October 28, 2019

In September, scientists from the National Museum of Natural History at the Smithsonian Institution in Washington, D.C., announced the discovery of a new type of electric eel that has the strongest shock of all bioelectric animals: Electrophorus voltai. A native of South America’s Amazon River, E. voltai generates a powerful 860 volts, more than seven times the voltage of a typical American wall socket (120 volts). The eel is named after Alessandro Volta, the Italian physicist who invented the battery.

Electrophorus voltai, seen here, is one of two new electric eel species recently discovered in the Amazon. Credit: © L. Sousa

From 2014 to 2017, the Smithsonian team studied Amazonian electric eels with researchers from Brazil’s University of São Paulo. They tested the eels’ voltages and studied their muscle structures, body shapes, and DNA. To their surprise, the Amazon electric eel—long thought to be a single species, Electrophorus electricus—turned out to be three distinct species. E. electricus remained as the main type, but the team named E. voltai and another electric eel, Electrophorus varii, as new species. The three species differ in voltage as well as in head shape, sense organs, and distribution. E. electricus lives in northern Amazon basin waters, while E. voltai inhabits waters further south. E. varii swims among the slow-flowing lowland Amazon basin waters.

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The Amazon River, the world’s second longest river, is 4,000 miles (6,437 kilometers) long. Credit: WORLD BOOK map

The new findings reemphasize the incredible diversity of the Amazon River and rain forest, much of which is still unknown to science, as well as the importance of conservation and saving the region from deforestation, logging, and fires.

The electric eel stuns its enemies and prey with a powerful electric shock. The electricity-producing organs take up most of the body. The other inner organs lie just behind the head. Credit: © Andre Seale, Alamy Images

Electric eels are a long, narrow fish that produce strong electric discharges, or shocks. The animals are not true eels, but rather a type of knifefish.  An electric eel discharge can kill a fish and stun such potential predators as caimans (large reptiles) or humans. However, electric eels rarely harm people in the wild.

February 17, 2017

Earlier in February, scientists learned that the dorado catfish (also known as the dourada or gilded catfish) of South America has the longest migration of any freshwater fish. These large catfish live in the Amazon River Basin, and their migratory path takes them from the foothills of the Andes Mountains to the mouth of the Amazon River on the Atlantic Ocean, and then back again—a trip of more than 7,200 miles (11,600 kilometers)!

The long-traveling dorado catfish navigates the waters of the Amazon River Basin throughout its life. Credit: © Michael Goulding, Wildlife Conservation Society

Catfish have pairs of fleshy whiskerlike growths near the mouth. These growths, called barbels, resemble the whiskers of a cat. Catfish differ from most other fish in that they do not have scales. Catfish live in many different places around the world, and they come in a variety of shapes and sizes. The dorado catfish (Brachyplatystoma rousseauxii) is just one of several different species (kinds) of catfish that live in the Amazon River and its many tributaries. Dorados can grow beyond 6 feet (2 meters) in length and are grouped with other “goliath” catfish, so named because of their large proportions. Their lengthy migration has long been suspected, but only recently have the details of their epic migratory journey been confirmed. A study of the distribution of larvae, juveniles, and mature dorados showed where the fish tend to be at different stages of life. The study, published in the journal Scientific Reports, was a group effort led by the Wildlife Conservation Society’s Amazon Waters Initiative. Ronaldo Barthem from the Museu Paraense Emílio Goeldi in Brazil led the research team.

This map shows the remarkable migration of the dorado catfish through the Amazon River Basin. Credit: © Wildlife Conservation Society

Barthem and his team found that dorados spawn in the Andes waters of Colombia, Ecuador, Peru, and Bolivia. The larvae then begin a precarious journey downriver, finding their way through such tributaries as the Madeira, the Marañón, and the Rio Negro—all major rivers themselves—to the mighty Amazon itself. The young catfish grow as they travel eastward, following the currents across Brazil to the eventual end of the Amazon north of Marajó Island on the Atlantic coast. There the fish gather in the vast and organically rich waters of the estuary, feeding, growing, and maturing. After a couple years, the fish get the urge to travel back upriver, taking the long swim—another two years—back to the spawning grounds where the life cycle begins again.

Dorados also live in the rivers of Guyana and Venezuela, and they are an important food fish for people throughout the Amazon Basin. Dorado catfish are not an endangered species, but their migration becomes increasingly difficult each year because of damming, deforestation, mining, and pollution.

Salmon and eels are other fish known for their long migrations, but their routes combine saltwater and freshwater routes and still fall short of the long swim of el dorado.

July 18, 2016

It lives in your mouth, slicing your tongue with its razor-sharp jaws and sucking up blood. After your tongue withers away, it takes its place. You can only be thankful it stays, since you would slowly starve if it left. It is a tongue-eating louse, and, if fish could dream, it would haunt their nightmares.

An unlucky fish opens wide to display the tongue-eating louse. Credit: © Brian Saunders

The tongue-eating louse (Cymothoa exigua) is a parasitic marine isopod that infests the mouths and gill cavities of fish. A parasite is a living thing that feeds off another living thing, called a host. An isopod is a type of crustacean that commonly has seven pairs of legs. Tongue-eating lice are known to infest several kinds of fish, but their primary host is snapper.

Tongue-eating lice larvae swim into a fish’s gills and grow there, feeding on the animal’s blood, skin cells, and mucus. As they grow, the largest louse in the fish migrates into the fish’s mouth, anchoring itself with its strong legs. It then pierces the tongue with its sharp jaws and drinks the blood. Eventually, the tongue withers away because of a lack of blood. Rather than let its host die, the louse instead takes the place of the fish’s tongue, helping to hold prey in the fish’s mouth. The louse may continue to feed on blood or grab stray scraps of food, or it might not feed at all once it has reached full size. There are many species of isopods that parasitize fish, but Cymothoa exigua is the only one known to consume and replace a fish’s tongue.

All tongue-eating lice are born male. At some point during the process of destroying the tongue, the largest louse transforms to a female, and the smaller male lice mate with it. Researchers think the female only releases its larvae when the host fish is near other potential hosts, such as when fish swim in schools. Scientists are not sure what the female louse does after the next generation is sent out to infest more fish. Most marine isopods generally give birth to only one brood, so the female probably dies shortly after the larvae are released. Most marine biologists suspect that the fish slowly starves to death once its replacement tongue is gone, unable to effectively hold struggling prey in its mouth. It is possible, however, that another male living in the gills takes the female’s place (and sex), functionally becoming the fish’s new tongue.

Two things are certain. First, more work must be done to study this bizarre animal. Like other marine isopods, the tongue-eating louse infests several kinds of fish that people catch and eat. Consequently, it is important to understand the louse’s life cycle and any negative effects it has on its host. Second, we can thank our lucky stars the tongue-eating louse doesn’t infest humans!

March 28

The U.S. Environmental Protection Agency (EPA) released the results of a study on March 26 that revealed that 55 percent of rivers and streams in the United States were in poor biological condition for aquatic life. Nutrient pollution–phosphorus and nitrogen runoff from cities and farms that enters into rivers and streams–formed the  major problem within the 1.2 million miles (1.9 million kilometers) of U.S. waterways.

Large amounts of nutrients in water lead to problems for many animals that live in water. Unnaturally high levels of nitrogen and phosphorus–which are the major components of fertilizer in agriculture and waste in sewage systems–leads to a process known as eutrophication. The excess nutrients benefit algae and allow them to grow much faster. But, an overgrowth of algae, called algal bloom–decreases the amount of oxygen in the water, which leads to the death of fish and other aquatic life. In addition, some algal blooms create poisons (toxins) that are harmful or even fatal to humans.

Excess nutrients from such sources as fertilizers and untreated sewage upset the balance in aquatic systems. This process is called eutrophication. The algae grow faster than the fish can eat them. As more algae grow, more also die and block light needed by aquatic plants. Bacteria use up much oxygen consuming the excess dead algae. The oxygen level of the water drops, causing many aquatic plants and animals to die and decay, using still more oxygen. (WORLD BOOK illustration by Michael Yurkovic)

According to the EPA, their study indicated that the western United States had the best stream and river quality, with 42 percent in good condition. In the East and the plains, only 17 percent and 16 percent, respectively, of waterways were in good condition.

Additional World Book articles:

• Environmental pollution (2008 back in time)
• Water pollution

A website of interest:

• United States Environmental Protection Agency: Nutrient Pollution