By Jaime Enrique
Introduction to hammerhead sharks.
More on hammerhead sharks.
Taxonmy and Evolution.
Since sharks do not have mineralized bones and rarely fossilize, it is their teeth alone that are commonly found as fossils. The hammerheads seem closely related to thecarcharhinid sharks that evolved during the mid-Tertiary Period. According to DNA studies, the ancestor of the hammerheads probably lived in the Miocene epoch about 20 million years ago.
Using mitochondrial DNA, a phylogenetic tree of the hammerhead sharks showed the winghead shark as its most basal member. As the winghead shark has proportionately the largest "hammer" of the hammerhead sharks, this suggests that the first ancestral hammerhead sharks also had large hammers.
Reason for head shape.
The unknown species range from 0.9 to 6 m (3.0 to 19.7 ft) in length and weigh from 3 to 580 kg (6.6 to 1,278.7 lb). They are usually light gray and have a greenish tint to them. Their bellies are white which allows them to blend into the ocean when viewed from the bottom and sneak up on their prey. Their heads have lateral projections which give them a hammer-like shape.
Hammerheads have disproportionately small mouths and seem to do a lot of bottom-hunting. They are also known to form schools during the day, sometimes in groups of over 100. In the evening, like other sharks, they become solitary hunters.
Reproduction occurs only once a year for hammerhead sharks, and usually occurs with the male shark biting the female shark violently until she agrees to mate with him.The hammerhead sharks exhibit a viviparous mode of reproduction with females giving birth to live young. Like other sharks, fertilization is internal with the male transferringsperm to the female through one of two intromittent organs called claspers. The developing embryos are at first sustained by a yolk sac. When the supply of yolk is exhausted, the depleted yolk sac transforms into a structure analogous to a mammalian placenta (called a "yolk sac placenta" or "pseudoplacenta"), through which the mother delivers sustenance until birth. Once the baby sharks are born, they are not taken care of by the parents in any way. There is usually a litter of 12 to 15 pups; except for the Great Hammerhead which births litters of 20 to 40 pups. These baby sharks huddle together and swim toward warmer water until they are old enough and large enough to survive on their own.
In 2007, the bonnethead shark was found to be capable of asexual reproduction via automictic parthenogenesis, in which a female's ovum fuses with a polar body to form azygote without the need for a male. This was the first shark known to do this.