It doesn't take a particularly experienced naturalist to recognize that snails and slugs – gastropods – leave a trail of mucus wherever they go and that the gooey material is both secreted by the animals and intimately tied to their movement. What may be considerably less appreciated is the metabolic cost associated with producing such trails. Estimates of the energy needed for mucus production across a number of species suggest that approximately one-quarter of consumed energy is used in making these tracks. Given its high cost, investigators are interested in the potential role(s) of mucus secretion in marine and terrestrial gastropods beyond its role in locomotion. For example, might trails be used in food capture or navigation?Various gastropod species track trails laid down by other individuals, so Mark Davies and Janine Blackwell, from the University of Sunderland, recently performed a series of experiments to test the possibility that tracking snails may save energy by using another snail's mucus.
To test their hypothesis about energy savings in gastropod trail following,Davies and Blackwell used the widespread, intertidal common periwinkle, Littorina littorea, collected from Whitburn, UK. The investigators allowed individual `marker' snails to lay down single mucus trails on an array of connected microscope slides. In some cases they used `tracker' snails to create double trails consisting of a second layer of mucus by placing a second animal directly onto a marker snail's path, where it would secrete its own mucus trail onto the original marker's trail that it followed.
To quantify whether tracker snails secreted less mucus than marker snails,the biologists then flooded the glass slides with a fluorescent dye and used fluorescence microscopy to measure the thickness of single and double trails. All trails were convex in cross-section, and double trails were significantly thicker at their midpoint (on average 46.8 μm) than single trails (on average 35.4 μm). By estimating the area under curves that describe mucus thickness as a function of trail width, the investigators showed that double trails possess only 27% more mucus than single trails, suggesting that tracker snails secrete much less energetically costly material when they follow a previously laid trail.
Davies and Blackwell also studied the rate of mucus decay caused by environmental exposure, for example to waves and the weather, to find out if this would affect the trails laid down by tracker snails. To measure mucus decay, they took microscope slides with single and double trails to a midshore site and fixed them horizontally onto a frame attached to a flat rock, leaving them for one of four time periods: one tidal cycle, two tidal cycles, one week and 2.5 weeks. Again examining trails using fluorescence microscopy, they found that trail thickness decayed with increasing exposure time, such that by 2.5 weeks no signs of marker trails remained. Moreover, tracker snails placed onto marker trails secreted more mucus the longer the marker trail had been exposed, showing that the amount of marker mucus laid down affects how much mucus a tracker secretes.
Assuming mucus composition remains comparable between single and double trails, it is clear that tracker snails produce much less mucus than is found in the original paths laid by marker snails. Thus, in some ways like cross-country skiers, gastropods appear to gain energetic benefits when following in the track of another individual, and more recent trails confer greater benefits. I now have new-found respect for those snails I've observed in aquaria forging paths that others can follow.
