Most plants in tropical forests rely on animals to disperse their genes and seed dispersal by frugivores shape the distribution and composition of plant communities. In turn fruit constitute an important energy source for frugivores. Differences in morphological, physiological and behavioral traits among frugivores constrain fruit consumption and therefore seed dispersal. Recent evidence demonstrated that frugivorous birds and mammals impose selective pressure on fruit traits such as fruit size, color and odor.
Frugivorous fish are among the most important seed dispersers in South American floodplains, where annual floods last months. Water is also an important seed dispersal vector, although fish and water likely yield different plant distributions. Water currents move seeds downstream, while fish disperse seeds downstream, upstream and laterally across the floodplain.
Whether frugivorous fish impose selection on fruit traits remains unknown. A recent quantitative literature review revealed that fish are more likely to disperse seeds of fleshy fruits than those of dry fruits. If fish impose selective pressure on the development of fleshy fruits in wetlands, we predict that 1) production of fleshy fruits is high during the flood season, 2) fleshy fruit are equally frequent in flooded forests versus non-flooded forests, and 3) buoyancy is independent of fleshly tissues. Moreover, if fish impose selective pressure on other fruits traits aside fleshiness, we expect to detect traits or groups of traits with higher probability of seed dispersal versus mastication by fish.
In flooded and non-flooded forests in the Pantanal wetland, we assessed fruit production, morphological traits and buoyancy of ripe fruits. In addition we tested whether fruit shape, color and nutrient composition affect the probability of seed dispersal versus seed predation by fish.
During the flood season, fruit production was higher in flooded than in dry forests. Species producing fleshy fruits dominated in both flooded and dry forests, but more species with dry fruits occur in dry forests. We measured fruit traits of 50 species from flooded forests. 60% of fleshy fruits sank immediately after immersion in water, making seed dispersal by water unlikely. All seeds from which we manually removed pulp sank suggesting that secondary seed dispersal by water of seeds defecated by fish is unlikely. Fruit traits influenced the probability of seed dispersal. We concluded that frugivorous fish are important vectors of seed dispersal in the Pantanal wetland. Seed dispersal by fish likely favors the development of fruit traits among wetland fruits via natural selection.