(Cuvier and Valenciennes, 1828)
1. Distinctive characteristics of early developmental stages
(a) Eggs
Not found in the shallow turbid mangrove waters, but from the adjacent estuaries with relatively clear water and a moderate depth of 2-3 m.
(b) Larvae and early juveniles
Ambassis commersoni 1; Ambassis commersoni 2; Ambassis commersoni 3
Recently hatched larvae measure 0.9 mm in length and have 10 pre-anal and 15 post- anal myomeres. The eyes are devoid of pigment at this stage.
In pro-larvae measuring 1.2 mm in length, the mouth and alimentary canal are formed. The fin fold is continuous from the dorsal through to the ventral caudal region. Pigment forms a patch between the 1st and 18th myomeres and also in front of the oil globule.
Two days after hatching, the larvae measure 3-9 mm in length and possess an air bladder. The paired fins and median fins are well developed. Pigment is present as a line between the 11th and 24th myomeres at the lower margin of the body. Two pigment patches are also present between the 1st and 3rd myomeres.
Post-larvae measuring between 5-7.5 mm in length are transparent with a yellowish tinge colouration. The abdomen and air bladder are silvery. Black pigment concentrates around the alimentary canal and air bladder. Above the eyes there are two prominent chromatophores. The first dorsal fin has seven spines (of which the first spine is the shortest). The second dorsal fin has a spine and 10 rays. The paired pelvic fins have spines and 5 rays in each. The anal fin has 3 spines and 10 rays.
The early juveniles measuring 25 mm, have a continuous lateral line without interruption. The gap between the 1st and 2nd dorsal has yet to become wider. Preoperculum serrations are yet to become pronounced.
2. Distinguishing characteristics of early developmental stages in similar species occurring in the mangroves
(a) Early juveniles of Ambassis gymnocephalus
Lateral line disrupted. (Not disrupted in A. commersoni ).
3. Salient biological characteristics
This species grows to a maximum length of 100 mm (in S.L.) and 26 g in weight. Seven stages of maturity were described. During March to May, fish with stage V gonads were observed to be more abundant whereas during August to September, both mature and spent individuals were collected in good numbers from the Southeast Indian coasts. The fish matures for the first time at a size of 51-61 mm T.L. Females weighing 3.1-26 g have 13,000-36,800 eggs in the gonads.
4. Salient ecological information
Among the species of the Ambassis genus, certain species dwell exclusively in fresh water, whereas A. commersoni is an euryhaline brackish water species penetrating even fresh water. It is a carnivore preferably feeding on mysids, Acetes and other shrimp juveniles in the mangrove ecosystem. There is no information about larval feeding. It reproduces well in brackish water and the baby fish of this species serve as forage fish for many marine carnivorous juveniles in the mangroves.
5. Evolutionary significance
On the coastal plain of the western part of the Bay of Bengal, the Ambassis genus or Ambassidae family is represented by a limited number of species (less than 5 species in both freshwater and brackish water habitats) whereas in the eastern part of the Bay of Bengal (which receives relatively more rain from the monsoon and is endowed with diverse biotopes) the genus/family is represented by over 10 species – thus reflecting a high degree of breeding.
6. Capture fisheries
It constitutes one of the important estuarine fisheries in Southeast Indian brackish waters. It is active during the night and caught more at the night catch than during the day. It is also a useful bait fish.
7. Culture fisheries
Because of its white coloured flesh and flavour, and better quality than the trash fish, this species is caught locally from brackish waters and used as feed (after cooking) for extensive farming of Penaeus monodon and other shrimps on the Southwest Coast of India.