(Gray, 1835)
1. Distinctive characters of early developmental stages
(After Vijayaraghavan, 1957; Whitehead, 1972; Bensam, 1983)
(a) Eggs
Thryssa hamiltonii 1; Thryssa hamiltonii 2
The transparent eggs though spherical are not perfectly spherical; yolk is segmented; the size of the eggs range from1.1 mm to 1.27 mm in diameter; the eggs have no oil globules and devoid of pigmentation; segmented germinal disc is present in early stages of the egg; spherical eggs become more irregular as the embryo develops; somites are formed in the embryo; the tail of the embryo emerges out of the yolk in the advanced stages of development within the egg.
(b) Larvae
Thryssa hamiltonii 3
The prolarvae measuring 3.7-4 mm length are transparent and without pigmentation, including the eyes. Yolk mass tapers posteriorly. Auditory vesicles clearly seen and pectoral fins develop as a small process above the yolk. Fin folds are continuous and entire. Post-larvae measuring 4.2 mm onwards have black pigments in the iris of the eye. Silvery appearance of the eye is evident from 4.5 mm onwards. Mouth is wide and subterminal. Pre-anal myomeres are 30 in number from 3.7 mm prolarva up to 30 mm post larva. Post anal myomere count vary between 16 and 17. Caudal fin rays start appearing in 4.5 mm post larva. Dorsal and anal fins start appearing in 6.8 mm post larva. The basals develop in dorsal and anal fins at 16 mm postlarva. Black branching chromatophores appear in the dorsal and ventral sides of the myomeres from the anal to the caudal tip when the larva measures 4.4 mm in length. Chromatophores line the dorsal and ventral walls of alimentary canal when the larva measures 5 mm in length. The black chromatophores in the myomeres of caudal region move to caudal fin folds, whereas, the chromatophores in the alimentary tract become lesser and scattered when the larva reaches a size of 16 mm in total length. Black chromatophores are seen in the lower half of caudal fin and few stellate pigments scattered along the alimentary canal are trapped inside the thin muscular wall.
(c) Juveniles (after Whitehead, 1972)
Maxilla short, net reaching to pectoral base; gill rakers 12-14; anal with 38-43 rays.
2. Distinctive characters of early developmental stages in similar species occurring in the mangroves
(a) Eggs of Thryssa mystax
Relatively smaller in diameter (0.8 to 0.85 mm).
(b) Larvae of Thryssa mystax
Pre-anal myomere count is 28. (30 in the case of T. hamiltonii ).
(c) Juveniles
1) Thryssa mystax
Maxilla reaching up to pectoral base.
2) Thryssa setirostris
Maxilla reaching beyond pelvic fin origin.
3) Thryssa vitrirostris
Gill raker serrae in distinct clumps; maxilla , reaching to 1/3 along pectoral; supra maxilla present.
3. Salient biological (including ecological) information
It is a pelagic species occurring in large shoals in coastal waters. It grows to a maximum size of 200 mm in length. Specimens measuring less than 130 mm commonly occur in mangrove waterways. the juveniles of the species dwell even at low saline waters of 5 ppt salinity in the mangrove waterways. Among the Thryssa species occurring in mangroves, this species has remarkable salinity tolerance, especially in juvenile stage. The eggs and larval forms occur only in lower reaches of mangroves where mixo-euhaline condition prevails. Juvenile fishes feed on sergestids and juvenile shrimps in the mangrove waterways. Adults in marine environment feed on planktonic organisms.