Animal profile

Where to see bigeye thresher sharks

Alopias superciliosus

The bigeye thresher is the thresher you do not get to meet. Like its relatives it hunts with its tail, whipping the enormously elongated upper lobe over its head to stun fish, but it does this several hundred metres down: it spends the day deep in near-darkness and rises toward the surface at night, one of the most pronounced daily vertical migrations of any shark. Its eyes are the adaptation that makes that possible — huge, and rotated so they look upward through the top of the skull, letting it pick out silhouettes against whatever light filters down from above. Pair those with the deep grooves running back over the head and it is unmistakable at close range, if you ever get close range. It is caught in numbers as longline bycatch and listed as Vulnerable, and divers see it so rarely that most sightings are single, half-glimpsed passes at the deep edge of a seamount dive.

Best places to see bigeye thresher sharks

How to identify a bigeye thresher shark

• The eyes settle it: enormous, and set so high they extend onto the top of the head, angled upward. No other thresher — and no other shark you will see — has eyes like that.
• Look for a deep horizontal groove running back above each gill slit, over the top of the head. The pelagic thresher lacks these, and they are the classic feature separating the two underwater.
• The tail is as long as the body, as in all threshers, but the first dorsal sits further back than a pelagic thresher’s and the snout is longer. If you are at 30 m at Kimud Shoal and something enormous-eyed goes past, this is the one worth reporting.

Meeting them responsibly

• Stay off the reef top and hold your depth. These sightings happen on deep seamount dives where the animals pass along the edge, and divers dropping below plan chasing a glimpse is the real hazard of this dive.
• No lights, no flash, no chasing. This is an animal adapted to almost no light, and the local etiquette at thresher sites — torches off, stay behind the line, let them come along the wall — exists because the sharks abandon disturbed cleaning stations.
• Report it properly. Tell your guide and get it logged: confirmed bigeye records from recreational dives are genuinely scarce data, and the species is Vulnerable.

Frequently asked questions

Can you dive with bigeye threshers?

Not reliably, anywhere. Malapascua in the Philippines is the only place in the world where threshers are dependably seen, and the animals there are pelagic threshers; bigeyes turn up only occasionally. Treat it as a rare bonus on a dive booked for something else.

How is it different from a pelagic thresher?

The eyes and the head grooves. A bigeye’s eyes are far larger and angled upward onto the top of the skull, and it has deep horizontal grooves above the gills that the pelagic thresher does not. It also grows larger and lives deeper.

Why is it so rarely seen?

Because it lives below diving depth for most of the day. It sits several hundred metres down in daylight and only moves shallower at night, so the overlap between where it is and where divers can be is very small.