Blue shark
The blue shark, also known as the great blue shark, is a species of requiem shark in the family Carcharhinidae which inhabits deep waters in the world's temperate and tropical oceans. It is the only species of genus Prionace. Averaging around and preferring cooler waters, the blue shark migrates long distances, such as from New England to South America. It is listed as Near Threatened by the IUCN.
Although generally lethargic, they can move very quickly. Blue sharks are viviparous and are noted for large litters of 25 to over 100 pups. They feed primarily on small fish and squid, although they can take larger prey. Some of the blue shark's predators include the killer whale and larger sharks like tiger sharks and the great white shark. Their maximum lifespan is still unknown, but it is believed that they can live up to 20 years. They are one of the most abundant pelagic sharks, with large numbers being caught by fisheries as bycatch on longlines and nets.
The earliest fossil teeth of this species are known from the Miocene of Sri Lanka. Other fossil teeth of this species are known from the Miocene and Pliocene of Chile, Italy, and possibly Belgium.
Taxonomy
A 2023 study suggested that the species should be reclassified into the genus Carcharhinus.Distribution and habitat
The blue shark is an oceanic and epipelagic shark found worldwide in deep temperate and tropical waters from the surface to about. In temperate seas it may approach shore, where it can be observed by divers; while in tropical waters, it inhabits greater depths. It lives as far north as Norway and as far south as Chile. 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. In the tropics, it spreads evenly between 20° N and 20° S. It prefers water temperatures between, but can be seen in water ranging from. Records from the Atlantic show a regular clockwise migration within the prevailing currents.Description
Blue sharks are light-bodied with long pectoral fins. 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 at maturity, whereas the larger females commonly grow to at maturity. Large specimens can grow to long. Occasionally, an outsized blue shark is reported, with one widely printed claim of a length of, but no shark even approaching this size has been scientifically documented. The blue shark is fairly elongated and slender in build and typically weighs from in males and from in large females. Occasionally, a female in excess of will weigh over. The heaviest reported weight for the species was. However, anecdotal claims exist for the species to exceptionally reach in weight, though these are not verified. The blue shark is also ectothermic and it has a unique sense of smell.Sensory
The five senses that blue sharks share with other members of the Carcharhinidae family are vision, hearing, lateral line, chemoreception, and electroreception. These senses allow them to perceive and react to a variety of biotic or abiotic stimuli in their immediate environment and across a different range of spatial scales.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 lateral position of the eyes in the head allows a cyclopean visual field of 360° in the vertical plane and between 308° and 338° in the horizontal plane.
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. These sharks are most sensitive to frequencies below roughly 100 Hz, but they can hear sounds up to roughly 1000 Hz.
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. It is used to determine the direction and speed of water currents as well as the vibrations produced by prey, predators, and similar species moving through the water.
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. Gustation is mainly related to feeding and involves using taste buds to process food and assess its palatability through direct contact, which usually results in a decision to swallow or reject it.
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. As they move through the Earth's magnetic field, they may also sense the weak electrical fields produced by nearby water currents or their own bodies, which can aid in navigation and orientation.
The ampullae of Lorenzini are electroreceptors 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. These sharks are most responsive to fluctuating electrical fields between 0.1 and 10 Hz, with peak sensitivity around 1 Hz.
Although the receptors primarily detect low-frequency alternating currents, they are particularly attracted to steady direct current electric fields. However, for the shark to detect a continuous DC voltage, it must move relative to the voltage source.
In the northeastern coast of the United States, it was discovered that blue sharks are able to maintain a straight course for hundreds of kilometres over many days. It appears that the only continuously available cue that could be used to accomplish this is the geomagnetic field.
Reproduction
Sexual maturity
Maturity is assessed by observing sexual products and the developmental stage of reproductive organs.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. To assess maturity, the clasper's inner length and degree of calcification are recorded:
Mature males have fully calcified claspers that extend beyond the inner margin of their pelvic fins.
Immature males have claspers that are either shorter or longer than the inner border but not fully calcified.
Mating behaviour
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. Female Blue sharks have evolved skin three times thicker than that of males to withstand the rigors of mating.Female blue sharks are classified as immature, subadult, or mature based on the size and development of their ovary, oviducal gland, and uterus.
Mature females have enlarged uterine walls, a fully differentiated oviducal gland, visible and enlarged Ovarian follicles and a right ovary separated and developed from the epigonal organ.
Immature females have an undifferentiated oviducal gland and uterus, a small right ovary lodged within the epigonal organ, and no visible follicles.
Reproductive strategy and lifecycle
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. 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. Females mature at five to six years of age, while males mature at four to five years. Research suggests that females may exhibit natal and reproductive philopatry, meaning they return to specific sites to give birth.Ecology
Migration
Blue sharks are a highly migratory species, travelling vast distances across temperate and tropical waters. Their migrations are influenced by seasonal changes, prey availability, and the need for optimal environmental conditions. These sharks move both horizontally and vertically. Their swimming behaviour varies depending on the time of day. During the day, blue sharks move at a mean rate of 1.2 kilometers per hour, with a mean swimming speed of 1.3 kilometers per hour. At night, their activity increases, with a mean movement rate of 1.8 kilometers per hour and a swimming speed of 2.8 kilometers per hour. These increases in speed often occur during brief dives, particularly at night when sharks exhibit more vertical movement, ranging from shallow waters to depths exceeding 100 meters.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. Blue sharks often swim near the surface in cooler months, but this behaviour decreases during the coldest or warmest months, likely due to surface temperature changes.