Isopods: The evolution from water to land

Isopods, or woodlice, are often mistaken as insects, related to other invertebrates such as beetles and other creepy crawly bugs. This is a misconception, as woodlice are members of the class Crustacea which actually makes them crustaceans. This means isopods are more closely related to crabs, lobsters, and shrimps than insects.

Despite living on this planet before our time, woodlice have a negative reputation as pests due to wandering into our homes and gathering in areas of damp.  Woodlice are completely harmless. They don't sting, bite or transmit any disease and they don't cause damage to home furnishings or structural materials. In evolutionary history isopods are not thought to have been monstrous, destructive or poisonous either. It seems our human perception of isopods has been unfair to these historic creatures.  

But how did a crustacean end up living on land and wander into our homes and gardens?

Terrestrial isopods (or land isopods) are the only group of crustacea almost entirely composed of terrestrial forms. This means terrestrial isopods are completely independent from the aquatic environment from which they originally arose. There is no developmental stage (egg, juvenile, etc.) that requires water and all biological activities can be conducted on land, in contrast with some amphibious crustaceans. Despite how fascinating this group of isopods are, the origin and evolution is still poorly known. 

Isopods have been on the planet for longer than human beings. Isopods are members of the suborder oniscidea which are difficult to fossilize. Their fossil record alone is fragmentary and not representative of their complete evolutionary history. However, isopods first appeared in the fossil record 300 million years ago in the Carboniferous period. The first isopod fossil species were primitive and short-tailed and were originally members of the suborder Phreatoicidea, which consisted of marine organisms. At this time, isopods were already widely distributed (from Western Europe to Eastern Asia) with several species.

It is thought there are over 10,000 identified species of isopod worldwide, with around 4,500 species found in marine environments, mostly on the seabed, 500 species in freshwater, and another 5,000 species on land. There are new species being discovered every day, so these figures are likely to increase. 

The conquest from water to land is one of the major ecological events in the history of life on Earth. Life on land requires many physiological and morphological adaptations. It is highly probable that all terrestrial isopods derived from marine ancestors.

It is too simplistic to state that terrestrialization occurred as a single event in the history of a lineage. This phenomenon has likely occurred in parallel multiple times, through several evolutionary trials and may have often been unsuccessful. 

The fossil history of isopods shows a wide and interesting gradation of adaptations; morphological, physiological and behavioural. The morphological data support the hypothesis that woodlice have moved abruptly from marine to land environments without going through fresh water or a semi-terrestrial phase. For example, insects seem to have come to land via freshwater stage, whereas woodlice seem to have come directly from the marine world.

The question is why? The answer is unknown. Scientists have long hypothesised that the evolutionary trigger may have been due to an increase in predators, changes in environment i.e. tectonic plate movement, volcanic eruptions, poisonous gasses and/or decline in food sources. 

An evolutionary goal for the isopod was to adapt to absorb oxygen out of water. Like their ocean ancestors, terrestrial isopods have gills. However, gills work great in water but on land are much more of a liability. Isopods typically are unable to survive in dry lands. This is because they dry out causing their gills not to function properly leading to fatal suffocation. That’s why you usually only find them in damp areas, like under a dead log. If they start to overheat and dry out, isopods will even roll into a ball to protect the remaining moisture within their gills, as well as a means for protection.

A decline in oxygen levels may have led to the development of new adaptations for life on land such as females developing brood pouches rather than dispersing larvae in the water. 

An isopod's respiratory system is not as efficient as some other terrestrial insects. However, some species of isopods have evolved further to dry land by forming folds in the surface of their first two pairs of gills eventually turned into hollow branched structures, almost like tiny lungs.

Whilst it is unclear about the how and the why terrestrial isopods evolved, terrestrial isopods are clearly naturally great colonisers of new localities, but many species may have been transported during human migration.

Interestingly, isopods are part of human evolution. Historians have found that humans from the past kept small pouches containing woodlice around their necks. It is believed isopods were ingested when humans experienced a stomach-ache. This may have been due to the calcium carbonate found within an isopod's exoskeleton which can help to neutralise stomach acid.