We tend to think of all leaf litter as being equal, that our isopods will break it down, and will thrive on it. But leaf litter contains a lot of molecules and compounds, some of which are better for our colonies than others.
Isopods are invertebrates that rely on gut microorganisms to help digest complex plant compounds like tannins.
What are Tannins?
Tannins are a group of chemicals that are present in a lot of plant material. They're best known for their ability to bind to proteins, meaning they are used to preserve animal hides, a process known as tanning. Unless you're a fish keeper, then you probably immediately thought about blackwater habitats and botanicals.
The issue is that for most species tannins are toxic - they bind to proteins and minerals and reduce them from their diet. Equally, some animals need small amounts of tannins to prevent conditions such as bloat.
Some animals have specific enzymes to help digest or at least reduce the impact of tannins. These tannases are produced by yeasts, fungi and other microorganisms in their gut. It's likely the isopods get these from either eating the faeces of other isopods or from their diet.
There are two main groups of tannins, hydrolysing and condensing. Condensing tannins are generally too large to be absorbed into the bloodstream, although they can interfere with diet, whilst hydrolysing tannins are a wider range of molecules - some of which can make it into the bloodstream, and tend to cause more issues, especially if fed in large amounts or over a long time period.
In general, and I want to make it clear this is an extreme generalisation and more is being found out each year, condensing tannins build up in the leaves over the year whilst hydrolysing tannins reduce, so that when many species of leaves fall from the trees they are far lower in hydrolysing tannins than condensing tannins, and so safer for isopods to eat.
Cave-Dwelling, Giant, and Other Specialist Isopods
Cave species are sometimes said to handle tannins worse than surface ones, on the reasoning that a cave diet of fungi, droppings and moss carries few of them. Nobody has tested that in any isopod, and no wild diet has been published for any of the premium species the hobby keeps, so treat it as reasoning rather than a finding: offer a mixed litter and let them choose.
For most other species, they will have evolved and adapted to be able to consume tannin-rich foods, although we still should take some precautions.
Other Sources of Tannins for Terrestrial Isopods
Leaves, fruit, vegetables, some species of terrestrial algae and similar can all contain varying amounts of tannins. Fish food also often has a vegetable base, such as wheat or other plant matter, and that will contain tannins. This is fine for most species of isopod, but any species sensitive to tannins in their diet might struggle.
Improving Your Terrestrial Isopods' Diet
A study (link.springer.com/article/10.1023/A:1020962105931) fed tannic acid, a hydrolysing tannin, to Porcellio scaber and found that a little over half (55%) passed through unchanged, while the rest was broken down or oxidised in the gut, and the tannins sharply reduced the microbes in the hindgut, a change the author discusses as potentially harmful to digestion.
A further study (doi.org/10.1046/j.1365-2435.2002.00669.x) measured feeding, digestion and growth in three woodlouse species from the edge of salt marshes in the south-eastern United States, fed on oak, pine and rush litter and on diets laced with tannins and other phenolics. It concluded that fully terrestrial woodlice are better adapted to the range of phenolics in flowering-plant litter than species living at the top of the shore. An earlier study of coastal isopods found the ability to digest phenolics best developed in one of the more marine species, and its authors suggest the trait evolved in species with a phenolic-rich diet, on land or in the sea.
What this means is that in an ideal world, you'd feed leaves that come from later in the growing season. One tip is to gather the leaves in the autumn, leave them to dry out completely somewhere, and then store them for use throughout the year.
Woodlice also choose what they eat: a 2002 review of their feeding notes that they prefer decaying litter, which is more palatable and easier to digest, to fresh, and find acceptable food through chemical cues, at a distance and on contact. So a crucial part of this - and other micronutrients and anti-nutrients - is to feed a variety of foodstuffs and allow your isopods to select what they want to eat.
Are There Any Positives in Tannins?
Tannins are plant defences, and give many of our favourite foods and drinks their distinct flavours - tea, wine and more offer many benefits because of the tannins. They have antibacterial, antifungal, and antiparasitic qualities. But there are different tannins, and in small amounts they add huge benefits, while in large amounts (such as in leaf litter) they might be an issue.
The other thing is not to keep things too clean for colonies - to ensure the younger members of the colony can get the microbes they need to help them digest this sort of foodstuff from other colony members.
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