Biology of the Bigeye Thresher
|Field Marks||Upper caudal lobe long and curving, nearly as long as rest of shark; no labial furrows at mouth corners; lateral teeth without basal cusplets; eyes huge, oval and expanded onto top of head; a deep groove behind eyes, creating a peculiar 'helmeted' appearance (more pronounced in large individuals); pectoral fins falcate with broad tips; first dorsal fin base closer to pelvic fin origins than pectoral fin free rear tips; creamy color of abdomen not extending over pectoral fin bases as conspicuous patch. Color in life purple to violet-grey above, shading to paler on flanks, with metallic luster.|
|Size||Pups are 27.5-42 in (70-106 cm) long at birth; average individuals are 9-13 ft (2.7-4 m) in length and about 350 lb (160 kg) in weight; maximum length is 15.9 ft (4.84 m).|
|Range||Virtually circumtropical in oceanic and coastal areas.|
|Habitat||In coastal waters over continental shelves, sometimes in shallow waters close inshore, and on the high seas far from land; sometimes caught near the bottom in deep water; known depth range from the surface to at least 1,640 ft (500 m), habitually deeper than the Common Thresher (Alopias vulpinus). Infrequent or rare in the Mediterranean Sea, but regularly caught off Cape Hatteras, North Carolina, from April-June. Warm bodied, with well-developed lateral and orbital retia mirablia, able to maintain a body temperature 3.8-9.1 °F (1.8-4.3 °C) above ambient.|
|Feeding||Preys primarily on pelagic fishes, including mackerels, herrings, and small billfishes, but also takes bottom fishes (such as whiting and hake), oceanic squids (such as the bioluminescent deep-sea squid, Lycoteuthis diadema, and ommastrephid 'flying' squids), and crab megalopae (possibly from other prey); in the Mediterranean, strong association with schools of Frigate Mackerel (Auxis rochei). Apparently stuns prey with its long caudal fin, as individuals are often tail-hooked on longlines, some of which have been found to contain bait in their stomachs.|
|Reproduction||Ovoviviparous, oophagous; number of pups usually 2 per litter, but sometimes up to 4; no defined pupping season or known nursery areas in northwestern Indian Ocean; no data on gestation period.|
|Age & Growth||Males mature at a length of about 9-9.4 ft (2.7-2.9 m) and an age of 9-10 yrs, females at a length of about 11-11.2 ft (3.3-3.4 m) and an age of 12-13 years; estimated longevity of males is 19 yrs, of females 20 yrs.|
|Danger to Humans||Minimal.|
|Utilization||Often caught in oceanic longline fisheries operated by Russia and Japan, especially in the northwestern Indian Ocean and Central Pacific; an important component of the Cuban longline fishery, where best catches occur at night using Cylumes (chemical lights) as attractants; occasionally taken incidentally in the Sicilian Channel, often thrown back due to low commercial value of meat; has recently been taken in considerable numbers off northeastern US as 'nuisance' bycatch of longline fishery for swordfish; also taken in fixed bottom and pelagic gillnets, in trawls, and with sportfishing gear (rod and reel); not considered a sport fish, but IGFA all-tackle record is 802 lb (363.8 kg), taken off Tutukaka, New Zealand, in February 1981. Meat is mushy, of unpleasant texture, not highly regarded as food, but sometimes consumed fresh, smoked, dried and salted; liver processed for vitamins, skin for leather, and fins for shark-fin soup.|
|Remarks||Sometimes caught with lampreys adhering to skin near the cloaca.|
When contemplating a given organism, it is always important to consider it in the context of its natural environment. Otherwise, one's impressions are likely to be misleading. The Bigeye Thresher Shark provides a prime example of this caveat. Seen hauled up on a boat deck or fishing dock, the Bigeye Thresher seems downright goofy-looking.
One of the first things one notices are its eyes. As the shark's vernacular name suggests, they are huge: their vertical diameter can be up to 4 inches (10 centimetres) in adult specimens and as much as 9% of the standard length (distance from the tip of the snout to the upper precaudal pit) in juveniles, giving the Bigeye Thresher the distinction of having the largest eyes in proportion to its body of any non-avian vertebrate. The form of its eyes is odd, too. Rather than being more-or-less circular in outline, this species' eyes are shaped vaguely like an upside-down pear, being about a third taller than wide and with the broader upper part extending partly onto the dorsal surface of the head. Further, its pupils are permanently mounted upward, giving the shark a decidedly conceited look (hence its species name, superciliosus). But its eyes are only the beginning of the Bigeye Thresher's apparent oddness.
A deep indentation extends forward from the fifth gill slit 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 specimens. Then there is the matter of coloration. Compared with many teleosts, sharks are generally rather drab creatures — while some may sport attractive markings, their background color is generally grey, greyish-blue, greyish- or yellowish-brown. Even the brilliant ultramarine flanks of the Blue Shark (Prionace glauca) fade to a leaden grey upon death. Until recently, it was thought that the Bigeye Thresher was simply dark grey above, paler below. But fresh-caught specimens have revealed yet another surprise about this strange-looking fish: in life, Bigeye Threshers are deep midnight purple on the back, fading to greyish violet on the flanks, and a pale cream color on the undersurfaces.
All of these features — combined with unusually long, pointed fins and the characteristic sickle-shaped upper caudal lobe (which is proportionally shorter than in other species of thresher shark) — contribute toward making the Bigeye Thresher resemble a spiky, purple other-worldly creature that seems both surprised and somewhat disgusted at the indignity of being abducted from its liquid universe. So what do we know about where and how this alien creature goes about its normal, fishy business if left unmolested?
Not much. Most of what we do know about the Bigeye Thresher is deduced from specimens collected incidentally in commercial or test fisheries. From catch records, we know that this species is patchily distributed in the western Indian Ocean, where it is the most abundant species in some plankton-enriched areas; overall, it comprises an average of 10% of the total high-seas shark catch. Off northwestern Cuba, the Bigeye Thresher is abundant year-round and the third most frequently caught of 11 commercially harvested species; overall, it comprises some 20% of the total longline catch, with a slight peak in catch rate in the fall from September to October and the poorest catches occurring in spring from March to June. In their 1992 study of large pelagic fishes caught on surface longlines set for Swordfish (Xiphias gladius) in the tropical eastern Atlantic, ichthyologists Juan Carlos Rey and Ramon Muñoz-Chápuli noted an interesting negative correlation between the Bigeye Thresher and the Blue Shark (Prionace glauca): where one species was caught, the other was conspicuously absent.
In the Cuban longline fishery, the best catches of Bigeye Thresher Shark occur at night, using Cylumes (chemical lights) as visual attractants. This fact suggests that the Bigeye Thresher relies at least somewhat on its eyesight to locate prey — although whether this species regularly includes bioluminescent prey in its diet or has learned that non-luminescent prey often aggregate near light sources is not yet clear. Bigeye Threshers are also occasionally caught at night by sport and commercial anglers fishing for Swordfish near the surface off southeastern Florida. Many of the Bigeye Threshers caught in this fishery are foul-hooked through the tail, suggesting that these sharks use their hyperextended caudal fins to stun prey. However, in the southeastern Florida Swordfish longline fishery, the hook is usually attached to a nylon monofilament leader specifically designed to avoid catching sharks. Thus, the low incidence of mouth-hooked Bigeye Threshers could reflect losses due to biting through the nylon leader rather than a preference for stunning prey with the caudal fin. However, in the western North Atlantic off North Carolina, a 1976 study by fishery biologists Charles E. Stillwell and John G. Casey reported that numerous Bigeye Threshers taken by experimental longline (lacking a long, nylon leader) had in their stomachs two or more stolen baitfish and several of these sharks were foul-hooked through the tail. These findings strongly suggest that these Bigeye Threshers used their whip-like caudal fins to dislodge several baits from the longline before becoming caught themselves.
Catch data have also revealed that the Bigeye Thresher is a rather deep-dwelling species, occurring from the surface down to 820 feet (250 metres) in the Mediterranean, and down to at least 1,640 feet (500 metres) elsewhere. Furthermore, catch records also indicate that the Bigeye Thresher typically inhabits deeper water than the Common Thresher (Alopias vulpinus). This vertical segregation may be related to differences in these species' abilities to thermoregulate. Lateral rete mirablia (elaborate networks of blood vessels arranged along the flanks so to conserve body heat rather than radiate it away to the surrounding water) have been described in the swimming muscles of both Bigeye and Common Threshers, but the system is less well developed in the latter species. Fish thermal ecologist Francis G. Carey and his co-workers measured the swimming muscle temperature of two Bigeye Thresher Sharks and compared these readings to the temperature of the water surface at the same location where each was caught. Carey's team found that the sharks' body temperatures were 3.8 and 9.1 °F (1.8 and 4.3 °C) higher than the water temperature. Unfortunately, no one has yet measured the body temperature of a Common Thresher, but — given its lesser development relative to the Bigeye Thresher — it is predicted to be only mildly elevated above ambient water temperature. The Bigeye Thresher has, in addition, a well developed orbital retia — a kind of 'brain heater', which may prevent the predator from becoming addled in the mind-numbing chill at depth.
Keeping its wits sharp may be of particular adaptive value to the Bigeye Thresher Shark. The teeth of this species are larger and more obliquely-cusped than in other threshers, which suggests that it may tackle some large and tough-to-catch food items. Stomach contents from the Bigeye Thresher have revealed that it is quite gastronomically adventurous. Known prey of the Bigeye Thresher includes not only small-to-medium-sized schooling pelagic fishes such as mackerels (family Scombridae) and herrings (Clupeidae), but also cryptic bottom fishes such as whiting and hakes (Gadidae), large, toothy predatory teleosts such as lancetfishes (Alepisauridae), and even small billfishes (Istiophoridae). The Bigeye Thresher also feeds heavily on oceanic squids, such as the bioluminescent deep-sea squid, Lycoteuthis diadema, and ommastrephid squids such as short-finned squids of the genus Illex. Short-finned squids comprise some 75 to 80% of the cephalopod diet of the Swordfish in the Florida Current, which may explain why Bigeye Threshers in this region are so often caught on longlines set for the more lucrative spear-nosed teleost. In the Mediterranean Sea, the Bigeye Thresher demonstrates a strong association with schools of Frigate Mackerel (Auxis rochei), suggesting the possibility that it may follow schools of its prey like a wolf tracking a herd of sheep from pasture to pasture and feeding when the need takes it.
Putting all the pieces of direct and circumstantial evidence together reveals a dramatic image of the Bigeye Thresher earning a living in a difficult and demanding environment:
At a depth of some 300 feet (100 m), a solidly built, muscular torpedo sculls languidly through the velvety blackness. Despite the marrow-numbing chill, the hunter's boneless body is kept warm by vascular shunts just beneath an armor-like skin that redirect metabolic heat toward the core rather than losing it to the external environment. It's huge, upward mounted eyes scan the surface for any hint that might betray the presence of a potential meal on the fin. There! A faint movement far above: the swirling silhouette of a school of mackerel, careening in formation like a disciplined, well-practiced regiment. With a flick of its top-heavy caudal fin, the Bigeye Thresher rises surfaceward, through chill layers of midnight purple, indigo, and sailor blue. As her 12-foot (3.7-metre) body punches through the thermocline, the thresher's long, pointed fins adjust her trim with consummate skill and delicate precision. The water is brilliant ultramarine now. She can see her quarry clearly, and they can see her. Instinctively, the mackerel draw together, those on the outer perimeter desperately seeking shelter by diving toward the center of the herd.
For a predator that would pluck its prey from the water, the writhing swarm of scaly bodies presents only a hopelessly confused mass, with no clear target. But for the Bigeye Thresher, this prey response is most opportune. Within 20 feet (6 metres) of the school, she accelerates sharply, her jaws gaping rhythmically as though in anticipation. As the shark closes rapidly, the mackerel school packs ever tighter. Exposed in the featureless vastness of the open sea, it is their only defense. As her snout pierces the edge of the school, the thresher's broad, falcate pectoral fins curl downward and rotate outward, bracing her body in the water column. At the same instant, she bows her body into a sharp U-shape and powerfully lashes her scythe-like caudal fin into the panic-stricken mob. ZWAACK!! ZWAACK!! ZWAACK!! In rapid succession, the hard, flat tail-tip slices through water and flesh. About 20 mackerels are killed or mortally injured in the attack. Some fish were cut neatly in two, others fluttered weakly — broken and bent — before performing a final shudder, while still others exploded in a brownish-orange chrysanthemum of fish fluff. The surviving majority of the school speed away from their fallen members. As the scent of blood and fish oils reaches her nostrils, the Bigeye Thresher begins to feed.
Her feeding is as casual as the attack had been violent. With delicate control, the Bigeye Thresher circles, deftly scooping up dead and dying mackerel with a sideways swipe of her jaws as her tail trails behind like a lethal exclamation mark. As if by magic, a barrage of seabirds plunge downward, fracturing the undulating mirror surface with feathery explosions, greedily snapping up stray morsels before the thresher can swallow them. As the shark finishes picking off the last worthwhile remnants of her meal, tiny fishes — too small to warrant her attention — swarm in to consume the leftover tattered bits of flesh, bone, and scales. Within a few minutes it is all over and an eerie silence permeates this vast blue wilderness. The mackerel school is long gone. The seabirds have abandoned the drowned feast for aerial swooping and raucous squawking. All that remains to mark the passing of the hapless score of fish who died here is a slowly sinking constellation of silvery scales, scintillating brightly as they are kissed by flickering shafts of underwater sunlight. Silently and unhurried, the Bigeye Thresher descends back to the depths from which she rose, there wait until her next meal swims overhead.
Seen in the context of its natural environment, the Bigeye Thresher is very different from the crude cartoon suggested by a mis-shapen and inanimate carcass lying on a deck or dock. It is a formidable predator, elegantly adapted to making a living in the chill vastness of the ocean depths. For only sculling silent and free through its native habitat can this remarkable shark be appreciated for the living marvel that it is.