Marbled Swamp Eel

Fascinating facts about the Marbled Swamp Eel

• A marbled swamp eel can grow to a maximum length of about 150 cm, while most adults are around 50 cm.
• When out of water, the eel can breathe using the blood-rich lining of its mouth and pharynx, while in water it uses its gills.
• In laboratory tests, a marbled swamp eel survived more than six months in a burrow that was slowly drying out, periodically crawling through the tunnel to stay moist. Its endurance shows how fish can outlast droughts that would kill most aquatic life.
• The species starts life as either a primary male or a female, and females transition into secondary males at a length of 45–60 cm.
• The eel can live upstream of large waterfalls and is a major predator of tadpoles in those locations.

At a glance

About the Marbled Swamp Eel

The marbled swamp eel, Synbranchus marmoratus, is a sleek, elongated fish that can reach up to 150 cm (60 in) but most adults linger around 50 cm (20 in). Its cylindrical body lacks pectoral and pelvic fins, and the dorsal and anal fins are reduced to vestiges, giving it a serpentine silhouette that glides through the tangled roots and leaf litter of Central and South American waterways. Beneath its mottled, marbled skin lies a mouth lined with a richly vascularised membrane, a secondary respiratory surface that kicks in when the eel abandons the water to explore the damp earth of riverbanks. By day it burrows deep enough to stay below the water table, emerging at night to hunt; by night it becomes a voracious predator, snapping up frogs, tadpoles, fish, spiders, insects and other invertebrates with equal relish. Its nocturnal forays into shallow pools and dense vegetation are punctuated by daring descents into burrows, where it can survive months of drying conditions, sustained by the oxygen-rich lining of its mouth and pharynx. A remarkable twist in its life history is sequential hermaphroditism: juveniles may start as primary males or females, and those that become females later transform into secondary males at 45–60 cm (18–24 in), a strategy that fuels rapid colonisation of new or harsh habitats. The secondary males dig and guard nests, ensuring the next generation of these adaptable eels can continue their secretive, amphibious existence.

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