(Bleeker)
1. Distinctive characteristics of early developmental stages
(a) Eggs
No information.
(b) Larva (Based on 4.48 mm specimens). (after Agate et al., 1991)
Johnius macropterus 1 (Courtesy: A-Termvidchakorn, Thailand)
The larva has a blunt mental barbel in the chin, without any pore on the tip. The second part of the dorsal fin has one spine and 30 soft rays. The caudal fin is round in shape (rhomboid in juveniles and adults). A lateral line has not yet developed. The length of the mental barbel, the height of the first spine of the spinous dorsal, the height of the first spine of the anal fin and the size of the pectoral fins are proportionately higher in the post-larva than in adults. On the other hand, the second spine of the anal fin is proportionately larger in fully grown fish than in the post-larva. The preoperculum region has five spine-like serrations of which 4 are located at the angular region. The mouth is almost terminal or mildly sub-terminal in the post-larval stage (whilst it is distinctly sub-terminal in adults).
2. Distinguishing characteristics of early stages in similar species occurring in the mangroves
(a) Early juveniles of Dendrophysa russelli
The mental barbel on the chin is relatively longer and more pointed than in J. macropterus. The spinous dorsal and nape are highly pigmented (not so in J. macropterus ). The second part of the dorsal fin has 25-28 soft rays (27-34 soft rays in the case of J. macropterus ). (This species is very common in the mangroves of India and Thailand (Jeyaseelan, 1981 and Agate et al., 1991).
(b) Early juveniles of Johnius dussumieri
A blunt mental barbel is present on the chin, but the second part of the dorsal fin has one spine and 23-26 soft rays (there are 27-34 soft rays in Johnius macropterus ) (J. dussumieri occur in the mangroves of Singapore and Thailand (Gomez, 1980 and Agate et al., 1991).)
(c) Early juveniles of Nibea albida
A pair of small tapering barbels are present on the chin (a single barbel in the case of Johnius macropterus ). Even the adults of this species occur in the shallow mangroves of southeast India (Jeyaseelan, 1981).)
3. Distinguishing characteristics of early stages in similar species occurring in the marine environment
(a) Juveniles of Umbrina canariensis
The mental barbel has a pore at the tip (no pore present at the tip of the barbel in J. macropterus ). The second part of the dorsal fin has a spine and 27-29 soft rays (27-34 soft rays in J. macropterus ). The body is highly pigmented with 9 bands (not so in J. macropterus ). The spinous dorsal fin is fully black, while the soft dorsal, anal and pelvic fins have black bands (not so in J. macropterus ).
(b) Juveniles of Umbrina ronchus
The mental barbel is present but with a pore at the tip (no pore at the tip of barbel in J. macropterus ). The second part of the dorsal fin has one spine and 23-25 soft rays (27-34 soft rays in J. macropterus ). The caudal fin is sub-truncated (the caudal fin is almost round in post-larvae and rhomboid in adults in the case of Johnius macropterus ).
4. Salient biological characteristics
This species grows to a maximum size of 250 mm in total length. It is a carnivore which feeds on benthic worms and crustaceans.
5. Salient ecological and evolution related information
Among the mangroves of Indo-Pacific, the larvae and juveniles of this species were recorded only from the mangroves of Thailand. The adults with a sub-terminal mouth are benthic feeders, while the larvae with a nearly terminal mouth could be a planktonic/pelagic feeder.
In general, the sciaenids with well developed air bladders are demersal fishes and are mainly caught in bottom trawls. The sciaenidae family is noted for its diversity and species richness. The complexity of the family in evolving into many genera and species may be attributed to its greater degree of association with brackish water (with widely fluctuating environmental variables) rather than with the typically constant marine environment (Jeyaseelan et al., 1991 and Jeyaseelan and Krishnamurthy, 1981).
In the mangroves of the Indo-Pacific, over 15 species of Sciaenidae have been reported so far, of which the juveniles of Johnius macropterus occur rarely in Indian and Thailand mangroves (Jeyaseelan, 1981 and Agate et al., 1991). However, it is interesting to note that the larvae of this species occur in the mangroves of Thailand.