SWAMP AND SPINY EELS: Synbranchiformes

PHYSICAL CHARACTERISTICS

Swamp and spiny eels reach a length of 8 to 48 inches (20 to 150 centimeters). Although they look like eels, these fishes are not related to true eels. Scales are either absent or very small. These fishes do not have a swim bladder, an internal sac that fishes use to control their position in the water. The dorsal (DOOR-suhl) and anal (AY-nuhl) fins are low and join each other at the tip of the tail. The pelvic fins, if present, are small and on the throat. The dorsal fin is the one along the midline of the back. The anal fin is the one along the midline of the belly. The pelvic fins, the rear pair, correspond to the rear legs of four-footed animals.

GEOGRAPHIC RANGE

Swamp and spiny eels are native to Africa, Asia, Australia, South America, and North America only as far north as Mexico. They have been introduced in Florida, Georgia, and Hawaii in the United States.

HABITAT

Swamp and spiny eels live in swamps, caves, and sluggish freshwater and in water with a low salt content. They live in leaf litter and mats of fine tree roots along the banks. Swamp eels can move overland, and some can live out of water for a long time. Some species are burrowers and dig holes in which to live. Four species are found only in caves.

DIET

Swamp and spiny eels eat bottom-dwelling fishes and invertebrates (in-VER-teh-brehts), or animals without a backbone, especially larvae (LAR-vee), which are animals in an early stage that must change form before becoming adults.

BEHAVIOR AND REPRODUCTION

Some swamp and spiny eels can breathe air with special structures in the throat. They usually are active only at night. In some species of swamp eels, adult males grow a head hump and are larger than females. These fishes lay about forty eggs at a time. The eggs have a pair of long threads for attaching themselves to the bottom material. Reproduction takes place during the wet season. Females probably spawn, or release eggs, more than once but only in one season. These fishes do not have a long life span.

SWAMP AND SPINY EELS AND PEOPLE

In some parts of Asia, swamp eels and one species of spiny eel are valued as food and are sometimes kept in ponds or rice fields.

CONSERVATION STATUS

The World Conservation Union (IUCN) lists one species of swamp and spiny eels as Endangered, or facing a very high risk of extinction in the wild.

MARBLED SWAMP EEL (Synbranchus marmoratus): SPECIES ACCOUNT

Physical characteristics: Marbled swamp eels grow to about 59 inches (150 centimeters). They have a long cylindrical body, no pectoral or pelvic fins, and very small dorsal and anal fins. The pectoral (PECK-ter-uhl) fins, the front pair, correspond to the front legs of four-footed animals.

Geographic range: Marbled swamp eels live in North and South America from Mexico to central Argentina.

Habitat: Marbled swamp eels live in clear or murky freshwater and in water with a low salt content in a variety of habitats, including streams, canals, ponds, and rice fields.

Diet: Marbled swamp eels eat fish and invertebrates and are considered aggressive predators (PREH-duh-ters), or animals that hunt and kill other animals for food.

Behavior and reproduction: Marbled swamp eels are active at night, usually at the edge of the water. They can travel over land for long distances. They also burrow, especially during the dry season. During the time in the burrow the metabolism slows, but the fish still may flee if disturbed. After the first rains, marbled swamp eels return to larger bodies of water. Many marbled swamp eels function first as a female and then as a male. Others remain young females. Marbled swamp eels use external fertilization (FUR-teh-lih-zay-shun), meaning egg and sperm unite outside the body.

Marbled swamp eels and people: Marbled swamp eels sometimes appear in public aquariums, although they do not make a good exhibit because of their burrowing behavior.

Conservation status: Marbled swamp eels are not threatened or endangered. ∎

FOR MORE INFORMATION

Books:

Berra, Tim M. Freshwater Fish Distribution. San Diego, CA: Academic Press, 2001.

Web sites:

Bricking, Erica M. "Asian Swamp Eel (Monopterus Albus)." Introduced Species Summary Project. http://www.columbia.edu/itc/cerc/danoffburg/invasion_bio/inv_spp_summ/Monopterus_albus.html (accessed on October 14, 2004).

"Frequently Asked Questions about the Asian Swamp Eel." United States Geological Survey. http://cars.er.usgs.gov/Nonindigenous_Species/Swamp_eel_FAQs/swamp_eel_faqs.html (accessed on October 14, 2004).