Bigeye Thresher

Fascinating facts about the Bigeye Thresher

• A 10 cm (3.9 in) eye diameter, housed in keyhole-shaped sockets that can rotate upward, allows this shark to see silhouettes in dim light.
• During daylight it hangs between 300 m and 500 m deep where the water is 6 °C to 12 °C, then darts up to less than 100 m at night to feed. That daily 15–16 °C temperature swing is buffered by a special rete mirabile around its brain and eyes, a built-in thermal shield. The adaptation shows how sharks can brave extreme thermal shifts that would cripple other fish.
• Up to half its body length is taken up by a broad upper lobe of the caudal fin, which is broader than in other threshers, and it can deliver whip-like strikes that stun schooling fish; longline hooks often snag the shark by the caudal fin.
• Females give birth to just two pups after a year-round pregnancy, each newborn measuring about 1.35–1.4 m (4.4–4.6 ft). A female may produce only 20 offspring in her entire life, making the species exceptionally vulnerable to over-fishing. This low reproductive output highlights why even modest catches can push populations toward collapse.
• A single individual was documented moving 2,767 km (1,719 mi) straight from New York to the Gulf of Mexico.

At a glance

About the Bigeye Thresher

The bigeye thresher (Alopias superciliosus) is a striking thresher shark whose name comes from its enormous, upward-rotating eyes set in keyhole-shaped sockets. Adult eyes can be up to 10 cm (3.9 in) across, giving the shark a helmet-like silhouette accentuated by deep lateral grooves above the eyes. Nearly half its 3.3–4.0 m (10.8–13.1 ft) length is the elongated upper lobe of its caudal fin, broader than in other threshers, while its deep metallic violet-to-purplish brown back fades to creamy white below. This species inhabits temperate and tropical waters worldwide, ranging from New York to Brazil in the Atlantic, the Mediterranean, the Indian Ocean coasts of South Africa and Madagascar, and the Pacific from Japan to Hawaii and New Zealand. It prefers surface temperatures of 16–25 °C (61–77 °F) but dives to 723 m (2,372 ft) where it encounters 5 °C (41 °F) water, performing a daily vertical migration that carries it from deep, cooler layers by day to the sunlit surface at night to hunt. Bigeye threshers feed on an eclectic menu of schooling forage fish, benthic hake, pelagic lancetfish, squid and even crab megalopae, stunning prey with whip-like strikes of their massive tail. Their large, upward-facing eyes and binocular vision allow them to spot silhouettes in dim light, while a specialized rete mirabile circulates blood around the brain and eyes, buffering them against the 15–16 °C (59–61 °F) temperature swings of their migrations. Reproduction is ovoviviparous with typically two oophagous pups per litter; females mature at 3.3–3.6 m (11–12 ft) and may produce only about 20 offspring in a lifetime, a factor that makes them especially vulnerable to over-exploitation.

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