After 14 years of studying whale sharks I’ve never actually written up the stats on their world-beating amazingness in a single article. It’s time to change that.

Whale sharks are the world’s biggest fish. In fact, they’re the largest cold-blooded animal. Some truly gigantic whale sharks have been documented in fisheries. One, caught off Taiwan, was 20 m length and 34 tons in weight. Another shark from the same area weighed 42 tons, though its length apparently wasn’t recorded. An 18.8 m whale shark was landed in India, too. The second largest fish is the tiddly little basking shark, which only grows to around 12 m. Bless their ickle fins. 

Thresher sharks are one of those animals, like flying snakes or walrus, that are obviously made up. Except they aren’t.

Thresher sharks are big. Common threshers, the largest of the three thresher species, grow to about six metres long. Pelagic threshers, the smallest member of the family, still reach over three metres in length. What’s unusual in threshers is that, like a royal wedding dress, half their length is tail. Somehow, they manage to make this look graceful, as opposed to appearing like they’re accidentally dragging toilet paper around.

How do I know? Well, I can’t really speak to royal weddings, but I can vouch for the sharks. I’ve spent several weeks at Malapascua Island, in the Philippines, which is the best place in the world to dive with thresher sharks. 

People are often apprehensive about diving with large sharks. Given that we’re a terrestrial ape that evolved on African savannahs, this is not entirely unreasonable. Thresher sharks, though, are adorable. They pose no danger whatsoever to people… and the permanently anxious expression on their face makes them look like Eeyore.  Seriously.

However, this is a very human-centric (possibly a Simon-centric) perspective. For a sardine, thresher sharks are excessively deadly ninja sharks. But we’ll get to that later. Outside the Philippines, pelagic thresher sharks are rarely seen by divers, and most biological information on the species has come from fisheries studies. These sharks grow slowly, reaching adulthood at about 10 years in males and 13 in females, and live to at least 24.

Thresher reproduction is really interesting. Only two pups are born per litter, one from each uterus. The developing embryos have a unique way of obtaining nutrition. The mother continues producing eggs through her pregnancy, and these yolk-filled egg capsules provide food for the pups as they grow. This reproductive mode is termed oophagy, literally “egg-eating”. The pups are free-swimming at birth, and almost half the length of their mother, at 1.4 metres. 

Contrary to popular myth, divers already know that most sharks aren’t interested in people. In fact, they’ll purposefully avoid us. That’s why “shark dives” often use bait; they have to overcome this natural inhibition. Monad Shoal, a huge seamount off Malapascua Island, is a pleasant exception to that norm. The shoal hosts several thresher shark “cleaning stations”.

Cleaner fishes, in this case blue-streaked cleaner wrasse and moon wrasse, like to hang out on particular patches of reef. These “stations” are like a health-clinic-slash-day-spa. For fish. Sharks accumulate nasty little external parasites over time. These can cause chronic disease, developmental problems, and issues with respiration if they attach to the gills. Normal wear and tear from the sharks’ active predatory lives also result in minor injuries and dead skin, which can lead to infection.

Specialist cleaner fish eat these parasites, and the dead tissue, providing a useful service to the sharks – while gaining an easy meal themselves. Everybody wins. The sharks know that. When the threshers want to be cleaned, they’ll lower their tail and circle over the station to signal that they’d like some attention (and that they, as a fish-eating shark, mean no harm). They’ll slow right down to make it easier for the small cleaners to do a thorough inspection.

The near-daily presence of sharks at these stations has enabled Dr Simon Oliver and the other researchers in the Thresher Shark Research and Conservation Project team, operating from Malapascua, to conduct almost all of the behavioural research published on the species done to date. It also makes Monad Shoal a world-class dive site.

I’ve been up to Malapascua twice now. I’ll definitely go again. The island is great. The only challenge? The thresher shark dives are often… early. Painfully early. Like, 4.45 am early. Why go through such punishment? Cleaner fishes get hangry, that’s why. They sleep overnight, then wake up with empty stomachs. Scientists think the sharks are likely to get a better clean from these highly motivated little fish, and the sharks seem to agree. Some cleaning activity does occur throughout the day, but the mornings are the best time to visit.

The diving itself is easy. The water is warm – 28ºC or so – and, while it can be a deep dive, as he cleaning stations are on the edge of the shoal in 25-30 m depth. It’s fine for any Advanced-qualified diver. It’s a bit different to a usual shark dive though. Like the Mona Lisa, the thresher sharks have to be roped off from visitors, for their own protection.

Shark diving provides around 80% of Malapascua’s income. However, increased tourism comes with its own pressures. Reef damage from divers and anchors has destroyed several cleaning stations on Monad Shoal. More recently, mooring buoys were put in place for boats, and the remaining cleaning stations were roped-off to divers. This may seem like an extreme measure, but it’s clearly justified (and still allows for fantastic viewing of the sharks). 

With a good operator (check out www.greenfins.net), I view diving with the thresher sharks at Malapascua as an excellent example of positive ecotourism that helps to protect these globally threatened sharks, while also benefiting the local economy. This case study of sustainable tourism even helped to get all three thresher sharks listed on Appendix II of the Convention on International Trade in Endangered Species in 2017.

All positive. But let’s hear from those sardines again… Dr Simon Oliver led another study, at Pescador Island off Cebu (about 175 km south of Monad Shoal), that used underwater video footage to examine thresher shark hunting strategies for their preferred local prey, the aforementioned sardines.

Fish don’t like to be eaten. Indeed, they actively try to avoid it. Even for fast-swimming and agile sharks, chasing down their prey on a one-on-one basis is energy-intensive and often results in failure. Schools of fish are far more attractive to a predator, but individual sardines clearly feel safer in numbers. However, this schooling behaviour does have a downside: it places a large number of baitfish close together.

Several predators take advantage of this behaviour. Billfish, such as marlin and sailfish, will swim in fast and slash their elongated bills from side-to-side. Killer whales (orca) use tail-slaps to stun or kill their prey. Thresher sharks have those long, sophisticated, not-at-all-like-toilet-paper tails, which we now know they use like a catapult.

Frame-by-frame video analysis reveals the sharks’ strategy. They lunge forward, then splay the big wide pectoral fins on the sides of their body to hit the brakes. This stalls their forward momentum, allowing them to deliver an overhead tail-slap at measured speeds of over 20 meters per second (45 miles per hour). That’s so fast that the tip of the tail literally causes water to bubble.

This results in considerable splattage. Up to seven sardines were seen killed by a single strike. Although threshers always look concerned in photos, it’s the small fish that should be worried. Ninja sharks are real.

Column by Dr Simon J Pierce.