Published online 23 March 2011 | Nature | doi:10.1038/news.2011.177

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Fish travelling through flood waters disperse seeds.

The fish Colossoma macropomum may be a key seed-dispersal vector in the Amazon. Visual&Written SL/Alamy

Massive Amazonian characid fish may carry seeds more than five kilometres across forest flood plains, researchers say.

Although fish have long been suspected of having an important role in seed distribution, proof of their ability to carry fertile seeds such distances has been lacking.

Jill Anderson, an evolutionary ecologist at Duke University in North Carolina, and her team had previously discovered thousands of seeds in the guts of Colossoma macropomum fish in Peru's Pacaya-Samiria National Reserve1. However, it was not clear how far the creatures might carry these seeds, nor whether they deposited them in areas where such seeds might grow.

To answer these questions, Anderson, then at Cornell University in Ithaca, New York, and her colleagues radio-tracked 24 of the animals during three flood seasons at the reserve and found that the location of wild fish varied by as much as 5.9 km. Combining this with data from captive fish on how long seeds are retained in their guts, the authors predict that C. macropomum probably have a mean dispersal distance of 337–552 metres and can carry seeds up to 5.5 km.

These are among the greatest distances reported for frugivores, the researchers note in a paper published today in Proceedings of the Royal Society B2, and put the fish on a par with other long-distance seed movers of the animal world — African hornbills and Asian elephants.

Crucially, the team's modelling work also suggests that the bulk of the seeds are distributed on the flood plains — where they are likely to germinate — rather than in permanent bodies of water such as lakes.

"I would not be surprised if fish are the most important dispersion vector [in this habitat]," says Anderson, whose interest in the topic was piqued when a research base she was working on was flooded and invaded by fish.

Far-flung fish

"Seed dispersal by fishes has been under-appreciated in the past, although written about for decades as of great potential value in this important ecological role," says Michael Horn, a biologist who studies this subject at California State University, Fullerton. Many fish may provide a hugely important link between terrestrial and aquatic ecosystems, and not only in the Amazon.

In the African tropics, for example, it is likely that fish distribute grass seeds, and fish in North America and Europe probably also move seeds around. However, both of these are woefully understudied, Horn notes, in part because it is much easier to study seed distribution by birds and terrestrial mammals. Horn's own research has shown an involvement for fish in distributing fig tree seeds in Costa Rica3.

"The whole field is fertile for further study," says Horn.

Further still?

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Even having established C. macropomum as among the top players in the distribution league, Anderson says that the team's numbers are probably "very, very much an underestimate".

The study predicts that larger fish will distribute seeds further. However, most of the team's radio-tracked fish did not come close to the maximum reported size for C. macropomum, which can reach a whopping 30 kg.

Overfishing in the Amazon region has devastated C. macropomum 's numbers and pushed the average size of the population down, a worrying development given the emerging proof of the importance of these animals to the ecosystem (see also: Fish help to spread forest seeds).

"In some areas populations have declined by 90%," says Anderson. "Overfishing could really alter the seed distribution of these habitats."