3.2 Elasmobranchii (Sharks and rays)
A total of 52 samples were sequenced belonging to 5 order, 8 families, 13 genera and 21 species (Table 1). Among these 21 species, two species have been found as endangered (EN), five as vulnerable (VU) and seven as near threatened (NT). One species was least concern (LC), two were not evaluated (NE) and the remaining four species were data deficient (DD) (IUCN, 2020).Chiloscylliumburmensis andC. hasseltiiwere reported as two new records (Ahmed et al., 2019b) in this area. The sequence analysis indicated the average nucleotide frequencies to be- A: 25.90%, T:32.70%, G:16.20% and C:25.20%. The base composition analysis for the COI sequence showed that the average percent T content was the highest and the average percent G content was the lowest; the AT content (58.60%) was higher than the GC content (41.40%). The GC contents at the first, second and third codon positions for the Elasmobranchii were 53.30%, 43.60% and 27.70%, respectively. At the first codon position, the usage of T (20.00%) was the lowest, and the usages of the other bases were 23.50%, 27.40% and 29.50% for C, A and G respectively. At the second codon position, the content of T (42.00%) was highest, and the percentage of the other bases were 29.40%, 14.50% and 14.20% for C, A and G respectively. At the third codon position, the base usage was T: 37.00%, C: 22.80%, A: 35.70% and G: 4.90%; the G content being the lowest, exhibited a clear pattern of anti-G bias.
The K2P genetic distances within each taxonomic level are summarized in Table 2. The average genetic distance within species, genus, family and order were 1.2±0.0007%, 6.07±0.014%, 11.08±0.018% and 14.68±0.04%, respectively. The NJ tree clearly distinguished all the species and the species belonging to 8 families were represented by eight distinct clades (Fig. 1).