CHUBS, SHINERS HAVE A RELATIONSHIP

by Whit Gibbons

April 30, 2017

I received an email with a photo of two kinds of fish in a stream in Georgia. It brought back memories of a fascinating phenomenon I first wrote about more than 20 years ago. The fish are bluehead chubs and yellowfin shiners. Their relationship is intriguing.

Julie Wallin, a University of Georgia graduate student, conducted research on why yellowfin shiners get chummy with bluehead chubs. Both live in clear, cool streams of the Southeast. One question she had 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. No chubs, no yellowfin shiners. Next question. What do chubs get in return?

The bluehead chub is a small fish common to gravelly streams. The gravel is critical because bluehead chubs construct their nests from stones. Several males work together, picking up pieces of small rocks in their mouths and carrying them to a communal nest site. The females congregate in small schools around the pile of little stones (probably making sure the project is done correctly).

Once the rocks are satisfactorily arranged, the males clear out spawning pits at the upstream edge. A male will tread water at a spawning pit in hopes of having a female release her eggs, which the male immediately fertilizes. If all goes well, the fertilized eggs settle in the gravel nest. Even after spawning has occurred and eggs are sitting on the sandy bottom of the stream, the males continue remodeling: picking up stones and moving them around in the nest. This is not a make-work exercise. It 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 picks up stones in its mouth to build a nest of rocks would certainly qualify. But that’s only part of the reproductive story. The other part involves yellowfin shiners and explains why these stream fishes occur together. During the chubs’ nest-building activities, the smaller shiners gather in the area. While the chubs are constructing the nest, hundreds of yellowfin shiners form an enormous, hyperactive school in the water above them. The male chubs for the most part go on about their business, paying little attention to the onlookers.

Yellowfin shiners do not congregate around the gravel nest of the chubs merely because they enjoy watching other fish work. Instead, they are inspecting the construction because they will lay their eggs in the newly built chub nest. The nest built amid gravel by one fish becomes a safe harbor for the eggs of another. In fact, yellowfin shiners lay their eggs only in the gravel nests built by chubs. One interpretation of the behavior is a symbiotic relationship between the two species: shiners cannot reproduce unless chubs build a nest for them. Symbiosis is a relationship between unrelated species in which one or both benefit from the association.

Do the chubs gain anything from the relationship? Julie's studies showed that shiners potentially contribute to the survival of chubs in two ways. One is through creating a confusion effect around and above the nest. Snakes, turtles or predatory 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 evolutionarily sufficient for them to tolerate the presence of a bunch of annoying shiners at their nesting site.

A seemingly simple biological phenomenon of minnows nesting in a stream may represent an entrenched and intricate ecological relationship. We must be careful when we tamper with our natural environment. We may be affecting more species than we realize. I appreciate an email revealing that others find such observations as spellbinding as I do.

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