I. Overview of Discussion
B. Sensory Systems of Modern Sharks
C. Sharks of the Caribbean
The evolutionary record of modern sharks is still poorly understood partially because of their physical structure. Unlike most fish, shark skeletons are made primarily of cartilage instead of bone. Cartilage is a tough, flexible tissue that does not lend itself to fossilization as bone does. Therefore, scientists have had to rely on the few bony parts (such as teeth) of shark skeletons that have been preserved in fossilized form to determine the origin of these creatures. Obviously, this presents an incomplete picture of shark development, since characteristics such as skull shape and fin structure cannot be analyzed reliably over time.
The evolution of sharks can be traced back to the development of backboned or vertebrate animals during the Cambrian Period between 600 and 500 million years ago. The earliest fishes were found in 480 million year-old Ordovician rocks. By the end of the Silurian period (approximately 435 million years ago), two major groups of fishes were established: the Agnatha or jawless fishes, and the Gnathostomata or jawed fishes. Throughout the Silurian period, the jawless fishes were the most abundant, although primitive bony and cartilaginous jawed fished had begun to appear. Fossils of the earliest sharks were found in rocks from the Silurian Period.
No one can say for certain what species may have been the ancestors of modern sharks, but scientists have put forth theories regarding the type of fish that sharks probably evolved from. This fish was probably small with a long, slender body, one dorsal fin but no fin spines, paired pectoral fins and at least 7 pairs of gill supports. Its mouth would have been at the front of its body or slightly beneath it. The fish would have been covered with small scales; these would vary in size and shape according to their position on its body. Its mouth would also probably have been lined with scales.
The history of shark development from ancestral forms is confusing and very complex. Scientists have not found a set of fossil remains showing a smooth transition from species to species. What scientists do know is that modern sharks did not rise to dominance until after the Jurassic period (between 195 and 140 million years ago). At this time, many of the more ancient shark species became extinct for unknown reasons. Some Jurassic sharks are closely related to species alive today, which means that the histories of many present-day shark families stretch back 135 million years or more. Evidence indicates that all modern sharks were present in the world's seas between 25 and 5 million years ago.
III. Sensory Systems of Modern Sharks
Today, there are 344 known species of sharks, all of which possess remarkable sensory capabilities.
Sharks have excellent vision and a keen sense of smell. One experiment showed that sharks are capable of detecting dilutions of one part tuna flesh to 25 million parts of seawater. This ability seems to be heightened during periods of extreme hunger. In an experiment conducted in the 1960's, starved blacktip sharks responded to concentrations of one part grouper flesh to 10 billion parts of seawater.
Sharks also possess vibration and electromagnetic senses that are unique in the animal world. It has been demonstrated that sharks locate prey in the open seas by detecting the minute electric fields generated by all living creatures.
Skin pores on a shark's snout lead to the ampullae of Lorenzini, which are sensory organs that enable sharks to detect very weak electric fields. These organs allow sharks to find any prey that may be hidden under sand or along the sea floor.
IV. Sharks of the Caribbean
Of the 344 shark species known today, at least 68 have occurred in the waters of the Caribbean. We most likely will never see the vast majority of these sharks, since many of them swim in deep waters near the bottom. Local sharks range in size from the Green lanternshark, with an average length of 23 cm for males and 21 cm for females, to the Whale shark, with an average length of 8 m for males and 9 meters for females. The largest whale shark on record was 12.65 meters long and weighed 21.5 tonnes.
Some other local species that might be familiar to many of us include the Caribbean reef shark, the Blacktip and Oceanic whitetip shark, the bull shark, shortfin mako, lemon shark, the thresher, the tiger shark and the Great Hammerhead.
Reader's Digest. 1986. Sharks: Silent Hunters of the Deep. Sydney, Australia: Reader's Digest.
Morrissey, J.F. and H.V. Butcher. 1988. "The natural history of the tiger shark." Sea Frontiers v. 34 (5): 264-271.
Klimley, A.P. 1996. "Dances with sharks." Natural History v. 105 (11): 54-55.
Maisey, J.G. 1998. "Voracious Evolution." Natural History v. 107 (5): 38-41.
Cardone, B.J. 1994. "Sharks you should know: the Tiger Shark." Skin Diver v. 43 (12): 28+.
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