Sardines are generally characterized with a constricted, silvery, streamline body, protruding scales and a single soft-rayed dorsal fin. They have bilobed, non-functional lungs and are known for their oily flesh, which is a great source of omega-3 (Luceño et al., 2014 and Mahrus et al., 2012). Sardines are generally identified by the relationship between their body depth and standard length, the presence or absence of colored spots, colored lines and fleshy outgrowth located behind the gill cover.
According to Willete et. al(2011), they can be differentiated with other small pelagic fishes by their rounded upper lip and two pronounced supramaxilla at the proximal end of the mouth. Their dorsal fin is located at the midpoint of the body, their anal fin found anterior to the dorsal fin, and their pelvic fin behind the origin of the dorsal fin. Depending on the species, sardines possess six to eight pelvic fin-rays with the last two anal fin-rays that may or may not be enlarged. In addition, continuous or discontinuous striations across the center of the scales, the number of scutes on the belly (from 28-34), and the number of gill rakers (from 26-253) located on the lower half of the first gill arc is crucial in distinguishing similar sardine species from one another.
According to Whitehead (1985), S. lemuru has a subcylindrical, elongated body with a depth of 30% of its standard length, and a rounded belly. Distinguishing features of S.
lemuru from other clupeids in the eastern Indian Ocean and Western Pacific are the presence of nine pelvic fin rays, presence of a faint golden spot behind the gill opening followed by a faint-golden midlateral line and lastly, the presence of a distinct black spot at hind border of the gill cover.As stated in the study conducted by Willette and Santos (2012), a nearly resembling species of S. lemuru is its relative from the sub-family Clupeinae, S. longiceps.
These two can be differentiated from all other Indo-Pacific species by the presence of nine pelvic fin-rays (one unbranched and the eight branched), the combination of high count of lower gill-rakers, absence of black spot at the dorsal-fin origin, presence of the black spot at the posterior opercular margin, and a faint golden stripe down the flanks preceded by a faint gold spot located at the operculum. They can be further distinguished from each other by the counts of gill-rakers in the lower limb of the gill arch; for S. lemuru, it possess 77-188 gill-rakers, while S. longiceps have 150-253 gill-rakers, but typically 180-253 gill-rakers.
The length of the head (26-29% of standard length in S. lemuru, while 29-30% of standard length in S. longiceps) can also be a distinguishing factor between the two.