Biology and Behaviour of the Oceanic Whitetip (Carcharhinus longimanus = [maou])

by Ben S. Roesch


Oceanic Whitetip (Carcharhinus longimanus = [maou])

One of the more conspicuous sharks alive today is the oceanic whitetip (Carcharhinus longimanus = [maou]). While many members of the family Carcharhinidae (to which the genus Carcharhinus belongs, along with about 11 other similar-looking genera) appear very similar to each other, this stocky shark stands out, primarily because of its fins. It has a large rounded dorsal fin and very long (hence its specific name longimanus) paddle-like pectoral fins; all of these fins have a tip of whitish color on their ends, hence the species' common name. (It is has been suggested that the white markings may serve as a recognition signal, so that members of the species can recognize one of their own, and will not begin to hunt eachother!). Sometimes, the white tips (which are sometimes also apparent on their caudal, second dorsal, pelvic and anal fins) are accompanied by black markings. In younger specimens, the pelvic and pectoral fins are often tipped with black, and these juveniles often have saddle-shaped markings between the first and second dorsal fins (Ellis, 1994). The underside of the shark is always whitish, but the color of the dorsal half of the body sometimes varies slightly from location to location. While it is usually grey-bronze above, in the Red Sea, the back is sometimes browner; in the Indian Ocean the back is greyer; and off Hawaii the back is more of a pale beige (Stafford-Deitsch, 1988).

The whitetip can grow quite large; gigantic specimens may reach 3.5-3.95 m (11-13 ft), but most are less than 3 m (10 ft). Data on weights of whitetips is scarce, but Backus et al. (1956) wrote that they weighed a 2.04 m long male at 60-65 kg (135-145 lb).

The whitetip has a short blunt snout, and its powerful jaws are filled with sharp serrated teeth. As in many other carcharhinids (members of the family Carcharhinidae), the shape of the teeth in the oceanic whitetip differ in the upper and lower jaws. In the upper jaw, the teeth are broad, triangular and completely serrated, but the teeth in the lower jaw are much more pointed (and not so broad and triangular), and only have serrations on a small portion nearer to the tip (Compagno, 1984). The shape of its teeth suggest that they are useful in taking large chunks out of big prey; the pointy lower teeth pin and hold on to the prey, while the serrated triangular upper teeth "saw" out a piece of flesh.

Many sharks have rather limited geographical ranges, but the whitetip is found worldwide in epipelagic (the upper 200 m of the open sea) tropical and subtropical waters. It is most common in waters between 20 o North and 20 o South, and (corresponding) water temperatures between 18 o Celsius to 28 o C. It appears to prefer water temperatures of above 20 o C, though it has been caught in 15 o C water. It will leave waters that cool below this temperature, such as the Gulf of Mexico in winter. Though it is usually found well offshore in areas of deep waters, occasionally it is found near land, in waters as shallow as 37 m (120 ft). This occurs primarily at mid-ocean islands such as Hawaii (or near continents where the continental shelf is narrow), where deep water is nearby (Compagno, 1984). Where it does exist, the whitetip is very abundant. Lineaweaver and Backus (1969) wrote: "[it is] extraordinarily abundant, perhaps the most abundant large animal, large being over 100 pounds [45 kg], on the face of the earth." It is because of this abundancy that whitetips are often the first to appear at mid-ocean disasters, and do so in great numbers, an unfortunate thing for the victims in the water. In the World Wars this was a major concern, as boats were torpedoed and sunk (and planes shot down) in great numbers. It is believed that whitetips were responsible for the deaths of many men when the steamship Nova Scotia was torpedoed and sunk by a German submarine off northern Natal, South Africa in WWII (Bass, D'Aubrey & Kistnasamy, 1973). Of the 900 men on board (of which 750 were Italian prisoners of war) only 192 survived, many probably victims of sharks, in what survivors described as a "feeding frenzy" (Reader's Digest, 1989).

The behaviour of C. longimanus is to say the least, unpredictable. It has been sometimes been described as being slow, lazy, and stubborn, but it has also been described as aggressive and dangerous (Ellis, 1983). What we may be seeing is the difference between excited predatory behaviour and normal "cruising" behaviour. No pelagic shark stops swimming (1), so most of the time they slowly cruise along at the surface - it would be a great waste of energy to speed about all the time, especially because there is no need. However, when the sharks sense food, they go into "predatory mode" which is marked by increased speed and aggressiveness. At "the dinner table", whitetips often dominate other pelagic sharks, like the silky shark (Carcharhinus falciformis), as long as the other shark is not much bigger than the whitetip! In the case of the silky shark, whitetips are less agile and speedy, so if the food item drifts away from the whitetip, the silky shark may use its nimbleness to snatch the food and make a quick getaway (Compagno, 1984).

Whitetips are very inquisitive, persistent and bold when it comes to checking out possible food sources, and will investigate almost anything they come across, including divers! (Interestingly, they are very cautious of hooked baits). This is understandable when you think about the open ocean; if you were to jump into the middle of the Pacific and look around, you probably wouldnÕt see many fishes or other marine animals, because population densities are low, and localized in schools of fish etc. Therefore the whitetip is an opportunistic feeder, checking out any objects in the water to see if it might be palatable. This all means that divers in the open ocean must be very wary of the whitetip, and that you had better watch out if shipwrecked or stranded somehow in the open ocean (see above for the case of the Nova Scotia). Certainly, if the whitetip was a coastal species (like the white shark) it would be a very real danger.

However, whitetips are usually after oceanic cephalopods and bony fishes, including lancetfish, oarfish, threadfins, barracuda, jacks, dolphinfish, marlin, various species of squid, and tuna, skipjack and other scombrids (mackerels, including tunas). Whitetips have also been found to eat sea birds, sea turtles, marine gastropods (snails, limpets, periwinkles etc.), crustaceans, carrion from marine mammals, and even garbage. Finally, they occasionally eat stingrays (probably the pelagic stingray [Dasyatis violacea]). Predatory behaviour and techniques has been noted in two occasions: In the first occurrence, whitetips were seen feeding on a dense school of threadfins by slowly taking bites out of the school, as a person would eat an apple. Another more amazing feeding technique was the observation of whitetips feeding on tight schools of small tuna, who were feeding frenzily on schools of sardines near the surface. For 30 minutes, whitetips were seen cruising erratically through the aggregation with jaws open wide, without attempting to bite or chase any of the tuna. Some of these whitetips were later caught, and in their stomachs were some of the feeding tunas; thus, it has been suggested that the whitetips merely swam through the school (which probably didn't acknowledge the shark's presence, thanks to the feeding frenzy), waiting for the tunas to swim into their gaping jaws, before biting! (Compagno, 1984)

One of the more interesting facets of the whitetip's behaviour is its association with the shortfin pilot whale (Globicephala macrorhynchus) off Hawaii, as described by Jeremy Stafford-Deitsch (1988). Whitetips are often found swimming along with the big pods of pilot whales, and often follow the whales when they dive down into the depths. The reason whitetips hang around pilot whales is not known, but it is possible that the sharks follow the whales and feed along on the squid that the whales actively seek out (this means the sharks donÕt have to go find their food, they just follow the whales, which are great squid finders). This behaviour probably occurs not only off Hawaii, but anywhere pilot whales and whitetips share habitat.

Data on the reproductive biology of the whitetip tells us that they are viviparous (eggs hatch inside the mother, and the young are born alive), with the young being nourished by a placental yolk sac. Litter sizes range from 1-15, with increasing numbers being proportional to the size of the mother. As far as is known from records from the northwestern Atlantic and southwestern Indian Ocean, it appears whitetips mate and give birth in the early summer, with a gestation period which lasts about one year. In the central Pacific, there appears to be little distinction between mating/birthing seasons, as females with small embryos have been found year-round (Compagno, 1984).

The role of the whitetip in the open ocean habitat is obvious: it is an apex predator, feeding on a variety of animals lower in the food web. Like any apex predator, it keeps populations of prey animals in check, and therefore is an essential part of the ecology (and the ecological balance) of its environment. The species does not seem to be endangered in any way, despite the fact that it is regularly caught on open ocean longlines (and then used for meat, fins [for shark-fin soup], hides, fishmeal and vitamins [extracted from its liver]). Hopefully its status will remain this way, so it can continue to help maintain the ecological balance in the open ocean, and continue to amaze us with its remarkable behaviour and lifestyles.

Notes:

(1) Most sharks are denser than water, and if they stop swimming, they will sink. This is not an option for pelagic sharksbecause pelagic sharks occur where the bottom is very deep (so resting on the sea floor is not possible); and the shark would suffer from oxygen dificiency. Some sharks, especially those that live on the sea floor, have small openings behind their eyes called spiracles which allow the shark to intake water to pass over its gills, without the shark having to move. But in many pelagic species the spiracle is virtually nonexistent, and the shark must continually swim to keep oxygen-rich water flowing over its gills.

Bibliography:

Backus, Richard H., Stewart Springer and Edgar L. Arnold, Jr. 1956. "A contribution to the natural history of the white-tip shark, Pterolamiops longimanus (Poey)." Deep-Sea Research vol. 3, pp. 178-188.

Bass, A.J., J.D. D'Aubrey & N. Kistnasamy. 1973. "Sharks of the east coast of southern Africa. 1. The genus Carcharhinus (Carcharhinidae)." Invest. Rep. Oceanogr. Res. Inst., Durban, no. 33, 168 pp.

Compagno, Leonard J.V. 1984. FAO Species Catalogue, Vol. 4, Parts 1 and 2, Sharks of the World. (Rome: FAO), 655 pp.

Ellis, Richard. 1976. The Book of Sharks. (New York: Grosset & Dunlap), 320 pp.

Lineaweaver, Thomas H., III, and Richard H. Backus. 1969. The Natural History of Sharks. (Philadelphia: Lippincourt), 256 pp.

Reader's Digest. 1989. Sharks: Silent Hunters of the Deep. (Sydney: Reader's Digest), 208 pp.

Stafford-Deitsch, Jeremy. 1988. Shark: A Photographer's Story. (San Francisco: Sierra Club Books), 200 pp.


All Rights Reserved: Ben S. Roesch, 1997.