While slugs are known for being slow and slimy, they've found success in living all over the world. Slugs and snails (otherwise known as gastropods) are found in freshwater, saltwater and on land, and are probably a great group to teach us how animals adapt to different lifestyles. One group of slugs (called Acochlidium) possesses a strange net of branched tubules that extend from the open cavity surrounding the heart, known as the pericardium. When these structures, called ‘dorsal vessels’ were first discovered, it was assumed that they were connected to the heart, thus replacing gills. To determine whether the Acochlidium slugs have a secret gill, or if the tubules function in some other way, Timea Neusser from Ludwig Maximilian University of Munich and colleagues from Bavarian State Collection of Zoology in Germany, studied the anatomy and function of four freshwater slug species to better understand the adaptations that make it possible for these animals to shift from life in the salty ocean to freshwater.
The researchers gathered four species of bottom-dwelling Acochlidium freshwater slugs from streams and rivers on various Indo-Pacific islands. Then, they collected tiny slices of tissue samples from the slugs’ excretory and circulatory systems, viewed them under a microscope and stitched together the images to form 3D digital versions of the organs. With the reconstructions in hand, the team examined and compared the organs of the circulatory and excretory systems of these freshwater species with the organs of other slugs that live in saltwater.
Contrary to what was believed previously, the team discovered that the net of branched tubules close to the heart of Acochlidium is not actually homologous to gills. Instead, it has an entirely different physiological function. Freshwater and marine animals face very different challenges to maintain a healthy water balance in their bodies. Where ocean water is dense with salt (and animals have to prioritize their water intake and excrete concentrated, salty urine), freshwater does not contain much salt, so animals must excrete a lot of dilute urine. It turns out that the dorsal vessels that connect to the cavity that surrounds the heart work to pull salts from the freshwater that the slug ingests. The tubules are lined with a thin membrane of special cells that helps to retain valuable salts in the body and also increases the surface area of this cavity, which increases pressure in the cavity surrounding the heart. This helps to move lots of haemolymph (slug blood) to the kidney to produce urine, an adaptation that allows these freshwater slugs to expel large volumes of water from the body while keeping the salts that their bodies need to stay healthy. Interestingly, the tubule structure found in a related nudibranch (Elysia) that lives in salt water does not have a thin membrane lining increasing its surface area; therefore, the structure is not capable of filtering fluid or moving it quickly to the kidney. This is a great example of how similar-looking structures can function very differently in related animals.
Comparing the structure and function of the body parts and organs of animals that are closely related but live in very different environments can give us important insights. We can understand more about the adaptations that allow animals to shift to these different habitats, like how these slugs use very different organs to deal with the pressures of living in the salty ocean or in a freshwater stream. Studies such as this one also show us that we have only scratched the surface in discovering the diverse ways that animals function – even in the possession of unique organs.
