12 April, 2014

Making sense of shark senses


If you have ever been asked, which sense is the most important sense to sharks?   Keep reading…

The good people at Mote Marine Lab have done it again.  They seem to always crank out the good stuff, but this study is simply exemplary.  Here’s an overly simplified version of the bits I liked the most – for all the bits, go read it yourself!

They collected several individuals of three species of shark - blacktips C.limbatus, bonnetheads S.tiburo, and nurses G.Cirratum.  They also collected live prey items from the diets of each (blacktip+nurse = pinfish L.rhomboides and bonnethead = pink shrimp F.duorarum).  Each species’ general 'norm' of how they go about detecting, tracking, orientating, striking and capturing their prey was established.  Researchers then blocked some of four senses (smell, vision, electroreception, lateral line) one at a time or in combination and observed how the individuals adapted to these changes and shifted their behaviours from the established ‘norm’ of all senses firing.     

When it comes to the initial detection of prey, the sense of smell reined king for all three species.  While blacktips and bonnetheads would still capture the prey when they eventually came into visual contact with it, they did not detect and track the prey from a distance as they would normally.  For nurse sharks, sense of smell was an absolute necessity.  With their nares blocked, nurse sharks sat at the bottom of the tank and failed to feed entirely.  When smell and vision were blocked, blacktips and bonnethead also failed to feed.  This, of course, makes sense.  Nurse sharks primarily hunt at night or hunt prey animals hidden from view in reefs, whereas blacktips and bonnetheads have light available to them.  So guess who performed the best when vision and lateral line were blocked?  Yep, the nurse shark.  Who did the worst?  Bonnetheads.

The sense of electroreception was linked with successfully capturing prey.  When blocked, the sharks could still detect, track, orientate and strike at the prey, but would simply forget to open their jaws in time to capture it, even if the prey actually touched the shark! 

SourceA. A bonnethead, Sphyrna tiburo, with all senses intact opens the mouth to capture shrimp using ram-biting. B. The same bonnethead fails to open the mouth when electroreception is blocked and misses the shrimp, despite making tactile contact with the prey. doi:10.1371/journal.pone.0093036.g006

Vision and lateral line seemed to go hand in hand:  “…animals with simultaneous vision and lateral line blocks did not orient or strike, even when they were within electrosensory prey detection range.”  Thus, “these results suggest that sharks do not recognize electrical cues alone and prey, but require an additional visual or olfactory cue.”

In conclusion:
Our results demonstrate that sharks are capable of attending to multiple sensory cues simultaneously, switching sensory modalities in a hierarchical fashion as they approach their prey, and substituting alternate sensory cues, when necessary, to accomplish behavioural tasks.  This flexibility in behaviour suggests that sharks are well adapted to success, even in the face of a changing environment and evolutionary advancements in prey defenses including chemical, visual, and mechanical camouflage. Gardiner et al. 2014

The bottom line?  Sharks are not the super simple one-sensory fish that some may want you to believe that they are.  Elegant behavioural research, bravo! 

It would be very interesting, but impossible, to do a similar study on large migratory sharks to see what senses they may use to navigate over long (say, Dyer Island to Mozambique) distances, but alas...

Well done Mote lab!



2 comments:

  1. So how do you block the sensory motors of the shark, and would these deter or work as an anti shark device like a Sharkshield?

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    1. They blocked the electrosense by painting a particular substance over the ampullae of Lorenzini and blocked the lateral line sense by soaking sharks in another particular chemical substance. Both of these would eventually wear off the shark and they would regain their sense (check the actual paper and under "Methods" it says the exact chemicals used). So, not a practical shark deterring method, and as the paper says, even with both of these blocked, sharks could still sniff out and search out potential prey.

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