Macrognathus is a genus of spiny eels in the family Mastacembelidae, native to fresh and brackish waters across South and Southeast Asia — India, Myanmar, Thailand, Cambodia, Laos, Vietnam, Malaysia and Indonesia all host species in the atlas. Despite the name, these are not true eels (order Anguilliformes) but a separate, eel-shaped lineage: an elongate, laterally compressed body, no pelvic fins, a pointed, fleshy proboscis over the mouth, and a row of small, isolated spines running along the back ahead of the dorsal fin that gives the group its common name. Members span a wide size range within the genus, from modest stream species up to the 60+ cm giants recorded in the atlas.
The best-known aquarium species is the peacock eel, M. siamensis, a long-standing staple of the spiny-eel hobby prized for its sinuous swimming, nocturnal habits and appetite for snails and worms; the barred M. pancalus and belted M. circumcinctus also see some trade. Spiny eels are burrowers by nature and do best with soft substrate to dig into and a tight-fitting lid, since they are accomplished escape artists. Most Macrognathus remain Not Evaluated or Least Concern, but one, M. pentophthalmos — the Sri Lanka spiny eel — is listed Critically Endangered, a reminder that this otherwise resilient-looking group has at least one genuinely imperilled member.