Mexican blind cave tetras, Astyanax jordani, have all the earmarks of troglomorphy, or the physical adaptations to living in cave environments: loss of pigmentation, degeneration or loss of eyes while other sense organs become more keen, and the ability to live on very little food.
Stacy says that these fish are true albinos, although, of course, you won’t see any pink eyes. What’s interesting, he continues, is that the fish are born with developing eyes, but the sight organs degenerate and skin grows over them. "The juveniles do respond to light, or more accurately, shadows, through the pineal gland—also called the ‘third eye’ because it has photoreceptor cells—in the top of the skull," he says. "But as the fish mature and the skull bones thicken, they no longer sense light." You will notice that some of the tetras seem to have tiny eyespots in the usual area, but this is strictly individual variation in the overgrowth of the eyes, and they are sightless.
How do they negotiate the stalactites and stalagmites of their flooded cave habitat then? Like all fishes, they possess lateral lines, one on either side of the body, starting around the gills and running to the base of the tail. These cave tetras also have them on the face. Lateral lines contain organs that detect electrical impulses, and they also help fish stay upright, avoid collisions and locate prey.
The pickings are slim where the sun doesn’t shine—there are no photosynthetic plants to support a diverse food web of herbivores and carnivores. So these cave dwellers have evolved more highly refined olfactory/taste organs to detect their tiny invertebrate prey, along with a slower, more efficient metabolism to get by on less food, than their sighted counterparts.
Which brings us to perhaps the most fascinating facet of these fish: According to FishBase, the authoritative database of fish information, A. jordani "recently evolved from the surface form of Astyanax mexicanus." A. mexicanus, the Mexican tetra, is a sighted, surface-dwelling species. Kathy Lee, collection manager and our go-to person on taxonomy, explains, "A. jordani probably became a separate species through either allopatric or parapatric speciation." Big words for a basic evolutionary mechanism—isolation—but with fine lines of distinction. She continues, "Allopatry occurs when a population is separated by a geographic barrier and it evolves into a separate species due to isolation. A famous example of allopatric speciation is Darwin’s Galapagos finches. Parapatry occurs when a segment of the population enters a new niche"—perhaps a predator-free cave—"and becomes isolated, and a separate species evolves because of the isolation."
Kathy adds that A. jordani is one of her favorites. "I had one as a kid. I also had red and green gravel in the aquarium, and, because the fish didn’t have any pigment, every so often you could see a red or green spot moving through its digestive system. Fortunately, it always returned the gravel to its original place!"
The rock-filtered, nutrient-deficient groundwater in cave environments is incredibly pure—important to fishes and people alike. A. jordani is considered vulnerable to extinction because of habitat loss in its limited range. Populations are threatened, says Stacy, due to the diversion of water from cenotes, or sinkholes, that open onto the large bodies of underground fresh water that the Mexican blind cave tetras call home.
As Shedd’s shadowy cave tetra habitat goes completely dark, spend time appreciating these fish who go about their business of living in the total absence of light.
For more info and tickets to Jazzin' at the Shedd click here
Posted by Karen Furnweger, web editor