We Know Sharks Can Bite, But What Else Do We Know About Them?

The summer beach news of 2001 has made me a little wary of walking in the ocean in anything but ankle deep water! Determined to combat this hesitancy, I decided to research this fearful creature and add to my knowledge base.

I limited my study to the great white shark, Carcharodon carcharias, and would like to share this information with you. We will consider its distribution, physiology, and of course, genetics.

This shark is found worldwide in temperate and cooler waters. The shark is not truly pelagic (ocean crossing), but is capable of transoceanic trips. In the United States, it is frequently sighted from Maine to New York and along the length of the California coast. It is also frequently found around South Africa and Australia.

The physiology of C. carcharias is very unusual. The great whites exhibit regional endothermy, which is the ability to keep the temperature of certain body regions above the temperature of the surrounding water. This is accomplished through a network of capillaries called the rete mirabile located near the brain, eyes, muscles, stomach, and viscera. Regional endothermy enhances the over all predatory and physiological efficiency of the shark, particularly digestion. Efficient digestion is necessary due to the shark’s high blubber diet. Blubber is high in energy, but slow to digest.

Significant ecological data are difficult to obtain due to the rarity of the fish and its large size. An article in the July issue of Nature detailed a study using genetic information to better define the dispersal of the fish. The genetic study was undertaken to illustrate that the dispersal of C. carcharias is sex-biased, with non-roving females (philopatric) and roving males. The results of the genetic studies along with the shark’s long lifespan and late maturation suggest a similarity more to marine mammals than to other fish.

The study involved sharks collected off the coasts of Australia, New Zealand and South Africa. The study looked at mitochondrial DNA (mtDNA) that is inherited only from the female and nuclear DNA that is inherited from males and females.

Statistical analysis of the mtDNA determined there were two distinct genetic lines: 1) Australian and New Zealand sharks and 2) South African sharks. The information indicated that these two populations have been long separated.

In contrast, population genetic analysis on the nuclear DNA revealed no significant differences in the populations. These results provide strong support that female sharks are less likely to cross ocean basins than are males.

The ecological significance of this study is that over fishing of shark populations that have little immigration from outside populations and whose females are non-roving may lead to threatening declines of shark numbers.