To get upstream, snails hitch a ride
In the South Pacific there’s a species of snail that never got the message that hitchhiking can be dangerous. Young snails ride on the back of a larger relative to migrate upstream.
Yasunori Kano of the University of Miyazaki in Japan observed the behavior by specimens of Neritina asperulata in the Solomon Islands and Vanuatu. Juvenile snails, each about an eighth of an inch in size, were attached to the shells
of subadults of N. pulligera. about an inch long.
N. asperulata are born in freshwater but as larvae are swept downstream to the ocean. They must then migrate back upstream, often five miles or more. Riding on the back of a larger snail increases the success rate for this migration by speeding up the process, though it still probably takes one to two years to travel about 2.5 miles, Kano says in Biology Letters. The behaviour may also provide protection from predators.
While the juveniles of other snail species are known to attach themselves to larger snails, Kano writes, in these cases the behaviour seems to be optional — there are plenty of free-living juveniles around. N. asperulata, on the other hand, hitches a ride from necessity. Kano writes that it’s the first reported case of hitchhiking ``that shifts the cost of migration onto other organisms. ”— New York Times News Service
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