Diagnosis: Ambassis ambassis has the following combination of characters: a single supraorbital spine; rostral spine absent; preopercle ridge and rear margin serrate; interopercle edge smooth except for few small serrae at angle; 2-3 rows of cheek scales; predorsal scales 13-18; lateral line continuous (normally); lower gill rakers 19-23; pectoral-fin rays 14-15 (Ref. 50956).

Description: Dorsal fin with 8 spines and 9-10 soft rays; anal fin with 3 spines and 9-11 soft rays; pectoral fin with 14-15 rays (Ref. 50956, 52193). Lateral line usually continuous, with 27-31 scales when so, very infrequently 1-2 scales without tubes interrupt lateral line below juncture of dorsal fins; vertical scale rows 29-32; horizontal scale rows 9-10; cheek scale rows 2-3; predorsal scales 13-18 (Ref. 50956). Gill rakers 7-10 + 19-23 (Ref. 50956). A single supraorbital spine; preorbital ridge smooth in early juveniles, becoming serrate by about 35-40 mm standard length; preorbital edge with 5-12 serrae; retrorse rostral spine absent; preopercle ridge with 7-22 serrae; lower edge of preopercle with 12-26 serrae, these extending dorsally along rear margin of preopercle, its entire height in specimens larger than 30 mm standard length; interopercle smooth except for 3-10 minute serrae at angle; teeth in single row on palatines; in three patches on vomer in larger specimens (Ref. 50956).

Colouration: Live colouration translucent greenish-brown dorsally, head and abdomen silvery; scales with minute melanophores; membrane between second and third dorsal and anal spines dusky, black at tip; black lineations along bases of unpaired fins; lobes of caudal fin dusky toward upper and lower margins (Ref. 50956).