CHUBS AND YELLOWFIN SHINERS HELP EACH OTHER
Many species have symbiotic relationships. Symbiosis refers to a relationship with an unrelated species in which one or both benefit from the association. Ecologists are notorious for making up new terms for different symbiotic associations; one of the oldest and most basic is mutualism, a situation in which both species profit from the interaction. In some of these relationships, the benefits for one of the species may not be obvious. Some fascinating ecological studies have focused on determining how such species profit from these relationships.
Yellowfin shiners clearly benefit from a relationship with chubs, based on a discovery made by University of Georgia graduate student Julie Wallin, who conducted studies on the two species of minnows, which live in clear, cool streams of the Southeast. One question that she addressed was whether yellowfin shiners are dependent on bluehead chubs to the point that they cannot live without them. She concluded that the shiners are totally dependent on the chubs to build a nest for them to lay their eggs. What do chubs get in return?
she needed a stream not severely affected by urban, agricultural, or industrial
pollution, she conducted her study on the Department of Energy's Savannah
River Site. The site's protection from disturbance by the public makes
it an ideal location to conduct ecological field research. The bluehead
chub is a small fish common to streams with gravel. The gravel is critical
because bluehead chubs construct their nests from small stones. Several
males work together, picking up pieces of gravel in their mouths and carrying
them to the nest site. Females congregate around the pile of stones, and
the males build spawning pits at the upstream edge. A spawning pit is
a cleared area over which the females release their eggs. The waiting
males fertilize the eggs, which eventually settle in the gravel nest.
After spawning has occurred, the males continue to rearrange the furniture,
picking up stones and moving them around in the nest. This prevents the
nest from accumulating silt and provides aeration to the developing embryos.
Yellowfin shiners do not congregate around the gravel nest of the chubs simply because they like to watch other fish work. Instead, they are inspecting the construction because they also lay their eggs in the newly built chub nest. The gravel nest built by one fish becomes a safe harbor for the eggs of another. In fact, yellowfin shiners lay their eggs only in the gravel nests constructed by chubs. One interpretation of this phenomenon is that shiners cannot reproduce unless chubs build a nest for them.
the chubs gain from the relationship? Julie's studies showed that the
shiners potentially contribute to the survival of chubs in two ways. One
is through creating a confusion effect around and above the nest. Predators,
such as snakes, kingfishers, and other fish might catch a shiner rather
than a chub. In addition, shiner eggs mixed in with those of chubs lower
the chance of something eating a chub egg. The gain by the chubs is subtle
but apparently sufficient for them to tolerate the presence of a bunch
of annoying shiners at their nesting site.