The Natural Georgia Series: Atlanta's Urban Wildlife

Perilous Experiment: The Asian Rice Eel in Georgia

By Wayne C. Starnes, Richard T. Bryant, and Greg C. Greer

Let us consider doing a vast experiment on the waters of our state. This experiment will be especially risky because, once it is begun, it cannot be stopped by anyone. The aquatic environment could be irreparably damaged in ways we can only imagine, and the experiment has absolutely no perceivable benefits. There is no way to predict the experiment's outcome because nothing quite like it has been done before.

Few people would condone such a misguided venture. Yet, this very experiment is currently underway in Georgia. It began with the release of an exotic species, the Asian Rice Eel, Monopterus albus, into the Chattahoochee River system. Exotics are species that are not native to an area, and, as a result, can greatly upset the balance of a natural ecosystem. The Asian Rice Eel, probably released from a pet-owner's aquarium, has the potential to do just that.

The Chattahoochee Nature Center, with its peaceful woods, winding nature trails, log cabin-style visitor's center, and three picturesque ponds, is an unlikely setting for an experiment with peril. Located north of Atlanta adjacent to a wetland along the north bank of the Chattahoochee River, the center is a valuable island of beauty lodged between the rapidly growing urban centers of Atlanta, Roswell, and Marietta. In this increasingly crowded urban area, the center plays an increasingly important role as a place for school children to visit and learn about nature. They can walk the trails, feed the ducks, collect tiny invertebrates from the ponds with dip nets, see bald eagles, view interpretive exhibits, and learn about nature. And now, too, perhaps they can learn important lessons about a newcomer to this peaceful scene.

While visiting the Nature Center in September 1994, a tiny, eel-like creature, barely 2 inches long and swimming in a shallow dish, came to our attention. Puzzling over the dish, we learned that the little creature had been dipped from the smallest of the three impoundments at the center by a youngster earlier that day. We also learned that, on rare occasions, others had been dipped from the pond over the previous four or five years. Barely thicker than a pencil lead, details of the mystery animal were difficult to discern. Any fins, legs, gill slits, mouth shape, or other characters that might give a hint of its identity were scarcely if at all visible without the aid of a microscope. It clearly was not a worm or a snake but rather a fish or amphibian. But was it a very young eel or lamprey, or a tiny eel-like salamander, such as a siren?

The American Eel

In much of Georgia, an eel would seem a good bet. The American Eel, Anguilla rostrata, is a freshwater species that penetrates far into the continental waters of North America. All true eels spawn at sea, even though a few, including the American Eel and its kin on other continents, enter freshwaters. The American eel spawns in the ocean, somewhere near the Atlantic's vast Sargasso Sea, and has a semitransparent, ribbonlike larval stage, termed a leptocephalus. These larvae drift hundreds or thousands of miles before transforming into a transparent juvenile stage, called a glass eel, as they finally near coastal waters. Leptocephali may reach over 3 inches but actually shrink to a length closer to 2 inches upon transformation.

As the young eels near or enter the mouths of rivers, they darken into a grayish elver stage before, at about 4 inches, becoming the more familiar yellowish color that they maintain as juveniles and most of their adult life. They may remain in freshwaters or estuaries for as long as 20 years before reaching full maturity. During this time, while many individuals remain in lower reaches of rivers or estuaries, many others penetrate far inland to the headwaters of Atlantic and Gulf tributary rivers.

After total sexual maturation occurs, the eels turn a silvery color, their eyes become much larger, and a spawning migration to sea occurs. A few of the largest may exceed 3 feet in length. Adults ostensibly die after spawning. This reverse spawning migration, the opposite of the more familiar anadromous spawning migrations of salmon, shad, or striped bass, that live as adults in the sea but enter rivers to spawn, is termed a catadromous migration.

Although most true eels, other than the American Eel and its kin on other continents, do not enter freshwaters, all spawn at sea and have the unique leptocephalus stage. This life cycle is important to remember for discussions that follow. Eels are extremely diverse, with about 15 families and nearly 750 species known. Some, like the American Eel and its kin, have paired pectoral fins, while others, such as the familiar morays, have none.

While American Eels are renowned for their prowess at surmounting obstacles in the paths of their upstream migrations, few if any have been able to negotiate the several large dams on the Apalachicola and Chattahoochee rivers in recent decades and reach the Atlanta area. This fact, and the very small size of the squirming little creature in the dish, greatly reduced the probability that it was an American Eel that had somehow made its way to this scenic little locale so far removed from the coast. So, as we worked to identify the creature, lamprey or amphibian were our other logical choices among critters native to this region of Georgia.

Lampreys are extraordinarily primitive vertebrates with eel-like bodies, no paired fins, and a sucker disklike mouth. Some have rasping teeth and are, as adults, parasitic on other fishes, sucking blood from them for nourishment. The native freshwater lamprey species cause few fatalities among their hosts as they have evolved a balanced host-parasite relationship. On the other hand, the large, anadromous Sea Lamprey, Petromyzon marinus, once it gained entrance to the Great Lakes via human-constructed canals and established nonnative, landlocked populations, became a serious threat to populations of the native Lake Trout, Salvelinus namaycush.

There are also several nonparasitic lamprey species that do not feed as adults but simply spawn and die. All lampreys, whether parasitic as adults or not, spend their early lives as ammocoetes, a blind larval stage that burrows into leaf litter or beneath banks and derives nourishment from the muck in which it lives. The nonparasitic Southern Brook Lamprey, Ichthyomyzon gagei, is native to the Chattahoochee system but, even to the naked eye, the head and mouth of the tiny critter in the dish didn't look anything like a lamprey.

A Closer Look

When we took our first look at the little stranger beneath a microscope, what we saw was both shocking and totally unexpected. We had seen that face before—or one very much like it—not on any North American creature but, instead, on South American swamp eels during our previous work in that region. We knew now that this little specimen and its fellows that had been dipped up in recent years heralded an invasion by alien creatures. They represented an established population of exotic swamp eels, family Synbranchidae, that are naturally distributed through parts of Asia, Africa, and the tropical Americas. None were previously known from North American waters north of southern Mexico, but they were now clearly thriving in Georgia!

Synbranchid eels are not true eels at all—far from it. They just bear an incredible resemblance to them. While true eels, among the over 480 or so families of fishes currently recognized, are classified low among the more primitive bony fishes, synbranchids, based on skeletal, muscular, and other characteristics, are classified up near much more advanced spiny-rayed groups, such as sticklebacks and the perchlike fishes. Internally, certain skeletal and other features differ markedly from those of true eels. Further, they lack the characteristic leptocephalus larval stage of true eels and do not spawn in the ocean.

The body forms of synbranchids and true eels have probably converged on one another over millions of years because of similar habits, such as burrowing and life in crevices. Synbranchids no longer possess pectoral fins, though they show skeletal vestiges of having had them in the distant past. They have only a tiny soft fringe representing dorsal and anal fins.

Under currently published concepts, the swamp eel family consists of about 15 species in four genera. Upon further scrutiny of the little Georgia specimen and comparison to appropriate literature, it became apparent that it represented a population of the Asian Rice Eel, Monopterus albus, that was somehow now thriving halfway around the globe from its native waters. The Asian Rice Eel, as currently construed, might actually consist of several highly similar species but much further taxonomic study will be required to resolve this question. For now, the best handle for the creature we found is Asian Rice Eel. It is so named because of its success in the flooded rice paddies of Asia where it is considered an important food fish and a delicacy at meal time.

This finding fueled intense curiosity about the status of the population of aliens—how many were there, how large do they grow, what were they eating, how far had they spread? In July 1995, we attempted to collect a series of rice eels, thoroughly working much of the shoreline of the smallest impoundment with fine mesh dip nets. Load after load of muddy vegetation and debris was searched for the squirming little eels to no avail. Finally, we encountered what must have been a nest beneath an undercut bank lined with grass roots. Fourteen tiny rice eels, barely an inch long, lay writhing in the net. As happy as we were to finally locate these little critters, we also knew it confirmed our fears that an eel invasion was in full swing, and they were reproducing quite successfully.

We had several juvenile rice eels up to 4 inches in length that had been collected sporadically over the months since discovery, but we had never seen an adult. Publications state that rice eels may reach over 3 feet; we very much wanted to know where the big boys were in this population. Since fishing is not allowed in the Nature Center's ponds, no adult eels had ever been snagged by anglers to our knowledge. Baited minnow traps failed to capture any rice eels, and dragging small seines through the vegetation-choked pond was ineffective. It was time to invoke some technology!

Electroshocking fishing rigs are known to be extremely effective for collecting American Eels, so we reasoned they might be similarly effective for rice eels. These rigs, which may be small backpack units or large boat-mounted shockers, project a field of DC current into the water that interacts with the sensory and muscular systems of fishes so that they are involuntarily attracted to the positive pole. The fish is momentarily stunned and incapacitated, facilitating its capture. Once out of the current, most fish quickly recover and swim away. Electroshockers are an extremely effective tool for sampling difficult habitats without causing undue mortality. But, when using electroshocking gear, with all that voltage in the water, one must wear insulation in the form of rubber waders and gloves to avoid receiving a radically different hairdo.

In August 1996, technology arrived on the backs of Mike Spencer and John Biagi, Georgia Department of Natural Resources biologists with an interest in nonnative species who lent their equipment and expertise to the cause. Properly suited up in rubber attire, we waded into the waters of the smallest pond on the Nature Center grounds. And yes, in spite of their distant relationship to true eels, rice eels act the same way when current is applied to the water. Within seconds of beginning shocking, the bright orange belly of a large rice eel, turgid from electrotetanus, rolled to the surface and was easily dipped into a waiting bucket. It was a striking sight that put the exclamation point on the suspended line of suspicion that had grown from that first little fellow in the dish.

Despite the extremely turbid waters of the pond, 15 adult and smaller rice eels were dipped within a few minutes; many more doubtless escaped detection beneath the murk. Moving on to the two larger impoundments on the center grounds, we further confirmed our suspicions that the rice eels had dispersed among all three ponds. Nine additional specimens were collected in just a few minutes. The largest was 2 feet in length with several others approaching that size.

Once placed in containers, the rice eels recovered almost immediately. Once they calmed from their initial attempts to escape their new confinement, they could be seen occasionally rising slowly to the top, just penetrating the surface with the very tip of their snouts. Remember this, too, for it bears much relevance to later discussion. A few eels were kept alive for experiments with possible control measures by Spencer and Biagi. The remainder were preserved and returned by Wayne Starnes, curator of fishes, at the North Carolina State Museum of Natural Sciences in Raleigh, North Carolina for further study.

In September 1997, we returned to the Nature Center ponds with Mike Spencer. Again, rice eels were easily shocked from the three impoundments. Several more were preserved for study and some were kept alive for educational programs, both at the Nature Center and the museum in Raleigh. At this time, and in 1996, we thoroughly electroshocked selected reaches of the marshlands adjacent to the Chattahoochee River into which the impoundments drain; no rice eels rolled to the surface. We further shocked backwaters and marshland along the river and lower portions of tributaries upstream and downstream of the center as well as some nearby small impoundments. No rice eels were detected beyond the waters of the Nature Center.

Amazing Creatures

Despite their unwelcome presence, one cannot help but be in awe of the amazing repertoire of adaptations rice eels possess—a veritable evolutionary bag of tricks. First of all, remember the surface-dappling behavior exhibited by the rice eels we placed in containers? Well, these fish are air breathers.

The Asian Rice Eel and several other species of the Asian-African genus Monopterus are among the most highly derived of air breathing fishes, even surpassing other synbranchids. The gills of rice eels are highly modified, having the normal gill filaments greatly reduced and, in their stead, numerous fleshy folds of mucus-laden tissue. These folds increase surface area and are highly invested with blood vessels, served by specialized arterial arrangements, to promote easy gas exchange, much like a lung. And, instead of having rigid gill covers like most fishes, rice eels have an inflatable, baglike cavity surrounding the gills. Further, the gill slit is limited to the underside of the head enabling air bubbles to be easily trapped in the gill chamber.

When rice eels stick their snouts above the surface, they take in a large volume of air and hold it in the chamber for up to several minutes, exchanging carbon dioxide for oxygen before exhaling and rising for another breath. In this way they are able to tolerate the anoxic conditions that often dominate their favored swampy habitats in drier seasons and are perfectly capable of living in wet mud if necessary. In fact, under captive conditions, a rice eel survived out of water for seven months, on damp towels, with no food, at normal room temperatures. And, as if the special gills are not enough, laboratory experiments show this fish is capable of obtaining one-fourth of its oxygen through its skin.

The primary haunts of rice eels are burrows that may go several feet beneath banks and have chambers above water level where they can take air making them virtually invulnerable to capture by predators. They can live in this habitat for many months until wetter times return. Rice eels are scaleless and have highly developed slime organs in the skin, much more so than most other fish groups. They are exceedingly slippery, and very difficult to hold. This adaptation provides lubrication to reduce friction and protect them both against injury and capture, prevents desiccation, and probably helps facilitate the exchange of gases through the skin.

If rice eels don't like where they are living, they may simply just pick up and move over land. In Indonesia, under humid conditions, they are documented to abandon habitats and move in groups of 15 to 20 some distances over land to new haunts during the night or early morning hours, especially just prior to the breeding season. Under captive conditions, with controlled experiments, it was demonstrated that rice eels leave the water frequently when food density becomes low, when their own population density becomes high (even if food remains plentiful), when large competing predators are present that are not aggressive towards them, or when other large aggressive fishes are present.

In addition to possible quests for new breeding grounds to colonize, these may be the conditions under which rice eels most often migrate to new surroundings. In the wild, they do not move to new habitats just because their home happens to dry up. Instead, they just wait for more water in their puddles or burrows as noted above.

Under captive conditions, rice eels are voracious feeders. We have had several rice eels in captivity over the past couple of years, including five sharing a shallow aquarium at the museum in Raleigh. It is an eerie sight to walk into a room and have five ominous heads rise to the surface, orient to your presence, and follow you about the room with almost demanding expectancy. The limits of their gluttony are hard to establish. In nature, rice eels are effective predators on a wide variety of small prey items, including fish, shrimp, crayfish, insects, worms, frogs, tadpoles, and frog eggs. In the aquarium, they will take one nightcrawler or minnow after another.

A rice eel typically lurks in a crevice or burrow until an unwary quarry is in its sights (and probably its range of smell). With a great smacking sound, the unfortunate one is violently sucked into the rice eel's throat by a sudden vacuum created by rapid expansion of the massive gill chamber. Critters too large to suck in straight away are struck violently, and some part of them is sucked into the mouth. The prey is somehow wedged or braced whereupon the rice eel spins rapidly until the prey item is wrenched apart. Similar feeding behavior is exhibited by true eels and even crocodiles.

In the aquarium, the doubly unfortunate prey item is one that falls victim to two rice eels at once–which promptly face each other and spin at high speeds in opposite directions until the victim is torn to pieces. In nature, one might speculate that the prey is snatched and wedged into the entrance of the burrow and reduced to smaller pieces while the eel spins in the tunnel.

Rice eels are truly amphibious fishes. In the wild, they are demonstrably capable of leaving the water and feeding on land, again, if necessary, by pinning the prey against the ground and spinning rapidly until it is in small enough pieces to ingest. It has been speculated that, out of the water medium, suction feeding could not occur. However, we have fed rice eels out of water and, in conjunction with a small strike, they are quite capable of sucking in smaller food items.

If you aren't impressed yet, wait until you hear about the rice eels' sexuality! Aside from their air-breathing capabilities, perhaps the most remarkable attribute of these fishes is their transsexual life-style. In many populations, all young are hatched as females. After maturing and functioning as females, the rice eels pass through a nonfunctional stage that lasts up to a year or more, and then they transform into large males. These rulers of the labyrinths in the mud, themselves perhaps former members of some large male's harem, apparently acquire such an assemblage with which they breed. They are thought to harbor and guard eggs and young near the mouths of their burrows in a bubble nest. Few potential predators on the young are likely to be successful with the big male lying in wait. Rice eels, then, are probably highly successful parents much of the time.

The sex reversal phenomenon is found in all other members of the swamp eel family, Synbranchidae, and is certainly not unheard of in other fishes as well. In fact, it occurs in several other families of fishes unrelated to synbranchids. However, not all populations of rice eels are the same. In some populations, some of the young are apparently hatched as males (called primary males) and function as such throughout their life while the remainder are hatched as females. But these females, as described above, transform into secondary males, and all members of the population that survive into adulthood eventually become large males.

The Experiment—Where Did It Come From, Where Will It Go?

Clearly, we are up against an incredible and potentially formidable invader to Georgia's waters. The rice eel's presence and fate in these waters is an experiment in nature. And just how was such an experiment launched? Probably completely unwittingly by some well-meaning pet owner whose rice eels, interesting and entertaining as young, soon became non grata because of the voracious behavior described above.

The Chattahoochee Nature Center is frequented by many people. Most doubtless have some love of nature and probably many maintain aquaria in their homes. These aquarists, for the most part, are probably conscientious about any pets they dispose of. However, others, although misguided, think the humane thing to do is release their pets into local waters. The Nature Center ponds, known to so many and so accessible, is unfortunately perhaps the logical choice of such persons.

Rice eels, though not a major import, are easily obtainable in the aquarium trade. We purchased a 6-inch eel for only $7.99 from a local fish store in Atlanta. Without extensive genetic studies, it is hard to say from just how many rice eels the Nature Center populations stem. They are probably the descendants of very few dumpees, perhaps just a single pair. It is tempting to consider the possibility that a few transsexual fishes may be capable of self-fertilization at some point in their lives. However, there is no evidence so far that this is possible in rice eels.

And just what will the experiment's outcome be? How will the rice eels fare? How far will they go? First, they are clearly here to stay unless they are eliminated through some intrinsic limitation thus far unknown to us. While rice eels occur over much of the tropics of Asia, they are also temperate animals, occurring well northward into China. They have already withstood several of the coldest winters that Georgia has ever experienced and have flourished in their wake. Heavy freezes and ice cover have not daunted our invader. At our latitude, and perhaps even farther north, climatic elements are not a factor.

We are apparently as powerless to control the rice eel experiment as we are the climatic elements. No measures currently known to fisheries biologists can come close to eradicating the invader. Traditional fish-collecting chemicals that are safe for humans, such as rotenone, act on fishes by blocking oxygen uptake from water by the gills. Since rice eels breathe air rather than water, they are completely protected from this threat. Lethal amounts of chlorine and rotenone were tried on captive specimens by Mike Spencer at the Georgia DNR lab, but the invaders seemingly showed disdain and simply crawled out of the water until the water quality improved. With their labyrinths of under-bank and above-water chambers, no chemical is going to reach much of the population. Draining the impoundments is also not an effective option. Remember those dried swamps in Indonesia?

The impoundments could probably be drained, chemicals applied, and the whole area burned and enough rice eels would probably survive to repopulate in short order. Could one "drown" this fish? After all, they are air breathers. Rice eels are capable of respiring while confined beneath water for indefinite periods if a reasonable amount of oxygen is present in the water. If it isn't, remember their habitats include those back door burrows with air chambers. And, as effective as electroshocking is, even if thousands were removed by this method at great expense and effort, a few survivors would doubtless survive in remote burrows and ultimately prevail.

"Unfortunately, the illegal release of rice eels into ponds at the Chattahoochee Nature Center is a perfect example of an introduction that cannot be corrected," says Mike Spencer, chief of fisheries for the Georgia Department of Natural Resources. "Because rice eels tolerate cold water, breathe air, make deep burrows, survive desiccation, and can move over land, there are no techniques we can use to eradicate them from the ponds. Also, their close proximity to the Chattahoochee River, their movement into several ponds and ditches on the Nature Center, and the fact that they have been there for many years make it highly likely that they have already left the ponds and are in the river. We now have to wait and see what impact this unfortunate, illegal release will have on native fish and wildlife in the Chattahoochee River drainage."

No one knows whether or not Georgia's rice eel population is still confined to the Nature Center's grounds. In just a few years, these fishes dispersed among the center's three impoundments. This was not an easy feat—at least not for the average fish. The two larger impoundments, of several acres each, drain one to the other. But the upper lake is about 50 feet higher than the lower. If the invaders were first established in the more accessible lower impoundment, then they scaled a nearly vertical rocky spillway to colonize the upper pond. The small third impoundment drains independently to the Chattahoochee River from the larger two. Barring a helping hand from humans, our skilled travelers either dispersed overland or made an end run through the overflow marshes along the river. Given how difficult the eels are to capture, the hitchhiking hypothesis seems unlikely.

The prospect of the end run through the marshes drives home the realization of the rice eels' potential for dispersal. Situated as they are at the Chattahoochee's margin, the center's impoundments are a virtual springboard to the rest of the drainage. The fact that we located no rice eels in the marshes or nearby tributaries is not conclusive evidence that they have not ventured to new waters. The Chattahoochee's waters run cold by the center due to the discharges of Buford Dam a few miles upstream. Although the rice eel is a temperate beast and capable of withstanding very cold waters, perhaps if left to its preferences, it may not enter cooler water frequently. Only time will tell if, by chance or design, rice eels will stray to new haunts or actively seek them—or whether the cool waters will indefinitely curtail these ventures.

If they escape these bonds, how far may they go? The uplands of Georgia probably do not present much of the preferred habitat of the rice eel. Though they are quite capable of using streams for dispersal and inhabiting the more sluggish portions, there is nothing like a good swamp for a rice eel. But if, after many years, the swamp-lover is able to make it to Georgia's Coastal Plain, then it should know no bounds. And, to make matters worse, it could meet its kin coming from the south. Recent discoveries have revealed other rice eel invasions underway in the waterways of Florida. Populations are now known from tributaries to Tampa Bay and in waterways north of Miami. The opportunities for dispersal in Florida's interconnected maze of waters are unlimited. Again, only the rice eel's own intrinsic limitations, if it has any, may keep it from one day, maybe a hundred or more years from now, spreading throughout our coastal lowlands from Texas to Chesapeake Bay.

Only time will tell of the impacts that this intrepid interloper may have on our aquatic habitats. We know it can live many months with no food if need be, but we also know its appetite if given its way. We do not know what the center's eels have been eating and have no historic data to assess changes in the pond's populations of native species. Most of the dozen rice eel specimens examined thus far have had little food in their stomachs, but these were not collected at night when this predator is probably most active. However, one specimen contained a crayfish and another the partially digested remains of two 3-inch fishes, a Bluegill and a probable juvenile Largemouth Bass. Fish in the smaller impoundment are now sparse. Electroshocking most of the shore revealed the presence of few bass or sunfish. On the other hand, in the larger, deeper impoundments, good populations of Bluegill appear to remain.

The rice eel did not evolve in North America. It is an exotic species and thus has no natural built-in balance with the ecosystems and coinhabitants it finds here. It may exploit to detriment or, somehow, fit in; the latter is rarely the case with exotics.

How about any possible benefits? One could argue that, chopped up in sections and sauteed in delicious sauces as they are in Asia, rice eels could become a delectable food fish. If enough Georgians actually develop sufficiently broad and daring palates to embrace the exotic eel, only a few people would probably spend the time required to entice the edible-sized eels from their burrows. This marginal benefit is far surpassed by their potential to do harm to our native fishes and aquatic ecosystems.

So only one thing is sure. The rice eel will do what it wants to do within the limits of its capabilities, which are more than considerable. We can only stand by helplessly and watch the experiment unfold.

The Lesson

What can we learn from this experiment? Never mind the possible outcomes. We should first learn, like a nuclear scientist, that potential runaway experiments, with no controls, are to be feared and avoided. Introductions of exotic species are just that. There are numerous examples of nonnative fish or invertebrate species that have severely impacted native and more desirable faunas and floras. In the case of pet fishes, because most are tropical, ninety-nine times out of a hundred, there are not dire consequences for releasing them in Georgia waters. However, inevitably, this type of unconscious experiment will blow up in our faces given enough repetitions. The next potential explosion may be smoldering along the bank of the Chattahoochee River.

Read and add comments about this page

Go back to previous page. Go to Atlanta's Urban Wildlife contents page. Go to Sherpa Guides home.

[ Previous Topic | Next Topic ]