(Valenciennes, 1847)
1. Distinctive characters of early developmental stages
(a) Eggs and prolarvae
Not reported from the mangrove waterways.
(b) Post larvae and early juveniles (Based on 10.8 to 31 mm total length specimens)
Zenarchopterus dispar 1
Dorsal half of the body pale with less pigmentation and at times yellowish/light brownish in colour. The ventral half of the body darkly pigmented. The early juveniles are hardy with stout and stumpy body. Lower jaw slightly elongated in post larvae and protruded well and measuring 20 to 22% of body length in early juveniles. Early juveniles and juveniles do not possess fimbriated nasal papilla. Scales yet to develop. Dorsal fin is darkly pigmented and with 10 to 11 rays. Anal fin has 11 to 12 rays. Sexual dimorphism (6th and 7th anal rays of males thick and enlarged and 4th dorsal ray elongated) not discernible at this stage. Pelvic fins originate very closer to anal fin than to pectoral. Caudal fin round.
2. Distinguishing characters of early developmental stages in similar species occurring in the mangroves
(a) Early juveniles of Hyporhamphus (Hyporhamphus) limbatus
Caudal fin slightly forked (emarginate - not strongly forked) and with a black border. Anal fin with relatively more (13-17) rays. (In Z. dispar the anal fin has 11-12 rays. Caudal fin is round and the entire dorsal fin is black in colour). Dorsal fin has 13-16 rays. (In Z. dispar the dorsal fin has 10-11 rays only).
(b) Early juveniles of Rhynchorhamphus georgii and R. malabaricus
Fimbriate nasal papilla is present. (Collette, 1984b and Collette, 1976) (Absent in Z. dispar ).
(c) Early juveniles and juveniles of other species of the genus Hemiramphus
Caudal fin deeply forked (caudal fin round in Z. dispar ). Scales if developed are absent on snout. Preorbital ridge absent (Collette, 1984b).
3. Salient biological characteristics
(a) Size
Maximum length is 170 mm.
(b) Sexual dimorphism
Males possess enlarged 6th and 7th anal fin rays.
4. Salient ecological information
It occurs in rivers and estuaries including the mangrove waterways of India, Malaysia, Thailand and Singapore. It is a 'viviparous' half-beak. Moreover, the early juveniles when possessing feeble locomotor power are neustonic and float on the surface of estuarine mangroves in lower reaches, mimicking like twigs of mangrove vegetation to avoid predation.
5. Salient fishery related information
It was reported as commercially important species in India (Jhingran, 1983).