(Bloch, 1786)
1. Distinctive characteristics of early developmental stages
(a) Eggs and pro-larvae
Not yet recorded from the mangroves.
(b) Post-larvae and early juveniles
Triacanthus biaculeatus 1
The post larvae measuring around 4 mm in size possess a spinous dorsal fin with five spines of which the first spine is very strong and the second spine is relatively very short and thin. The second dorsal fin has over 22 rays. The anal fin has 18 rays. Pelvic spines are visible at this stage. The caudal fin is round in shape whereas in juveniles and adults it is strongly bifurcated.
Triacanthus biaculeatus 2
Early juveniles measuring around 18 mm possess a laterally compressed body and 8 or 9 small vertical bands below the dorsal fins. In addition, the region between the pectoral fin and the abdomen is highly pigmented. A prominent first spine of the spinous dorsal and two prominent pelvic spines, each one on each side, are present. In specimens measuring around 30 mm, the second spine of the spinous dorsal is very small and measures about one fifth the height of the first dorsal spine.
The spinous dorsal possesses a dark spot in the membrane portion between the first and second spine (in adults the entire membrane portion of the spinous dorsal is darkly pigmented). Around 12-14 narrow vertical bars made of dark pigmentation are present along the body. The caudal peduncle is long and the caudal fin is moderately bifurcated. The height of the second dorsal and anal fins are proportionately higher when compared to those of adults.
2. Distinguishing characteristics of early developmental stages in similar species occurring in the mangroves
(a) Post-larvae of Pseudotriacanthus strigilifer
Pseudotriacanthus strigilifer 1; Pseudotriacanthus strigilifer 2
The second spine of the spinous dorsal is more than half the length of the first spine. Dark pigmentation connects the base of the first dorsal spine and the anus with a broad patch in the middle of the body. Pigmentation is also present at the middle portion of the base of the soft dorsal, besides lining the upper and lower portions of the caudal fin. The caudal fin is almost rhomboid at this stage (and becomes well bifurcated in adults).
3. Salient biological characteristics
T. biaculeatus grows to a maximum size of 300 mm in length in coastal waters. This species feeds on benthic invertebrates in a marine environment (after Fischer and Bianchi, 1984). In mangrove waters, the juveniles and early juveniles of this species feed on algal weeds and epiphytic diatoms. Crustacean appendages were also observed in the gut content of this species.
4. Salient ecological information
This species dwells in up to 60 m depth in coastal and estuarine waters. In shallow mangroves, it occurs in the algal weed and sea grass biotopes within the lower reaches.
5. Capture fisheries and utilization
Bottom trawls and gill-nets are used for fishing from the seas. It is marketed in a fresh condition but is the least preferred among certain groups of people in India and considered as a trash fish.