(Linnaeus)
1. Distinctive characteristics of early developmental stages
(a) Eggs and Larvae
Not occurring in the mangroves.
(b) Early juveniles
Siganus javus 1
The procumbent spine located in front of the spinous dorsal fin is hidden in the nape. Pearly spots in the upper half of the body extend up to the soft dorsal fin. The difference in length of dorsal fin spines is negligible at this stage. The lower half of the body possesses pale coloured wavy (undulating) lines. In certain specimens, dark pigmentation occurs near the hypural plate and the base of the soft dorsal.
2. Distinguishing characteristics of early juveniles in similar species occurring in the mangroves
(a) Siganus canaliculatus
Silvery undulating lines are absent in the lower half of the body (present in S. javus ). A black blotch is present at the shoulder (absent in S. javus ).
3. Salient biological characteristics
It grows to a maximum size of 450 mm in total length in coastal waters. The maximum total length recorded for this species in shallow mangroves was 175 mm. It is a herbivore, feeding on algal weeds, epiphytic diatoms and Cyanophyceae in the algal and sea-grass beds of the mangrove environment. Phytoplankters, copepods and detritus were also found in the gut content of juveniles collected from the lower reaches of the mangroves.
Based on the early juveniles recruitment studies in the Southeast Indian mangroves, spread over a 4 year period, this species seems to breed over an extended period except during the southwest monsoon (June-August) and a brief pause during February and October. Year to year fluctuations in the abundance of hatchlings and a mild deviation from the normal spawning seasons are not uncommon in the Southeast Indian region.
4. Salient ecological information
This species is primarily a demersal herbivore. It shows wider geographic distribution from the Gulf through the coast of India, Malaysia, Thailand, Indonesia, the Philippines and up to Papua New Guinea in the south and northwards up to the southern most islands of Japan. Among the 25 known species of siganids from the world oceans, only 6 species occur in global mangroves. The coral reef associated Philippine mangroves support relatively more species of siganids (up to 5 spp.). In the Southeast Indian mangroves (Pichavaram mangroves) three species occur of which S. javus is the dominant one and S. lineatus being a rare species. It survives in low saline conditions relatively better than S. canaliculatus . The juveniles occur up to mixo-mesohaline conditions of salinity in mangrove waters (with lower limits of salinity being 5 ppt). It is caught by trawls, gill-nets and traps from coastal waters (Fischer and Bianchi, 1984). In addition, the juveniles in schools are subjected to undersized destructive fishing by drag-nets, stake-nets and cast-nets, especially during the northeast monsoon months in the mangrove waters (Jeyaseelan, 1981). It is a candidate species for brackish water herbivorous fish culture as wild hatchlings are available from the mangroves in sufficient numbers seasonally for initiating small scale farming. Information regarding the contribution by this species towards capture and culture fisheries from different countries and environments are lacking.