Did you know that there are several different animal and plant species that are known only to exist in or around the Ozark Mountains of Northern Arkansas, Southern Missouri, and extending westward just across the borders of Oklahoma and Kansas. Most of them have very close relatives that are more widespread, but ours have become so closely adapted with their unique ecosystem that they are now considered a separate species. One such organism is the Ozark Cavefish.

Amblyopsis rosae

Ozark Cavefish By MDC by USFWSmidwest

Image Credit

The Ozark Cavefish is a small species that can only be found in a small cluster of 11 counties distributed among 3 US states. Their widest distribution is found in Missouri, where there are known populations in Barry, Greene, Jasper, Lawrence, Newton, and Stone counties. There are also populations located in Delaware, Mayes, and Ottawa counties in Oklahoma, as well as Benton and Madison counties in Arkansas. Their largest population is located in Cave Springs Cave in Cave Springs, Arkansas. At one time, there were verified populations in as many as 52 different caves in the Ozarks, but that number has since dropped to 15 or 16. An exact population size hasn’t been determined, but the International Union for Conservation of Nature estimates that there are over 250, and it is thought that those numbers are slowly decreasing along with their distribution. As such, they are listed on the IUCN Red List as a Near Threatened species.

Tunnel Cave Falls, Indian Creek, Arkansas by Thomas Shahan

Image Credit

A Good Omen

As the name implies, Ozark Cavefish live in caverns and caves within the Ozark Plateaus, and they have become highly adapted to their unique environment. Early settlers in the area actually considered them a good omen. They thought that the presence of cavefish in the streams that fed their water supply was a sign that the water was good to drink. While they may not have understood the science behind their belief, it was pretty spot on! The Ozark Cavefish live in underground streams that are fed by groundwater that bubbles up through the underlying rock rather than surface water that drains into the caves from above. This water gets filtered as as it percolates upward, so it is extremely clean and keeps a constant temperature range of 12.8 - 15.6 ^oC (55.04^o - 60.8^o F). The cavefish have become so dependent on this crisp, clean water that even the slightest of changes can decimate the population. So, the settlers’ observations were correct: if the cavefish can survive, then the water is very clean and suitable for drinking.

Ozark Cavefish closeup By Missouri Dept. of Conservation by USFWSmidwest

Image Credit

Use It Or Lose It

The cool, clean streams that the Ozark Cavefish call home are usually found deep within the caverns and caves where no sunlight can penetrate.
The cavefish have existed in this ecosystem for so long that their bodies have made several major adaptations. Food is scarce in the dark caves. As a result, they are very small fish, maxing out at about 2 ¹/₄ inches long, and they have developed a very slow metabolism, which affords them a fairly long lifespan of 6 - 10 years. They have a slower reproduction rate than other fish, perhaps due to difficulties finding a mate in the darkness. Over time, they have quit producing eyeballs and optic nerves, opting instead to divert those resources to more useful features like an increased sensitivity to sound, smell, and vibrations in the water. Its head and fins have become elongated, and neuromasts (sensory organs associated with their lateral line) enlarged have become more exposed to external stimuli. The combined effects allow the cavefish to navigate in the darkness and maneuver quickly to avoid obstacles in the darkness. They have also quit producing skin pigment, so they appear transparent white to light pinkish in color, which is probably where the rosae part of the scientific name came from.

Gray Bat by U.S. Fish & Wildlife Service

Image Credit

What Cavefish Eat

Being totally blind and living in a pitch dark environment, you can imagine that the Ozark Cavefish would have some difficulty finding nutrition. How do they find nutrition in an ecosystem where plants, primary producers, can not exist? There must be a different way for energy to be introduced into the dank recesses of the caves.

The answer lies in poop. The Ozark Cavefish is most commonly found in caves that house Myotis grisescens, the gray bat. Bats are like the energy liaisons that travel between these deep caves ecosystems and the outside world. They leave at night by the hordes in search of food, only to return to shelter down for the day. It is the mounds of guano that they leave in the caves, along with the decaying corpses of dead bats that act as the entry point for energy. Various insects, fungi, and other decomposers break down these dregs, stripping them of every last bit of sustenance. They then pass it along to the other denizens of the caverns when they are eaten. This energy exchange continues all the way down the line to our cavefish. Their menu consists primarily of copepods, which make up anywhere from 70% - 90% of their diet. They are also known to eat crayfish, salamanders, isopods, amphipods, and even other cavefish.

Reproduction In Ozark Cavefish

It is believed that the Ozark Cavefish mates between February and April, when spring floods raise the water levels in the cave and trigger the spawn. They won’t become sexually mature until the age of 4 years. On top of that, it is estimated that about 20% of the females in a population won’t even produce ova in a given year, due to a lack of food available in the cave; however, it also ensures a small, stable population This only exacerbates the problem of finding a viable mate in the dark. To overcome this difficulty, both male and female fish will mate with multiple other individuals, if they are able to locate them to increase the odds of successful fertilization; a trait known as polygynandry. Those who are successful will only lay about 20 large eggs. Researchers aren’t sure what they do with those eggs, but they hypothesize that, after fertilization, the females store them in their gill cavities for the entire 4 - 5 month incubation period.

The Future of the Ozark Cavefish

The future is uncertain for the Ozark Cavefish. At one time, they were known to exist in 52 different caves, but that number has dwindled to 41. Of those, the majority haven’t had a single fish sighted within the past 6 years. Another 6 have shown population declines, and 10 have a stable population. The rest are classified as unknown, and may even be unoccupied. Most of the problems that the species faces are human-related. The biggest threat is groundwater pollution from nonpoint sources, especially agriculture. Additionally, sinkholes and the filling of cave entrances have caused problems, as have spelunkers who disturb the fish’s habitat.

Another potential problem for the Ozark Cavefish is the decline of the cave’s major point source of energy, the gray bat. While there are over a million individuals left in the wild, 90% of them only inhabit 8 different caves during hibernation. Studies also show that their population has been in decline due to destruction of habitat, pesticide use, and disturbance of their breeding and hibernating grounds by spelunkers.

Hopefully, with current conservation efforts, the Ozark Cavefish will be able to make a comeback. Prior to being listed as threatened in 1984, they were collected for study and even for the aquarium fish trade. Now, special care is being taken to protect their habitat and educate the public about how to help preserve the species. Extra care is also being taken to protect the recharge areas around the caves where water enters the ecosystem. In 1991, the U.S. Fish & Wildlife Service opened the Ozark Cavefish National Wildlife Preserve to in Missouri to protect the fish population there, and several other caves have been posted with signs that notify the public that federally protected species inhabit them. Others caves have even had their entrances barred up to keep humans out. Perhaps, if we can leave them alone, the Ozark Cavefish will enjoy a long lasting existence.