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
More on animal migration:
- Losing Nemo - acid oceans prevent baby clownfish from finding home
- Bats: compasses, tongues and memories
- How sharks, penguins and bacteria find food in the big, wide ocean
- March of the locusts - individuals start moving to avoid cannibals
The hitchhiking snail is a special sort of parasite, and one that Kano thinks has never been described before.
I'm confused by this, because phoresy (smaller animals hitching a lift on larger animals) is a common and well-studied process in invertebrates, including a number of cases of specialised phoretic connections (such as lice riding on louse-flies, or botflies laying their eggs on house flies so that they'll be carried to the larva's cattle host). I tried looking up the source to see what it said, but the DOI doesn't work yet.
I believe humans have also developed into donkey parasites.
How interesting! What do the little snails eat during those months?
I had the same reaction as Christopher Taylor. Lots of organisms disperse themselves at the expense of others, not least plants whose seeds are tucked into a burr or other kind of sticker that clings to fur or feathers. And lice... The snails are a cool example, and I was glad to read about them, but this is hardly a phenomenon that "has never been described before."
also same reaction as J. Dusheck and Christopher Taylor, what about the hummingbirds who hitch rides across the gulf of Mexico in the feathers of geese?
Those are all good points. This is a direct quote from the paper, although I'm more than happy for people to point out if it's wrong:
To my knowledge, this is the first reported case of âhitchhikingâ behaviour that shifts the cost of migration onto other organisms while reaping the benefit. This case is, I believe, unique, not only among diadromous animals with marine and freshwater periods, but also among other forms of obligatory migration.
I hate to beat a dead horse...so instead I will make a redundant point. Sphaeriid clams (=pea/fingernail) clams are a little smaller than their names would suggest and live in various freshwater habitats and some species specialize in temporary waterbodies. I had heard that they can be dispersed by water birds and frogs but I was sort of dubious about it. It just seemed like one of those accidental incidents magnified by over exaggerated hypothetical claims until someone showed me a Northern Leopard Frog, hundreds of meters from water, with a clam clamped onto its toe. I'm a believer now.
But why is the larger snail making the journey?
Schistosomiasis is spread by a parasitic worm that uses humans to distribute eggs into bodies of water near the snails in which they develop into the flukes which infect more people.