Shark Page 13
The blacktip reef shark (Carcharhinus melanopterus) is also widespread in the tropical waters of the Indo-Pacific, its common name derived from the distinctive black tips of the first dorsal fin and lower caudal lobe. It is similar in size to the whitetip reef shark, reaching a length of about 1.8 metres and weighing up to 45 kilograms. It is a streamlined, agile shark with classic countershading: grey upper body and white lower body, with a dark stripe along each flank. The blacktip is a pursuit predator and often hunts in a pack, rounding up shoals of fish which are then consumed in a feeding frenzy. Like the whitetip reef shark, the blacktip has a very small range, and frequently inhabits shallow mangrove and lagoon waters, its dorsal tips exposed as it cruises. Along with its diet of fishes, crustaceans and cephalopods, a northern Australian study found that almost a quarter of blacktip reef sharks’ stomachs contained terrestrial snakes—clear evidence of its extremely shallow water lifestyle.
Unfortunately, any healthy juvenile blacktip unlucky enough to be netted may be kept alive to gratify the human desire to possess the unusual and rare:
Amongst the ‘real shark’ looking sharks offered in the aquarium trade (other than those Nurse Sharks, Leopards, Epaulettes, Bamboo, Catsharks . . . that spend so much time ‘just sitting on the bottom’) is the Requiem Shark (family Carcharhinidae) member Carcharhinus melanopterus . . . Unfortunately, this shark is entirely unsuitable for home aquarium use, requiring a pool-sized enclosure (thousands of gallons) to do well. Though folks can/do keep small specimens of the Blacktip Reef Shark, aka the Reef Blacktip Shark in much smaller systems for a time, invariably these are short term successes, with the specimens almost always dying ‘mysteriously’, crashing into the tank’s side, or jumping out. As with all the other 49 species of Requiem Sharks, you’re encouraged to go visit them in the ocean or Public Aquariums.20
The grey reef shark (Carcharhinus amblyrhynchos) shares the same Indo-Pacific habitat as the whitetip and blacktip reef sharks, and is similar to the latter except that its upper body is silvery-bronze. The teeth are triangular and serrated in the upper jaw and narrow and generally smooth-edged in the lower jaw. Social and tending to school, it is not only the largest of the three, exceeding two metres in length, but probably the fastest, with a top speed approaching 50 kilometres per hour. Its range takes in coral shallows as well as deeper areas near the drop-offs to deep water. Shark researcher Leonard Compagno has studied the relationship between the three species:
. . . [the grey reef shark] shows microhabitat separation from the blacktip reef shark; around islands where both species occur, the blacktip occupies shallow flats while the grey reef shark is usually found in deeper areas, but where the blacktip is absent the grey reef shark is commonly found on the flats . . . [The grey reef shark] complements the whitetip shark, as it is far more adept at catching off-bottom fish than the whitetip, but the latter is far more competent in extracting prey from crevices and holes in reefs.21
Compagno also describes the grey reef shark’s threat display, as investigated by behavioural researcher Donald R. Nelson:
This consists of an exaggerated swimming pattern in which the shark wags its head and tail in broad sweeps, arches its back, lifts its head, depresses its pectoral fins and sometimes swims in a horizontal spiral. The display varies in intensity from merely a component of flight from the accosting diver to a series of figure-8 loops in front of the aggressor. Using a small shark-shaped ‘Shark Observation Submersible’ to approach grey reef sharks, Dr Donald R. Nelson was able to elicit threat display from the sharks while other divers filmed the behaviour from a safe distance. When persistently approached by the sub, some of the displaying sharks fled, but a few terminated the display and attacked the sub at high speed, biting one or more times and then fleeing. The speed of the attacks and the damage to the sub was impressive, and is a mute warning that these sharks should be treated with respect and not cornered or harassed by divers. The threat-display behaviour of this shark is thought by some researchers to possibly intimidate potential predators on it.22
The Caribbean reef shark (Carcharhinus perezi) can grow to a little under three metres and shares many of the physical and behavioural characteristics of other reef shark species. It is, however, distinguished by its restricted geographical range—the reefs and adjacent waters of eastern South America north to the east coast of Florida, the Gulf of Mexico and the Bahamas.
Caribbean reef sharks played a central role in a shark-tracking project which used surgically implanted acoustic transmitters to monitor the movements of shark populations and to determine the threats posed to the animals by commercial fishing. This Pew Institute for Ocean Science project took place over four years at Glover’s Reef Marine Reserve (GRMR), an atoll 45 kilometres off the coast of Belize, home to twelve species of sharks and rays.
A Marine Protected Area (MPA) is one in which fishing is prohibited. Obviously, sharks have no concept of such boundaries and an MPA is of little use if it doesn’t protect the species most at risk, hence the importance of tracking. The Pew project also set out to determine to what extent the coral itself is dependent for its health on the animals, including predatory sharks, which share its habitat.
Results showed a strong tendency to ‘site fidelity’, but also confirmed the unpredictability of shark behaviour. Caribbean reef shark 3348, an adult male, was tracked as follows:
Although he routinely patrols the eastern reef slope and visits the east side of Middle Caye on an almost daily basis, 3348 occasionally disappears from Glover’s Reef. On one such trip, for about four days in July 2004, he swam across deep, open water to Lighthouse Reef (30 km to the northeast of Glover’s Reef) . . . Despite this, 3348, like all other Caribbean reefs tracked to date, spends many days at GRMR and does not seasonally emigrate from this location for any length of time.23
Their unpredictability is one of the characteristics which make sharks so fascinating to researchers, but it also complicates the already difficult task of promoting, let alone enforcing, conservation in the oceans.
The false catshark (Pseudotriakis microdon)
This is a rare ground shark which grows to about three metres. Although sometimes found in moderate depths, it generally lives in very deep waters, to about 1500 metres. Its usual range is in the northern hemisphere waters, although it is occasionally found in the Indian and Pacific Oceans. The false cat is named for its elongated cat-like eyes. It is dark pinky brown in colour, with a long snout, large spiracles and hundreds of rows of tiny teeth in a large mouth with a huge gape. The low first dorsal fin extends for a long way along the back, but the fins, musculature and skin are all flabby, indicating a sluggish swimmer. The pectoral fins are small and lobe-like.The stomach contents of specimens taken in the Pacific included deepwater eels, grenadiers, mackerel, lanternfish, squid and octopus, but also pufferfishes, which are normally restricted to shallow waters. Researchers speculate that the sharks may have scavenged dead pufferfishes that had sunk to the ocean floor.
In 2004 a false catshark was recorded for the first time in eastern Australian waters, by a commercial fishing vessel undertaking exploratory species targeting in the Coral Sea. It was a mature male, measuring 277 centimetres. Interestingly, in that same year a slightly larger female became the first recorded in nearby Indonesian waters when it was taken near the island of Lombok.24
The lemon sharks
The lemon shark has been intensively researched, because it is easily kept in captivity and not difficult to study in the wild. The Bimini islands off Florida’s coast are home to the Bimini Biological Field Station, whose biologists have for many years specialised in lemon shark research, because the species spends its entire life cycle of about 25 years in the local waters. This whaler shark is fusiform and grows to about three metres, weighing 120 kilograms or more, and is in many respects a ‘typical’ shark. It has prominent, firm, broad-based pointed fins and a large powerful tail. The body is variously described as moderately stout, stocky, or robust, al
l of which apply to the whaler clan. (This uniform similarity is why it is difficult to differentiate between many of the whaler sharks.) Lemon sharks have a widespread global distribution, fringing the continental shallows of the western and central Pacific Ocean, the Indian Ocean, the Atlantic and the Gulf of Mexico. There are currently two recognised species, Negaprion brevirostris, common in the western Atlantic, and Negaprion acutidens, found in Australasian waters. These sharks have a yellowy-brown dorsal colouring with a paler yellow underside. Juvenile lemon sharks are born in litters of up to about fourteen, the pups then developing in mangrove shallows, which are both rich in prey and relatively safe from predators. The eyes of juvenile lemon sharks are adapted to cope with the tannin-coloured water of these shallows, which alternates between bright and dark. And research on lemon shark eyes has had broader results:
Perhaps the most ecologically significant discovery about shark retinas was revealed in a fascinating 1991 paper by Robert Hueter. Hueter discovered that the Lemon Shark has a broad, horizontal band that lies across the equator of the retina and is disproportionally rich in cones. Based on a similar retinal band in the Lion (Panthera leo), Cheetah (Acinonyx jubatus), Thompson’s Gazelle (Gazella rufifrons), Wildebeast [sic] (Connochaetes taurinus), and other mammals of the African plains, this so-called ‘visual streak’ probably grants the Lemon and other sharks a particularly clear view of the underwater horizon. As a shark’s potential prey, rivals, and mates are most likely to first appear on the horizon at the limit of visibility, the adaptive (survival) value of the visual streak is easy to imagine.25
Unfortunately, worldwide destruction of mangrove areas, primarily for coastal development or aquaculture farming, is affecting lemon sharks, as well as every other marine species reliant upon these critical bodies of water.
Juveniles swim together and this group behaviour persists into adulthood. As the animals mature they move into the open waters, with a home range of some hundreds of square kilometres. The lemon shark is a tropical water apex predator, fast in pursuit with long, thin, sharp, finely serrated teeth ideal for grabbing a variety of prey: teleosts, rays, smaller sharks and seabirds resting on the ocean surface.
Oceanic whitetip shark (Carcharhinus longimanus)
This pelagic species averages about three metres. The heaviest recorded specimen to date weighed 167.4 kilograms. It is one of the most abundant, cosmopolitan and widely distributed pelagic sharks. Highly migratory, it is found in circumglobal warm waters from as far south as south-western Australia to northern Atlantic waters off southern Canada. The tips of the huge dorsal, pectoral and upper caudal fins are generously splotched with white (hence its common name), contrasting with the evenly denticled bronze-grey skin of the upper body. Longimanus translates as ‘long hand’, a good description of its winglike pectoral fins.
The oceanic whitetip has a short, broad snout. Its powerful jaws contain about 60 teeth. Those in the upper jaw are broad at the base, heavily triangulated and serrated, those in the lower jaw thinner and sharper, with little or no serration. The shark seizes large teleosts such as tuna, barracuda and marlin by thrusting the gaping jaws forward, impaling the prey on the lower teeth while the upper teeth engage in sawing and tearing actions to remove a large chunk of flesh. The oceanic whitetip will swallow smaller prey whole and, although solitary, will occasionally hunt as a pack, herding school fish into tight balls and feeding communally. They have also been observed associating with shortfin pilot whale pods which are expert at hunting one of the whitetips’ prey, squid. They regularly follow tuna fishing boats, which likewise lead them to prey. They also have a predilection for garbage dumped from ships.
The most common adjectives used to describe the whitetip are slow, lazy, aggressive, dominant, inquisitive and persistent. Certainly, with its bulky, slightly humpbacked body and large, broad fins, the whitetip is less slimline than the truly fast big sharks: ‘Reports have described swimming behavior in open waters at or near the surface of the water as moving slowly with the huge pectoral fins spread widely’.26 This is not to say, however, that it is a ‘slow’ shark. Like an eagle or vulture gliding in a thermal, the whitetip conserves its energy, and has plenty of muscle and tailpower in reserve to chase and strike.
The common and probably mistaken belief is that the great white shark is the number one maneater. The oceanic whitetip, whose domain is the upper pelagic layer, is an opportunistic predator and invariably the first to arrive, and in greatest numbers, at shipwreck sites. It is highly likely that over the centuries whitetips have eaten many more shipwreck and other marine disaster victims than any other species of shark.
Galapagos shark (Carcharhinus galapagensis)
The shark’s common name suggests a particular affinity with the Galápagos group of islands in the Pacific Ocean west of Chile, famed for their uniquely isolated flora and fauna and the double geographic rarity of being sandless desert islands. While this requiem shark is common in that area, the name derives only from the fact that a specimen caught there early in the twentieth century was the first of the species to be described. It grows to about three metres and weighs about 85 kilograms.
Like the oceanic whitetip, the Galapagos shark is circumglobal in tropical waters but the similarities end there. The Galapagos shark is slimmer, with straighter, more pointed dorsal and pectoral fins, a longish snout and uniformly brown-grey skin with white underside. Its worldwide distribution is described as ‘patchy’. This is because it has evolved as an oceanic island specialist, preferring the clear, shallow inshore waters often characterised by rocky bottoms and fast-flowing currents found around deepwater islands. It generally feeds on bottom dwellers—teleosts, cephalopods, sharks and rays—and has similar defence/threat postures to those of the grey reef shark. Those sharks which inhabit the waters around the Galápagos group also prey on sealions and marine iguanas.
Although these sharks have been tracked swimming between islands, it seems that their home ranges are based firmly on islands, some well known—Hawaii, Lord Howe, Madagascar, Cape Verde, Bermuda and the Virgin Islands—others less so, such as Kermadec Island off New Zealand, the Revillagigedo Islands off Mexico, Sao Tome and Principe off Nigeria, the Tuamoto Archipelago, Clipperton . . . not to mention St Peter and St Paul’s Rocks.
The Mid-Atlantic Ridge, the planet’s longest mountain range, occasionally breaks the ocean’s surface to form islands such as Ascension, St Helena and the outcrop of St Peter and St Paul’s Rocks. Brazilian national territory, the Rocks consist of nine tiny outcrops totalling less than two hectares. They are the peak tips of a 4000-metre submarine mountain equidistant between the east coast of South America and the bulge of western Africa. Some ten million years old, they are rich in magnesium and iron, a visible manifestation of Earth’s active mantle. Their geological volatility and the wave action around them mean that they are changing shape constantly, and the life forms associated with them are themselves in a permanent state of adaptation. Algaes grow on corals and pridotite (a non-volcanic rock) that form near-vertical undersea dropoffs. The outcrops support only mosses, primitive grasses, insects and spiders, and were described by Charles Darwin in 1831 as ‘from a distance of a brilliantly white colour . . . owing to the dung of a vast multitude of seafowl’.27
Researchers are not sure why the Galapagos shark favoured these rocks. An early twentieth-century expedition by the Scottish National Antarctic Expedition, led by William Spiers Bruce, recorded significant shark activity: ‘Bruce and his party had hoped to land but the swell was too strong. [Medical officer] Pirie jumped ashore but fell into the sea. He managed to scramble back aboard as the crew fended off sharks with their oars’.28
In 1980, a Cambridge Expedition survey found that the Rocks ‘support one of the densest shark populations in the Atlantic Ocean . . . Since [American navigator Amano] Delano’s visit in 1799 . . . visitors to St Paul’s have remarked on the extraordinary number of sharks surrounding the Rocks’.29 The same researchers recorded that
Galapagos sharks were ‘unusually common’,30 making them the dominant resident predator. But just twenty years later, a Brazilian survey found a dramatically altered ecosystem. During the course of four expeditions between 1999 and 2001, involving 47 days of diving to 62 metres, and rock pool surveying, researchers observed many fish species noted in the Cambridge survey, but they did not see a single Galapagos shark.
Why would the apex predator at one of the planet’s most remote landforms vanish from its pre-eminent niche? The Brazilian surveyors’ gloomy assessment was that while the past abundance of Galapagos sharks could be ‘partly attributed to the lack of fishing activity’,31 since the end of the 1970s ‘fishing pressure has increased greatly, and sharks are now targeted due to the high commercial value of their fins. The pelagic fishes on which sharks feed are also targeted by the fishing industry and this may also have contributed to an apparent population decline of C. galapagensis’.32
Blue shark (Prionace glauca)
The blue shark was one of only two shark species recorded in the Brazilian survey of St Peter and St Paul’s Rocks (the other being the silky shark (Carcharhinus falciformis)). Yet far from being rock and reef specialists these two sharks, along with the oceanic whitetip, make up the trio of tropical and temperate pelagic requiem sharks considered to be the most abundant big sharks of their kind.
The blue shark’s circumglobal range is vast, extending considerably further south and north than that of the oceanic whitetip. This shark is also considered to be the greatest traveller, undertaking migrations of hundreds and even thousands of kilometres. Adults grow to almost four metres, and weigh up to 200 kilograms. The website of the Florida Museum of Natural History claims that ‘they are rumored to get as large as 20 feet [six metres]’.33