Science of Water : Concepts and Applications

182 The Science of Water: Concepts and Applications
Use of hooks
or suckers
Sticky undersurface
FIGURE 6.18 Adaptations to stream current.
Attachment to firm
substrate
Adaptations
to
stream current
Flattened and
streamlined bodies
Positive thigmotaxis
Positive rheotaxis
All nektons such as fi sh, amphibians, and insect larvae exhibit this adaptation. Some
organisms have fl attened bodies, which enable them to stay under rocks and in narrow
places. Examples are water penny, a beetle larva, mayfl y, and stone fl y nymphs.
5. Positive rheotaxis (rheo: current; taxis: arrangement)—An inherent behavioral trait of
stream animals (especially those capable of swimming) is to orient themselves upstream
and swim against the current.
6. Positive thigmotaxis (thigmo: touch, contact)—Another inherent behavior pattern for
many stream animals is to cling close to a surface or keep the body in close contact with the
surface. This is the reason that stonefl y nymphs (when removed from one environment and
placed into another) will attempt to cling to just about anything, including each other.
It would take an entire text to describe the great number of adaptations made by aquatic organisms
to their surroundings in streams. For our purposes, instead, we cover those special adaptations
that are germane to this discussion. The important thing to remember is that there are several basic
ways for an aquatic organism to adapt to its environment.
Types of Adaptive Changes
Adaptive changes are classed as genotypic, phenotypic, behavioral, or ontogenic.
1. Genotypic changes—Tend to be great enough to separate closely related animals into
species, such as mutations or recombination of genes. A salmonid is an example that has
evolved a subterminal mouth (i.e., below the snout) to eat from the benthos.
2. Phenotypic changes—These are the changes that an organism might make during its
lifetime to better utilize its environment (e.g., a fi sh that changes sex from female to male
because of an absence of males).
3. Behavioral changes—These changes have little to do with body structure or type: a fi sh
might spend more time under an overhang to hide from predators.
4. Ontogenetic change—This change takes place as an organism grows and matures (e.g.,
a coho salmon that inhabits streams when young, and migrates to the sea when older,
changing its body chemistry to allow it to tolerate saltwater).