Understanding how animals navigate under water is not only fascinating in its own right, it also contributes to instrumentation designs of underwater vehicles, robots and surface vessels, impacts management of fisheries, and helps protect the marine environment. Sharks have been chosen to demonstrate how they navigate. While they can not detect a drop of blood a mile away, as often stated, sharks do have impressive prey tracking capabilities. Sharks are important in fisheries worldwide and have been severely depleted in recent decades, often taken as unwanted by-catch in other fisheries. Yet, they are essential top predators needed to maintaining a healthy ecosystem. This research project will show how sharks use all their senses in hunting behavior, starting with initial prey detection, through tracking and locating, and ending with striking their prey. For more complete understanding, we compare a few shark species that appear to use their senses differently mostly because they specialize in different prey in different habitats. A team of experts in sensory and shark biology, using unique testing facilities in Massachusetts and Florida, has been assembled including graduate students being trained in the many technical approaches needed for work on live sharks. The research directly involves undergraduate and high school students and provides extensive outreach to other students of all ages and to the public in general. The accumulated knowledge should lead to a model of shark navigation and predation that can be used for the conservation of sharks, protection of humans, and the engineering design of underwater steering algorithms. The inevitable presentation of this research in future television programs and video documentaries will disseminate new knowledge to the public at large, both of sharks and of rigorous science.
How animals sense and locate their prey – and particularly how they use multiple senses like vision and smell to integrate sensory information – has long been a topic of interest to the public as well as research scientists. Although we have known something about this for animals that live on land or in the air, our knowledge of animals that live in the sea has been limited on this subject. To address this, we conducted research on one of the sea's most well-adapted predators – the shark. Through a series of experiments on live sharks in the laboratory, we challenged these animals to show us how they detect, localize, get to, and feed on their prey, and how they use their multiple senses together to accomplish these tasks. To accomplish these goals, we examined prey tracking and capture in four different shark species: the smooth dogfish, the nurse shark, the blacktip shark and the bonnethead. These four species have differing behaviors and ecologies in that some are more bottom-dwelling while others are more active swimmers in the water column. The senses we tested were vision, smell, the lateral line, and also electroreception. The lateral line essentially detects what is often termed "distant touch", the vibrations that are given off by prey, as well as water motion in the environment. Electroreception is mediated by organs termed the ampullae of Lorenzini, allowing the sharks to detect weak electric fields surrounding their prey. By selectively blocking each sense one at a time, and in combination, we were able to see how sharks use their various senses for orienting to, striking at, and capturing their prey. The results were revealing with regards to how these different species of sharks hunt their prey. The first finding, utilizing the smooth dogfish, was that when sharks initially detect odors, they do not orient to them based on differences in concentration, but based on differences in the timing of odor arrival at the two nostrils. By turning towards the side that receives the first, rather than the strongest, odor cue, sharks can steer into a patch or plume of odor. This finding may have implications for the evolution of the strange head shape of the hammerhead family – the more widely spaced nostrils may although these sharks to orient to patches at flatter angles than other pointed-nosed shark species. In the other phases of feeding behavior, each species showed a different preference for sensory information depending on their behavior and ecology. For example, bonnetheads and blacktip sharks could detect prey using vision or smell, while the nurse shark required smell and would not feed if smell was blocked. All shark species tracked the odor plume emanating from prey using smell in combination with either vision or the lateral line, but the nurse shark could also track using smell combined with touch. In orienting and striking, bonnethead required vision, but the blacktip shark could use vision or, from a closer range, the lateral line. The nurse shark could strike using vision, the lateral line or electroreception. Finally, to determine when to move the jaws to capture the prey, all three species used the electrosensory system, but the nurse shark and blacktip shark could also use touch, while the bonnethead could not. These three species took advantage of all their available senses. By blocking senses in different combinations, we found that when some of the sharks' normal sensory cues were unavailable, they were still capable of successfully detecting, tracking and capturing their prey by switching to alternate sensory information. This means that sharks are adaptable in using whatever sensory information is available to find their prey – a good thing in a successful predator. Our research can now be applied to the design of underwater robotic vehicles that must navigate to some type of target. Furthermore, the research brings into question the effects various man-made events, such as noise or chemical pollution, could have on the ecology of sharks and their ability to successfully feed. In the end, this research taught us a lot about how sharks work, how the senses of animals are integrated in performing complex tasks, and how we can apply this information to engineer better tools for society's use.
