by Whit Gibbons

September 14, 2008

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?

Because 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.

The nest-building behaviors of many animals are intriguing. And a fish that uses its mouth to build a nest out of rocks would certainly fall within that category. But that's only part of the chub's reproductive story. The other part involves yellowfin shiners, stream fishes that occur only in streams where bluehead chubs live. During the chub nest-building activities, these smaller minnows gather in the area. While the chubs are constructing the nest, hundreds of yellowfin shiners form an enormous, frenzied school in the water above them. The male chubs for the most part go on about their business, paying little attention to their uninvited audience.

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.

What do 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.

The conservation implications are that even a seemingly simple biological phenomenon, such as minnows nesting in a stream, may actually represent a deep-rooted and intricate ecological relationship. The lesson for us is that we must be careful when we tamper with our natural environment. We may be affecting more species than we realize.

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