(Bloch)
1. Distinctive characters of early developmental stages (Largely after Bensam, 1983)
(a) Eggs
Nematalosa nasus egg
The eggs are spherical, transparent and pelagic. The diameter of the egg is almost 1 mm. Egg contains eight oil globules of varying size (0.035 to 0.08 mm diameter). Oil globules are yellow in colour and are concentrated near the tail. Yolk is round and not much coarsely segmented. Perivitelline space is narrow. Embryo hatches after 55 hours and just prior to hatching, pigment spots appear on the dorsal side of embryo.
(b) Larvae
Nematalosa nasus 1; Nematalosa nasus 2
The prolarva measures 3 mm in length. Yolk sac is globular. Oil globules ventral in position in the segmented yolk. Yolk sac round off posteriorly and tapering gradually into alimentary canal. Dorsal fin fold has black pigments. Pre-anal myomere count is 35. Post-anal myomeres are not discernible due to imperfect myosepta and the existence of vacuoles in notochord. Prolarva possesses the remnant of egg membrane around the yolk. In 9 mm larva, the dorsal fin contains 8 rays and anal fin about 15 rays. There are 12 pigment spots on each side of the larva, all black, one in front of of opercular opening, 2 on the posterior side of pectoral fins, 6 in foregut region, 2 in midgut and one above vent.
2. Distinguishing characters of early developmental stages of similar species occurring in the adjacent marine environment
(a) Egg of Anodontostoma chacunda
Anodontostoma chacunda egg
Yolk is relatively coarsely segmented (relatively finely segmented in N. nasus ). Size of the egg is 0.95 mm in diameter. The shape is spherical. A wide range of 6 to 12 small oil globules are present. (Only 8 oil globules in N. nasus ). Perivitelline space is very narrow. Fully developed embryo in the egg occupies more than 80% of the yolk surface (only about 70% in the case of N. nasus ). The embryo hatches in 12 hours in tropical environment. (In N. nasus the embryo hatches out after 55 hours in tropical environment).
(b) Larvae of Anodontostoma chacunda (After Delsman, 1926c and Delsman, 1933b)
Anodontostoma chacunda 1; Anodontostoma chacunda 2
The newly hatched larva measures 2.7 mm (3 mm in the case of N. nasus ). Prolarva possesses a total of 44 to 45 myomeres of which 37 are pre-anal and the rest are post-anal in location. (However, in the adults, the vertebral count is just 41 only. As a general rule, myomere count in the early life history of fishes represent the vertebral count of adults. Further studies will bring more information regarding clear identity of this species). Post-anal myomeres are countable and vary from 7 to 8. (In the case of N. nasus , the post-anal myomeres of early larvae are not clearly countable). Yolk in the prolarva still has seqments and oil globules are located around ventral part of the yolk. In the prolarva yolk is not lined by egg membrane. (In the case of N. nasus remnant egg membrane borders the yolk).
In the second day after hatch ing itself the larvae measure around 5.1 mm and is without yolk. (In the case of N. nasus , the absorption of yolk is delayed considerably).
(c) Early juveniles and juveniles of Anodontostoma chacunda
Early juveniles and juveniles do not have filamentous last ray in dorsal fin. (In the case of N. nasus the last ray of dorsal fin is filamentous). The juveniles (and even adults) possess a single series of pre-dorsal scales. (In the case of N. nasus , the pre-dorsal scales are in overlapping double series).
3. Salient biological characteristics
(a) Size
Maximum length 24.5 cm in brackish water and marine environment.
(b) Food and feeding habits of juveniles and adults in the mangrove environment
Detritus, silt, benthic diatoms, planktonic diatoms, planktonic copepods, nematodes, tintinnids and foraminiferans. (Jeyaseelan, 1981)
(c) Reproductive biology
Six maturity stages of ovary identified through ova diameter studies. It spawns only once a year during a definite short period. It spawns during October - November in West coast of India (Annigeri, 1963). It spawns during north-east monsoon (October - December) or post monsoon (February - March) seasons in south east Indian region, depending upon favourable climatological conditions prevailing in different years.
4. Salient ecological information
(a) Habitat and abundance
Adults are marine. Juveniles and adults frequently visit the mangroves and brackish water. Spawning takes place in the sea, very much closer to the shore. Lunar high tides bring the eggs with developing embryo to the mangrove estuaries. Eggs with advanced embryo and larvae occur sporadically in the lower reaches of mangroves during breeding season. Juveniles about 50 mm size were recorded from 'commercial' catch from the mangroves of south east coast of India during November, January and February in different years. Adults constitute considerable fishery in the mangroves and estuaries of India. Nematalosa nasus is fairly common in Indian waters, whereas, its closely allied species namely Anodontostoma chacunda , though occurring in commercial catches of mangroves and seas of south east India, is very rare. On the contrary, in South East Asian waters (Indonesia, Thailand, Malaysia etc.) A. chacunda is fairly common, whereas, N. nasus is rare in commercial catch. Specimens were occasionally recorded from salt pans with salinity up to 87.25 ppt.
(b) Geographic distribution
Persian Gulf, India, Indo Malay Archipelago, Hong Kong, China, and northern part of Australia.
(c) Behaviour
It is a pelagic, shoaling fish with bottom feeding habit. To cope with bottom feeding habit, this species has inferior month and gizzard in the alimentary system.
(d) Distribution in the mangrove environment
The juveniles and adults occur in waters of wide range of salinity (from mixo-oligohaline to mixo-euhaline condition). It also occurs in waters of wide range of turbidity (from 10 g to 900 g/m^3 seston content) in the mangroves. Though this species occur round the year in all the zones of the mangroves, the early developmental stages are restricted to lower reaches, that too either during monsoonal or post monsoonal months. (Jeyaseelan, 1981)
(e) Ecology of parasitism in the mangroves
A copepod parasite namely, Lernanthropus rubiginosus was reported from this species in the mangroves (Jeyaseelan, 1981). Isopod parasites also occur frequently in the adults from the mangroves. (Jeyaseelan, 1981)