Hammerhead Sharks - Research Paper Example

23 July Hammerhead sharks Hammerhead sharks are unique among all elasmobranchs because of the presence of a laterally expanded and dorso-ventrally compressed head which looks somewhat like a flattened hammer. 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 olfactory rosettes in the sphyrnid species. This provides an enhanced ability to resolve odor gradients. Also, many sphyrnid species have pre-narial grooves that greatly increase the volume of water sampled by the nares. This increases the probability of odorant encounter. However, it was found that scalloped hammerhead sharks do not possess a greater amount of olfactory epithelial surface area than the carcharhiniform sandbar sharks (Kajiura et al., Morphology 253). Therefore, the scientists have concluded that sphyrnid sharks might not possess any greater olfactory acuity than carcharhinids. Despite this, there are clear olfactory advantages to the cephalofoil head morphology. 3. Electrosensory Pore Distribution Sharks have electrical sensors in their nose and heads called ampullae of Lorenzini, which can detect weak electric emissions from other sea life. Because hammerheads have broad, flat heads, the ampullae are spread out over a greater surface area, giving the shark the ability to cover more ground and sense its prey easier. This theory is strengthened by the fact that hammerhead sharks troll the bottom of the floor and can find camouflaged stingrays buried beneath the sand. Scientists compared the morphology and distribution of electro-sensory pores on two sphyrnids - the scalloped hammerhead (Sphyrna lewini) and the bonnethead (S. tiburo) with the distribution of electro-sensory pores in a carcharhinid - the sandbar shark (Carcharhinus plumbeus). They found that Sphyrna lewini pups had a higher density of electro-sensory pores per unit area compared to Carcharhinus plumbeus pups. The study demonstrated that the greater number of ampullae, the higher pore density and the larger sampling area of the head equips the sharks with morphologically enhanced electro-receptive compared to comparably sized carcharhinids (Kajiura et al., Morphology 253). 4. Increased Maneuvering Capabilities Some scientists claim that the cephalofoil helps the hammerhead in providing hydrodynamic lift and increases maneuvering capabilities when it swims in the water, just like the wing of an airplane (Kajiura et al., Maneuvering 19). They tested this hypothesis by determining whether two species of hammerheads (Sphyrna tiburo and Sphyrna lewini) turn more sharply, more often, and with greater velocity than a closely related carcharhinid shark (Carcharhinus plumbeus). However, the findings reveal that although hammerheads have better maneuvering capabilities than other sharks, they do not roll their body during turns, negating the possibility that the cephalofoil acts as a steering wing ((Kajiura et al., Maneuvering 19). The hammerhead sharks demonstrated greater lateral flexure in a turn than carcharhinids. The scientists believe that this flexibility may be due to cross sectional shape rather than number of vertebrae. 5. Electroreception Scientists have also hypothesized that the cephalofoil helps hammerhead sharks in enhanced electroreception to catch their prey. The responses of juvenile scalloped hammerhead sharks (Sphyrna lewini) and sandbar sharks (Carcharhinus plumbeus) were compared to determine if scalloped hammerhead sharks sampled a greater area of the substratum than similarly sized sandbar sharks. The sensitivity of both species to the prey-simulating electric fields was also compared. The results showed that hammerheads typically demonstrated a pivot orientation in which the edge of the cephalofoil closest to dipole remained stationary while the shark bent its trunk to orient to the center of the dipole (light-science.com). In contrast, sandbar sharks swam in a broader arc toward the center of the dipole. The different orientation patterns are attributed to the hydrodynamic properties of the cephalofoil, which enables the hammerheads to execute sharp turns at high speed (light-science.com). The greater trunk width of the sandbar sharks prevented them from demonstrating the same degree of flexibility. Although the cephalofoil does not appear to provide a greater sensitivity to the dipole, it provides a greater lateral search area, which may increase the probability of prey encounter, and enhanced maneuverability, which may aid in prey capture (light-science.com). In conclusion, scientists have made several hypotheses regarding the function of the cephalofoil in hammerhead sharks. They have also conducted numerous studies to test these hypotheses some of which have had encouraging results. Based on the studies that have been conducted thus far, scientists have identified some of the functions of the cephalofoil which include - enhanced binocular field, olfactory advantages, enhanced electro-receptive capability and greater lateral search area. However, further research is needed to explore other possible functions of the cephalofoil and to strengthen the facts that have already been gathered. Works Cited Aquatic Community. Hammerhead Shark. Aquatic Community. n.d. Web. 22 Jul. 2011. http://www.aquaticcommunity.com/sharkfish/hammerheadshark.php Center for International Earth Science Information Network. How Policy-Makers are Responding to Global Climate Change. Columbia University. Aug. 1992. Web. 18 Jul. 2011. http://www.ciesin.columbia.edu/docs/iucc201/fs201.html Kajiura, S.M., Forni, J.B. & Summers, A.P. “Olfactory Morphology of Carcharhinid and Sphyrnid Sharks: Does the Cephalofoil Confer a Sensory Advantage?” Journal of Morphology 264 (2005): 253–263. Print. Kajiura, S.M., Forni, J.B. & Summers, A.P. “Maneuvering in juvenile carcharhinid and sphyrnid sharks: the role of the hammerhead shark cephalofoil.” Zoology 106 (2003): 19–28. Print. Light-Science.com. Why the hammerhead shark's head is in the shape it's in. Light-Science.com. n.d. Web. 20 Jul. 2011. http://www.light-science.com/hammerhead.html McComb, D.M., Tricas, T.C. & Kajiura, S.M. “Enhanced Visual Fields in Hammerhead Sharks.” The Journal of Experimental Biology 212 (2009): 4010-4018. Print. National Geography. Hammerhead Shark. National Geography Wild. Aug. 2009. Web. 21 Jul. 2011. http://animals.nationalgeographic.com/animals/fish/hammerhead-shark/ University of Florida. Bonnethead. Florida Museum of Natural History. n.d. Web. 22 Jul. 2011. http://www.flmnh.ufl.edu/fish/gallery/descript/bonnethead/bonnethead.html Read More