Smalleye Pygmy Shark

Among the smallest of extant sharks, the smalleye pygmy shark attains a maximum recorded length of 22 cm (8.7 in). It is shaped like a cigar and has a bulbous, pointed snout. The eyes are relatively small, with their diameters measuring 43–66% as long as the snout (compared to 61–82% in S. laticaudus). The eyes of this species also differ from S. laticaudus in that the upper rim of the eye socket is chevron-shaped rather than nearly straight. The nostrils lack substantially expanded skin flaps in front. The mouth is nearly transverse and bears thin lips; a pair of papillae (nipple-like structures) is on the upper lip that is absent in S. laticaudus. There are 20–27 upper tooth rows and 18–23 lower tooth rows. The upper teeth are slender and upright. The larger, broader lower teeth have angled and knife-like cusps, and interlock to form a continuous cutting surface. The five pairs of gill slits are tiny and uniform. The two Squaliolus species are the only sharks that have a spine on the first dorsal fin, but not the second. The spine is usually exposed in males and covered by skin in females. The tiny first dorsal fin originates about over the rear tip of the pectoral fin. The second dorsal fin is long and low, and originates over the front half of the pelvic fin bases. The pectoral fins are short with rounded margins, and the pelvic fins are long and low. The anal fin is absent. …

The smalleye pygmy shark appears to be widely distributed, but in patches, in the western Pacific Ocean, having been reported from off southern Japan, the Philippines, and northern and eastern Australia. It inhabits the upper and middle layers of the water column near land, at depths of 150–2,000 m (490–6,560 ft). It conducts a diel vertical migration, spending the day in deeper water and rising to shallower waters at night.

Little is known of the natural history of the smalleye pygmy shark. It feeds primarily on midwater squid, krill, shrimps, and small bony fishes such as lanternfish. The photophores on its underside may serve to camouflage its silhouette from predators by matching the downwelling light, a strategy known as "counter-illumination". Reproduction is aplacental viviparous as in other members of its family, with the young being born under 10 cm (3.9 in) long. Males attain sexual maturity around 15 cm (5.9 in) long.