Geography

Hammerhead Sharks

41000 The characteristic hammer shaped head of these ocean predators is called cephalofoil and is the reason why they are called hammerhead sharks. The nostrils and eyes are located at the tip of the extensions. All hammerhead sharks belong to the genus Sphyrna. There are nine identified species of Hammerhead shark and all have the characteristic projections on both sides of the head. The largest Hammerhead species, the Great Hammerhead shark, can grow up to 20 feet in length and weigh up to 1,000 pounds, although smaller sizes are more common (National Geography). The smallest of this species is the Bonnethead, which can reach an average length of 3-4 feet and can weigh up to 24 pounds (University of Florida). Cephalofoil: Structural Variations There are two main theories regarding the development of the cephalofoil. Some scientists believe that the shape evolved gradually over numerous generations, while others suggest that it was a sudden mutation that rapidly proved to be an advantage (Aquatic Community). The shape of the cephalofoil has been found to vary considerably among the various species of Hammerhead sharks. The great hammerhead has a cephalofoil that is broad and nearly flat across the front, with a single shallow notch in the center. The scalloped hammerhead is arched more and has a pronounced center notch with two matching notches on either side, giving it a scalloped appearance. The smooth hammerhead is smooth with no notches but a slight broad arch. The cephalofoil of a bonnethead is rounded at the front and resembles a shovel more than a hammer. Function of Cephalofoil Researchers are not sure about the function of cephalofoil in hammerhead sharks, but they have a few theories, some of which hold up well under research. The prominent theories which have been tested are listed below: 1. Enhanced binocular field In this study, scientist hypothesized that one of the main features of the cephalofoil is to enhance frontal binocularity (McComb et al. 4010). As part of this study, they compared the visual fields of three hammerhead species: the bonnethead shark (Sphyrna tiburo), the scalloped hammerhead shark (Sphyrna lewini) and the winghead shark (Eusphyra blochii) with the visual field of two carcharhinid species: the lemon shark (Negaprion brevirostris) and the blacknose shark (Carcharhinus acronotus). Scientists also quantified the eye rotation and head yaw of these sharks in order to determine if species compensate for large blind areas anterior to the head. The study revealed that the winghead shark possessed the largest anterior binocular overlap which was nearly four times larger than that of the lemon and blacknose sharks (McComb 4013). The binocular overlap in the scalloped hammerhead sharks was greater than the bonnethead sharks and carcharhinid species (McComb et al. 4013). However, the bonnethead shark did not differ from the carcharhinids (McComb et al. 4013). The hammerhead species did not demonstrate greater eye rotation in the anterior or posterior direction. However, both the scalloped hammerhead and bonnethead sharks exhibited greater head yaw during swimming than the lemon and blacknose sharks, indicating a behavioral compensation for the anterior blind area (McComb et al. 4013). The results indicate that hammerhead species have larger binocular overlap compared to the carcharhinid sharks which is consistent with the ‘enhanced binocular field’ hypothesis. 2. Head Morphology Scientists have conducted a comparative morphology test to determine if the sphyrnid cephalofoil offers better stereo-olfaction, increases olfactory acuity and samples a greater volume of the medium compared to the carcharhiniform sharks (Kajiura et al., Morphology 253). The broadly spaced nares provides significantly greater separation between the

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