Bigeye thresher

What it is

This shark belongs to the thresher family and is distinct for its enormous eyes and keyhole-shaped sockets. These organs can rotate upward, aiding vision in dim light. A pair of deep grooves runs from above the eyes to the gills, creating a helmet-like look. The upper tail lobe is extremely long, often making up half the total body length. Adults typically measure between 3.3 and 4.0 metres, with a maximum recorded length of 4.9 metres. Their skin is metallic violet or purplish brown on top and creamy white underneath.

Where it lives

The species has a nearly global distribution in tropical and temperate seas. It inhabits the western Atlantic from New York to Brazil, and the eastern Atlantic from Portugal to Angola. In the Pacific, it ranges from Japan and Australia to California and Hawaii. It lives over continental shelves and in open oceans, occasionally entering shallow coastal zones. While it prefers surface temperatures of 16 to 25 degrees Celsius, it dives into much colder depths.

How it lives

It performs daily vertical migrations, staying in deep water during the day and rising to feed at night. This behaviour helps it avoid predators and find prey in low light. The diet includes schooling fish like mackerel, hake, and lancetfish, as well as squid. It uses its long tail to stun victims before eating them. Larger sharks and marine mammals may hunt this species. It competes ecologically with blue sharks, often appearing where they are absent.

Breeding and life cycle

Females give birth to live young after an internal development period. Litters usually contain two pups, one from each uterus. Embryos feed on unfertilized eggs produced by the mother, a process called oophagy. There is no specific breeding season, and females may remain pregnant year-round. Newborns are about 1.4 metres long. Males mature at nine to ten years, while females take twelve to fourteen years. The maximum lifespan is approximately twenty years.

Fish and people

Commercial longline fisheries catch this shark for its fins, liver oil, and skin. The meat is often dried or smoked but is not highly prized due to its texture. Recreational anglers pursue it as a game fish in places like New Zealand and South Africa. It poses little threat to divers. However, its low reproductive rate makes it sensitive to overfishing. Conservation groups list it as vulnerable due to these pressures.

Status and threats

The International Union for Conservation of Nature classifies the species as Vulnerable. This rating reflects its susceptibility to population decline from commercial fishing. Its slow growth and few offspring limit recovery rates. In New Zealand, local authorities consider it not threatened. Global catches remain significant, particularly in Taiwanese and Cuban waters, raising concerns about long-term sustainability.

Questions people ask about Bigeye thresher

Good reading and sources

• Bigeye thresher on WikipediaEncyclopedia
• More photos on Wikimedia CommonsPhotos
• Wikidata recordOpen data