(Shaw)
1. Distinctive characteristics of early developmental stages
(a) Eggs
Not yet reported from the mangrove waters.
(b) Larvae
Eleutheronema tetradactylum 1 (Courtesy: A. Termvidchakorn, Thailand)
In larvae measuring around 7.5 mm in length, the mouth is large with prominent upper and lower jaws. The cleft of the mouth is located slightly towards the ventral profile of the head. The lower lip does not show partial development at this stage. The pectoral fins are not yet fully developed. The spinous portion of the anal fin is traceable to some extent but not the spinous portion of the 1st and 2nd dorsal fins (which look like rays). The caudal fin is somewhat truncated. There are 9-10 pre-anal myomeres and 15-16 post-anal myomeres at this stage. As the larval form grows, the pre-anal myomere count increases and the corresponding post-anal myomere count decreases within the given range due to the posterior shifting of the anus. A black pigment spot is present at the nape region of the larvae (and at limes in the upper jaw).
Eleutheronema tetradactylum 2 (after Venkataramanujam, 1975)
In 16 mm post-larvae, the mouth becomes inferior, and 4 free pectoral filamentous rays are traceable. The caudal fin becomes moderately forked.
(c) Early juveniles
Eleutheronema tetradactylum 3
Early juveniles measuring over 20 mm possess dark chromatophores in the occipital region, the first dorsal, the distance between the first and second dorsal, and in the pelvic fins. Also, chromatophores are present from the base of the anal fin to the caudal fin in rows on either side of the body. Free pectoral rays do not reach beyond the pectoral base. The posterior tip of the upper and lower lobes of the caudal fin are somehow produced to appear deeply forked. As in adults, the lower lip is not fully developed.
2. Distinguishing characteristics of early developmental stages in similar species occurring in the adjacent marine environment
(a) Post- larvae of Polynemus sextarius (after Bensam, 1983)
The pre-anal and post-anal myomere count varies between 5-7 and 16-18 respectively (9-10 and 15-16 respectively in E. tetradactylum).
(b) Early juveniles and juveniles
1) Polynemus sextarius
There are six free rays in the lower pectorals of P. sextarius. A black blotch is present just behind the shoulder in juveniles and adults (only 4 free rays in the lower pectoral and no blotch in the shoulder in the case of E. tetradactylum).
2) Polynemus indicus
The body is blackish with 5 free pectoral filaments extending beyond the pelvic fin base. The caudal fin lobes are highly developed and the caudal fin is deeply lunate (in the case of E. tetradactylum the body is light greenish-blue dorsally and yellowish (golden) ventrally. The caudal fin lobes are somewhat developed in young ones and not produced in adults of E. tetradactylum. Only 4 short free pectoral filaments are present in E. tetradactylum which do not reach beyond the pelvic fin base). The pectoral fins are blackish in colour (not so in E. tetradactylum). There are 5 free pectoral filaments (only 4 in E. tetradactylum).
3) Polynemus heptadactylus
The lower portion of the pectorals possesses 7 free filaments (only 4 in the case of E. tetradactylum). The pectoral fin is blackish in colour (not so in E. tetradactylum).
4) Polynemus sexfilis
The lower portion of the pectorals have 6 free filaments (only 4 in the case of E. tetradactylum). The pectoral fins are black in colour (not so in E. tetradactylum).
5) Polynemus paradiseus
The lower pectorals have 7 free filamentous rays of which the upper 3 are the longest, extending as far as 200% of the body length (only 4 free pectoral filaments not extending beyond the pelvic fin base in the case of E. tetradactylum).
3. Salient biological characteristics (after Fischer and Bianchi, 1984; Bal and Rao, 1990; Jhingran and Gopalakrishnan, 1974; Jhingran and Natarajan, 1973; Kagwade, 1970; Karandikar and Palekar, 1951)
It grows to a maximum size of 2 metres in length and weighs over 145 kg. In natural waters it grows between 190-300 mm in length at the end of the first year (depending upon the fertility of the environment) and over 370 mm at the end of the second year.
This species is a voracious carnivore and at times cannibalistic. Juveniles measuring up to 3 cm feed on cape pods, mysids and lucifers. Young ones measuring between 3-6 cm feed on planktonic crustaceans, mysids, small prawns, shrimps and fish larvae. Specimens measuring above 6 cm feed on shrimps, polychaetes, decapods, stomatopods and fish.
Males attain first maturity at a size of 243 mm (225 mm according to some other reports) and females at 285 mm. Hermaphroditism is occasionally reported in this species. It has an extended breeding season (almost round the year) on the Maharashtra coast with peak spawning during January-April and July-September. On the Tamil Nadu and Orissa coasts the peak spawning season for this species was reported to be during the northeast monsoon (October-December). However, based on the recruitment pattern of the early stages of fish in the mangroves, it seems to breed during February-April and August-September off the Southeast Indian coast. According to certain reports, this species spawns at the mouth of large rivers – however this needs further confirmation. The ripe ova are transparent, measuring between 0.7-1.0 mm and possess an oil globule.
4. Salient ecological information (after Jeyaseelan, 1981; Fischer and Bianchi, 1984; Bal and Rao, 1990; Kagwade, 1970)
This species prefers prey (especially shrimps) from rich shallow muddy coasts. The juveniles up to 215 mm in length use shallow mangroves as a nursery ground and exhibits a tolerance towards a wide range of salinity fluctuations. Specimens of over 215 mm move to coastal waters for further growth. It is distributed throughout the Indo-Pacific region except the East coast of Africa.
5. Capture fisheries and utilization (after Fischer and Bianchi, 1984 and Bal and Rao, 1990)
Among the 9 species of Polynemids reported from the Indian waters, next to P. indicus, E. tetradactylum constitutes an important fishery (on the contrary both species occur in the mangroves, but E. tetradactylum is dominant among the two in the mangrove waters). E. tetradactylum constitutes a good fishery in certain estuarine mangroves (eg. the Hooghly Matlah estuaries), probably because of the absence of any coastal-cum-offshore trawling grounds in the adjacent region. On the other hand, it constitutes a good fishery in the coastal-cum-offshore trawling grounds on the northwest coast of India.
This demersal carnivore is captured by bottom set gill-nets, bottom trawls, beach seines, boat seines and long lines. It is a highly prized species with firm white flesh. Marketed mostly fresh and occasionally sun-dried.
6. Culture fisheries (after Jhingran and Gopalakrishnan, 1974 and Jhingran and Natarajan, 1973)
This species was cultured in the brackish water impoundments of India and Bangladesh in an extensive style. It was also cultured in fresh water experimentally after proper acclimatization. This ‘white salmon’ has many desirable qualities for promotion in brackish water and sea water aquafarming and the first progressive step is to establish a hatchery production technology. It is a candidate species for intensive farming especially in cages (its potential had not yet been realized by the aquaculturists). It can also be cultured along with Tilapia in brackish water ponds. This species can serve as a useful genetic resource for ‘rotation crop’ in shrimp farms to check disease outbreaks.