Having previously mentioned land and air in considering seed dispersal by animals, we couldn’t ignore sea – well, aquatic more generally – so we naturally turn to fish. Seed dispersal by fish, icthyochory, is important in aquatic habitats* , and can remove seeds up to 5495 m from the parent plant. If we think of fish and their environment we probably have in mind permanently submerged areas.
However, fish can also be an important seed dispersal vehicle in habitats that aren’t permanently aquatic. Indeed, land-based plants can benefit greatly from fish that disperse their seeds during periods of flooding. This has been well shown by Bianca Weiss et al. in the Amazon Forest in areas known as várzea. Várzea is the name applied to forested land in South America that is inundated by overflow from rivers – such as the Amazon – to a depth of 6 to 7 m (and up to 15 m) for up to 10 months each year. Under such a regime one might imagine that more typical land-based animals would be hard put to help the plants disperse their seeds. No matter, this important service is provided by fish that occupy the temporarily-flooded environment. During their in-depth study, Weiss et al. captured 1915 fish and removed 3,092 whole seeds (of 18 spp., representing 17 genera and 15 families) from the digestive tracts of the 148 individuals that contained seeds. Of those pisceans, Triportheus albus was the most abundant species, presenting the highest number of seed-swallowing individuals; T. auritus, T. angulatus, and Colossoma macropomum were also abundant and contained appreciable amounts of seeds in their digestive tracts.
Amongst the plants, Cecropia spp. showed the highest number of ingested seeds (1,492) and the highest frequency of occurrence in the fishes’ digestive tracts (FOI – frequency of ingestion, 28.3% of seeds recovered). Laetia corymbulosa (259 seeds) was the second in frequency of occurrence (FOI, 23.4%), although it represented fewer ingested seeds than Ficus insipida** (number of seeds = 893, FOI 17.2%). OK, seeds were found, but were they viable and able to germinate? Seeds of 16 spp. germinated after removal from fish guts. Although only 148 individual fish (8% of the total number examined) contained seeds, that still has the potential to be a significant seed-dispersal pathway in this habitat. And, since many of the fish species involved are migratory, they contribute to the plants’ colonization of new areas, which facilitates gene flow between subpopulations distributed along the rivers.
* Such habitats include the oceans, which contain seed-bearing angiosperms – the so-called seagrasses. I know you’re now wondering whether animals are involved in seed-dispersal of these oceanophytes. So, by way of an answer, please see Fig. 2 on p. 116 of Robert Orth et al’s Chapter 5 “Seagrass Seeds and Dispersal Strategies” in Seagrasses: Biology, Ecology and Conservation, and the text on pp. 122 and 127, which mentions that fish – and manatees, dugongs, waterfowl, and turtles – may mediate seed dispersal of seagrasses.
** Ficus is the genus that contains the edible fig – Ficus carica, the ‘fruit’ [technically a multiple fruit body or syncomium] of which famously has a laxative effect in humans. Ficus insipida also has a laxative effect on humans. Whether it also has such an outcome upon the fish that consume its seeds, I don’t know. But if it did, that would be a most useful property that could assist discharge of the seeds from the digestive tract of the fish – which ‘expulsion-assisting action’ may also be a benefit to the seeds of other – non-laxative – plant species that had been ingested. An idea not to be ‘poo-pooed’ without further investigation?
*** For more on involvement of fish in seed dispersal, why not have a look at the related item on the ever-interesting In Defense [sic(!)] of Plants blog?[This is part 5 of a series celebrating the creatively imaginative and enterprising ways in which plants dupe poor unsuspecting animals into doing their sexual bidding…]