Nematode worms have a really shitty commute. It turns out that in order to get from place to place, these worms get inside a slug’s guts, sit tight while they’re being transported, and are then excreted at their new destination. All the while managing to avoid dying in this less-than-friendly internal environment.

Nematodes, including Caenorhabditis elegans, are one of the most intensively studied animal models in biological research. These worms are small, transparent, and easy to grow in a lab, and have a simple nervous system. Though they’re fundamental to biological research, lead researcher Dr Hinrich Schulenburg, from Christian-Albrechts University in Germany, tells IFLScience that there is a surprising lack of information about the ecology of the worm.

“We study the worms in artificial conditions, but may be missing some crucial information on certain traits that are only expressed properly in their natural context,” Schulenburg says.

These nematodes reside in short-lived habitats like rotting plant material, which has many microbes that they can feed on. Although we know that these habitats are temporary, and that the nematodes need to move, little is known about how they make this important journey from one residence to another.

To find out more, Schulenburg and his research team collected over 600 slugs and over 400 other invertebrates, which included flies, centipedes, spiders, beetles and locusts. They then dissected these invertebrates and looked for the worms. Researchers found that nematode worms frequently hitched a ride inside slugs, woodlice and centipedes.

“We were really surprised that these worms can go into the gut of the slug and survive in a very hostile environment. Slugs have a ‘grinder’ organ that can crush and destroy whatever they take up, but some of these worms can pass through this, persist in the gut and are secreted in the feces alive,” Schulenburg says.

^{The worm Caenorhabditis elegans. Image credit: Antje Thomas and Hinrich Schulenburg/Kiel University.}

The study, published in the open access journal BMC Ecology, suggests that these worms can stay in the slug’s guts for one day. Though it remains unclear exactly how the nematode worms enter and leave the slug without any obvious harm, Schulenburg suggests that the worms may have evolved specific mechanisms that allow them to survive.

There are other advantages to using slugs as a means of public transport. The slugs provide a humid environment, which prevents the worms from drying out too quickly, allowing them to survive the journey. The slugs also have many microbes in their intestines that provide the worms with snacks during the journey. It’s unclear whether the worm has any control over when it’s excreted, Schulenburg says, but his research team hope to investigate this in the future.

Karin Kiontke, postdoctoral researcher from New York University, who studies nematode worms extensively but was not involved in this research, tells IFLScience that the study “contributes important information” about the natural life history of these worms. For decades, Kiontke says that “hardly anybody cared about their natural life history,” but insists that “things are different now.”

She explains that the advance of DNA sequencing technologies has sparked an interest in using C.elegans and other nematode worms in comparative studies, and also comparing specific laboratory strains of C. elegans.

Kiontke explains that though scientists know an incredible amount of detail about C. elegans in the labs, including their preferred oxygen concentration and temperatures, and which chemicals they avoid, these features "can only be fully understood in the correct ecological context."

Schulenburg's study and others like it are helping us to develop this important understanding. Researchers will go on to investigate whether this is a parasitic lifestyle or otherwise, and if the worms can inhabit the slugs over the winter.