The Galapagos shark ( Carcharhinus galapagensis) is a species of requiem shark found worldwide, particularly favoring clear reef environments around oceanic islands, where it is often the most abundant shark species. This article delves into the diet of the Galapagos shark, along with its habitat, behavior, and interactions with humans.
Identification and Distribution
The Galapagos shark is a large species, often reaching 3.0 m (9.8 ft) in length. It has a typical fusiform "reef shark" shape. It is very difficult to distinguish from the dusky shark (C. obscurus) and the grey reef shark (C. amblyrhynchos). It can be distinguished from the dusky shark in having taller first and second dorsal fins and larger teeth. It can be distinguished from the grey reef shark in having a less robust body and less pointed first dorsal fin tip. However, these characters can be difficult to discern in the field.
This shark is found mainly off tropical oceanic islands. In the Atlantic Ocean, it occurs around Bermuda, the Virgin Islands, Madeira, Cape Verde, Ascension Island, Saint Helena and São Tomé Island. In the Indian Ocean, it is known from Walter's Shoal off southern Madagascar. In the Pacific Ocean, it occurs around Lord Howe Island, the Marianas Islands, the Marshall Islands, the Kermadec Islands, Tupai, the Tuamotu Archipelago, the Juan Fernández Islands, the Hawaiian Islands, the Galapagos Islands, Cocos Island, the Revillagigedo Islands, Clipperton Island, and Malpelo. The Galapagos shark is generally found over continental and insular shelves near the coast, preferring rugged reef habitats with clear water and strong converging currents. It is also known to form groups around rocky islets and seamounts.
Physical Characteristics
One of the largest species in its genus, the Galapagos shark commonly reaches 3.0 m (9.8 ft) long, with maximum lengths recorded from 3.3 m (11 ft) to 3.7 m (12 ft). The maximum recorded weight is 195 kg (430 lb) for a 3.0 m (9.8 ft) long female. This species has a slender, streamlined body typical of the requiem sharks. The snout is wide and rounded, with indistinct anterior nasal flaps. The eyes are round and of medium size. The mouth usually contains 14 tooth rows (range 13-15) on either side of both jaws, plus one tooth at the symphysis (where the jaw halves meet). The first dorsal fin is tall and moderately falcate (sickle-shaped), with the origin over the pectoral fin rear tips. It is followed by a low midline ridge running to the second dorsal fin. The second dorsal fin originates over the anal fin. The pectoral fins are large with pointed tips. The coloration is brownish gray above and white below, with a faint white stripe on the sides. The edges of the fins are darker but not prominently marked.
Diet and Feeding Behavior
The Galapagos shark is an active predator often encountered in large groups. The primary food of Galapagos sharks is benthic bony fishes (including eels, sea bass, flatfish, flatheads, and triggerfish) and octopuses. They also occasionally take surface-dwelling prey such as mackerel, flyingfish and squid. As the sharks grow larger, they consume increasing numbers of elasmobranchs (rays and smaller sharks, including of their own species) and crustaceans, as well as indigestible items such as leaves, coral, rocks, and garbage.
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At the Galapagos Islands, this species has been observed attacking Galapagos fur seals (Arctophoca galapagoensis) and sea lions (Zalophus wollebaeki), and marine iguanas (Amblyrhynchus cristatus). While collecting fishes at Clipperton Island, Limbaugh (1963) noted that juvenile Galapagos sharks surrounded the boat, with multiple individuals rushing at virtually anything trailing in the water and striking the boat bottom, oars, and marker buoys.
Social Behavior
During group interactions, Galapagos sharks are dominant to blacktip sharks (C. limbatus) but deferent to silvertip sharks (C. albimarginatus) of equal size. When confronted or cornered, the Galapagos shark may perform a threat display similar to that of the grey reef shark, in which the shark performs an exaggerated, rolling swimming motion while arching its back, lowering its pectoral fins, puffing out its gills, and gaping its jaw. The shark may also swing its head from side to side, so as to keep the perceived threat within its field of vision.
Reproduction
Like other requiem sharks, the Galapagos shark exhibits a viviparous mode of reproduction, in which the developing embryos are sustained by a placental connection formed from the depleted yolk sac. Females bear young once every 2-3 years. Mating takes place from January to March, at which time scars caused by male courtship bites appear on the females. The gestation period is estimated to be around one year; the spring following impregnation, females move into shallow nursery areas and give birth to 4-16 pups. The size at birth has been reported to be 61-80 cm (2.00-2.62 ft), though observations of free-swimming juveniles as small as 57 cm (1.87 ft) long in the eastern Pacific suggest that birth size varies geographically. Juvenile sharks remain in shallow water to avoid predation by larger adults. Males mature at 2.1-2.5 m (6.9-8.2 ft) long and 6-8 years old, while females mature at 2.2-2.5 m (7.2-8.2 ft) long and 7-9 years old. Neither sex is thought to reproduce until 10 years of age.
Interactions with Humans
Inquisitive and persistent, the Galapagos shark is regarded as potentially dangerous to humans. However, several live-aboard boats take divers to Wolf and Darwin, the northernmost Galapagos islands, every week specifically to dive in open water with these sharks where they and the scalloped hammerheads accumulate in numbers, and only a few incidents have been reported. They are known to approach close to swimmers, showing interest in swim fins or hands, and are drawn in large numbers by fishing activities. Fitzroy (1839) observed off St. Paul's Rocks that "as soon as a fish was caught, a rush of voracious sharks was made at him, notwithstanding blows of oars and boat hooks, the ravenous monsters could not be deterred from seizing and taking away more than half the fish that were hooked". Limbaugh (1963) reported that at Clipperton Island "at first, the small sharks circled at a distance, but gradually they approached and became more aggressive … various popular methods for repelling sharks proved unsuccessful".
As of 2008, the Galapagos shark has been confirmed to have attacked three people: one fatal attack in the Virgin Islands; a second fatal attack in the Virgin Islands, at Magens Bay on the north shore of St. Thomas; and a third non-fatal, attack off Bermuda. February 2018 saw a non-fatal shark attack in the Galapagos islands that shark photographer Jeremy Stafford-Deitsch suggested may have been carried out by a Galapagos shark, but the species remains unconfirmed.
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