Description
Distinctive features of the silky shark include its small first dorsal fin and large pectoral fins.Slim and streamlined, the silky shark has a fairly long, rounded snout with barely developed flaps of skin in front of the nostrils. The circular, medium-sized eyes are equipped with nictitating membranes (protective third eyelids). Short, shallow furrows are present at the corners of the mouth. There are 14–16 and 13–17 tooth rows on either side of the upper and lower jaws respectively (typically 15 for both). The upper teeth are triangular and strongly serrated, with a notch in the posterior edge; they are erect at the center and become more oblique towards the sides. The lower teeth are narrow, erect, and smooth-edged. The five pairs of gill slits are moderate in length.
The dorsal and pectoral fins are distinctive and help to distinguish the silky shark from similar species. The first dorsal fin is relatively small, measuring less than a tenth as high as the shark is long, and originates behind the free rear tips of the pectoral fins. It has a rounded apex, an "S"-shaped rear margin, and a free rear tip about half as long as the fin is tall. The second dorsal fin is tiny, smaller than the anal fin, with a drawn-out free rear tip up to three times as long as the fin is tall. There is a narrow dorsal ridge running between the dorsal fins. The pectoral fins are narrow and falcate (sickle-shaped), and particularly long in adults. The anal fin originates slightly ahead of the second dorsal fin and has a deep notch in the posterior margin. The caudal fin is fairly high with a well-developed lower lobe.
The skin is densely covered by minute, overlapping dermal denticles. Each dermal denticle is diamond-shaped and bears horizontal ridges leading to posterior marginal teeth, which increase in number as the shark grows. The back is metallic golden-brown to dark gray and the belly is snowy white, which extends onto the flank as a faint lighter stripe. The fins (except for the first dorsal) darken at the tips; this is more obvious in young sharks. The coloration quickly fades to a dull gray after death. One of the larger members of its genus, the silky shark commonly reaches a length of 2.5 m (8.2 ft), with a maximum recorded length and weight of 3.5 m (11 ft) and 346 kg (760 lb) respectively. Females grow larger than males.
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