A fascinating new study has revealed the interplay of the different senses that sharks use during predation, highlighting how different shark species use a combination of senses to guide the animal’s behavior from initial detection to attack.
Although sensing in aquatic animals is well documented, previous research has primarily focused on investigating one sense at a time. Since it is evident that sharks use multiple senses during hunting, scientists wanted to investigate not only how particular senses guide different stages of hunting, but also if sensing differed between different shark species. The results, which have been published in the journal PLOS ONE, demonstrated that while there were species differences which reflect the preferred capture strategies adopted by the sharks, the animals were actually able to switch between the senses used if one or a combination of senses were deprived.
Although different shark species capture prey in distinct ways, the behavioral hunting sequence is fairly uniform. First the shark detects the prey, followed by tracking to the area in which the prey is residing. Then the shark will orient itself toward the prey and strike, which if successful will result in prey capture. Different senses seem to dominate during these phases, providing more information as it becomes available to guide the attack.
The researchers chose three shark species with distinct habitats and capture mechanisms for this study; blacktips (Carcharhinus limbatus) which are ram-feeders, bonnetheads (Sphyrna tiburo) which are ram-biters, and finally nurse sharks (Ginglymostoma cirratum) which are suction-feeders. They temporarily blocked the senses of the sharks and observed them as they swam toward prey placed in the tanks. They blocked smell by using nose plugs, vision by covering the eyes, and the lateral line, which is a sensory system that detects water motions and pressure gradients which could be caused by prey movement, with antibiotics. They also blocked electroreception by placing insulating material over their snouts, which is where the pores that sense electric fields given off by animals are found. This allowed the team to discern the involvement of senses in guiding the different phases of predation.
The team found that despite different specializations, when the sharks approached prey from downstream, all of them detected the prey using their sense of smell. If approaching from upstream, however, they could use their sense of vision. When smell was blocked, only the nurse shark could not detect the prey and thus failed to feed. This is possibly because nurse sharks often feed in the dark, picking prey out from between rocks, meaning they can’t rely on vision. Although the other sharks could successfully capture prey when their noses were blocked, they had to be close enough to see it before they attacked. Therefore when vision and smell were blocked simultaneously, neither the blacktips nor the bonnetheads could detect the prey and thus did not capture it.
When vision alone was blocked, the sharks could still orient toward the prey using their lateral lines, albeit striking was slower and occurred at closer distances to the prey. This suggested vision was required for orientation of long distance strikes. When vision and the lateral line were blocked, however, only the nurse sharks could locate the prey. Nurse sharks often use touch to detect the directionality of water movement by contacting objects such as rocks, which could be used to aim the shark toward the prey.
Although detection of the electric field given off by the prey seemed to trigger jaw opening, this alone was found to be insufficient to precipitate striking. When vision and the lateral line were blocked, the sharks failed to open their jaws and strike despite electroreception still being active. This is probably because electroreception only functions over very short distances. When electroreception was blocked, the sharks usually could not capture their prey, although sometimes if they touched the prey this triggered jaw opening in the blacktips and nurse sharks.
These results highlighted how sharks exploit the many signals present in their aquatic environment, combining senses to provide more detailed information during hunting. It is so far the most detailed experiment of how sharks utilize their senses during different stages of attack, and demonstrates the differences across certain shark species. The fact that compensatory sensing can occur when one or more sense is blocked suggests that some measures used to deter sharks may not be sufficient. For example, shark-repellent wetsuits recently developed may not be enough to prevent attacks alone. This flexibility also demonstrates that sharks are well adapted to succeed despite constantly changing environments and prey, for example as prey becomes more camouflaged. However, their success as a species can only go so far, since they are still susceptible to over fishing and many shark species face dwindling populations.
Check out this YouTube video below from the Mote Marine Laboratory of sharks catching and missing prey.