Elongated body, somewhat compressed posteriorly with large, dorsally flattened head.Eyes are positioned on top of the head. Mouth is large and upward-facing with a protrusible lower jaw.Presence of a venomous spine behind the operculum (gill cover).Some species, including U. guttatus, have an electric organ behind the eyes.Typically grayish to brown with dark or pale spots; camouflage pattern adapted to sandy or muddy substrates.