Geena Zebrasky – Features Writer

When you walk outside on a rainy day, you’re often greeted by the smell of worms in the humid air. As you walk, you’re watching your feet, playing worm-watch so that you won’t accidentally be the determining factor in one of these pink invertebrates’ futures.
The reason that we have to play hopscotch everytime it rains is actually a little unclear. It was originally thought that the rain saturated the soil with too much water, and since worms “breathe” through their skin, scientists thought the need for quicker oxygen acquisition drove worms out of their burrows to the surface. However, earthworms can still get enough oxygen through water, and in fact, require moisture in order for respiration to occur. That’s why you see so many worms once the sun comes back out—they dry out and die because they can’t respire when they’re dry.
Earthworms can’t drown like we can, and can survive several days fully submerged. Because of this, some scientists don’t buy the need-for-oxygen claim. Rather, they argue that earthworms experience the vibration of the rain drops on the soil surface in a similar way to predator vibrations. Earthworms have receptors in their epidermis that are very sensitive, and they often come to the surface to avoid predators that disturb the soil, like moles.
Earthworms belong to the phylum Annelida, which is a subphylum of the greater Lophotrocozoans and also includes mollusks, brachiopods, and others. This phylum is crazy diverse, with over 22,000 living species! The bulk of the diversity isn’t actually terrestrial species, but rather is found in the polychaetes: marine worms. Annelids are fascinating invertebrates, and display so many unique adaptations to their environments, from color to size to feeding habits. Going over all of this diversity could take an entire book, so we’ll just focus on earthworms that you can find here in Minnesota.
Earthworms, like all annelids, are segmented. These segments form one after the other, from growth ends located at the hind end of the worm. This means that the “youngest” segment of a worm is always towards their back end. Their bodies are covered in a cuticle that is made of collagen secreted by the cells beneath, which both protects their bodies as they burrow and helps with staying moist. They eat soil, detritus, fungi, and any other things that can be found in and on the soil. Food passes through the esophagus into a crop and then a gizzard, that has stones to help grind up the food (like chickens!). Just like us, worms have a closed circulatory system, but instead of one heart, they have five heart-like arches at the front of their bodies.
Alright, enough about worm anatomy—what about where they live? Earthworms are often grouped into three main categories: those that are non-burrowing, that live at the soil surface and eat leaf litter; those that are topsoil dwelling and feed on soil at the upper layer; and those that construct deep vertical burrows that they use to visit the surface to get food. All of this burrowing activity mixes and allows oxygen to move through the soil, which is good for making organic, rich soils. In fact, if all the topsoil they had turned up was layered on the Earth, it would cover Earth’s land mass in a layer 300 miles deep.
Earthworms actually aren’t native to Minnesota—all of the known terrestrial earthworms are non-native invasive species originating from Europe or Asia. In some cases, they’re beneficial (like aerating the soil), but in other cases, earthworms actually represent a problem in ecosystems. Earthworms’ burrows in forests create large pores, allowing water and nutrients to move much quicker through the soil than the ecosystem’s plants had originally adapted to. Our northern forest soils after the glacial period developed thick organic layers, with a carpet of leaf litter that’s actually really beneficial for the organisms that live there. However, when Earthworms are introduced, they consume this layer and the accelerated mixing of the organic layer can lead to quicker decomposition, leaching, and even soil compaction in the long-run. The removal of this leaf litter can also change the forest makeup, switching the plants that grow on the forest floor and subsequently affecting the organisms that needed the original plants as part of their habitat.
With worms so prevalent, is it too late for our forests? No—earthworms move really slowly, and only live about seven years, so if we’re careful not to introduce them, we can keep areas worm-free.
Invertebrates make up over 95% of all animal species, and annelids are just a little slice of this spineless world. With such a wide range of species, earthworms represent a small, but important bit of the annelids—and hopefully you find yourself asking more questions about the world of worms next time you’re out walking in the rain!