The total length of
Escualosa thoracata for all collected samples ranged from 7.0 to 11.5 cm with a coefficient of variance of 9.31%. The maximum total length 11.5 cm was recorded in male specimens from Mumbai in the west coast. Among the representatives from the East coast, the maximum total length was reported from a specimen from Chennai (10.1 cm) (Table 2). The total length, standard length, body weightand maximum body depth showed a significant difference between the two coasts and the four locations but was not found in the sexes. After regression analysis with standard length, four morphometric traits such as the eye diameter, pre-pelvic length, preanal lengthand caudal depth indicated significant variance between west and east coasts. Between locations within the coasts, the F-test proved substantial difference for four traits such as head snout length, postorbital length, dorsal base length, body depthand caudal depth. For all traits, F-test depicted that there was no significant variation among sexes. The result indicated that a maximum co-efficient of variation is observed in snout length (13.55%) followed by postorbital length (12.59%), body depth (11.81%). The lowest coefficient of variation was recorded in fork length (8.15%). The eye diameter in the east coast ranged from 0.34 to 0.62 mm with a mean of 0.49 mm, whereas the range and mean in west coast from 0.39 to 0.70 and 0.52 mm respectively. The mean caudal depth of fishes from the west coast was 0.78 mm and 0.65 mm for the east coast. The mean head length was lowest for the east coast and highest for the west coast. Comparative variability among locations of total length, caudal depth, snout lengthand eye diameter of
Escualosa thoracata is shown in Fig 1, 2, 3 and 4. Descriptive statistics of meristic traits is given in Table 3. Out of six characters, five showed the significant variations between east coast and the west coast. The traits which did not show coast-wise and location-wise variation was the number of pelvic fin rays. Five traits,
i.e., NDFR, NPFR, NAFR, PREPES and POPES, show significant variation in the east and west coast. In the PCA analysis of the meristic traits of
E. thoracata, 68.45% of total variations were explained by the first three principal components together. 34.68% of total variations contributed by PC 1, 21.45% by PC 2 and 15.33% contributed by PC3. Only one trait showed significant loading on PC1, which was POPES and NAFR loaded significantly to the PC2 and NDFR and NAFR loaded significantly on PC3 (Table 4).
The analysis clearly shows that the white sardine stocks from two locations on the east coast,
i.e. Kolkata and Chennai, vary from those on the west coast. Apart from that, the stocks from Kolkata and Chennai significantly differed in two traits such as NDFRand NAFR. On the west coast, Kochi and Mumbai stocks also showed significant variation in three features such as NPFR, PREPES and POPES. While NPeFR did not show significant variation coast-wise, location wise and sex-wise also. Morphometric analysis revealed significant heterogeneity among the
E. thoracata, which is used to separate stock on the Indian coast. The average length of white sardine samples collected from the west coast was significantly higher than the east coast. Also, there was a significant difference in body weight and maximum body depth of fishesand fishes from the west coast were found to be heavier and deeper. The Arabian Sea is one of the world’s most productive areas of the ocean due to a wide range of geo-climatic phenomenon like upwelling, mixing of waterand lateral advection
(Kumar et al., 2009).
In contrast, it is traditionally considered a region of lesser biological productivity in the Bay of Bengaland recent measurements for phytoplankton C^14 uptake also support this fact
(Kumar et al., 2002). The productive Arabian Sea provides favourable conditions for the continuous supply of food for fishes such as white sardine. This might be the possible reason for this fish characterized by high growth parameters. The stock characteristics of
E. thoracata were studied from the north-west coast of India (
Raje, 1994,
Prajapat, 2015,
Rahangdale et al., 2016), south-west coast of India
(Abdusamand et al., (2018) and central west coast of India
(Gurjar et al., 2021).
Masuda and Tsokamato (1996) found the development of pigment in the retina and rod formation corresponding to the light intensity and morphological evolution in respect to phototaxis and rheotaxis in the striped jack
Pseudocaranx dentex.
Higs and Fuiman (1996) found the relationship between light intensity and eye diameter for schooling in several species. They identified that there is a strong correlation between light intensity and eye diameter.
Matthews (1988) studied and depicted that the variation in eye diameter can be analysed to the developmental changes in fishes in early stages corresponding to the light intensity in their habitat. It may reflect differences in turbidity of the habitat.
Miyazaki et al., (2000) analyzed reduced penetration of solar radiation in the Bay of Bengal due to the large quantities of sediment influx. Hence, variation in light penetration and resultant light intensity in both seas is attributed to variation in eye diameter in fishes from India’s east and west coasts and associated adaptive developmental changes in early stages in the species.
Imre et al., (2002) demonstrated that microhabitats differing in water velocity showed morphological variation in the caudal area in brook charr observed deeper caudal peduncle in fishes from turbulent waters. The water turbulence in the Arabian Sea is considerably lower than the Bay of Bengal. the variation in caudal peduncle depth in
E. thoracata indicates the possibility of a phenotypic plasticity in response to different hydrological conditions in the Arabian Sea and Bay of Bengal.
Plasticity in overall body shape and natural habitat associated with different morphological divergence is well known in fishes, including intraspecific differences. Hence, it was concluded that physical characteristics of living habitats can determine biological evolutionary and ecological driving changes in the morphological characteristics of habitual fish populations. Polymorphisms involve diversifying behavioral, morphological or life-history traits in populations (
Smith and Skulason, 1996). Such polymorphisms are more common in vertebrate populations than initially thought (
Robinson and Wilson, 1994;
Wimberger,1994;
Skulason and Smith,1995).
Thompson (1991) analyzed that phenotypic plasticity is the genotype ability to respond to alternative environmental conditions to produce a difference in phenotypes. The study of morphometric parameters of
E. thoaracata clearly showed the variation of fishes from Arabian Sea and Bay of Bengal, though in a small magnitude, but surely enough to be considered as subpopulations which suggests the need for different type of strategies to manage the resources. Morphometric analyses have been used as a robust tool for discriminating biological groups (
De La Cruz-Agüero et al., 2004). The Principal component analysis (PCA) showed significant variation between the Bay of Bengal and Arabian sea populations. The PCA of meristic traits depicted the separation of Mumbai population. The meristic characteristics were responsible for the differentiation of the Mumbai population. The results obtained by the two methods, morphometric and the meristic analyses, were correlated and conformed.
