The hidden diversity

When we ask ourselves which animal groups are key to the good state of natural ecosystems, we tend to think, for example, of bees, since the pollination tasks they carry out so effectively are vital, as we well know. Without going any further, a significant percentage of human crops depend on their labor. However, there are other groups of animals that, because they live hidden inside the ground, we barely notice them. This is the case of earthworms, which play an important role in our soils, being the basis of a multitude of food chains and responsible for the proper functioning of the soil ecosystem. In addition, a group of them are recyclers of organic waste, being able to transform this waste into fertilizers with high added value. Their role in agriculture has already attracted the attention of Egyptian civilizations and Cleopatra elevated them to the category of minor gods. Darwin in 1881 dedicated a treatise to them and called them the intestines of the Earth. We are going to call them hidden diversity.

Already in 1826, the French zoologist Savigny impressed those attending a conference at the French Academy of Sciences, when he stated that the worms they saw in Parisian gardens when it rained were not a single worm, but that they were at least 27 species. Well, although it may seem surprising, only In the Iberian Peninsula there are about three hundred different species of earthworms. It is true that we tend to think of worms as animals that appear in orchards when grooves are dug to plant the first seeds or as fishing bait, but they perform functions that, at the very least, are on a par with those carried out by bees. Without going any further, they represent more than 50% of the biomass present in temperate terrestrial ecosystems, which implies that their number, even if they are hidden underground, is immense.

Because of their daily activities covering the floor up and down and also horizontally, they modify the habitat in which they are found in a significant way: they promote soil aeration, improve the ability of water to penetrate the soil and fertilize the deeper horizons of the soil by fertilizing it with their faeces, since many of them, even if they live inside the soil, feed on organic remains, such as leaf litter that falls from trees. On the other hand, they are the basis of many food chains, being a key element in the flow of energy in the terrestrial ecosystems where they live. Not only do the birds that tirelessly peck at the ground feed on them, but they also provide a very nutritious food for those who have the necessary methods to access their underground galleries or to find them under leaf litter. Hedgehogs, badgers, rodents, foxes, snakes and toads are some of those that choose worms in their daily menu.

Given their importance and diversity, it is logical to think that research by this group is necessary to learn more about it and understand the benefits they bring to ecosystems. It is especially necessary today, given that the possible effects of climate change in the Iberian Peninsula, which tend towards aridity and desertification of habitats, can substantially compromise and affect the distribution and survival of many of these species. Well, it is precisely, to learn more about earthworms, their ecology, their genetics and their distribution, that the group of Soil Zoology at the Complutense University of Madrid and that he is working on a Research Project funded by the Ministry of Science and Innovation dedicated to earthworms and which, currently, is finishing the volume of Iberian Fauna XII: Oligochaeta, published by the MNCN.

The UCM Soil Zoology Group, led by Dr. Dolores Trigo Aza, is responsible for compiling all available information on Iberian earthworm species and capturing it in the future volume of Iberian Fauna XII. Not only that, but they carry out numerous scientific projects that expand knowledge about these species, which are often endemic to the Iberian Peninsula, to be included in the catalog. From the subtle morphological differences between species belonging to the same genus, which are sometimes limited to the position of one or two sexual pores, to the Genome sequencing of certain species to get to know in depth how far diversity goes in the Peninsula, since there are complexes of cryptic species, which are morphologically and ecologically identical and only differ at the genetic level. Just as species that were considered different because of their morphological and anatomical characters, when applying molecular tools, they are included in the same taxonomic group.

The publication of the volume of Iberian Fauna XII will represent a key breakthrough in the study of earthworms and an important tool for researchers around the world. In the bulk of the volume we can find detailed sheets on each species, which indicate their distribution, their ecology and an exhaustive description of their most important morphological characters. Along with each file, a schematic representation of each specimen is included, in which the most characteristic morphological characters appear for identification. It is, in this section of the scientific illustration, in which I had the pleasure and privilege of participating, making illustrations of the almost 300 species present in the Peninsula.

To complete this blog post, I would like, very briefly, to explain my process to digitally capture the different species and their most important characters. It all starts, ideally, with a sexually mature individual of the study species, which is preserved in ethanol and stored in the collection of oligochaetes of the Faculty of Biological Sciences of the UCM. The specimen is studied under a binocular magnifying glass connected to a camera with which different images of the individual are taken, focusing on the external diagnostic characters most useful for identification. Detailed photos are also taken of some of the most important structures when making the illustration, such as the type of prostomy (structure that covers the mouth dorsally), the type of cracks, the position and type of clitellum (integumentary widening of several rings during reproduction), the glandular atriums of the male pores (widening of shapes and variable extension around the male pore) or the position of the first dorsal pore.

The next step is to capture the information about the species that the photo reflects in the illustration that will finally appear in the Volume. Since the intention is to create illustrations that combine realism and a more schematic perspective, the key characters are specified in such a way that they are visually recognizable to the researcher, while maintaining the shape of the body and the proportion between the different structures that the specimen presents. For this, it is very important to have taken the necessary measurements of the individual, which will be reflected as a scale in the illustration.

Finally, details such as the different folds, striations and papillae that the different rings can present are added, which gives a more interesting dimension to the drawing, since, being in black and white, in pursuit of the clarity of the different diagnostic characters, it lacks the conspicuousness of a color illustration, which, on the other hand, is not a defining feature in these animals, except in exceptional cases.

So, if you are ever asked Which earthworm is your favorite, you can answer that it is, for example, Compostelandrilus cyaneus, the commonly called Leonese earthworm, which is endemic to the province of León and cite Volume XII of Iberian Fauna: Oligochaeta, for more interesting visual references.

Raúl Navarro Zurro, Biodiversity