For long journeys, the shell of a snail hardly seems like the ideal public transport. That is, of course, unless you're an even smaller snail...
Yasunori Kano from the University of Miyazaki has found that the babies of Neritina asperulata, a tiny snail just 3 millimetres across, hitchhike on the back of a larger species Neritina pulligera. This living bus is about 2 centimetres long, and dwarfs its passengers by more than seven times.
The hitchhiking snail is a special sort of parasite, and one that Kano thinks has never been described before. They don't use their hosts as a snack, a home, an incubator or a foster parent - they simply treat them as a vehicle. Other parasites may unwittingly migrate in the bodies of their hosts, but there's no evidence that these travels are intentional. N.asperulata, on the other hand, is completely dependent on the movements of other host snails. Without them, it would never get to the small rivers it needs to complete its life cycle.
The animal world is full of famous migrants from salmon to spiny lobsters to Arctic terns. In comparison, the journey of the neritinid snails may seem less epic, but it's all a question of scale. As larvae, the snails spend their lives at sea. When they mature, the youngsters settle at the mouths of rivers and start their long trek upstream, often in large groups. There, they will find relative safety from predators and higher concentrations of the algae they eat.
But this future home is a long way away, several kilometres from the mouth of the river. Even for larger snail species, the distance is equivalent to a 300 mile hike for a human. For the tiny N.asperulata, it's even longer. It does, however, seem necessary for Kano only ever found adults in fast-flowing, upriver streams.
By studying snails in the Solomon Islands and the Republic of Vanuatu, Kano found virtually all of the N.asperulata's tiny youngsters were found on the backs of larger snails, who were carrying anywhere between 1 and 16 hangers-on.
The youngsters clearly have a knack for finding potential transport. Kano removed 22 hitchhikers and placed them in a container together with their old host, a new one, an empty shell and a similarly-sized stone. All the youngsters made a bee-line for either one of the living snails, ignoring the useless shell or stone.
The large snail shells are coated in a layer of calcium carbonate, and once their passengers were removed, Kano saw small circular etchings left behind on these coats. These take days or even weeks to form, which implies that once the small snails hang on, they don't let go for some time. That supports the idea that they are attaching themselves to cadge a lift, rather than to, say, feed on algae growing on their host.
Kano thinks that N.asperulata has developed a couple of subtle adaptations to suit a youth spent hitchhiking. Unlike species that make the trip themselves, its shell has a continuous rim, free from any uneven edges, lips or interruptions that would get in the way of a secure foothold. It also grows very slowly at first, presumably so that it's harder to dislodge or that it doesn't overload its vehicle. Juveniles all share the same small size, and the shells of adults have a clear division where their growth has accelerated, presumably at the point where they hop off their living taxi.
Based on their growth rate and their top speed, Kano calculated that the juvenile snails would have taken 1.5 years to make the journey themselves, even if they had started from the uppermost part of the estuary. As it happens, hitching a ride cut down the journey time to a mere 3-4 months. They have found a way to shift the costs of their otherwise arduous journey onto a larger cousin.
Reference: Biology Letters doi:10.1098/rsbl.2009.0191
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