Blue Catfish: Seed Dispersal Agents?
Blue Catfish are a large-river catfish that has been introduced to several Atlantic slope rivers. They are now abundant in many rivers and reservoirs along the Atlantic slope. Some entities have labeled Blue Catfish as “invasive” and resolved that all practicable methods should be employed to minimize negative effects on indigenous flora and fauna. However, the fish may not be all bad and, in addition to the recreational and commercial harvest of the Blue Catfish in Atlantic slope rivers, there are many other unexplored effects.
American Lotus Nelumbo lutea
In a serendipitous observation, we saw a germinating Lotus seed Nelumbo in a tank of captive Blue Catfish. We did not put the Lotus seed in the tank; rather it was transported in the gut of the Blue Catfish and expelled with the feces during acclimation. Other investigators have described the role of fish in seed dispersal (Gottsberger 1978; Goulding 1980; Horn 1997; Kubitzki and Ziburski 1994). Many seeds have hard outer coverings that allow dormancy and let the seed survive until conditions are right for germination and early plant survival (Goulding 1980). However, we were surprised to discover that no investigators had explored the role of the Blue Catfish, an opportunistic omnivore, in seed dispersal and germination. Therefore, we conducted a pilot experiment to demonstrate whether the seed of the Lotus would germinate after passage through the gut of the Blue Catfish.
The experimental Blue Catfish were held at the Aquaculture Center at Virginia Tech. Seeds of Lotus plants were acquired from a local supplier. We randomly assigned seeds to one of three treatment groups: (1) control group, (2) artificially scarified, and (3) passage through Blue Catfish. We had 10 seeds per treatment group. Experimental units were nylon bags, one for each treatment group, which floated in the tank. We used sandpaper and artificially scarified one side of the lotus seed for treatment 2. This breaks the hard seed coating and allows water to permeate the seed. Duration of the experiment was 10 days. After ten days we examined seeds for signs of germination. Seeds that were artificially scarified or passed through the gut had higher germination rate than the control group. None of the control group seeds showed signs of germination after the allotted time. Ultimately, we found that Lotus seeds passing through the gut of a catfish germinate at a higher rate than seeds that receive no treatment. It is likely that passage through the gastrointestinal tract degrades the hard seed coating thus expediting the process.
Aquatic plants play a very important role for the ecosystem converting nutrients, providing shade, food, and cover, and stabilizing pH (Capers et al. 2010). In South America over 200 species of fish consume fruits and seeds, underscoring the importance of floodplain resources to biotic communities in large rivers (Gottsberger 1978). Several of these fishes pass viable seeds through their digestive tract, thereby acting as potential seed-dispersal agents (Horn 1997). Many plants, such as the Lotus, produce floating seeds that may be adaptations to take advantage of fishes for seed dispersal (Araujo-Lima
and Goulding 1997).
Seed-eating South American Characin
The Lotus seed is a very hard nut and due to the hard coating the nut is almost completely impermeable to water. The seed remains viable for many years even when conditions are ideal for sprouting. Some South Americans characins have strongly developed dentition that may break the hard nut walls and aids in germination (Goulding 1980). Blue Catfish eat just about anything, including Lotus seeds that fall in the water. Our pilot study confirms that the Blue Catfish can speed up germination of Lotus seeds, and it is likely that Blue Catfish could expedite germination of other plant seeds too. Blue Catfish are known to make long migrations, and may be ecologically valuable as seed dispersal agents within their native range, though this has yet to be explored.
Araujo-Lima, C., and Goulding, M. 1997. So fruitful a fish. Columbia University Press,
New York, 191 pp.
Capers, R. S., Selsky, R., and Bugbee, G. J. 2010. The relative importance of local
conditions and regional processes in structuring aquatic plant communities. Freshwater
Gottsberger, G. 1978. Seed dispersal by fish in the inundated regions of Humaita,
Amazonia. Biotropica 10:170-183.
Goulding, M. 1980. The fishes and the forest: explorations in Amazonian life history.
University of California Press, Berkely, California.
Horn, M. H. 1997. Evidence for dispersal of fig seeds by the fruit-eating characid fish
brycon guatemalensis regan in a costa rican tropical rain forest. Oecologia 109:259-264.
Kubitzki, K., & Ziburski, A. 1994. Seed dispersal in flood plain forests of Amazonia.