Monad Shoal, a large seamount off Malapascua Island in the Philippines, is the only place in the world where pelagic thresher sharks are consistently seen. They’re amazing creatures.
Pelagic threshers are a large shark, growing to almost 4 m in total length, including their oversized tail. That tail looks a little odd in photos, but most people who’ve seen them underwater rate threshers amongst the most graceful and balletic of all sharks.
While I can sympathise with that point of view, and they’re certainly no danger to people, I personally prefer to think of them as deceptively violent ninja sharks. But I’ll get to that later.
Thresher shark biology
Pelagic threshers are one of three species of thresher sharks. They’re more tropical than the common and bigeye thresher species, and only found in the Indian and Pacific oceans.
Most knowledge on pelagic threshers has come from fisheries studies. They’re relatively fast-growing for a large shark, reaching adulthood at 7-9 years, with an estimated lifespan of 16.
Only two pups are born per litter, one from each uterus. Developing embryos have a unique means 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. The strategy is termed oophagy, literally “egg eating”.
The pups are free-swimming at birth, and almost half the length of their mother at 1.3 – 1.9 m.
Thresher sharks tend to look concerned.
Outside the Philippines, pelagic thresher sharks are rarely seen by divers. The behaviour of the species is poorly-known. That’s not unusual.
Contrary to popular myth, divers know that most sharks are completely disinterested in people. In fact, sharks often purposefully avoid them. That’s why most “shark dives” use bait to overcome this natural inhibition.
Monad Shoal, which hosts several thresher shark cleaning stations, is a pleasant exception to that norm. Thanks to Dr Simon Oliver and the other researchers at the Thresher Shark Research and Conservation Project team, operating from Malapascua, several studies have been published on the species.
Why do they swim to Monad Shoal?
Cleaning stations are like a fish health clinic. Sharks accumulate unpleasant little external parasites over time, which can cause chronic disease, developmental problems and issues with respiration if they attach to the gills. Normal wear and tear from their active predatory lives also results in minor injuries and dead skin, which could lead to infection.
Specialist cleaner fish pick off these parasites and remove dead tissue, providing a useful service to the sharks, while also obtaining a meal for themselves.
At Monad Shoal, both blue-streaked cleaner wrasse and moon wrasse service the sharks. Obviously, sharks eat fish. You can see how this could create an issue for the cleaners. When the threshers turn up, they lower their tail and circle over the cleaner fish’s “stations” in a specific posture to explicitly signal harmless intent and a desire to be cleaned. The sharks also slow right down to make it easier for the small cleaners to do a thorough inspection.
It appears that the cleaner fish focus on the small parasitic flatworms that are often found around the shark’s pelvic area.
Remote video was used to monitor cleaner fish inspections of thresher sharks at Monad Shoal. One red line represents 20 inspections.
Normally, reef fish accumulate parasites overnight while they’re sleeping, so they clean during the day. Thresher sharks don’t stop swimming, and they’re most active at night, but they also clean during the daytime – particularly early in the morning. Why?
Well, the cleaner fishes do sleep overnight. When they wake up, with empty stomachs, they’re hungry. The thresher sharks probably get a better clean from these highly motivated, possibly hangry cleaner fishes first thing in the day. (That makes for a very early start to the diving day in Malapascua, but I assure you it’s worth it.)
Cleaning is an important activity for these pelagic threshers. Not much is known of their activities away from the shoal, but presumably they’re off hunting. Studies from the Eastern Pacific found that much of their diet was composed of squid, particularly the large and somewhat fearsome Humboldt squid, and mesopelagic fish species.
Males are thought to forage more in offshore waters, with females possibly more coastal as adults.
That tail though
That all seems normal enough. So what’s the tail for? I’m so glad you asked.
A study at Pescador Island, off Cebu in the Philippines, used video footage to examine thresher shark hunting strategies for their preferred local prey, sardines.
Turns out, fish don’t like to be eaten. Even for fast-swimming sharks, chasing down their prey on a one-on-one basis is energy intensive, and often results in failure. Where these fish are in a school, and individuals are difficult to target, life is even tougher for a hungry shark.
However, this schooling behaviour also has a downside: it places a large number of baitfish in close proximity to one another.
Frame-by-frame video analysis reveals the sharks’ strategy. They lunge forward, then use their large pectoral fins to stall their whole body and deliver an overhead tail-slap at measured speeds of up to more than 30 metres per second. That’s so fast that the tip of the tail literally causes water to bubble. Up to seven sardines were killed in a single strike.
So there we have it. Although threshers look rather concerned in photos, it’s the small fish that should be worried. Deceptively violent ninja sharks.
Told you so.
Read more at the Thresher Shark Research and Conservation Project website and Facebook page.
Much of the work published from Monad Shoal to date was part of Simon Oliver’s PhD study.
The scientific figures and a lot of the general information presented here came from two open access scientific papers:
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