When startled, do all fish respond the same way? A few fish, like Mexican cavefish, have evolved in unique environments without any predators. To see how this lack of predation impacts escape responses that are highly stereotyped across fish species, scientists explored this tiny fish to determine if there are evolved differences in them. Findings reveal that the dramatic ecological differences between cave and river environments contribute to differences in escape behavior in blind cavefish and river-dwelling surface cavefish.
A few fish, like Mexican cavefish, Astyanax mexicanus, have evolved in unique environments without any predators. To determine how this lack of predation impacts escape responses that are highly stereotyped across fish species, researchers from Florida Atlantic University’s Charles E. Schmidt College of Science and Harriet L. Wilkes Honors College explored the tiny A. mexicanus to determine if there are evolved difference in the species. A. mexicanus exist as surface fish that inhabit rivers in Mexico and Southern Texas and as more than 30 geographically isolated cave-dwelling populations of the same species.
The ecology of caves differs dramatically from the surface habitat, resulting in distinct morphological and behavioral phenotypes in A. mexicanus. Cave populations live in environments without light, which is thought to contribute to the evolution of albinism, eye-loss, and circadian rhythm. Because they lack predators they might also lack the selective pressure to avoid predators. Dramatic differences in these cavefish populations combined with the robust ecological differences suggest that the startle reflex could indeed differ between populations of A. mexicanus.
To put this theory to the test, researchers elicited «C-start» responses using acoustic stimuli and high-speed videography in multiple cavefish populations and compared responses to eyed surface fish. The C-start escape response represents a primary mechanism for predator avoidance in fish and amphibians. C-start gets its name from the «c-shaped» curve a fish’s body forms during the first stage of the escape response, which is followed by a smaller counter-bend and then rapid swimming.
Results of the study, published in a special issue of the Journal of Experimental Zoology: Molecular and Developmental Evolution on the evolution of Mexican cavefish, support the idea that ecological differences between cave and river environments contribute to differences in A. mexicanus escape behaviors. Findings provide a platform for investigating the evolution of neural circuits contributing to sensory-motor integration and support using A. mexicanus as a model to investigate the evolution of escape behavior.
In diverse fish species, acoustic stimuli activate Mauthner neurons, which initiate a C-start escape response. Mauthner cells receive input from multiple sensory modalities including visual, olfactory, and mechanosensory systems. To compare C-start kinematics between surface fish and cavefish, researchers examined and quantified response latency, maximum change in orientation (referred to as «peak bend angle») and angular speed. They used six-day post-fertilization larvae instead of adult surface fish and cavefish to eliminate the confounding variable of learned behavior.
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Materials provided by Florida Atlantic University. Original written by Gisele Galoustian. Note: Content may be edited for style and length.