Deep Sea: the Twilight Zone and Beyond
Bigeye Thresher Shark
Its huge, upward-mounted eyes immediately mark the Bigeye Thresher Shark (Alopias superciliosus) as a deep-sea predator that hunts its prey from far below. The species part of the Bigeye Thresher’s scientific name means “conceited”. But — like every other aspect of this shark’s strange appearance — its unusual ocular arrangement makes perfect sense in the context of how it lives.
Just the Facts:
Habitat: Open Ocean, Deep Sea
Depth: surface to at least 1,640 ft (500 m)
Distribution: Central Pacific, Tropical Eastern Pacific, Western North Atlantic, Caribbean, Argentinean, Eastern North Atlantic/ Mediterranean, West African, Southern African, Madagascaran, Arabian, South East Asian, Australian/New Zealand, Northern Australian, Japanese
As its vernacular name suggests, the eyes of the Bigeye Thresher are among its most conspicuous features — and they are absolutely amazing. They are shaped like an upside-down pear and extend partway onto the top of the head. The iris and pupil of each eye is restricted to the widest, uppermost part of the ‘pear’, fixed in an exaggerated and permanent eye-roll. Their vertical height is about 4 inches (10 centimetres), making them proportionately the largest eyes of any non-avian vertebrate. The Bigeye Thresher’s visual system is clearly adapted to detecting from considerable depths as much faintly downwelling light as possible.
To help it function effectively in the chill depths, the Bigeye Thresher is partially warm-bodied. Like a few other lamnoid sharks — including the Shortfin Mako (Isurus oxyrinchus), White Shark (Carcharodon carcharias), and the closely related Common Thresher (Alopias vulpinus) — the Bigeye Thresher has elaborate networks of tiny blood vessels extending along its flanks, arranged in a way that conserves body heat rather than radiating it to the surrounding water. With this system, a Bigeye Thresher can maintain its body 3 to 8 degrees Fahrenheit (2 to 4 degrees Celsius) warmer than the water through which it swims. A similar arrangement of tiny blood vessels shunts to the brain heat generated by the muscles that rotate the eyeball, possibly preventing the Bigeye Thresher from becoming addled by the mind-numbing cold at depth. This would represent a significant advantage for an active predator that must remain sharp and focused on its lethal purpose.
The Bigeye Thresher is a broad-spectrum predator, taking a wide variety of prey. Its teeth are larger and more obliquely cusped than those of other threshers, enabling it to tackle just about anything it encounters. Known prey of the Bigeye Thresher includes not only small-to-medium-sized schooling pelagic fishes — such as mackerels and herrings (families Scombridae and Clupeidae, respectively) — but also cryptic bottom fishes — such as whitings and hakes (Gadidae) — large, toothy predatory teleosts — such as lancetfishes (Alepisauridae) — and even small billfishes (Istiophoridae). In the Mediterranean Sea, the Bigeye Thresher is strongly associated with schools of Frigate Mackerel (Auxis rochei), which suggests that it may follow schools of its prey like a wolf tracking a herd of sheep from pasture to pasture, feeding whenever the need arises. The Bigeye Thresher also feeds heavily on oceanic squids — such as short-finned squids of the genus Ilex and the bioluminescent deep-sea species, Lycoteuthis diadema. Clearly, the Bigeye Thresher hunts over a wide range of depths and can catch just about anything it encounters.
Exactly how the Bigeye Thresher captures its prey has never been observed, but there are a few tantalizing clues. The upper lobe of its caudal fin is proportionately shorter than that of other threshers, but — like them — is supported by modified vertebrae with flattened cartilaginous processes above and below. Such modifications convert the Bigeye Thresher’s tail into a structure well suited to striking prey, stunning individual animals before grasping each with the teeth. This scenario is supported by the fact that Bigeye Threshers are often tail-hooked on deep-sea longlines and, when their stomach contents are examined, sometimes contain bait from the same longline on which they were caught. Therefore, it seems likely that, like other threshers, the Bigeye uses its hyperextended upper caudal lobe like a buggywhip to knock-out individual fish or squid before swallowing them.
Much of the Bigeye Thresher’s diet broadly overlaps that of the Blue Shark (Prionace glauca). A study of large pelagic fishes caught on surface longlines set for Swordfish (Xiphias gladius) revealed an interesting negative correlation between Blue Sharks and Bigeye Threshers: where one is caught, the other is conspicuously absent. This may reflect some kind of competitive exclusion between these two species or simply be an artifact of the time of day that the longlines were sampled. In the Cuban longline fishery, the best catches of Bigeye Thresher occur at night, using cylumes (chemical lights) as visual attractants. This finding suggests that the Bigeye Thresher relies, at least in part, on its eyesight to locate prey.
Putting all the pieces together creates a vivid scenario of how the Bigeye Thresher uses its various endowments to locate and capture prey. It probably cruises at a depth of 300 feet (100 metres) or so, scanning the surface for the silhouette of a school of fishes or squid with its huge, upward-mounted eyes. The Bigeye Thresher’s modified circulation keeps its swimming muscles and brain warm, ready to explode upward to ambush its prey from below. Striking with its elongate, structurally reinforced upper caudal lobe, it lashes repeatedly at the huddling prey until a dozen or more are knocked-out, killed, or otherwise incapacitated. The Bigeye Thresher then circles to casually scoop up its victims, grasping and chopping them into bite-sized pieces using its knife-like teeth. Then it retreats to the depths, ready to begin the well-rehearsed drama all over again.
All of this seems perfectly feasible, but there is one conspicuous aspect of the Bigeye Thresher’s topography that is not so easy to explain. A deep indentation extends forward from the fifth gill on either side, the two furrows meeting at the mid-line of the head just behind the eyes. These head grooves demarcate the base of a massive muscular crest that extends smoothly backward, giving the Bigeye Thresher a peculiar ‘helmeted’ appearance. This effect is obvious even in embryos, but is most pronounced in very large individuals. This suggests to me an interesting possibility: perhaps, like the antlers of rutting deer, the ‘helmet’ of large Bigeye Thresher Sharks indicates sexual maturity, announcing an individual’s health, vigor, and suitability as a mate. Who among us can possibly know what constitutes ‘good-looking’ to a shark with its eyes permanently rolled-upward?