Geography 316: Biogeography
In progress 12/12/2003
of the Basking Shark
Cynthia Comerford, student in Geography 316 Fall 2003
Thank you for visiting our site. This web page was written by a student in Geography 316: Biogeography and edited by the instructor, Barbara Holzman, PhD. All photos and maps are posted with specific copyright permission for the express use of education on these web pages. The students have tried to be as accurate as possible with the information provided and sources and references are cited at the end of each page.
Species Name: Cetorhinus maximus
Species: Cetorhinus maximus
Description of Species:
Sometimes mistaken for a sea monster, the sight of the second largest fish in the world could send shivers down your spine. However, unlike the Great White, its deadly cousin, the Basking Shark, is a gentle giant .This docile creature is an extremely large cartilaginous shark. In length, the average size is between 23 and 30 feet (7 to 9 m) and it weighs up to 8580 pounds (3900 kg). Its face has a large snout, which comes to a rounded point at the end. Its teeth are tiny, rounded and abundant. The Basking Shark has one hundred teeth per row (Robins 1997). Its mouth is very wide; the edges stretch far across its face, so that it almost looks as if the Basking Shark is smiling at you. The size of its open mouth is enormous. The Basking Shark's most distinguishable features are its very long gill slits, which surround its entire head. The body of the shark is a grayish brown color. The belly is a little paler. The Basking Shark's body is very wide and has a cumbersome look to it. Its first dorsal fin is the largest. It has a dull triangular shape. The second dorsal and anal fins are approximately half the size of the first dorsal fin (UN FAO 2003).
The Basking Shark is completely harmless. Although the exact interactions between the Basking Shark, other life forms and its physical environment are not entirely known, it has never been known to attack a human (Steel 1985). Until recently, there have been few studies done on their biogeography. The Basking Shark differs greatly from other Lamniformes, especially in its eating habits. The Basking Shark is a placid plankton eater (Shark Trust 2003). Its diet consists of small planktonic organisms, small copepods, barnacles, decapods' larvae, and fish eggs (UN FAO 2003). The Basking Shark feeds itself by swimming with its mouth wide open. As the shark feeds, it channels up to 2000 tons of water per hour (Shark Trust 2003). There is usually a half a ton of food in the Basking Shark's stomach at any given time (UN FAO 2003). The massive amount of plankton it consumes might contribute to keeping a balance of oxygen in the ocean.
The home range of the Basking Shark varies from boreal
waters to warm waters near continents (UN FAO 2003). It Copyright © 1998-2000 The Boating Information Bureau
frequently is found in temperate waters, close to the continental shelf; it has been known to enter bays and estuaries and swim close to land. It has also been found well offshore, in the open ocean. It is a slow-swimming shark, often seen traveling in pairs. When sighted, the Basking Shark is frequently "basking" near the surface of the water.
Like many other features of the Basking Shark, the entire reproductive process is not fully understood. The sharks have a very slow growth rate and do not sexually mature until sometime between 12 and 20 years (UNEP 2003). Basking Sharks are said to conceive while basking on the surface of the water (Robins 1997). The sharks appear to return annually to coastal sites for feeding and mating, where there is distinct sexual separation (Shark Trust 2003). The mating activities take place close to shore off headlands, islands, and bays (UNEP 2003).The female sharks have many eggs in their ovaries and are ovoviviparous. This means that the shark produces eggs that hatch within its body without obtaining nourishment from it. For nourishment, sibling cannibalism takes places between embryos in the uterus. (UN FAO 2003). The actual length of the Basking Shark's gestation period is not fully understood. Many scientists believe it to be 3 years or longer, but conflicting sources also say around 15 months (UN FAO 2003). The female shark gives birth to live babies. Only one pregnant female shark has been caught, so it is believed that the females go into hiding in the deep ocean when they are pregnant. The average number of young that a shark gives birth to at one time is unknown. The one female that was caught was carrying six young. The life span of the Basking Shark is uncertain - scientists have estimated that it's anywhere from 20 to 100 years.
An important behavioral trait of the Basking Shark is that it is a migratory species. In the summer it moves to higher latitudes and in the winter it vanishes (Robins 1997). This disappearance in the wintertime suggests a movement to deeper waters where the shark hibernates or goes into an "energy efficient mode" (Aidan 2003).
A significant physical trait is that the Basking Shark sheds its gill rakers. Gill rakers are cartilaginous projections which point forward and inward from the gill arches in the mouth of the shark, which help the shark in feeding. This may be an adaptation to reserve energy. It is thought that when plankton levels fall below a vital level, the Basking Sharks lose their gill rakers. It is believed that the Basking Sharks then hibernate on the bottom of the continental shelf until their gill rakers renew and plankton levels have been restored (UN FAO 2003). Another noteworthy physical trait is the Basking Shark's dependence on the passing of water through its pharynx during swimming, for filtration of food (UN FAO 2003).
There are four main Basking Shark products used by man. They are liver oil, fins, cartilage and meat. The value of liver oil has been declining, but significant quantities still enter international trade. This is especially true of northern European counties. The fins have high value in Asian markets; they are used for shark fin soup. The cartilage is sold as herbal medicine in health food stores. The meat is used for food in Norway and other eastern and northern European countries (UNEP 2003).
The distribution of the Basking shark is wide-ranging, even though the species is only frequently observed in a few areas. Its emergence and disappearance in different areas mystifies scientists (Tennesen 1993/1994). It is most commonly found off the coast of the United Kingdom and Northern Ireland. The basking shark prefers the western Atlantic, the western Mediterranean Sea, and other temperate oceans globally. It is almost never found in the Tropics, and if seen there, is usually dead or stranded (UNEP 2003).
Geographic Distribution of the Basking Shark
C. Knickle, L. Billingsley & K. DiVittorio Ichthyology: The Florida Museum of Natural History
In the Northwest, specifically off the coast of California, Basking sharks are observed in the spring and winter. The most common area for sightings is from the Santa Barbara Channel to the Monterey Bay (Squire 1990). Seasonally, Basking sharks are usually found where plankton levels are high, which led scientist to believe this was the primary reason for their migration. In California, the Central Coast is a major upwelling area and high phytoplankton concentrations are found in the summer (Squire 1990). Conversely, the sharks are found off the Central Coast in the winter. This has led scientists to believe the Basking sharks convergence off the coast of California is due to mating rituals and not feeding practices (Squire 1990).
The Basking sharks are thought to be extremely migratory, but their exact patterns are not known (UNEP 2003). In the North Atlantic, they are seen during the spring and summer months and movement of the species takes place from deep to shallow water. In the North Atlantic, Basking sharks are never seen in the winter (UNEP 2203). This may be due to migratory patterns or hibernation.
The first evidence of sharks dates back approximately 425 millions years ago during the Silurian period. Many people associate sharks with primitive creatures, but they have made several significant adaptive radiations which make them quite different than their ancient ancestors (Martin 2003). What is unique about sharks is they have survived at least five mass extinctions throughout their evolution, which many other species have not. The first significant evolutionary change can be traced back to the Carboniferous period. At the end of the Devonian period, many of the fish that swam in the ocean died out before the start of the Carboniferous period (Martin 2003). This gave sharks the opportunity to flourish during this time period .Over 45 different families of sharks evolved during this period (Martin2003). This expansion of the sharks was short lived according to the geologic time scale. During the Permian Period, there was a mass extinction and 99% of the marine mammals went extinct (Martin 2003). Some shark species were able to survive this catastrophe. The sharks that survived eventually evolve into the modern sharks we know today.
GEOLOGIC TIME SCALE OF SHARKS
|ERA||Period||Epoch||Approximate Duration||Started||Life forms|
|Cenozoic||Quaternary||Holocene||Still Going||Current||Human Civilization|
|Pleistocene||2-4||2-4||Plants and animals living today - Many extinctions|
|Milocean||18||23||FIRST MEGALODO, GREAT WHITE, HAMMERHEAD|
|Oligocene||12||35||RADIATION OF MODERN SHARKS|
|METER COMES AND HITS EARTH|
|Mesozoic||Cretaceous||81||176||FIRST LAMNOID SHARKS Extinction of Dinosaurs - Dominance of Mammal|
|Jurassic||62||208||FIRST BATOIDS SHARKS Birds|
|Triassic||37||245||FIRST NEOSELACHIANS SHARKS Dinosaurs. Reptiles are getting big - dominate|
|Paleozoic||Permian||45||290||True Reptiles - Many Extinctions|
|Carboniferous||72||362||GOLDEN AGE OF SHARKS -Terrestrial Vegetation and land animals|
|Silurian||32||440||FIRST SHARKS - Land Invertebrates|
|Ordovician||75||510||Vertebrate Salt Water - Fresh Water Fish|
|Cambrian||60||570||Primitive Marine Invertebrates,|
The Basking Shark belongs to the cartilaginous fish class Chondrichthyes. The first class of Chondrichthyes swam in the ocean approximately 400 million years ago during the Devonian Period. Basking Sharks belong to Elasmobranchii, or the sharks and rays subclass. The subclass Elasmobranchii contains plate gill sharks. Associated with the Elasmobranchii are the earliest recognizable sharks, which had terminal jaws attached to the skull and poorly formed vertebrae (Moss 1984). This class was able to make it through the period of mass extinction and eventually give rise to the modern day Basking Shark. During the Jurassic and Cretaceous periods, sharks made some of their most important adaptive radiations. These adaptations include an anal fin and a stronger vertebral column representative of the way modern sharks look today. Significant adaptive radiation continued through the Jurassic Period, including enhanced features in tooth and jaw structure, and in the sensory, nervous and reproductive systems (Moss 1984).
|During the Tertiary Period, approximately 65 millions years ago, aggressive sharks modified behavior to become more passive (Martin 2003). This was the beginning of filter feeding sharks. The order Orectolobiformes gave rise to the Lamniformes, which were the Mackerel Sharks. From the Lamniformes, two distinct families descended, the Basking (Cetorhinus maximus) and Megamouth (Megachasma pelagios) sharks (Martin2003). There are 16 species in this Lamniformes order, and although the order is small, it is one of the most diverse (Martin 2003). Two sharks, including the Basking Shark, are plankton eaters. The rest of the species in the order are fast-swimming predators associated with man-eating sharks (Steel 1985). Some of the sharks in this order include Great White Sharks, Whale Sharks and the Goblin Shark. Physical characteristics of the Lamniformes include two spineless dorsal fins, gill slits around the head, and a large mouth that expands beyond the eyes (Steel 1985). The Basking Shark is in the Cetorhinidae family of Lamniformes. In this family, there is only one single genus and species. The Basking Shark was discovered by Johann Gunneru, who was a Norwegian naturalist in 1765. He first called it Squalus maximus, which was later changed to Cetorhinus maximus, the Basking Shark (Shark Trust Organization 2003).|
Classification of Cartilaginous Fish Classification of Lamniformes
Other interesting issues:
|The Basking Shark got its common name because it is often seen "basking" near the surface of the water. Basking Sharks come up to the surface to feed and also mate. This habit of basking has also made the fish susceptible to harpoon fisheries. The Basking Shark is considered a vulnerable species, based on past records of declining populations. It is suggested that there has been a decrease in populations between 50 and 90 percent in the last decade(UNEP 2003). This is due to overfishing and lack of management of the species. The Basking Shark's natural history makes it an especially vulnerable target (UNEP 2003). It has a very long growth rate, takes a very long time to sexually mature and has a long gestation period, all of which make its difficult to recover from over-exploitation. The demand for Basking Shark products has sharply risen in the last ten years. The shark's fins can stretch up to six feet and have sold for more than $16,000 in China (Mack 2003). The Basking Shark is also heavily fished for its liver. The British Government has petitioned to include the Basking Shark on Appendix II of the Convention on International Trade in Endangered Species (CITIES), but have not been successful.|
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