SHINERS HAVE A RELATIONSHIP
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.
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?
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).
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
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 thats
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.
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.
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.
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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