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.
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 and other specialist isopods
My only major concern in terms of tannins in their diet is certain cave-dwelling species. Living off fungi, bat droppings, moss and similar things, these isopods won't be exposed to many sources of tannins. Therefore it's possible that they won't be able to host the microorganisms that they need to break down these compounds. But if they have evolved to live both inside and outside the cave environment they may be able to break down leaves.
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
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 isopods diet
A study (https://link.springer.com/article/10.1023/A:1020962105931) looked at feeding hydrolysing tannins to Porcellio scaber and found that a little over half the hydrolysing tannins passed through unchanged, but the rest interacted with the digestive system and caused changes in the gut microbes, and not a positive change.
A further study (https://besjournals.onlinelibrary.wiley.com/doi/full/10.1046/j.1365-2435.2002.00669.x) looked at the feeding preferences and tannin digestion of a variety of isopods. They found that more terrestrial species such as armadillidium could digest tannins better than semi-terrestrial species.
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.
The above study also showed that isopods can make a choice in what they eat, selecting the best diet for themselves. 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 proffer 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, but in large amounts, such as in leaf litter, they might be an issue.
The other thing is not to keep things too clear 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.