3.8. Elasmobranch species in the Mediterranean Sea
Sharks, together with skates and rays, make up the subclass Elasmobranchii of the class Chondrichthyes, cartilaginous fish; they represent less than 5% of marine fish species. The warm waters of the Mediterranean Sea are home to at least 79 species of elasmobranch fish (45 shark species and 34 ray species). The most represented is the Carcharhiniformes or ground sharks, including blacktip reef sharks, blue sharks, and hammerhead sharks. Although they live in both coastal and deep bathymetric areas, in the last century there has been an alarming decline of their presence in different areas. With very saline, warm waters and little tidal movement, the Mediterranean Sea is home to an abundance of sea creatures, giving the sharks plenty of food to live off. Unfortunately, sharks are largely misunderstood, as most of them are completely harmless and, despite their integral part in the ocean’s delicate ecosystems, they are being fished to extinction.
The great white shark (Carcharodon carcharias) can be found in the coastal surface waters of all the major oceans. It is notable for its size, with female individuals growing to 6.1 m in length. The lifespan of great white sharks is estimated to be as long as 70 years or more making it one of the longest lived cartilaginous fish currently known. Great white sharks can swim at speeds of over 56 km/h and can swim to depths of 1.200 m. It has no known natural predators other than, on very rare occasions, the killer whale. The great white shark is arguably the world’s largest known extant macropredatory fish, and is one of the primary predators of marine mammals. It is also known to prey upon a variety of other marine animals, including fish and seabirds. Although these formidable creatures certainly do strike fear, you should know that, in the Mediterranean Sea, there have only been a total of 31 attacks against people over the last 200 years, and most of those attacks did not result in fatalities.
The common thresher (Alopias vulpinus) is the largest species of thresher shark reaching some 6 m in length. About half of its length consists of the elongated upper lobe of its caudal fin. The common thresher is distributed worldwide in tropical and temperate waters, though it prefers cooler temperatures. It can be found both close to shore and in the open ocean, from the surface to a depth of 550 m. It is seasonally migratory and spends summers at lower latitudes. The long upper tail fin lobe of the common thresher is used to strike and incapacitate prey. This species feeds mainly on small schooling forage fishes such as herrings and anchovies. Before striking, the sharks compact schools of prey by swimming around them and splashing the water with their tails, often in pairs or small groups. It is a fast, strong swimmer that has been known to leap clear of the water, and possesses physiological adaptations that allow it to maintain an internal body temperature warmer than that of the surrounding sea water. It is highly valued by commercial fishers for its meat, fins, hide, and liver oil; large numbers are taken by longline and gillnet fisheries throughout its range.
The blue shark (Prionace glauca) is a species of requiem shark, in the family Carcharhinidae, that inhabits deep waters in the world’s temperate and tropical oceans. 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. They 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. Squid are important prey for blue sharks, but their diet includes other invertebrates, such as cuttlefish and pelagic octopuses, as well as lobster, shrimp, crab, a large number of bony fishes, small sharks, mammalian carrion and occasional sea birds. It can swim at fast speeds, allowing it to catch up with prey easily.
The scalloped hammerhead (Sphyrna lewini) is a species of hammerhead shark. The most distinguishing characteristic of this shark is the ‘hammer’ on its head. The shark’s eyes and nostrils are at the tips of the extensions. Many, but not necessarily mutually exclusive functions have been proposed for the cephalofoil including sensory reception, maneuvering, and prey manipulation. The scalloped hammerhead is a coastal pelagic species; it occurs over continental and insular shelves and in nearby deeper water. It primarily lives in warm, temperate, and tropical coastal waters all around the globe. These sharks have a very high metabolic rate, governing behavior in acquiring food. These sharks are often seen during the night, day, and morning in big schools, sometimes numbering hundreds, most likely because large groups can obtain food easier than singles or small groups.
The basking shark (Cetorhinus maximus) is the second-largest living shark, after the whale shark. Adults typically reach 6–8 m in length. They are usually greyish-brown, with mottled skin. The basking shark is a cosmopolitan migratory species, found in all the world’s temperate oceans. A slow-moving filter feeder, its common name derives from its habit of feeding at the surface, appearing to be basking in the warmer water there. It has anatomical adaptations for filter-feeding, such as a greatly enlarged mouth and highly developed gill rakers. The basking shark has long been a commercially important fish, as a source of food, shark fin, animal feed, and shark liver oil. Overexploitation has reduced its populations to the point where some have disappeared and others need protection.
The giant devil ray (Mobula mobular) is a large marine vertebrate and can reach up to 5.2 m in disc width (DW), although specimens of about 3 m DW are most common. This large epipelagic batoid fish inhabits the entire Mediterranean Sea and possibly the adjacent Atlantic waters. Due to its geographic distribution and rare records outside the Mediterranean, it is considered as an endemic elasmobranch in the region. Information on the biology of the giant devil ray is scarce. It mostly inhabits deep pelagic waters where it feeds on plankton, predominantly krill (pelagic crustaceans) and small schooling fish. Giant devil rays are ovoviviparous, meaning that only one large egg is developed inside body of the female. After more than a year and a half, a young fish is “born”. At this time, young fish can have a DW of over a meter! Throughout its range, the giant devil ray is believed to live in low numbers although population estimates are unavailable. The giant devil ray is listed as Endangered on the IUCN Red list.
3.8.1 Elasmobranchs: Key features
The term elasmobranch comes from the Greek word "elasma" meanning metal plate and "branch", the Latin word for gill. It refers mainly to sharks, rays and skates, which are cartilaginous fishes. There are numerous characteristics of elasmobranchs. Firstly, their skeleton is made of cartilage rather than bones. They have rigid dorsal fins and spiracles to aid in breathing. Furthermore, elasmobranchs have five to seven gill openings on each side. Their upper jaw is not fused to their skull and they have several rows of teeth which are continually replaced. The details of this jaw anatomy vary between species and help distinguish the different elasmobranch clades. Additionally, they don't have swim bladders, but instead their large livers are full of oil to provide buoyancy. This stored oil may also function as a nutrient when food is scarce. Deep sea sharks are usually targeted for their oil, because the livers of these species can weigh up to 20% of their total weight. Elasmobranchii reproduce sexually with internal fertilization, either bear live young or lay eggs. Moreover, Elasmobranchii class include over 1000 species including the southern stingray, whale shark, basking shark and the shortfin mako shark. The classification of elasmobranchs has undergone revision again and again due to continuous research and studies. Recent molecular studies have found that skates and rays are different enough from all of the sharks that they should be in their own group under elasmobranchs. Finally, they widely get distributed throughout tropical and temperate waters.
3.8.2 Mediterranean Sea - Home to an abundance of sharks and rays
Many would think that since shark attacks are not common in the Mediterranean sea,sharks are also rare.That is far from true. Mediterranean sea is thought to be home for 47 shark species like:Angelshark,Blue shark,Great white shark,Kitefin shark,Longfin mako,Sandbar shark,Scalloped hammerhead,Great hammerhead,Shortnose spurdog,Thresher shark.The most dangerous one is the Great White,however they are seen seldomly.On the other hand,it is not characteristic of blue sharks to harm people and when it happens, it is an gainst anglers.Most of them can be found in depths bigger than 200m.Therefore only researchers and observers are possible to see them.The country that has the most recorded attacks is Italy.Sharks that have Mediterranean as a natural habitat have to cope with problems like overfishing,pollution and habitat destruction and a great number of species is on the verge of extinction.As far as Mediterrranean rays are concerned,their length can be up to 6,5m and their weight 1300kg.On their heads they have 2 big horns so as to capture the preys easily.Rays eat mainly small fishes and molluscs.They have a huge mouth that contains a great number of tiny teeth and their body is often dark colours.Moreover,rays have the ability to jump vertically straight out of the water.Also,in the base of their tail they have jagged thorns.Just like sharks,rays too face extinction problems.Until now,it has been observed that 8 species of rays are in serious condition of being in danger from Mediterranean sea.Actually,it is a fact that the bigger the aquatic creature is,the more alarming its rate of extinction is.However,people should start thinking ways to overcome this new problem.In conclusion,scientists believe that it is everyone's responsibility to protect all the animals in this planet.
3.8.3 Great white shark (Carcharodon carcharias) - Fearsome predator
A Great white shark is a species of large mackerel shark and can be found in almost all of major oceans. They are known for its size, their maximum amount being 6.1 m long. These massive creatures can leave up to 70 years making it one of the longest lived cartilaginous fish currently known.Male Great white sharks take 26 years to be sexualy mature while female sharks take 33 years to be able to produce offspring. They also have the ability to reach a speed of well over 56 km/h while swimming in depths of 1200 m. Although they are the the largest known extant macropredatory fish they do have a preditor but they do not face it very often, the killer whale. They are mostly known to feast on fish, seabirds or marine mammals. Despite the fact that this species of shark is responsable for the most recorded human bite invisents than any other shark they face many ecological challenges which has resulted in international pritection. For instance Grat white sharks live in almost all coastal and offshore waters which have water temperature between 12-24 degrees celcius whith grater concentrations in the United States, South Africa, Japan, Oceania, Chile and the Mediterrenean including Sea of Marmara and Bosphorus. A great white sharks appearance is very interesting. Their always hace white undersight whits gives it its name but their dorsal area can be blue brown or grey. The coloration makes it difficult for pray to spot the shark because it breaks up the sharks outline when seen from the sight. From above, the darker shade blends with the sea and from below it exposes a minimal silhouette against the sunligt.
3.8.4 Dark reputation: Humans aren't a regular item on the great white shark's menu. Phew!
First of all, the great white shark is a streamlined swimmer and a ferocious predator with three thousand teeth. This much-feared fish is described by a torpedo-shaped body, a pointed snout, a crescent-shaped tail, five gill slits, no fin spines, an anal fin, and three main fins: the dorsal fin (on its back) and two pectoral fins (on its sides). Every time the shark is near the surface, we are able to distinguish its dorsal fin and part of its tail. In addition, an important note about this kind of shark is the fact that only its underbelly is actually white; its top surface is gray to blue gray, something that helps it in hunting its prey. Furthermore, great white sharks are carnivorous, in other words, "meat eaters". Young great white sharks eat fish, rays and other sharks. Adults, instead, eat larger prey, including pinnipeds (sea lions and seals), small toothed whales (like belugas), otters and sea turtles. They also eat dead animals that they have found floating dead in the water, known as carrion. Humans, luckily for us, are not a part of their diet list. To be more specific, most great white sharks' attacks are not fatal. Statistically, their attacks constitute a part of about 1/2 to 1/3 of all 100 annual reported shark attacks, of which only 10-15 people had been found dead. Moreover, humans are not appropriate prey because the shark's digestion is too slow to cope with a human's high ratio of bone to muscle and fat. Accordingly, in most recorded shark bite incidents, great whites broke off contact after the first bite, something that is directly connected to what some researchers have stated. They have hypothesized that the reason the proportion of fatalities is low is not because sharks do not like human flesh, but because humans are often able to escape after the first bite. To sum up, eventhough they have a reputation as terrifying predators, great white sharks typically do not target humans as prey.
3.8.5 Common thresher shark (Alopias vulpinus) - "Fox shark", from a belief that it was especially cunning
The common thresher, or known as fox shark or swiveltail, is the largest membere of thresher shark family, it is reaching 6 meters in length and weight up to 500 kilogrammes. Their most distinctive feature is their very long tail which can reach to 3 meters long. This is used from the shark like a whip to stun their prey. They are fast swimmers, powerful enough to leap out of water and this makes them fierce predators.Alopias vulpinus live in costal and pelagic water, in tropical and temperate climates in all the world, deaphs of at least 366 metres.They favor the Pacific and Indian ocean .The species is mainly fed with small fish flocks and pelagic shooling fish such as sardines, mackerel or juveline tuna. The alopias vulpinus developing embryos feed on fettered eggs provided by their mother. Thresher shark are slow to mature, reaching sexual maturity between 8-14 years old.Femals give birth to 2-7 newborns after a nine month pregnancy. They may live for 20 years or more. It is a huge shark and it has relativly small teeth and a calm behaviour and therefore is not dangerous for people. Alopias vulpinus is considered a commercial species for its meat, liver and fins. Their liver can be processed for vitamins rich oil and the skin can be used for leather.However common thresher can become an endagered species when it is overfished because it has a low reproductive rate.The thresher shark is able to recover from the impact of fisheries fairly quickly. Also they can generate and maintain body heat better than most other sharks.
3.8.6 Blue shark (Prionace glauca) - Slender swimmer
Blue shark or Prionace glausa is a species of requiem shark, in the family of Carcharhinidae. They grow up to a size of 12.5ft(3.8m), and their maximum lifespan is still unknown although it is believed that they can live up to 20 years. Blue sharks are light-bodied with long pectoral fins. Like many other sharks, blue sharks are counter shaded: the top of the body is deep blue, lighter on the sides, and the underside is white Slender, with long pectoral fins. They have a narrow head, with a parabolic snout, and Big eyes. This species gives birth to up to 130 pups per litter but normally between 25 and 50 pups which are in a yolk sac-placenta, and the gestation period is between 9 and 12 months. This species prefers cold waters (7-16°C) but tolerates warmer water too (+21°C). Normally they swim very slow at the surface of the water, but they can be found down to a depth of around 350m. They feed primarily on small fishes, as well as invertebrates and carcasses. Also, sharks have been observed and documented working together as a "pack" to herd prey into a concentrated group from which they can easily feed. Great white sharks and the Tiger sharks, Killer whales have been reported to hunt blue sharks. It is estimated that 10 to 20 million of these sharks are killed each year because of fishing, for feeding purposes and leather production from the skin of the shark. Blue sharks rarely bite humans. From 1580 up until 2013, the blue shark was implicated in only 13 biting incidents, four of which ended up dead.
3.8.7 Scalloped hammerhead shark (Spyrna lewini) - One of the weirdest-looking sharks in the ocean
The Sphyrna lewini ,well known as Scalloped hammerhead shark, is one of the most weirdest-looking sharks in the whole ocean! To begin with this animal is a part of the family Sphyrnidae and the most distinguishing characteristic of this family is the 'hammer' on there head.The male Scalloped hammerhead shark weight about 29 kg (64 lb) and measure 1.5 to 1.8 m (4.9 to 5.9 ft), the females measure 2.5 m (8.2 ft) and weigh 80 kg (180 lb),those numbers stands only when both,male and female,attain sexual maturity.Scalloped hammerhead shark can often be seen during the night,day, and morning in big groups(numbering about hundreds),called schools.In addition the diet of the sharks consist primarily of fish(sardines, mackerel, and herring).Furthermore the reproduction of the hammerhead sharks require about 12 months .Hammerheads have a really interesting navigation system.They have a homing behavior to navigate in the ocean,that means that they move during the night and they ''scan'' the environment and they use it as a map.Its well worth to mention that they use a point-to-point type of school.Additionally Spyrna lewini has a coastal and offshore waters habitat,something that might sounds a little bit wired.The best places to see those wonderful animal,according to the scientist,is the western Atlantic Ocean, eastern Atlantic and in Western Australia coast .However this weird looking animal has cost lots of human lives.When you hear the world ''shark''automatically in your mind the word ''danger'' comes on,but Scallopped hammerhead shark is aggressive toward humans.Taking all the above points into considerations Scalloped hammerhead shark is a really weird but magnificent shark with lots of interesting facts.