Mainly marine; some species enter freshwater. Distribution: Indo-West Pacific; one species in the Mediterranean. Strongly compressed, slimy body. Scales small. Head naked, bearing bony ridges on upper surface. Gill membranes united with isthmus. Mouth small; very protractile. Pseudobranchiae absent. Dorsal fin with 8 or 9 spines that are somewhat elevated. Three spines on anal fin. Dorsal and anal fin spines with a locking mechanism. A scaly sheath at the base of dorsal and anal fins. All species have esophageal luminous organs. Also noted for their production of mucus (see common name). Common in shallow coastal waters and tidal creeks and easily caught by trawls or beach seines; important artisanal food fish.