Written by: Gabriele Vieira; Reviewed by: Olga Kildisheva
Seed dispersal plays a vital role in maintaining biodiversity and ensuring the ecological balance of forest ecosystems. While birds and monkeys are commonly associated with this process, in the seasonally flooded forests of the Amazon, an unexpected group also helps spread plant life: fish.
Many tropical tree species depend heavily on animals to transport their seeds. In fact, around 75% of them would struggle to survive in their natural state without these plant-animal interactions. In flooded tropical forests, numerous plants have adapted to the rhythms of rising and falling waters, timing their flowering and fruit production to coincide with the flood season.
Fish may have been the first vertebrate seed dispersers. During these seasonal floods, frugivorous fish venture into the forest in search of food, inadvertently aiding in seed dispersal. This process, known as ichthyochory, is especially important in such dynamic and complex landscapes. There, dispersal occurs not only through water currents (hydrochory) but also thanks to the crucial ecological role played by fish, which help maintain plant populations and promote the colonization of new areas.
Ichthyochory
Ichthyochory refers to the process of seed and fruit dispersal carried out by fish. This specific form of zoochory—in which animals act as dispersal agents—plays a crucial role in the regeneration and maintenance of aquatic ecosystems and seasonally flooded environments. One of the most iconic examples of such habitats is the igapó, a type of Amazonian floodplain forest that becomes submerged for several months each year due to the rise of blackwater rivers like the Rio Negro. In these nutrient-poor and waterlogged areas, many plant species have evolved to release their seeds during the flood season, taking advantage of water currents and aquatic fauna to spread their offspring.
Ecologically, ichthyochory supports the survival and diversity of many plant species that depend on water-based dispersal routes. Some fish swallow whole seeds or parts of fruits, and the seeds that pass through the digestive tract intact may be deposited far from their parent plant, where they can successfully germinate. This interaction is especially common among frugivorous fish, which are found in both freshwater and marine environments. Their feeding behavior not only nourishes them but also strengthens the connectivity between aquatic fauna and plant reproduction in flood-prone landscapes.
Key Species Involved
Researchers have identified at least 276 fish species as potential seed dispersers in tropical flooded forests. However, this estimate is based primarily on studies conducted in Neotropical floodplain areas, so the actual number may be even higher. In a study aimed at evaluating how different fish species contribute to seed dispersal in oligotrophic blackwater floodplain forests of the Amazon, a total of 41 fish species were found to contain seeds. Among them were Brycon amazonicus, Myloplus asterias, and Serrasalmus rhombeus (Weiss et al., 2023.).
Species of seed-dispersing fish found in the Uatumã River basin. Source: Weiss et al., 2023.
Another study showed that seed size negatively influences the number of fish species that act as dispersers, with only a few capable of dispersing larger seeds. Among Brycon species, the probability of dispersal increases with individual body size. Large-bodied frugivorous fish also tend to masticate seeds less frequently than smaller species, which favors seed survival. Moreover, they can disperse a wider range of seed sizes—including non-floating and disperser-limited large seeds—playing a key role in the regeneration of flooded forests (Correa et al., 2015).
Brycon amazonicus measuring 35 cm. Source: Correa et al., 2015.
This feeding behavior, central to ichthyochory, varies according to fish and seed traits, and can lead to both dispersal and predation. During the seasonal floods in the Amazon, frugivorous fish enter the flooded forests to feed on fruits and seeds that are either floating on the water surface, submerged, or attached to vegetation. By consuming whole fruits or seeds, these fish inadvertently disperse seeds when they excrete them intact in new locations, often far from the parent plant. However, depending on the fish species and seed characteristics, some seeds may be crushed or digested, resulting in seed predation rather than dispersal. This dynamic feeding behavior highlights the dual role of fish as both seed dispersers and seed predators, making them key players in the regeneration and maintenance of flooded forest ecosystems.
Conservation Challenges for Frugivorous Fish
Despite the ecological importance of ichthyochory, the frugivorous fish responsible for this ecosystem service remain largely overlooked in conservation strategies. Several factors pose direct threats to these aquatic seed dispersers, particularly large-bodied species such as Colossoma macropomum, which play a key role in dispersing large, non-buoyant seeds. Overfishing has significantly reduced their populations, diminishing the effectiveness of seed dispersal across vast areas of flooded forests.
In addition to fishing pressure, the degradation of aquatic habitats represents an increasing threat. Deforestation along riverbanks and floodplain forests compromises the availability of fruits during the flood season, directly affecting the diet and feeding behavior of these fish. The construction of dams and other infrastructure also disrupts natural flooding regimes, fragmenting ecological corridors and limiting access to areas traditionally used for foraging and seed dispersal.
Although these impacts are still insufficiently quantified, they highlight the vulnerability of a mutualistic interaction that sustains much of the biodiversity in tropical flooded forests. Therefore, including frugivorous fish in conservation policies and sustainable fisheries management is essential to ensure the continuity of the natural regeneration cycle that connects aquatic and terrestrial ecosystems in the Amazon.
References
Correa, S. B., Araujo, J. K., Penha, J. M. F., Nunes da Cunha, C., Stevenson, P. R., & Anderson, J. T. (2015). Overfishing disrupts an ancient mutualism between frugivorous fishes and plants in Neotropical wetlands. Biological Conservation, 191, 159–167. https://doi.org/10.1016/j.biocon.2015.06.019
Correa, S. B., Costa‐Pereira, R., Fleming, T., Goulding, M., & Anderson, J. T. (2015). Neotropical fish–fruit interactions: eco‐evolutionary dynamics and conservation. Biological Reviews, 90(4), 1263-1278. https://doi.org/10.1111/brv.12153
Weiss, B., D. Santana, F., Petene Calvi, G., Costa, G., Zuanon, J., & Piedade, M. T. F. (2024). Effectiveness of fish assemblage as seed dispersers in Amazon oligotrophic flooded forests. Austral Ecology, 49(1), e13330. https://doi.org/10.1111/aec.13330