Autores: Pablo Saccone, María Valentina Villar
Revisión: Rubén Quintana
ish with cartilaginous skeletons called chondrichthyans—comprising sharks, rays and ghost sharks—appeared over 450 million years ago. That is long before the first vertebrates dragged themselves onto land. Evidence of chondrichthyans first appeared in the fossil record by the end of the Silurian Period (450 to 420 million years ago) with the discovery of isolated fossil scales (cartilaginous skeletons do not fossilize). They are probably the most successful fish in natural history, as they have survived periods of mass extinctions occurred since their appearance.
Current species of cartilaginous fish can be divided into two groups: those who merged their jaw to the skull (called holocephalans), including ghost sharks and elephantfish; and those with flattened gills (commonly called elasmobranchs), like sharks and rays. Such skeleton allows them to be lighter than fish with bony skeletons, which improves the way they move through the water.
These fish are poikilothermic—“cold blooded”—as bony fish; i.e., organisms whose body temperature varies with changes in the environment. They have no swim bladder and because of this they must be constantly swimming or they sink. Furthermore, their liver, heavy and oily, also helps maintain buoyancy. There are more than 360 species of sharks in the world, of which only a few can live in both salt and fresh water. An example of this is the Carcharhinus leucas (Bull shark) that runs estuaries to reach fresh water, which is why they have adaptations to withstand changes in salinity; in South America, they can be found in the Amazon River and into northern Bolivia. Another example is the speartooth shark, Glyphis Glyphis, found in the tropical waters of the western Pacific Ocean. This little-known species is usually found in Malaysia, New Guinea and Australia.
Majestic predators that are at the top of the marine food chain and have a well-developed sense of smell, so they are able to perceive prey several meters away, making it easier for hunting.
Not as Bad as in the Movie
Most people think of sharks as dreadful predators, but statistics show that attacks on humans are actually very rare. We should bear in mind that the possibility of dying from flu is 1 in 63 and dying from a fall is 1 in 218, while dying by shark attack is 1 in 3,700,000. So, although shark attacks on humans exist, one thing is the bad reputation that man has created around their figure and quite another is what happens in reality. We need to know that there are carnivorous species that never attack humans and that both the whale shark, which is the largest, and the basking shark, which inhabit our waters, feed on plankton.
Kings of the Ocean
If we look back to the origins of the Spanish word tiburón, we must bear in mind that there are many assumptions of the root from which the word comes. One of them asserts that it is of Taíno origin (the ancient inhabitants of the Spanish island in the Caribbean, that is, what is today Haiti and the Dominican Republic). The book “Cristóbal Colón y los Taínos” (“Christopher Columbus and the Taíno People”) notes that the term “tiburón” is within the group of words from the Caribbean Arawak languages that at some point were incorporated into Spanish.
According to studies of Amerindian languages and cultures, the origin is uncertain, but in 1519 this term is adopted from the Tupi-Guarani language, derived from the word uperú (or iperú), to which it was added the letter T, which means “The”, resulting in the term T-uperú, and eventually it led to the Portuguese word Tubarão that led to the Spanish word Tiburón. Others say that this word comes from the Caribbean, where ti means land and burón means fish.
Formerly the Spaniards called them “jaquetón” (augmentative of “jaque”—check—, which means “threat”; today relegated to the world of chess). Many ancient peoples, such as Native Americans and tribes of Polynesia, considered the shark an animal related to survival, protection for themselves and their boats, and a symbol of leadership and domination. In New Zealand, this fish is traditionally regarded by the Maoris as the most important living being in the ocean, which is why they call it “king of the ocean”.
The Amazing Sharks
Majestic predators that are at the top of the marine food chain and have a well-developed sense of smell, so they are able to perceive prey several meters away, making it easier for hunting. Their sense of hearing is also well developed as they are able to detect changes in water pressure and movement of other animals over long distances. This is thanks to the presence of some organs in the head called ampullae of Lorenzini (which have an electroreceptor with sensitive hairs that are connected to nerve fibers) to help them find their prey using electromagnetic fields.
Like all fish, they have no eyelids, and their eyes are very sensitive since they allow them to see in very low light conditions.
Shark teeth have different shapes: in some species they have the shape of a punch and in others they are triangular. Their edges are often sawed thus facilitating sharks to rip, tear or cut their prey to pieces. Sharks have 4 rows of teeth characterized by not having roots. For this reason, teeth are embedded in the gums and not directly in the jaw, thus detaching or breaking easily, and being replaced by other teeth in the back rows.
Their skin is perfectly adapted to their living conditions since it is flexible and very resilient at the same time. This is because they have placoid scales (or dermal denticles) that minimize friction and viscosity of the aquatic environment, causing the shark swim easily, quickly and quietly. Because of this, top swimming teams copied the distinctive design of sharkskin in their competition swimsuits, thus being able to break several world records.
Sharks have developed internal fertilization; males have a modification in the pelvic fins called claspers, which they use to channel semen during mating. Females develop various methods of reproduction ranging from oviparous species, where large and well protected eggs are laid, to viviparous species that give birth to live young that have been nourished through a placenta in a manner analogous to human.
Their survival strategies are more reminiscent of mammals than fish themselves, presenting great longevity and slow growth. They also have a late maturity, low fecundity and an extended period of gestation where the number of offspring is small.
All these features make sharks very vulnerable, quickly reducing their population number in places where they are subjected to overfishing or other human impacts. Unfortunately, today there are several endangered species due to fishing in its different—commercial, sport or recreation—modalities. There is a big business that supports a high rate of shark catch for different purposes such as the development of medicines for the treatment of arthritis and other cartilage diseases, or for food purposes like shark fin soup. In addition, many die trapped in fishing nets and in those that are placed in beach areas to protect swimmers.
Getting to Know Our Folks
82 species of chondrichthyans are distributed along the coast of Argentina, of which 37 are sharks. There are two main groups: those that live in the Patagonian coast, all from temperate-cold, cold and deep waters, and those in the Buenos Aires coast, from temperate-warm waters. Of all sharks present on our shores, 55% live in coastal areas or the continental shelf. Below are the most studied native species that inhabit our seas:
School Shark (Galeorhinus galeus)
This shark is the most exploited by fisheries worldwide. Also known as “vitamin shark”—as it is the species with the greatest amount of vitamin A on the liver—, it weighs between 40 and 45 lbs. and can measure up to one and a half feet. The school shark is lead gray in color, getting lighter on the sides and ending with a white belly. It is a long-lived species, since specimens aged 40 have been recorded. Apparently it migrates for reproductive purposes between April and November, moving from southern Brazil to southern Patagonia to later give birth in the summer. Sexual maturity in males is at age 13 and females at age 18. It is a lecithotrophic viviparous species in which embryos feed exclusively on yolk. Pregnancy occurs every three years, lasting about twelve months. For this species, the maximum number of embryos observed is 41. These are retained in the oviduct until development is completed. The annual offspring production per female is between 10 and 15. This shark is fished commercially as a substitute for cod, and its meat is widely used in cooking. For this reason it is considered at high risk of extinction in the medium term, because it is very vulnerable to human impact.
Sand Tiger Shark (Carcharias taurus)
It is the largest shark of our waters as it exceeds 6.5 feet in length and can weigh up to 400 lbs. Another name for this species is grey nurse shark and in Uruguay it is known as “Sarda”. Sand tiger sharks are gray with green and yellow reflections and white belly. They move at a certain depth, near the bottom (epipelagic). To maintain their buoyancy, these sharks breathe on the surface and keep it in the stomach. They overwinter in the southern coasts of Brazil and reach the Argentine coast in the spring and summer, where they mate. This species begin to reproduce at 8 years and give birth to two pups after a gestation period of 9 to 12 months. Sand tiger sharks are viviparous and embryos initially feed on yolk and then on the mother’s eggs. Subsequently, the largest pup eats the smaller brothers (adelphophagy or intrauterine cannibalism, which is a kind of Matrotrophy, where the mother transfers nutritive material to the developing embryo after fertilization). It is the most endangered shark in our country.
Copper Shark (Carcharhinus brachyurus)
Of great speed and strength, this shark is also called “short snout”. Their body is robust yet elegant, ash brown colored with flashes sometimes bluish. Fins are the same color, while the underside is white. They can reach 8 feet in length with a weight of 330 lbs. It is believed that copper sharks overwinter near southern Brazil, away from the coast, and reach Mar del Plata and Necochea in spring, to later—December—reach Bahía San Blas bay for mating. This species lives up to 40 years and begins to reproduce at age 22. They have litters averaging 18 pups every two years. It is a placental viviparous species since nutrition is through blood vessels of a placenta in a manner similar to that of the umbilical cord that connects the walls of the yolk sac and uterine epithelium. Its population is threatened.
Broadnose Sevengill Shark (Notorynchus cepedianus)
They are large predators that inhabit temperate and cold waters from Brazil to the Strait of Magellan. Also called “spotted shark” and dogfish in cases of smaller specimens, they are light gray, with dark, round spots and a nearly white belly. Adults can reach up to 10 feet in length. Of a very aggressive nature, it is a lecithotrophic viviparous species. Their gestation period is 12 months and their fertility rate is the highest among all sharks. Pups are born in summer. This species is very vulnerable to human impact.
Narrownose Smooth-Hound (Mustelus schmitti)
It is one of the smallest shark species, with a similar appearance to that of the school shark. It can be found from Rio de Janeiro to the Patagonia. Their color is iridescent uniform leaden gray but darker on the back and splattered with small white dots. The reproductive cycle of this species is annual: in the months from November to December, females give birth and are then fertilized to later recommence gestation, which lasts ten months. It is a matrotrophic viviparous species with the peculiarity that the fertilized oocytes are surrounded by a single biomembrane. The yolk sac is consumed before midgestation. They can release between 2 and 13 embryos. The narrownose smooth-hound was declared critically endangered by the International Union for Conservation of Nature and Natural Resources (IUCN).
How to Continue?
We know sharks are large predators that are on the top of the food chain, but as its growth is slow, its gestation period is very long and its fertility rate is very low, they are very vulnerable to commercial and sport fishing. For this reason, conservation plans should be undertaken in the short term. Among them, education and awareness actions should be promoted in order to change the relationship between humans and sharks so that their populations can recover slowly.