A serpentine fish native to Caldoria. The eel has a large sucker-mouth at one end, but is otherwise undistinguished. Its skin is smooth and brown, and it grows up to five metres long, though three to four metres are more common. The eels inhabit the mineral-rich swamps of equatorial Caldoria, consuming electrical static. As they swim, electric arcs discharge from stalactites over the water and are absorbed through the skin of the mouth. The eels’ digestive systems act as natural capacitors, storing energy and regulating it to body functions as needed. Occasionally they supplement their energy supply through carrion eating. On the whole, they’re reasonably sluggish creatures, though they can move on land with surprising ease, having primitive lungs along with gills. They glide through water by rippling ambulatory muscles beneath the skin, and crawl on land by similar means. They bite when threatened, and sometimes release a mild discharge to shock enemies. Other than this, they’re harmless.

Seeking out energy-rich environments, the eels frequently make their way onto spacecraft. They attach to internal systems, being carried as stowaways to other planets. It’s due to this behaviour that the creature earned its common name in the wider galaxy. There are thousands of eel species on Caldoria, but this is the type known to offworlders. Thus it’s simply called “Caldorian eel”. Although they rarely deplete large energy sources, they can siphon enough power to impair integrated systems notably. Smaller devices might well be drained of power completely. Eels can survive out of their natural habitat without problem, so long as they have a power source to feed on. Thus they’re now established on worlds across the galaxy, though most settlements eliminate them swiftly when they’re found. The eels aren’t dangerous and breed slowly, but at lengths of up to five metres they’re often startling. Finding a serpent of that length in a maintenance tube can spoil an engineer’s day.

Klim Dokachin, aboard a derelict freighter brought to Starfleet Surplus Depot Z15, once found a 4-metre long Caldorian eel in a storage locker. He nursed it back to health and took it as a pet. As of 2368, he still had it. In that year he offered to show the eel to Counsellor Deanna Troi, having mentioned it in conversation.