According to a recent study, some fish eggs swallowed by ducks could eventually escape unscathed from their excrement. A mechanism that could promote the spread of several species, some of which may be invasive.
Around the world, fish have colonized isolated water bodies without human assistance. It has long been suggested that these events were, and still are, attended by water birds carrying eggs attached to their legs and feathers. After all, we know that these birds can help spread seeds or invertebrates by these means. However, the empirical support to prove it is lacking.
Recently, it has been suggested that endozoochory (internal transport via the intestine) could play a more important role in this distribution of species. Orsolya Vincze, an evolutionary biologist at the Debrecen Ecological Research Center (Hungary), tested this approach during a controlled experiment.
As part of this work, the details of which are reported by ScienceNews, the biologist and his team fed eight mallard ducks with thousands of eggs from two species of carp (common and Prussian) considered to be invasive.
Note that what is commonly called “fish eggs” or “roe” are actually the ovaries of the females intended to feed the developing embryos.
At the end of this study, 0.2% of the “eggs” ingested (18 out of 8,000) were actually released into the feces of birds. And surprise: they were all intact. In other words, all had emerged unscathed from the digestive process of their attackers. In addition, according to the study, all of them hatched normally thereafter.
A neglected dispersal mechanism
It is not yet known whether certain eggs can also survive in the wild in this way, these operations having taken place in the laboratory. But if this is indeed the case, it could favor the dispersion of certain invasive fish species, suggests the biologist.
Indeed, if in the context of this study some eggs were excreted within an hour, others still spent more than four hours in the digestive system of birds. Knowing that migrating ducks can travel tens or even hundreds of kilometers before releasing these eggs, the question arises.
The study thus identifies a possible mechanism for dispersing the eggs of soft-membrane fish hitherto neglected.
“Although the number of surviving eggs is small, their number can add up, making bird droppings an important vector for the propagation of certain fish species,” said the biologist. After all, a single carp can release hundreds of thousands of eggs at a time. And there are enough ducks and other water birds in the world to force-feed their eggs.”