader humidity guidance see our humidity for isopods article.

Waste Excretion: An Adaptation to Drought

Fascinating adaptation: woodlice expel waste primarily as ammonia gas through their cuticle rather than as liquid urine. This:

• Conserves water (no urinary water loss)
• Allows survival in drier conditions than would otherwise be possible
• Produces a faint ammonia odour around heavy concentrations of woodlice
• Distinguishes woodlice from most other crustaceans which excrete liquid waste

This is one of several adaptations that have made woodlice successful terrestrial colonisers despite their marine ancestry.

What Happens If a Woodlouse Falls in Water?

How long a woodlouse lasts varies with the species and the water, so treat any water it cannot climb out of as a risk:

• Briefly wetted - most species can survive it; they'll struggle but may right themselves on solid ground
• Trapped in water - they cannot take up enough oxygen to survive for long, and in time they drown
• Sea slaters in seawater splash - survivable, as they're adapted for splash zone
• Standard hobby species in a smooth-sided water dish - often cannot grip the surface to climb out

For captive setups, this means NEVER include water dishes or standing water in isopod enclosures. The drowning risk outweighs any humidity benefit.

Practical Implications for Keepers

Several practical points emerge from the "can't swim" reality:

No Water Dishes

The standard hobby practice - never include water dishes in isopod enclosures. They cause drownings and don't provide any benefit. Humidity is maintained through misting and substrate moisture.

Misting Approach

Mist substrate surface lightly, not soak. Aim for damp substrate, not waterlogged. Multiple light mists better than one heavy spraying.

Humidity Gradient

Most species benefit from one wetter end and one drier end of the enclosure, allowing self-regulation. Avoid creating uniformly soggy substrate throughout.

Drainage in Containers

Worth checking your enclosure design doesn't allow standing water to pool. Drainage layer at the bottom (false bottom or coarse substrate) helps prevent waterlogging.

Recovery if One Falls In

If you find a woodlouse fallen into a water source, gently remove with a soft brush or tissue. Place on dry substrate; they often recover if recovered quickly enough. Don't try to "dry" them with anything that could damage their cuticle.

In Summary

Woodlice:

• Cannot swim - definitively no
• Need humid air, not standing water
• Cannot survive long underwater, and drown in water they cannot climb out of
• Have respiratory adaptations for moist air, not water
• Have a few amphibious relatives (sea slaters etc.) but these are exceptions
• Evolved from marine ancestors but lost aquatic adaptations

This informs husbandry: maintain humidity through misting and substrate, never through water dishes. Browse our accessories collection for suitable substrate and humidity equipment, or our isopods collection for current UK stock.