/// Whitetip Reef Shark in Bunaken

24 Aug / 2011
Author: Two Fish Blog Tags: , Comments: 0

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IMG_1738 In Bunaken Island, North Sulawesi the Whitetip reef shark is one of our regular encounters.

Despite having a face that looks strangely like that of a disgruntled weasel, the Whitetip Reef Shark (Triaenodon obesus) is generally unaggressive toward humans who invade its environment. Although it often rests in caves during daylight hours, this species is probably the most commonly-encountered shark of the tropical Pacific. Indeed, for many divers and snorkellers, the phlegmatic Reef Whitetip is their only ambassador to sharkdom.

The Slender Whitetip Reef Shark

Despite its formal species name, obesus, the Whitetip Reef Shark is rather slender and delicately built. This body form grants it the ability to slither, eel-like into crevices in the reef, where this species is an absolute master at extracting prey. Quiescent during the day, Whitetip Reef Sharks become active and determined hunters at night. Moving over the reef face in loosely organized packs, these sharks systematically poke their blunt heads into each crack and crevice in the reef face in search of prey. Known prey of the Whitetip Reef Shark includes sleeping diurnal teleosts or hiding nocturnal creatures such as octopuses, soldierfishes, wrasses, and trumpetfishes, which are grasped with its small, tricuspid teeth. Using its ampullae of Lorenzini and uniquely tube-flapped nares, the Whitetip Reef detects its prey primarily by bioelectrical cues and scent. When a hunting Whitetip Reef locates a prey animal within a hole in the reef face, it violently twists and turns to push itself deep into the crevice. Some sharks actually squirm into a hole in one side of a coral head and exit through an opening on the other. During these zealous foraging bouts, Whitetip Reef Sharks have been observed breaking off pieces of coral — sometimes tearing their skin and fins. Although they are primarily nocturnal, Whitetip Reef Sharks can and do feed opportunistically by day.

whitetip_reef_shark In addition to electrical and olfactory cues, the Whitetip Reef Shark is also highly responsive to sounds and vibrations. In experiments conducted at the Marshall Islands and French Polynesia, Whitetip Reefs consistently responded to recorded sounds of struggling fish, feeding sharks, and even vocalizations of teleosts. At Eniwetok Atoll, Marshall Islands, artificially generated sounds were found to be most attractive to Whitetip Reef Sharks when they combined low frequency (25-100 Hertz) with varied intermittent pulses every 7.5 to 15 seconds. These sound characteristics correspond well to the irregular sounds and vibrations generated by a fish struggling on the end of a spear or on hook and line. In South Pacific areas where spearfishing is common, Whitetip Reef Sharks respond very rapidly to the sound of a speargun discharge, typically appearing within seconds. Although normally quite placid, this inquisitive species can become persistent and bold when faced with a diver playing or carrying a speared fish — sometimes dashing in to tear an impaled fish from the spear tip.

Due to the abundance of Whitetip Reef Sharks in coastal areas of the tropical Pacific and Indian Oceans, day-to-day life of this species is better known than that of most sharks. Whitetip Reefs are most often encountered during daylight hours while they rest quietly in underwater caves throughout much of the tropical Indo-Pacific or in lava tubes of Hawaiian reefs. But in some locations — such as at Cocos Island and near-shore waters off the Pacific coast of Costa Rica — this species is often seen lying stretched out on the sandy bottom, completely exposed in broad daylight. Sometimes, several of these gregarious sharks are seen lying side-by-side or even stacked on top of one another, like cord-wood. The significance of these diurnal al fresco gatherings is not known, but may have something to do with these sharks being cleaned by small wrasses and at least one species of goby.

The Reef Whitetip is one of the three most common sharks on Indo-Pacific coral reefs, the other two being the Blackfin Reef Shark (Carcharhinus melanopterus) and the Grey Reef Shark (Carcharhinus amblyrhynchos). Although these three sharks are widely distributed over the vast expanse of the tropical Pacific and Indian Oceans, their relative distribution over the reef profile is remarkably uniform from place to place. Blackfin Reef Sharks, especially juveniles, are typically found closest to shore, inhabiting the turbid lagoon shallows over sandy plains at depth from 0 to 50 feet (0 to 15 metres). Juvenile Grey Reef Sharks are found in the clearer, deeper waters of the back reef, while adults of this species typically patrol the reef crest and fore reef from the depth of about 65 feet (20 metres) down to a depth of about 330 feet (100 metres). Both species are most active at dawn and dusk, accomplishing most of their feeding during these twilight hours when schooling diurnal fishes are most vulnerable.

Neatly nestled between the crepuscular Blackfin Reef and Grey Reef sharks, Whitetip Reefs typically haunt the reef flats and shallower parts of the fore-reef at depths of 35 to 100 feet (10 to 30 metres). Although it can extend its range from the intertidal to at least 130 feet (40 metres), the Whitetip Reef manages to coexist with the Blackfin Reef and Grey Reef Shark by feeding primarily at night and specializing in extracting prey from cracks and crevices in the reef face that are all but inaccessible to these other sharks. By inhabiting different depths and ecological niches, the slender, weasel-faced Whitetip Reef Shark reduces competition for food resources with other sharks sharing its habitat.

SHARK FACTS

sharkid-whitetipreef-shark-features-600 Size:
Birth: 20-24 in (52-60 cm)
Maturity:males 40-41in (104-105cm), females 41-43in (105-109 cm)
Maximum: 5.6 ft (1.7 m)

Reproduction:
Maturity: unknown; at least 5 years in both sexes
Mode: viviparous, with a yolk-sac placenta
Gestation: 13 months
Pups: 1-5 (usually 2-3), probably every two years

Diet:
Juvenile: small teleost fishes, crabs, octopuses
Adult: teleost fishes, crabs, octopuses
Habitat: Sandy Plains, Rocky Reefs, Coral Reefs, Deep Sea

Depth: 3-1080 ft (1-330 m), often at 26-130 ft (8-40 m)

Distribution: Central Pacific, South Pacific, Tropical Eastern Pacific, Southern African, Madagascaran, Arabian, Indian, South East Asian, Western Australian, Northern Australian, Japanese

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