The tresher shark: the magic is in the tail.
The Tresher shark is a rare mostly solitary shark that, like other marine apex predators, has a difficult time in surviving. It’s meat and fins make it a favorite target for offshore fisheries, which has put it on the IUCN Red List of vulnerable species. Many UW photographers that have succeeded to ‘shoot’ Tiger, Oceanic and Great Hammerhead sharks from a short distance, have not yet been able to come close to a tresher. My only encounter was many years ago in the southern Red Sea when I saw it passing below me in the hazy blue along a steep drop off. The tresher is simply a too rare, distant and shy animal to get lured into a baited shark session. It also prefers deeper water than the zone in which UW photographers normally feel comfortable. Probably it’s just that detachedness, combined with the magic of its enormous tail that keeps stimulating our interest and curiosity in the creature.
Left: From top to bottom: the Common, Pelagic and Big eye tresher. Picture adapted from Alessandro de Madalena.
Anatomy and general characteristics Although genetics has become a more accurate measure of relatedness than appearance, the conventional Linnaean taxonomy is still a helpful tool in distinguishing between the 400 different shark species of the World. Going from general to more specific categories this implies: order, family, genus and species. So, the tresher shark belongs to the order of Lamniformes also known as mackerel sharks. Which includes the great white, basking, megamouth and mako shark. Within this order the treshers form the family of Alopiidae or ‘foxlike’ sharks. Like other mackerel sharks thresher sharks are ovoviparous, meaning that the young develop inside a weakly formed shell within the female. Along each of their flanks runs a strip of red aerobic muscle, which can contract powerfully for long periods, enabling the thresher shark to swim without fatigue.* Treshers feed mainly on small fishes like herring, anchovies and sardines. They can be found in deep waters as well as along reef drop-offs and coastlines in many places of the world. Like other Lamnid sharks, the thresher shark (at least one of the species, the common tresher, see below) has a network of blood vessels called rete mirabile that allows it to maintain its body temperature slightly above that of the surrounding water. Its a system they share with the tuna fish. The network ensures that metabolic heath produced by its strong red muscles is not lost in the surrounding seawater, but retained inward towards the core of the body.* The treshers pectoral fins are long, a bit backwardly curved and spread out, like in the Oceanic shark. But it most conspicuous part is its enormous whiplike tail.
The tail The tresher’s elongated upper lobe of the tail fin almost equals the length of the sharks body. The only shark with a tail of almost similar length is the variegated shark (Stegostoma varium, or Stegostoma fasciatum). Because it is covered with dark spots or stripes (juveniles) it is often also called zebra shark or leopard shark (not to be confused with Triakis semifasciata). But the variegated shark belongs to the different order of Carpet sharks. Like the nurse shark it has nasal barbels and prefers shallow water, in particular sandy bottoms. Variegated shark is an oviparous (egg laying) shark that is also commonly seen in the Red Sea.
Why has nature equipped the ‘foxies’ with such an strange and enormous tail? Is it just a caprice of evolution, or an attribute with a certain advantage for its survival? A similar question is often asked with respect to the hammerheads bizarre shaped head, and the swordfish with its elongated snout. A generally accepted view is that the tresher uses its tail as a weapon or instrument for striking fish. In some amazing video captures it can indeed be seen slapping its whiplike tail and slashing its way through schools of sardines, before returning to devour stunned and wounded victims.**** Crocodiles are probably one of the few land species that are known to use their tails with a similar purpose. Some eye witnesses have even reported seeing the tresher slapping seabirds sitting on the surface. Alternatively, its long and strong tail might also enhance its propulsive power and ability to make swift turns.
Three species There exist three different species of the genus Alopius: the common tresher (Alopius vulpinus), the pelagic tresher (Alopius pelagius) and the bigeye tresher (Alopius superciliaris). The outside differences between these species may not be directly obvious. But despite the overlap in their anatomy, general appearance, behavior and habitats there still seem to be some important differences that justify their separation (see also the insert).
Common tresher The body is blue-grey to dark grey or blackish on top, with silvery or coppery sides and white undersides. It is the largest tresher that can reach a length of 6 meters. The small mouth is arched and, unlike in other thresher sharks, has furrows (labial folds) at the corners. The first dorsal fin is tall and positioned slightly closer to the pectoral fins than the pelvic fins. In addition, the shark has a stout cylindrical body with a short head, rounded between eyes. It is seasonally migratory and spends summers at lower latitudes. Common threshers tend to be epipelagic (oceanic near the surface), and are common in coastal waters over continental shelves. They are virtually circumglobal in warm seas. Common threshers are regarded by recreational anglers as one of the strongest fighting sharks, together with the short-fin mako, often leaping out of the water when caught on a fishing line. In addition, the common tresher has a vascular heat exchange system (rete mirabile) thet serves to generate and retain body heat, using the energy produced by its strong aerobic muscle.*
Pelagic tresher The Pelagic tresher was hardly mentioned some 20 year ago, probably because in many early publications it was mistaken for the common tresher or just called ''tresher'. The slender pelagic thresher is the smallest member of its family, rarily exceeding 3 meters. It has small teeth and its first dorsal fin is relatively short and is placed halfway between the pectoral and pelvic fins. It has a conical head. Around the mouth corner its has no labial furrows like the common tresher. It can also be distinguished by the dark, rather than white, color over the bases of its pectoral fins. Curiously, the pelagic tresher seems to lack the vascular heat exchange system found in the common tresher to generate and retain body heat (see also the big eye tresher below).* Although encounters between divers and the pelagic tresher are rare, it has been regularly seen in the Red Sea along offshore reefs such as Daedalus or the litte Brother islands. There have also been regular sightings at Layang Layang and Sipadan in Malaysia and Monad shoal near Malapascua in the Philippines. A Malapascua in the Philippines pelagic treshers can be seen when they visit more shallow cleaning stations for their early morning grooming*** Clearly the best opportunity for UW photographers to get closer to these shy sharks (see also the picture of Noam Kortler on this weeks frontpage).
But the pelagic tresher also shares many traits with the common tresher. It is an active, strong swimmer and has been known to leap clear of the water. It is a wide-ranging Indo-Pacific Ocean shark, apparently highly migratory, with low fecundity (two pups/litter) and a low (2-4%) annual rate of population increase. Like the common tresher it is epipelagic, although the species is reportedly relatively often in some coastal localities. It might also visit deeper mesopelagic zones when it hunts for schools of fish.
The big eye tresher’s This species can be easily distinguished by its large eyes and a pair of deep grooves on the top of its head, from which its scientific name is derived. Its enormous eyes are placed in keyhole-shaped sockets that allow them to be rotated upward. The first dorsal fin is placed more backwards than the common tresher. It is also know to visit very deep layers of the ocean in the mesopelagic zone below 200 meters. The large eyes of the bigeye thresher are probably adapted for hunting in the low light conditions of these greater depths. It is one of the few sharks that conduct a diurnal vertical migration, staying in deep and cold water during the day and moving into warmer surface waters at night to feed**. This migration likely relates to finding prey at night and avoiding predators during the day. The sharks' daytime swimming patterns are usually steady, while at night they have a pattern of slow ascents and rapid descents. Similar to the pelagic tresher it seems to lack the vascular heat exchange system found in the common tresher to generate and retain body heat. These differences could relate to the fact that the red aerobic muscle in this species as well as the pelagic tresher runs closer the skin than in the common tresher, allowing less effective functioning of the heat preservation blood vessel network and thus less metabolic heat conservation*.
Survival Overfishing in targeted shark fisheries, by-catch in fishing gear targeting other species, and high levels of illegal and unregulated fishing have caused drastic reductions in the tresher’s populations. The common thresher is widely caught by offshore longline and gilnet fisheries and is especially vulnerable to fisheries exploitation because its epipelagic habitat occurs within the range of many largely unregulated and under-reported gillnet and longline fisheries.
References and links
Sharks of the Open Ocean: Biology, Fisheries and Conservation. (2008) Blackwell Publishing Ltd Editor(s): Merry D. Camhi, Ellen K. Pikitch, Elizabeth A. Babcock Published Online: 28 JAN 2009 Print ISBN: 9780632059959
Susan E. Smith, Randall C. Rasmussen, Darlene A. Ramon and Gregor M. Cailliet. The Biology and Ecology of Thresher Sharks (Alopiidae) (pages 60–68) (Chapter 4 in the Camhi et al. book)
Compagno, L.J.V. (2002). Sharks of the World: An Annotated and Illustrated Catalogue of Shark Species Known to Date (Volume 2). Rome: Food and Agricultural Organization. pp. 8
*Sepulveda, C.A., Wegner, N.C., Bernal, D. and Graham, J.B. (2005). "The red muscle morphology of the thresher sharks (family Alopiidae)". Journal of Experimental Biology. 208 (Pt 22): 4255–4261. doi:10.1242/jeb.01898. PMID 16272248.
**Weng, K.C. & Block, B.A. (2004). "Diel vertical migration of the bigeye thresher shark (Alopias superciliosus), a species possessing orbital retia mirabilia". Fishery Bulletin – National Oceanic and Atmospheric Administration. 102 (1): 221–229.1–83. ISBN 92-5-104543-7.