(Valenciennes, 1847)
1. Distinctive characters of early developmental stages
(a) Eggs.
Escualosa thoracata egg
Pelagic spherical eggs of 0.7 to 0.85 mm diameter. Oil globules 4 to 10 in number, of yellow colouring and concentrated on the ventral periphery of a colourless, coarsely segmented yolk initially and disbursed during the embryonic development. Perivitelline space fairly wide, occupying about one fourth of the radius (Devanesan and John, 1941; Nair, 1952a; Delsman, 1926c; Delsman, 1933b).
(b) Larvae
Escualosa thoracata 1
Escualosa thoracata 2
Newly hatched larvae of about 1-5 mm in length has a segmented yolk with about 8 to 10 oil globules. Immediately after hatching the larvae is reported to float upside down for a while. The yolk in the larva is rounded off at the hind end. The anus is situated backward. As the larva grows, oil globules decrease in size and disappear. Pre-anal myomeres are 33-34 in number and post-anal myomeres are 7-8 in number. The alimentary canal is straight. In larvae of 10 to 12 mm, around 6 rod shaped pigment patches are seen on the ventral side of the body and on the dorsal side of the alimentary canal. Between the pelvic and anal fins, the older larvae possess around 35 peculiar transverse folds in the inner wall of the gut.
(c) Early juveniles (Based on 44 mm specimen)
Escualosa thoracata 3
Early juveniles possess 18 pre-pelvic and 10 post-pelvic scutes. Dark pigment is present on the dorsal side of the head, from the dorsal fin base to the caudal fin base and on the base of the anal fin.
2. Distinguishing characters of similar species occurring in the adjacent marine environment
(a) Eggs of Anodontostoma chacunda
Eggs relatively bigger - over 1 mm in diameter and with 6-12 oil globules; egg membrane completely filled by the egg, thus making the perivitelline space very narrow.
3. Salient biological characteristics
(a) Maximum size and growth
Maximum length 12 cm. It grows from 2.5 cm to 7.2 cm in 6 to 7 months in natural waters.
(b) Food and feeding habits
Feeds principally on zooplankton and occasionally on phytoplankton in a marine environment. In the mangroves the juveniles and adults of 20 to 100 mm size feed on planktonic calanoid copepods, planktonic centric diatoma gastropod veligers, detritus, bivalve veligers, planktonic cyclopoid copepods, nauplii of crustaceans and copepodite stages of crustaceans in descending order of importance (Fischer and Bianchi, 1984).
(c) Reproductive Biology
Differing populations of this species spawn from October to February in shallow coastal waters with peak spawning between November and January. Right side gonad development is completely suppressed and the left side ovary and testis are functional. Each ovary comprises about 8000 eggs. Spawning occurs during midnight/early morning. Individual fish spawn only once in its life span of one year (Fischer and Bianchi, 1984; Bal and Rao, 1990).
4. Salient ecological information
(a) Habitat
Spawns in shallow coastal waters; embryonic eggs and larvae occur occasionally in the lower reaches of the mangroves in the breeding seasons. Studies in the lower reaches of the mangrove waterways of South East India on the catch per unit revealed that peak occurrence coincided with peak values of primary production (both gross and net) and net phytoplankton biomass during the post-summer season (June-July), although this species feeds principally on zooplankton in the mangroves. The juvenile and adult ingress of the species into the mangroves from the coastal waters coincided with a strong wind velocity period from the south-west monsoon (June-July) (Jeyaseelan, 1981).
(b) Geographic distribution
West Coast of India, throughout the Indo-Pacific up to the north-east and north-west coasts of Australia.
(c) Behaviour
It is a shoaling fish grazing on planktonic swarms which occurs generally with Hilsa kelee in the mangroves in bulk quantities during the south west monsoon (June-July).
(d) Fishery
This species is caught in boat seines, beach seines, gill-nets, lift-nets and shallow trawls in a marine environment. A cast-net is used principally to exploit this species from the mangroves. The fishing season is from July to March with a peak during September-November in the west coast of India. The mangrove fishery in the south east Indian region is restricted to June-July (Jeyaseelan, 1981, Fischer and Bianchi, 1984 and Bal and Rao, 1990).