Posted by jlwile on February 18, 2013
I have always been fascinated by sharks. In fact, of all the times I have been scuba diving, the dive I remember the most occurred off the coast of South Africa. I went with some marine biologists who had been studying sand tiger sharks (Carcharias taurus). As I initially sunk down into the water, I leveled out just a few feet from the bottom, and as I was checking my gauge to determine my depth, a 2-meter (6-feet) long shark swum right underneath me! We ended up seeing more than a dozen sharks of various sizes on that dive. It was incredible.
One of the things that is fascinating about sharks is the way they hunt. While they use their sense of smell (and to some degree their sense of sight), one of their main hunting techniques involves using their electrical sense. Yes, sharks can sense electrical fields, and they are quite good at it. As a recently published book on sharks says:1
They can detect the minute electrical currents generated by the nervous systems of prey by using electrical sensors called the ampullae of Lorenzini…These sophisticated sensors are very useful in finding prey buried under the sand.
Interestingly enough, these sophisticated sensors often develop while the shark is still an embryo. Is that just to get the shark ready for hunting its prey when it is fully developed, or could there be some use that the embryo has for sensing electrical fields? It doesn’t need to hunt, so for what purpose could it be using its electrical sensors?
Some Australian scientists decided to investigate this issue, and what they found is fascinating!
Most sharks are ovoviviparous. This means the embryo develops in an egg that is kept inside the mother’s body until it is ready to hatch. However, some are oviparous, which means the embryo develops in an egg that has been laid outside the mother’s body. The Australian scientists studied bamboo sharks (Chiloscyllium punctatum), which are oviparous. The embryo develops in a egg case, such as the one shown at the top of this post. Initially, the egg case is sealed, but in the later stages of development, the seal near the bottom of the egg case opens, allowing seawater to enter so the embryo can get a more ample supply of oxygen.
Once seawater is allowed to enter the egg case, the shark embryo can detect electrical fields coming from the outside world. The scientists decided to see if the shark embryo uses this ability in any way. They exposed 11 different bamboo shark embryos to varying electrical fields. Soon after the field was turned on, the embryos stopped breathing. Why would they do that? The motion of the gills while the embryo is breathing could be spotted by a predator, so the the researchers concluded that the embryos were “freezing” in an attempt to avoid detection.
This conclusion was strengthened by the fact that the embryos held their “freeze” the longest when the electrical fields were similar to what would be produced by an actual predator. They also found that the more developed the embryo was, the better its freeze response.2 If you go to the scientific paper (the link is provided in the reference) and scroll down, there is a video that shows you the freeze response as it happened in one trial of the experiment.
So in the end, it seems that the sophisticated electrical sensors in a shark allow it to hunt, but while it is still young and vulnerable, they also allow it to avoid predators. What an amazing design!
1. Alessandro De Maddelena with Walter Heim, Sharks of New England, Down East 2010, p. 33
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