The Smalleye Hammerhead or golden hammerhead (Sphyrna tudes) is a small species of hammerhead shark, family Sphyrnidae. Because of its abundance, the smalleye hammerhead is an economically important bycatch of artisanal gillnet fisheries throughout its range and is utilized as food. In recent years, overfishing has caused marked declines in its numbers off Trinidad, northern Brazil, and probably elsewhere. Coupled with its low reproductive rate, this has led the International Union for Conservation of Nature (IUCN) to list it under Vulnerable.
One of the smaller members of its family, the smalleye hammerhead can reach a length of 1.5 m (4.9 ft), though 1.2–1.3 m (3.9–4.3 ft) is more typical, and a weight of 9 kg (20 lb). The body is streamlined and fairly slender. The mallet-shaped cephalofoil is wide and long, with a span measuring 28–32% of the body length; the leading margin forms a broad arch with indentations in the middle and on either side. The cephalofoil of newborns are longer, more arched, and less indented in front than those of adults. The eyes, placed at the ends of the cephalofoil, are proportionately smaller than in other hammerheads and equipped with nictitating membranes (protective third eyelids). The nostrils are positioned just inside of the eyes, each with a well-developed groove running towards the center of the cephalofoil. The mouth is strongly curved, containing on either side 15–16 upper tooth rows and 15–17 lower tooth rows. The teeth have single narrow cusps with smooth or weakly serrated edges, that are angled in the upper jaw and upright in the lower jaw.
The first dorsal fin is tall and slightly falcate (sickle-shaped), originating behind the pectoral fin bases; its free rear tip lies over the origin of the pelvic fins. The second dorsal fin is smaller than the first but still rather large, with a concave trailing margin. The pelvic fins have nearly straight trailing margins. The anal fin is taller and longer than the second dorsal fin. The caudal fin has a well-developed lower lobe and a notch near the tip of the upper lobe. The dermal denticles are oval with five horizontal ridges leading to marginal teeth. The most distinctive trait of this species is its coloration: the back and dorsal fins are gray to yellowish gray, and the cephalofoil margins, flanks, underside, pectoral fins, pelvic fins, and anal fin are bright yellow to orange with a metallic or iridescent sheen. Newborn sharks are gray above, darkening on the first dorsal fin and upper caudal fin lobe, and whitish below. They gain a bright yellow cast on their undersides by a length of 45 cm (18 in), which turns to orange by a length of 50 cm (20 in). The golden color is brightest in sharks 55–70 cm (22–28 in) long, and tends to fade with the onset of sexual maturity.
This species inhabits inshore murky waters 5–40 m (16–130 ft) deep, over muddy bottoms. There is segregation by sex and age: newborns and juveniles under 40 cm (16 in) long are found in the shallowest waters, moving deeper after a few months of life. Adult females are mostly found at depths of 9–18 m (30–59 ft), while larger juveniles and adult males are mostly found at depths of 27–36 m (89–118 ft). This species is tolerant of brackish water and can be found over a salinity range of 20–34 ppt.
Young smalleye hammerheads under 67 cm (26 in) long feed predominantly on penaeid shrimp, mostly Xiphopenaeus kroyeri. Larger sharks feed mainly on bony fishes, especially ariid sea catfish and their eggs. This species has also been known to consume swimming crabs, squid, grunts, and newborn scalloped hammerheads.