On a recent trip to Maui my family and I found ourselves at the Maui Ocean Center. (Actually, I tend to turn up at the local aquarium of any city I’m visiting that has one – one of these days I really should start doing aquarium reviews!) In the humble opinion of both my son and me, the best part of this particular aquarium was the hammerhead shark tank. Hammerheads are one of those creatures that seem to make no sense at all, something that you might think was a practical joke if you didn’t see them actually swimming around (“hey, let’s send another fake skeleton over to Byron in oceanography, we had such fun with that jack-a-lope!”)
The strangeness of the hammerheads got me to wondering how on earth something so unusual and awkward-looking would evolve? What was it that lead to the evolution of eye sockets bulging out on either side of its head like a … well … like a hammer?
Initially I was wondered why the hammer evolved in this one case and not in any others. Well, as it turns out, there are actually several different species of hammerhead shark – eight or nine of them, to be inexact (the sources that I researched seem to vary on this point). And multiple species suggests that this evolutionary branch is not quite as rare as I thought. It is still uncommon, however, as there are 440 or so species of shark. So just why did the hammerhead evolve?
As it turns out, not much is known for certain with regard to its evolution. Only very recently have scientists determined with a fair degree of certainty and agreement that the hammerhead evolved its peculiar head shape to enhance its vision, probably to aid in the search for prey. The wide eye position gives the shark two distinct benefits – good binocular vision and full 360 degree vision vertically. The 360-degree vision in a vertical plane simply means the shark is able to see above and below itself at all times. Binocular vision has 4 beneficial characteristics: one eye can act as a spare in case the other is damaged, the two eyes give a greater field of view when the field of view of each individual eye is combined together, it enhances the ability to detect faint objects because the vision in each eye adds together, and it enables depth perception (via parallax) which in turn enables more accurate jugdement of distance.
I couldn’t, unfortunately, find anything to suggest why it was a beneficial adaptation for the hammerhead to be equipped with binocular vision when so many other shark species lack it. There are a few different theories for this, including some that I basically just made up. Maybe you can think of a few more?
- If hammerheads tend to live in murkier water, then better vision would help them locate prey (this is one I made up!)
- As a smaller species of shark, better vision might also help them see predators more readily (an actual theory)
- It could be that ancestors of hammerhead sharks were isolated from other shark species in some way and underwent a dramatic evolution in response to stresses in their environment that other related shark colonies did not face.
With more study, we may one day have some good theories as to why hammerheads evolved in this way. Sadly, hammerhead sharks, like most shark species today, are endangered in many areas. The decline of sharks in our lifetime is startling and shocking (the scalloped hammerhead population has declined by 99% just in our lifetime). Most of this decline is due to overfishing, particularly in Asian markets where sharks are sold for their fins which are made into shark fin soup. This conflict of culture and ecosystem is something that needs to be resolved if we hope to protect sharks from extinction.