The blue shark (Prionace glauca), also known as the great blue shark, is a requiem shark in the family Carcharhinidae inhabiting deep waters in the world's temperate and tropical oceans. It is the only species of genus Prionace. With its striking metallic blue back and white underside, this highly migratory shark has captured the fascination of scientists and ocean enthusiasts alike. This coloration is a classic example of countershading, a camouflage technique used by many sharks. This article delves into the dietary habits of this fascinating creature, exploring what it eats, how it hunts, and its role in the marine ecosystem.
Taxonomy and Physical Characteristics
The scientific name of the blue shark, Prionace glauca, offers insight into its physical characteristics. The first part of the name, Prionace, comes from the Greek words "prion" (saw) and "akis" (pointed), referring to the shark's sharp, serrated, triangular teeth. These teeth are perfectly adapted for gripping slippery prey like fish and squid.
Blue sharks are light-bodied with long pectoral fins, ideally suited to their pelagic lifestyle. Like many other sharks, blue sharks are countershaded: the top of the body is deep blue, lighter on the sides, and the underside is white. The male blue shark commonly grows to 1.82 to 2.82 m (6.0 to 9.3 ft) at maturity, whereas the larger females commonly grow to 2.2 to 3.3 m (7.2 to 10.8 ft) at maturity. Large specimens can grow to 3.8 m (12 ft) long. The blue shark is fairly elongated and slender in build and typically weighs from 27 to 55 kg (60 to 121 lb) in males and from 93 to 182 kg (205 to 401 lb) in large females. Occasionally, a female in excess of 3 metres (10 ft) will weigh over 204 kg (450 lb).
Habitat and Distribution
Blue sharks are found in all temperate and tropical waters, often in the open ocean from the surface to depths of 600m. As a highly migratory species, the blue shark undertakes huge yearly migrations, totalling over 9000km (5700+ miles)! Blue sharks are found off the coasts of every continent, except Antarctica. Its greatest Pacific concentrations occur between 20° and 50° North, but with strong seasonal fluctuations. They live as far north as Norway and as far south as Chile. In temperate seas it may approach shore, where it can be observed by divers; while in tropical waters, it inhabits greater depths.
Sensory Adaptations for Hunting
The five senses that blue sharks share with other members of the Carcharhinidae family are vision, hearing, lateral line, chemoreception, and electroreception. The well-developed eyes of blue sharks exhibit interspecific variations in their eye structure, which are characteristic of adaptations for vision in a variety of light environments, from the brightly lit surface waters to the darkness of the deep sea. The morphology of the inner ears of blue sharks is similar to that of other gnathostomes. It consists of a membranous labyrinth that is made up of three semicircular canals that are filled with fluid and arranged orthogonally, as well as three otolithic organs, which are the sacculus, utriculus, and lagena. The blue shark's lateral line is a mechanosensory structure that can detect particle motion. As such, it can react to mechanical disturbances caused by hydrodynamic stimuli that are not auditory. Blue sharks' chemosensory system is made up of gustation, and olfaction, which is a common chemical sense. Functions like intraspecific social interactions, communication, reproduction, and food detection are all linked to smell. Blue sharks can detect weak electrical potentials generated by inanimate objects and other animals through specialized receptors. These sharks use their electrical sense to locate and capture prey, as well as to avoid predators. The electroreceptors, known as the Ampullae of Lorenzini, are of the ampullary type and develop from the lateral line placodes. Each ampulla consists of a pore on the surface of the skin, connected to a narrow dermal chamber called an ampullary bulb by a small canal with a diameter of about one millimetre. Although the receptors primarily detect low-frequency alternating currents, they are particularly attracted to steady direct current electric fields.
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An Opportunistic Diet
Blue Sharks are known as opportunistic feeders. The diet of the blue shark is remarkably diverse, reflecting its wide distribution and migratory habits. While squid are the most important prey for blue sharks, their diet includes other invertebrates, such as cuttlefish, blanket octopuses, and pelagic octopuses, as well as lobster, shrimp, crab, a large number of bony fishes (such as long-snouted lancetfish, snake mackerel and oilfish), small sharks, mammalian carrion and occasional sea birds (such as great shearwaters). Whale and porpoise blubber and meat have been retrieved from the stomachs of captured specimens and they are known to take cod from trawl nets. This adaptability allows them to thrive in various marine environments.
Common Prey Items:
- Pelagic Fish: Hake, haddock, cod, mackerel, pollock, tuna, and swordfish are all part of the blue shark's diet.
- Cephalopods: Squid, cuttlefish, and octopuses are a significant food source.
- Invertebrates: Lobster, shrimp, and crab supplement their diet.
- Marine Carrion: They are known to scavenge on dead mammals, such as porpoises and whales.
- Seabirds: Occasionally, they prey on seabirds like great shearwaters.
Hunting Strategies and Behavior
Blue Sharks are known for being highly inquisitive and curious. When snorkelling or diving near them, they will often come close to investigate people. They come together in groups to feed, preying on shoals of fish or carrion (i.e., carcasses). They’re most active in the early morning and night, when they cruise slowly on the surface.
Blue sharks are most active at night, particularly in the early evening, with their lowest activity occurring during the early morning hours. During the day, they tend to remain around a depth of 30 meters, while at night they venture slightly deeper, around 40 meters. Most of their time is spent within a depth range of 18 to 42 meters, although they sometimes dive deeper. Their behaviour is also influenced by water temperature, preferring a narrow range of 14 to 16°C, though they are found in waters between 8.5 and 17.5°C.
Sharks have been observed and documented working together as a "pack" to herd prey into a concentrated group from which they can easily feed. Blue sharks may eat tuna, which have been observed taking advantage of the herding behaviour to opportunistically feed on escaping prey. The observed herding behaviour was undisturbed by different species of shark in the vicinity that normally would pursue the common prey. The blue shark can swim at fast speeds, allowing it to catch up with prey easily.
Predators and Parasites
Younger and smaller individuals may be eaten by larger sharks, such as the great white shark and the tiger shark. Orcas have been reported to hunt blue sharks. This shark may host several species of parasites. For example, the blue shark is a definitive host of the tetraphyllidean tapeworm, Pelichnibothrium speciosum (Prionacestus bipartitus). Despite having excellent binocular vision and the capacity to see ahead when pursuing prey, research indicates that blue sharks are not always adept at spotting predators approaching from behind. According to an experiment, a large predator's best attack angle when pursuing a blue shark is probably from the caudal direction. This puts the predator in a position to strike the caudal fin of the shark and immobilize it.
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Reproduction and Life Cycle
Five reproductive variables are examined for their relationship to body growth: presence or absence of semen in the ductus deferens ampullae, length and wet weight of the testicle, Size and rigidity of the claspers. Male blue sharks primarily court non-pregnant mature females since mating marks on females are common, appearing as several tiny incisions arranged in a semicircle on their Dorsal fins. These marks are the result of non-feeding bites during courtship and mating. Blue sharks are viviparous, with a yolk-sac placenta, giving birth to 4 to 135 pups per litter after a gestation period of 9 to 12 months. Females mature at five to six years of age, while males mature at four to five years. After birth, young sharks are left in specific nursery areas outside adult regions to develop independently. These nurseries offer a safe environment for newborns during their early months. For sharks, this species reaches sexual maturity quickly, around 5 years of age. Live pups are born in an average-sized litter of 35, with litters typically ranging in size from 20-50 pups. 135 pups, is the largest litter size recorded. The newborns measure 40-51 cm (16-20 ft).
Conservation Status and Threats
The Blue Shark is caught and consumed for meat across the globe. It may be surprising then that they are not currently classed as threatened. Their global status of ‘Near Threatened’ suggests that they should be closely monitored. However, blues are listed as Critically Endangered in the Mediterranean.
The IUCN Red List of Threatened Species currently lists the Blue shark as Near Threatened, with a decreasing population trend. The biggest threat to the blue shark is the commercial fishing industry. An estimated 20 million are killed annually. The majority of those as bycatch on long-lines, in trawls and gill nets. They are also targeted for their large fins in the shark fin trade. The sport fishing industry also targets the blue shark for the fight and is generally released after it’s landed. They are the most prevalent species found in the world shark fin trade.
Since the 1970s, blue shark populations have declined in the Atlantic and Indian oceans, and they have increased only slightly in the Pacific Ocean. These developments prompted the International Union for Conservation of Nature and Natural Resources (IUCN) to list the species as near threatened. Blue sharks are under threat because they are often caught as bycatch by commercial fishing fleets, which often go on to harvest the sharks’ meat and fins. Blue sharks are also sought-after by the sportfishing industry, which releases them after capturing them; however, this can sometimes result in the death of the shark.
Interactions with Humans
Blue shark meat is edible, but not widely sought after; it is consumed fresh, dried, smoked and salted and diverted for fishmeal. Blue sharks rarely bite humans.
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Blue sharks, like most pelagic sharks, tend to fare poorly in captivity. Blue sharks are relatively easy to feed and store in captivity, and the three primary issues appear to be transport, predation by larger sharks and trouble avoiding smooth surfaces in tanks. Small blue sharks, up to 1 m (3.3 ft) long, are relatively easy to transport to aquaria, but it is much more complicated to transport larger individuals. However, this typical small size when introduced to aquaria means that they are highly vulnerable to predation by other sharks that are often kept, such as bull, grey reef, sandbar and sand tiger sharks. For example, several blue sharks kept at Sea World San Diego initially did fairly well, but were eaten when bull sharks were added to their exhibit. Attempts of keeping blue sharks in tanks of various sizes, shapes and depths have shown that they have trouble avoiding walls, aquarium windows and other smooth surfaces, eventually leading to abrasions to the fins or snout, which may result in serious infections. To keep blue sharks, it is therefore necessary with tanks that allow for relatively long, optimum swimming paths where potential contact with smooth surfaces is kept at a minimum.