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.