(Linnaeus)
1. Distinctive characteristics of early developmental stages
(a) Eggs
No information from the mangroves.
(b) Larvae (tholichthys stage) (Based on 11-13 mm specimens) (Largely after Termvidchakorn, 1989 and Nair, 1952b)
Scatophagus argus 1 (Courtesy: A. Termvidchakorn, Thailand)
The body is somewhat oval in shape and very compressed. The head is very large and possesses bony plates. A strong shoulder spine (supra scapular spine) is present. The dorsal fin has 10 or 11 spines and 16-18 rays.
The anal fin has 4 spines and 14-15 rays. The pectoral fin has about 20 rays, while the caudal fin possesses 16 rays. The spinous dorsal fin is a bright purple colour with a brownish tinge posteriorly. Black chromatophores and red pigmented cell groups are present in the body.
As the tholichthys stage transforms into the early juvenile stage, the red pigmented cell groups form distinct red patches in five locations, all in the dorsal profile. They include (a) the nape, (b) the spinous dorsal origin, (c) the junction between the spinous and soft dorsal, (d) on the middle of the soft dorsal, and (e) at the end of the soft dorsal/beginning of the caudal peduncle.
2. Distinguishing characteristics of early stages in similar species occurring in the adjacent protected coastal waters
(a) Juveniles of Scatophagus tetracanthus
Four broad vertical cross bars are present (in the case of Scatophagus argus juveniles, numerous greenish black blotches are present in the upper portion of the body which is greenish in colour, while the blotches below the lateral line are relatively smaller).
3. Salient biological characteristics
It grows to a maximum size of 310 mm in length and feeds on detritus and small benthic invertebrates (Fischer and Bianchi, 1984). In South Indian mangroves, the juveniles of this species feed on algal weeds, Cyanophyceae, epiphytic diatoms, detritus, crab zoea and fish scales (Nair and Dharmamba, 1960).
4. Salient ecological information
It occurs in sheltered coastal waters, estuaries and mangroves and moves upstream to the freshwater zone. It is a school forming species. The tholichthys stage (a special larval stage in this species) is very hardy and occurs even in highly turbid waters with a wide variation range of salinity. This larval stage has a pointed spine in the shoulder for protection. The adult forms have spines in the fins which are mildly venomous and aid in self protection. In the lower reaches of the South Indian mangroves, the young ones of this species are often associated with algal and seaweed beds. This species is distributed throughout India, Sri Lanka, Malaysia, Singapore, Thailand, Indonesia, China, Taiwan and the northern part of Australia.
5. Capture fisheries and utilization
This species is caught from brackish waters (including the mangroves) by cast-net. It does not form a major fishery anywhere in its range of distribution. It is also caught with traps and gill-nets (Fischer and Bianchi, 1984). and is reported to be much sought after as food for its trout-like flavour (Anon, 1962). However, because of its scavenging habit, it does not fetch a remunerative price in local markets.
6. Culture fisheries
This species is reported to be amenable for brackish water and fresh water aquaculture but on a small scale level (Anon, 1962). On the other hand, it is also considered as a pest, entering the coastal ponds through sluice gates along with tidal waters.
It may prove to be good for polyculture systems because of its capability to utilize the niche hitherto unutilized through its scavenging feeding habit. However, detailed research is needed in this regard to confirm this concept and to assess its co-existence with other polyculture species. This species is an excellent aquarium fish in its juvenile stage, readily suitable for marine, brackish and fresh water aquaria.