An Amazonian catfish as a seed disperser: evidence from the flooded forests of the Tupé Reserve

In the vast, seasonally flooded forests of the Amazon, the boundary between land and water is not a fixed line but a shifting zone where two worlds merge. During the high-water months, rivers swell and spill over their banks, submerging the forest floor and turning the trees into islands in a liquid landscape. This annual flood creates a unique opportunity for the creatures that live there. Fish, which normally navigate the rivers and lakes, swim into the trees to find food that has fallen from the canopy. Among the most abundant resources available in these submerged forests are fruits and seeds. For decades, scientists have known that some fish eat these items, but a critical question remained: does the fish simply destroy the seed, or does it help the plant survive? This process, where animals move seeds from one place to another, is a vital engine for forest growth. If a fish eats a fruit and later releases the seed in a new location, still alive and ready to grow, it becomes a gardener of the forest, helping to spread plant life across the flooded landscape.

Researchers set out to investigate this relationship in the Tupé Reserve, a protected area of blackwater flooded forest near Manaus, Brazil. They focused on a specific type of catfish, the long-maned driftwood catfish, a common fish in these waters. The team wanted to know if this fish actually eats fruits and seeds, and more importantly, if those seeds could survive the journey through the fish's digestive system. To find out, they spent four months during the 2022 flood season, using nets to catch 554 of these catfish. Once caught, the fish were carefully examined in a laboratory. The scientists opened the fish and removed the contents of their stomachs and intestines, sorting through the remains to find any seeds that had been swallowed. They then took these recovered seeds and planted them in soil under controlled conditions to see if they would sprout.

The results showed that the catfish are indeed active consumers of forest fruit. Out of the 554 fish examined, 77 individuals had eaten seeds from nine different plant species. The most common meal was the fruit of the Ficus maxima, a type of fig tree, followed by the camu-camu berry. The fish had swallowed a total of 1,032 seeds. Crucially, the seeds were not just passing through as food; many of them were still alive. When the scientists planted the seeds that had passed through the fish, a significant number of them germinated. For instance, seeds from the Eugenia patrisii tree had the highest success rate, with more than half of the stomach-recovered seeds growing into seedlings. The study also looked at whether the size of the fish mattered. They compared the number of seeds eaten by small, young fish against large, adult fish and found no difference. Both groups ate roughly the same amount of fruit, suggesting that this behavior is part of the fish's life cycle from a young age, not just a habit of the adults.

One of the most interesting findings concerned how the seeds fared depending on where they were found inside the fish. Generally, seeds taken directly from the stomach performed better than those that had traveled further into the intestine. However, there was a notable exception with the Ficus maxima seeds. Those that made it all the way to the intestine actually sprouted faster than those taken from the stomach. This suggests that the digestive process might have helped break open the hard outer shell of the seed or changed its chemical state in a way that encouraged it to grow sooner. While the researchers did not test seeds that had never been eaten by a fish to compare them directly, the fact that so many seeds survived the trip and grew indicates that the fish is a functional partner in the forest's life cycle.

The study also highlighted that this catfish has a mouth structure that is well-suited for this role. Unlike some other fish that have strong, crushing teeth designed to grind food into dust, this catfish has smaller, softer teeth that are better at grasping and holding items. This gentle grip likely helps keep the seeds intact as they are swallowed, preventing them from being crushed before they can be dispersed. The research confirms that this fish acts as a seed disperser in the blackwater forests, a type of environment that is often nutrient-poor and less studied than the muddy, sediment-rich floodplains. By moving seeds through the water, the fish helps connect different parts of the forest, allowing plants to colonize new areas and maintain genetic diversity.

Ultimately, the work demonstrates that the long-maned driftwood catfish is more than just a consumer of fruit; it is a vital link in the Amazonian ecosystem. The fish eats the seeds, carries them through the flooded forest, and releases them in a new location where they can take root. This interaction helps ensure that the forest regenerates itself every year after the waters recede. The findings add a new piece to the puzzle of how these complex ecosystems function, showing that even fish that are not the most famous fruit-eaters play a significant role in keeping the forest alive. The study reinforces the idea that protecting these flooded forests and the fish that live within them is essential for the health of the entire Amazon basin.