The sustainable beneficial insect with two respiratory organs

Biologist Gela Preisfeld on a complex organism: the domestic woodlouse and its relatives

10,000 isopod species worldwide

They crawl in dark rooms with high humidity and are ideal humus builders in our compost heaps. There are around 10,000 species of them worldwide: Woodlice. Biologist Gela Preisfeld knows their characteristics and importance for the ecosystem and says: "Isopods actually live mainly in the sea, but they can also be found hidden in the soil in fresh water and on land. They are very adaptable and some isopods can even be found in the deep sea, where it is very cold, dark and lonely. There they are not as small as we know them, i.e. one to two centimetres, but can grow up to half a metre in size. We call this phenomenon deep-sea gigantism, when comparable species grow larger with increasing water depth than those in shallower water." Isopods form their own order, explains the biologist, which is called Isopoda , and this name already gives an indication of their appearance. "'Iso' means 'equal' and 'foot' comes from the Greek pus, genitive podos, meaning equal feet. And they have seven of them as equal pairs of walking legs, so they have fourteen legs."

Woodlice belong to the crustaceans

What surprises most people is the fact that isopods are not insects with the typical three pairs of legs, but are more highly developed crustaceans. Their body is flattened and somewhat broader, and the sides of their abdomen and back are therefore flat, so that the isopods offer little resistance when they move in the ground. Their head is also not as distinct as that of insects, but fused with part of the chest. We call this a cephalothorax (head-chest part). This is followed by the thorax (peraeon) and abdomen (abdomen or pleon). The legs are attached to the thoracic segments. "If we look at the animals, we can clearly recognise their segments with a layer of armour on top. There is a pair of legs on each segment in the thoracic region. In all bilaterally symmetrical animals, segments are repeating functional units with originally identical organs. We are also familiar with this from the annuli of earthworms or the human body: the so-called six-packs of muscles or the nervous system are also segmental. Many crustaceans also have legs, pleopods, on their abdomen, which are usually transformed into swimming legs. In isopods, however, they fulfil another important function."

49 German woodlouse species live in our native regions

Woodlice belong to the suborder Oniscidia. They emerged from the sea onto land around 160 million years ago, which means that as aquatic animals they have successfully mastered the transition to land, something that only a few species of amphipod have managed to do, explains Preisfeld. "There are 3640 species worldwide, we know this precisely because there is a register for these animals. There are 49 established species in Germany, and there are currently 10 known invasive species that have been introduced here." They live in different terrestrial habitats, but all need water to survive because their armour skin (cuticle) has no wax layer to protect them from drying out. In addition to seven different species of woodlouse, which, like armadillos, can curl up into a ball when in danger, we usually find the so-called woodlouse(Porcellio scaber) under stones, flower pots and cellars and the wall louse(Oniscus asellus) in our compost heaps.

The aquarium isopod - a relic from marine times

Woodlice can also reproduce on land. The female has a so-called brood pouch for this purpose. "You can see from this that the animals have already evolved quite far as crustaceans and have already adapted well to life on land," explains Preisfeld. "Our native crayfish, for example, normally carries its eggs and young under its belly and protects them with its legs. In the case of isopods, we are dealing with a very special adaptation. The mother animal brings the water into her birthing chamber, so to speak. This is an abdominal pouch, which we call a marsupium and is located between the legs. Interestingly, a marsupium is also found in marsupials, kangaroos and koalas. There, the one to two centimetre large young animals hatch from the birth canal into the mother's abdominal pouch, suck on a teat and stay there until they are big enough. This means that development is shifted from the uterus to the marsupium." In isopods, this abdominal pouch is only created at sexual maturity, after the second moult, and is only functionally similar to that of marsupials. The male then clings to the female and fertilises the eggs, which then develop in this abdominal pouch. "The female secretes a watery secretion and the eggs lie in it like in an aquarium. The larvae therefore develop in an aqueous environment, even though the animals live on land. This can take around a month and is a fantastic adaptation to the land habitat."

Long youth, short adult phase

"Most isopods live for two to three years, but this also depends on the species," says the biologist, "and they only become sexually mature after one to two years." An isopod sheds its skin up to 14 times before it is fully grown and eats the old carapace, as it still contains many nutrients. "Above all, they need calcium to make their carapace really strong, and nature is very economical in this respect."

Water guidance system enables them to breathe on land

Woodlice originally come from the sea and usually still breathe through their gills. As this seems quite impossible on land, the animals have developed something very special. Preisfeld explains: "They have gills in an unusual place, because they are located on the abdominal legs, the pleopods. Gills function in such a way that they absorb oxygen from the water. Simply because of the different partial pressure of oxygen in the body and in the water, oxygen is transported into the body by diffusion. With land isopods, however, you have to ask yourself where the water that wets the gills comes from. And this is where they have developed something quite fascinating, namely an ingenious water conduction system." There are small grooves on the belly and back that are designed in such a way that they can channel water and thus moisten the gills, which are protected by the folded parts of the pleopods. "But it's fascinating how this works," explains the scientist enthusiastically. "The isopod has a gland in the head area. A secretion flows out of this gland into these channels of the guidance system, which is similar to our urine. Nitrogen compounds are dissolved in the urine, which would be toxic to us in higher concentrations, so the body always tries to get rid of these nitrogen compounds, urea, uric acid or ammonia. We do this via the kidneys, bladder and urine, and the isopods allow the secretions they emit to pass through these channels. The ammonia from the secretion evaporates and the ammonia-free water then flows on to the pleopods, where the gills are located, so that they are moistened. The remaining water flows on to the anus and can be absorbed again. In this way, the animals can also live in dry environments because the secretion means that water is always available. Of course, dew drops, for example, can also flow over these channels."

Second respiratory organ

Although woodlice use their gills, our native woodlice have developed something else to adapt to land. "They have so-called tracheal lungs," explains Preisfeld, "which are also located on their hind legs, allowing them to absorb oxygen from the air. The air can enter when the woodlouse lifts its abdomen a little. The air is drawn in and when they lower it again, the used air can escape. So they have two forms of oxygen uptake."

Animal storage of pollutants

Scientists can also use woodlice to determine various pollutants that are stored in the soil because the animals store them. "Woodlice are actually used as test organisms for bioaccumulation studies. They are used to investigate the accumulation of foreign substances in the body that are absorbed via food, the air or the surrounding medium of water or soil," says the scientist. Such studies are carried out, for example, to determine the level of pollution in an area. The environmental risk can then be assessed. It is not yet known exactly why the animals store substances. In any case, they store substances in their bodies in a similar way to stockpiling, such as calcium, which they need to build up their armour, but which they only incorporate in due course. "The animals digest several times by eating their own faeces. This also results in a very intensive storage of harmful substances. They have actually developed this efficient method to store minerals, such as calcium, which are not easily accessible to them on land. Approximately 90% of all metal ions that occur in an isopod are actually stored there. That's why they are so popular as test organisms."

Woodlice, an all-round sustainable organism

Even if many people don't like the little creepy-crawlies, they have an important function in our ecosystem, the biologist concludes: "The isopods in particular are destructors (in ecology, an organism that breaks down organic substances, editor's note), i.e. they remineralise substances in the ecosystem. By breaking down plant material and fungi, some of which they digest several times, they make these substances available again for higher trophic levels. They are also food and prey for spiders, birds and hedgehogs. They should be left in the garden because they are beneficial insects. They are sustainable all round, so to speak."

Uwe Blass

Professor Dr Gela Preisfeld studied, obtained her doctorate and habilitated at Bielefeld University. After short research stays in Australia and a temporary position at the Goethe University in Frankfurt/Main, she accepted a professorship in Biology and its Didactics, Zoology at the University of Wuppertal in 2006.