by Marilyn Davis
 

Catching evolution in the act isn’t an everyday occurrence. But Ginny Adams, a doctoral student in zoology, is doing just that.

Working in Perry County, Mo., just across the Mississippi River from Carbondale, she’s studying a cave-dwelling fish species that’s in the process of becoming a true cavefish. 

The fish, called banded sculpin (Cottus carolinae), are common throughout the southeastern United States. They’re found in streams, springs, and caves—but almost everywhere, the cave populations look just like the surface populations. They are not specially adapted for cave life.

In Perry County, though, it’s another story. Sculpin found in caves look different from those found in surface waters, and the degree of change varies remarkably from one cave system to the next. "There may be more than one population that's unique," says Adams, a cavefish expert.

That adds up to an ideal chance to study evolution in action. "You usually have fully adapted cave animals," says Adams. "It’s very rare to find a transitional form, and to find it here, in your own backyard, is pretty neat." 

She adds, "In very few cases do you have the same species inside and outside the cave so that you can make comparisons. We’re getting the chance to really see how changes occur in the cave environment."

Banded sculpin are small, thick-bodied fish about 3 inches long. They're a mottled brown color, with darker gray-brown bands across the body and fins. But the cave-dwelling sculpin in Perry County have less pigment than the surface-dwellers; they often show only traces of the bands. Their eyes are smaller and recessed. Pores on their head that sense movement are greatly enlarged. And they have fewer rays in their pelvic fins—an adaptation common in cavefish, which sometimes lose their pelvic fins altogether, for unknown reasons. 

There are physiological differences too. For instance, Adams has found that the cave-dwelling sculpin have a slower metabolic rate, another hallmark of cave organisms.

Perry County is one of the cave capitals of the United States, with more than 630 recorded limestone caves. In 1991, a southern Illinois caver reported the unusual appearance of some of the sculpin to SIUC ichthyologist Brooks Burr. Two undergraduates under Burr’s direction, Jean Krejca and Regina Paul, did their senior thesis research on the fish. Paul studied their morphology—their physical appearance and structure. Krejca looked at population distributions of banded sculpin in Perry County and other locations.

Adams, who came to SIUC to work with Burr, has been building on these initial studies. She is studying not only the morphology of these fish, but also their population genetics, their physiology, and their life cycle.

There’s much more to morphology than outward appearance. Adams is delving into the fishes’ brain structure, trying to determine if the optic lobe is smaller in specimens that have smaller eyes, and if other brain structures are increasing in size. She’s also examining cross-sections of the skin photographed through a scanning electron microscope, looking for any changes in the shape or size of minute sensory structures.

Genetic analysis will tell Adams about the relationship between various cave-dwelling and surface-dwelling sculpin populations in Perry County, and how the sculpin in different cave systems are related to each other.

Different groups of sculpin "seem to have moved into the caves independently, but we’re seeing the same kinds of morphological changes," she adds. Some of the caves are not connected hydrologically—they’re in different watersheds—so if the sculpin populations that inhabit those caves are developing the same genetic adaptations, they’re doing it independently of each other. 

In that case, says Adams, "The whole idea of convergent evolution is something we can look at a little closer, which is hard to do." (Convergent evolution is the phenomenon whereby different populations or species evolve similar characteristics in response to similar environments.)

Genetic analysis will also help Adams learn if speciation is occurring. The cave-dwelling sculpin in Perry County may already have developed into a subspecies or even a separate species. "We’re calling it a grotto sculpin as a common name to distinguish it from the normal surface banded sculpin," says Adams. These unique populations may need conservation protection in the future.

Some of the adaptive changes in the sculpin are clearly genetic—like number of pelvic fin rays. That’s "hard-wired," as Adams says. But could others be environmentally induced within a fish’s lifespan? Keep a sculpin in a permanently dark environment and it’s possible that its eyes might atrophy or that it might lose pigment. Or its physiology might alter in various ways.

Adams and Linda Roman, a junior in zoology, are running a lab experiment to sort out some of these questions. Roman has a prestigious $5,000 undergraduate fellowship from the Cave Conservancy Foundation to underwrite the costs of the study.

The two collected sculpin from a cave in Perry County and from a surface stream in southern Illinois. Some fish from each group are being kept in a normal light environment (12 hours light, 12 hours dark) and some in a completely dark environment. Within each group, some fish are well supplied with food and the others are being kept in a low-nutrient setting typical of caves.

Adams and Roman will run the experiment for a year, looking to see what changes, if any, occur in the sculpin that are kept in an environment different from the one in which they were found. Roman is monitoring for physiological changes. For instance, when surface-dwelling sculpin are moved to a cave-like environment, will their metabolic rate slow down? Adams will track any morphological changes, such as loss or development of pigmentation.

But most of Adams’s research is being done in the field, which poses challenges. Reaching some cave populations of sculpin requires wading through cold, neck-high water or slogging on your belly through 100 feet of mud. ("That’s just tons of fun," Adams laughs.) She’s now learning rappelling to reach more-remote populations.

The payoffs are worth it, however. For instance, once Adams discovered clutches of sculpin eggs in one of the caves—a rare find, since only a few people have ever found eggs even for surface-dwelling banded sculpin populations. "It’s one of our more poorly understood species" in terms of reproduction and life history, she explains.

Adams tries to get out to the caves in all seasons to monitor the populations. Because caves fascinate so many people, she has no shortage of volunteers for field help. 

"It's been fun," she says, adding, "It's such a neat place to take people and show them a whole other world." 

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