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.
Introduction to Isopods
Isopods are a remarkably diverse group of crustaceans that have adapted to thrive in a wide range of habitats, from the deepest parts of the ocean to freshwater streams and even dry land. Among the most intriguing members of this group are the giant isopods, deep-sea animals that have captured the imagination of scientists and ocean enthusiasts alike. Isopods belong to the order Isopoda and share close evolutionary ties with other crustaceans such as crabs, shrimp, and lobsters.
What sets isopods apart is their distinctive body structure: they possess a rigid, segmented exoskeleton, seven pairs of walking legs, and two pairs of antennae, which help them navigate their environments and locate food. These animals play essential roles in their ecosystems, acting as both predators and prey for other animals, and contributing to the breakdown of organic matter. Whether living in the deep ocean, freshwater habitats, or as terrestrial isopods on land, isopods demonstrate an impressive ability to adapt and survive. The study of isopods, including the giant isopod, not only sheds light on the evolutionary history of crustaceans but also highlights the incredible diversity of life found in different habitats across the globe.
Isopod Evolution and Diversity
The evolutionary history of isopods is a fascinating story that stretches back hundreds of millions of years. Although the fossil record for isopods is incomplete—owing to the challenges of fossilizing their delicate bodies—scientists believe that these crustaceans first appeared around 300 million years ago during the Carboniferous period. Since then, isopods have undergone extensive diversification, giving rise to thousands of species that inhabit a variety of environments.
Some isopods have remained in marine environments, where they are known as marine isopods, while others have successfully colonized freshwater and even made the remarkable transition to life on land as terrestrial isopods. This move from water to land, known as terrestrialization, required significant evolutionary changes, including the development of specialized respiratory systems and adaptations to prevent water loss. Today, there are over 10,000 recognized species of isopods worldwide, with about 4,500 species living in the ocean, 500 in freshwater, and approximately 5,000 on land.
One of the most striking examples of isopod diversity is found in the deep sea, where the genus Bathynomus includes the giant deep sea isopod. These animals exhibit deep sea gigantism, growing much larger than their relatives in shallower waters or on land. The study of isopods, from marine to terrestrial forms, continues to provide scientists with valuable insights into the adaptability and resilience of crustaceans, as well as the evolutionary processes that have shaped life in habitats ranging from the ocean floor to dry land.
Cave dwelling, giant isopods, and other specialist isopods
My only major concern in terms of tannins in their diet is certain cave-dwelling species. These isopods have a specialized habitat with limited exposure to tannins. Their primary food sources include fungi, bat droppings, moss, and similar things, so they 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 for marine 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 (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.
