(Hornell)
1. Distinctive characters of early developmental stages
(a) Eggs and Prolarvae (Not from the mangroves) (Largely after Vijiyaraghavan, 1973)
Eggs occur in bunch (egg mass) and each egg mass contains over 200 eggs. Spawning usually occurs in neritic waters and at times unusually close to the shore near estuarine mangroves. The fishermen operating catamarans in the coromandel coast at times use mangrove vegetation as fish lure during the spawning season of this species. At the time of spawning, the fishes are trapped and the fish lure containing attached egg mass are left in the sea itself. Individual fertilized eggs are spherical with about 1.85 mm diameter. Eggs are colourless, transparent and possess a tough capsule. Three types of transparent filaments arise from the surface of the egg membrane for adhesion. They include:
1) One main anchoring filament which is long (103.5 mm length) and thick (0.36 mm diameter) arising from the basal pole of the egg.
2) Median filaments (3 to 5 in number) scattered irregularly over the zona radiata and restricted to basal half of egg's surface which measure 4.6 mm length and 0.017 mm diameter on an average. They aid in formation of central rope to hold the mass of eggs together and
3) Small filaments (5 to 12 in number) with 1.1 mm mean length and 0.009 mm mean diameter, which originate from the distal pale of the egg and free ends float. The yolk is colourless, filling the entire egg and fairly transparent. At times, few minute colourless oil globules of assorted size are scattered irregularly within the yolk. In some abnormal eggs, perivitelline space is large with 0.2 to 0.4 mm thickness. Four hours after fertilization, a blastular cap of 0.8 mm diameter (blastoderm stage) is formed at the distal pole, beneath the small filaments. Embryonic axis is formed six hours after blastoderm stage. In another 2 hours, blastopore is closed and Kupffer's vesicle is formed. Rate of embryonic development varies within the same batch. One day old embryo has 23 myomeres, extending over half the circumference of the egg. The embryo possesses eyes, otic vesicles and rudiments of pectoral fins. The feebly pulsating heart is situated under the head between eyes. The colourless blood is corposcular. Few greyish pigments are present between eyes and frontal lobes of brain. Two days old embryo develops prominent vitalline circulation. Heart lies anterior to the head and sinus venorus is pointing anteriorily. Embryo with 25 myomeres occupies two-thirds the circumference of the egg. Hatching occurs mostly during night. The incubation period is around 5 days. However, certain embryos hatch out much early due to differential growth rate and probable "shooting fry effect". The prolarvae within a day after hatching measure 3.5 - 4.2 mm length and swim actively by vigorously flapping the large transparent pectoral fins. The prolarva on the third day after hatching measures around 4 mm and on the fifth day it becomes post larva (with no yolk) and measure around 5 mm in total length.
(b) Post larva (Based on a single specimen measuring 6.5 mm T.L.)
Hirundichthys coromandelensis 1
Snout located terminally; Jaws are of equal length and not protruded; chin barbels absent; pectoral fins possess 17 rays and yet to reach dorsal fin origin; Pectoral fins not yet enlarges as 'wings'. A single dorsal fin which is located posteriorly has 9 rays and no pigmentation. Pelvic fin is large, abdominal in position and reaching anal fin origin. Pelvic fin origin slightly closer to anal fin origin than to pectoral fin insertion. Anal opening behind the middle of body; anal fin origin below the 3rd dorsal ray. Caudal fin truncate / rounded and ventral lobe of caudal fin yet to grow bigger in size. Lateral line (which runs closer to ventral side of the body) has not yet developed at this stage. The colour of the larva is yellowish brown. Stellate pigments occur densely throughout the entire body.
(c) Juveniles and Adults
Not yet recorded form the mangrove waterways.
2. Distinguishing characters of early developmental stages in similar species occurring in the adjacent neritic/oceanic waters
(a) Eggs of H. oxycephalus
Group of shortest filaments which arise from the pole opposite to main anchoring filament are much longer than those of H. coromandelensis and are more uniform in length.
(b) Larvae and early juveniles
1) Cypselurus poecilopterus
Lower jaw shorter than upper jaw. Anal fin with 7 - 9 rays. Enlarged pectoral fin with many dark pigmented spots.
2) Larvae and early juveniles of Exocoetus volitans
Ventral fins short. Anal fin has 14 rays. Dorsal fin relatively low in height.
3) Larvae and early juveniles of Parexocoetus mento and P. brachypterus
The dorsal fin high with more black pigmentation. In early juveniles, upper jaw is protrusible.
(c) Early juveniles and juveniles of Hirundichthys oxycephalus
The pectoral fins dark with small pale basal triangle and narrow pale margin. (In the case of H. coromandelensis , the dark pectoral fins possess broad pale transverse stripe.)
3. Salient biological characteristics
Maximum length is 190 mm in standard length, and 220 mm in total length. It seems to breed during post north-east monsoon months (January - February) and summer/ south west monsoon months (May-July) in western part of Bay of Bengal.
4. Salient ecological information
It is a coastal cum oceanic species whose larvae very rarely inhabit lower reaches of estuaries/brackishwaters, especially during spring high tides of post northeast monsoon season. Juveniles and adults do not invade mangroves. This species has gliding (flying) behaviour by using the pectoral fins which are used as 'wings'. Adults are pelagic and dwell in neritic and oceanic waters.
5. Capture fisheries
In coromandel coast, the fishes of this species are caught during breeding season by using twigs of trees as fish lure (including mangrove vegetation) - thus capitalizing the breeding behaviour of the fish for capture.