Linnaeus, 1758
1. Distinctive characteristics of early developmental stages
(a) Eggs and Larvae
No authentic information on specific species is available from the mangroves. Most of the mugilids, except certain small sized species (such as Liza parsia and Rajamugil oligolepis ), seem to breed in a marine environment. Induced breeding techniques provide some reliable information about the early development stages of selected aquaculture species (vide Liao, 1975). However, if the identification of broodstock is not correct, the offspring from ‘accidental hybridization’ may confuse the purity of both the information and genetic resources.
(b) Early juveniles (Based on 30 mm T.L. specimens)
Mugil cephalus 1
A blue metallic blotch is present at the base of the pectoral fin (as in adults). The pectoral fin tip does not reach the first dorsal fin origin. Adipose eyelids barely cover the edge of the iris (as the specimens grow, the eyelids cover most of the eye). The angle of the mandibles is acute. Teeth are present in both jaws. Fine pigment spots were reported in specimens measuring around 26 mm total in length. The alimentary canal possesses only two pyloric caeca (more numerous in other species of mugilids). (after Zismann, 1981).
2. Distinguishing characteristics of early juveniles/juveniles in similar species occurring in the mangroves and adjacent biotopes
(a) Liza macrolepis
Adipose eyelids are very thin, bordering the anterior and posterior rim of the iris in juveniles (after Fischer and Bianchi, 1984) (the early juveniles of L. subviridis have poorly developed adipose tissue around the eye, whereas the juveniles and adults have well developed adipose eyelids. In the case of L. vaigiensis , the adipose eyelids exhibit similarity with L. macrolepis ).
(b) Liza oligolepis
Relatively smaller in length and broader in depth. A rare species, probably the smallest among mugilids (not so in the case of L. subviridis and L. vaigiensis ).
(c) Liza parsia
The tip of the upper jaw (near the snout) lies above the horizontal line through the centre of the pupil (not so in the case of L. subviridis and L. vaigiensis ). Adipose eyelids well developed in the posterior iris. (after Fischer and Bianchi, 1984)
(d) Liza tade
The head is dorsoventrally more flattened, broader and somewhat pointed towards the snout. The second dorsal fin originates vertically through the posterior half of the anal fin base (not so in the case of L. subviridis and L. vaigiensis ).
(e) Valamugil buchannani and Valamugil seheli
Adipose eyelids are very thin and not discernible at times. The pectoral fins reach beyond the first dorsal fin origin. The second dorsal fin origin is on a vertical through the anal fin origin (not so in V. cunnesius ).
(f) Valamugil speigleri
The body silvery. Adipose eyelids are fairly well developed. Pectoral fins reach beyond the first dorsal fin origin and has a black axillary spot. The second dorsal fin origin is on a vertical though slightly behind the anal fin origin. The lower lip appears as an inverted ‘U’ of a relatively narrow width when viewed from the ventral side by closing the mouth (the lower lip is an inverted ‘V’ shape with a wider angle in V. cunnesius ). The margin of the first dorsal fin is at times darkly pigmented (not so in V. cunnesius ).
3. Salient biological characteristics
(a) Maximum Size
M. cephalus – 900 mm.
(b) Food and feeding habits
The gut contents of juveniles from the mangrove environment, of the 4 species of mullets described in this program, show the presence of detritus, algal filaments, epiphytic and benthic diatoms and silt particles. In addition to these items, L. subviridis and M. cephalus also feed on dinoflagellates, centric diatoms, bivalve veligers, egg mass of invertebrates, ciliates and nematodes (Jeyaseelan, 1981).
(c) First sexual maturity
Normally in tropical and sub-tropical climates, the length of males at first maturity is between 230-330 mm standard length (2 years old) and the corresponding values for females range from 250-400 mm (3 to 4 years) in different geographic locations. In temperate climates, the corresponding figures show a higher range (310-410 mm in standard length for males of 3 to 7 years and 404-570 mm for females of 4 to 8 years. In broodstock ponds with relatively higher density, the size at first maturity values are very low (as low as 200 mm S.L. for males and 240 mm S.L. for females). Under extreme conditions or exceptional circumstances, the size at first maturity for males was recorded as low as 130-160 mm in standard length. When grown in ponds receiving heated effluent, this species attained first maturity at a minimum age of one year with a standard length as low as 160 mm. On the contrary, in land-locked temperate seas, the age at first maturity is very high for males (7 years) and females (8 years). In fertile mangrove and the adjacent waters of South India, the males and females with ripe gonads occur at a minimum size of 310 mm and 390 mm (S.L) respectively. (after Zismann, 1981; Abraham et al., 1966a; Abraham et al., 1966b; Broadhead, 1953; Erman, 1959; Ezzat, 1965; Faougi, 1938; Gunter, 1945; Hubbs, 1921; Kestevan, 1942; Linder et al., 1975; Thomson, 1963; Wallace, 1974; Wimpenny, 1934; Brusle, 1981).
(d) Fecundity
Absolute fecundity values in M. cephalus vary from 1.2-2.8 million eggs (Thomson, 1963). Relative fecundity values for V. cunnesius vary between 15 and 57 eggs per gram body weight, while the absolute fecundity varied between 0.08 million and 0.14 million for fish of 150-200 mm in total length. (after Sarojini, 1958 and Wijeyaratne and Costa, 1988)
(e) Spawning ground
All the above mentioned four species spawn only in the sea.
(f) Spawning season
Mugli cephalus
This species spawns principally during October-January in most of the Indo-Pacific seas with an extension up to March-April in certain Eastern Pacific regions. However, in certain regions of the Eastern Indian ocean bordering the Australian coast, the spawning season in this species is advanced to March-May and July-September. In the Western Atlantic, breeding occurs in this species between September and December, while in Eastern Atlantic coasts, the commencement of breeding is slightly delayed (November - December/February). In different coasts of the Mediterranean and Black Sea, the principal breeding season lies between June and September with a slight advance and postponement (May-October). Besides the above mentioned generalized pattern of spawning seasons in this speciesâ deviations from the normal course were also reported sporadically in the literature, when various types of scientific tools were used from time to time to obtain the evidence. (For further details vide Brusle, 1981).
This ‘mangrove mullet’ (vide Marshall, 1964) spawns during October-December off the mangrove waters in the Southwest Bay of Bengal, during the northeast monsoon season. Long term studies, spread over 5 years, reveal that an advancement or postponement of the northeast monsoon from the normal course make the species spawn during different months within the monsoon season, while in a single year, the spawning season is restricted to a brief period (of less than 2 months) as evidence has shown by the recruitment of young ones with a small size range in the Southeast Indian coastal mangroves.
4. Salient ecological information
(a) Geographic distribution
Among the above 4 species of mullets described in this program, M. cephalus is distributed round the world along tropical and temperate seas and brackish waters. L. vaigiensis and V. cunnesius are distributed almost throughout the whole of the Indo-West Pacific region. L. subviridis shows relatively restricted geographic distribution (among the 4 spp) from the Persian Gulf through India, China and up to the northern part of Australia.
(b) Water salinity versus distribution
All four species of mullets were reported to enter freshwater (Fischer and Bianchi, 1984). However, in the present study area, excepting V. cunnesius , all three species enter ‘freshwater zones’ in the upper reaches of the mangroves/river systems probably in search of food. Among these 4 species, M. cephalus was reported to occur even in metahaline ponds with 87 ppt salinity (Fischer and Bianchi, 1984).
(c) Behaviour
Adults of M. cephalus perform breeding migration from brackish water environments towards the sea. The young ones of L. subviridis and V. cunnesius (2-5 cm group) move upstream in schools along the small channels within the mangroves during low tides by opposing the ebb tide.
5. Aquaculture
Among the four species of mugilids described in this program, M. cephalus and L. vaigiensis are promising species for aquaculture (however, all four species find their place as ‘candidate species’ for brackish water aquaculture in the literature). But from a practical point of view, V. cunnesius attains a maximum size of 85 mm, 125 mm, 155 mm and 190 mm in length during first, second, third and fourth years of age in natural waters – which indicates the poor genetic capability of this species. Moreover, V. cunnesius prefers to dwell in relatively higher saline water and hence is not suitable for brackish water aquaculture.
During 1990, M. cephalus alone contributed 12,526 tonnes towards global aquaculture production, of which 10,612 tonnes were contributions from brackish water environments and the rest from fresh water aquaculture (FAO, 1992).