Have you ever seen creatures living in isolated waterholes or rock pools and wondered how they got there? Some recent work* on one of our least mobile animal groups – snails – sheds some light.
A key aspect of ecology is studying where plants and animals are found. One of the related questions is – how did they get there? And in the modern day – what do people do that helps or hinders them getting there ie movement or dispersal pathways?
As far back as Darwin’s time, isolated freshwater ecosystems or “islands in a sea of land” were a particular point of interest in understanding dispersal. He was one of the early scientists to suggest birds may play a part in the dispersal of some aquatic species.
But lets take a step back. Clearly if you’re a good flier, runner or swimmer you can get around on your own. However, animals with limited mobility can’t rely on self-propulsion. These species can only colonise isolated locations via wind, water, or hitching a ride with larger, more mobile animals.
Research has shown that terrestrial and water birds can transport plants, seeds, algae, and invertebrates (snails, insects and the like) internally – if they are tough enough to survive the journey through the bird’s digestive system. Latching on to the outside of birds is another more scenic option.
Experiments with different species of snails and mallards (a type of waterbird) in the Netherlands* showed that snails could stick to the feathers, feet, and bills of birds. Drying out can be a problem, so snails attached with mud probably have the greatest potential for long-distance travel. These snails often stayed attached for 30 min but some snails stayed attached for up to 8 hours.
Given when snails are left to their own devices their dispersal is limited to only few km/year, dispersal by water birds may explain the often rapid colonization of new suitable habitat by aquatic snails. Not all snail dispersal is good news however – invasive aquatic snail species may also be helped along by birds. This is particularly an issue for species that carry parasites that can be passed on to humans and livestock.
* The full article on which this piece is based is: Prerequisites for flying snails: external transport potential of aquatic snails by waterbirds. By C. H. A. van Leeuwen and G. van der Velde. Freshwater Science 2012; 31(3):963–972