How the shark got her sixth and seventh sense

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Sight, smell, hearing, touch and taste. Sharks share all of these senses with us but have also evolved a sixth and seventh sense to meet the challenges of marine life . And no this is not what I mean.

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Well some sharks do…

The word shark can conjure up many images; from jaws with rows of razor sharp teeth to a triangular fin cutting through the ocean. Despite this, one of the most defining characteristics of sharks (and rays) is the presence of a sixth and seventh sense. These senses may be responsible for the success these animals have had in colonizing the oceans and for their persistence for an estimated 450 million years.

The sixth sense 

All living animals produce their own electrical field due to muscle contractions. Sharks can use changes in electrical fields and the associated electrical pulses to locate prey and even mates. This is particularly valuable when visibility is poor, for example in the deep sea. Sharks can even use this invaluable sixth sense to navigate the oceans during migration by tracing the earth’s electromagnet field.

A fish swimming (or anything for that matter) in the ocean will cause minor disruptions in the surrounding electrical field. These disturbances can be detected by sharks and rays by electroreceptors, specialized pores on the skin around their head and on the underside of their snout. These are called Ampullae of Lorenzini. A single shark can have thousands of these on their snout, with the hammerhead shark known to have over 3000. The unique shape of the hammerhead shark head is actually thought to be due to the presence of these electrosensors. The shape allows the sharks to have more of them and as a consequence it increases their success in finding prey.

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There are a lot of Ampullae of Lorenzini in this picture

The Ampullae of Lorenzini are tiny jelly-filled pores. When something causes a disruption in an electrical field, this information will be received by the them. The jelly-filled bulbs inside them vibrate when an electrical signal is detected and cilia at the bottom of the pore then transport this information to nerve cells, which in turn transmit it to the brain. The electrical pulses can not only convey the location of a prey source, but can be used to infer whether a fish is in distress. A fish swimming erratically will produce tell-tale electrical pulses which allow sharks and rays to target already distressed fish.

The seventh sense

This sense is known as the lateral line. It detects pressure changes and vibrations similarly to how human skin detects changes in wind direction. This sense gives  sharks, as well as all other fish,  spatial awareness by detecting small changes to ocean currents. It is extremely important in predatory behaviour, orientation and social schooling (shoals swimming together in coordination).

The lateral line sensory system is made up of modified scale and hair cells which form a series of pores. The pores fuse to form a canal underneath the skin (see diagram below) . Cupula are located in between the pores, and they respond to changes in the movement of water by bending sensory hair cells beneath them. Bundles of these hair cells are called the neuromast, and this is responsible for sending nerve impulses to the brain. These nerve impulses allow the fish to process the information and respond accordingly.

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Lateral line sensory system

Changes in the movement of ocean currents cause the neuromast to be moved around. Differences in the normal movement of the neuromast informs the fish of any structural changes in their environment as well as the proximity of other nearby animals.

Due to the similarity between the lateral line system and the Ampullae of Lorenzini, it is likely that the latter developed through modification of the former.

Both the lateral line system and the Ampullae of Lorenzini are extremely sensitive, and when touched can induce a temporary trance-like state in a shark, as you can see in the video below. Would this would be the perfect escape from a dangerous encounter with a shark?

 

Thanks for reading!

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Photo credit great white shark: <a href=”http://www.flickr.com/photos/57915000@N02/6783249270″>Great White Shark</a> via <a href=”http://photopin.com”>photopin</a&gt; <a href=”https://creativecommons.org/licenses/by-nc/2.0/”>(license)</a&gt;

Gif credit: http://giphy.com/gifs/horror-movie-bruce-willis-DvtsYOKrqPZg4

Photo credit hammerhead sharks: <a href=”http://www.flickr.com/photos/99024595@N00/21717747416″></a&gt; via <a href=”http://photopin.com”>photopin</a&gt; <a href=”https://creativecommons.org/licenses/by-nc-nd/2.0/”>(license)</a&gt;

Attribution for lateral line system diagram: By Thomas.haslwanter (Own work) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0)%5D, via Wikimedia Commons

 

 

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How the fish got his legs

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The history of life on earth has involved many spectacular metamorphoses. From the primeval soup containing the first life to the first single celled organisms, and then to the first multicellular organisms which made possible the evolution of the human body. One of the most spectacular of these metamorphoses was the evolution of terrestrial four-legged animals from fish. An important step in this transition from aquatic to terrestrial life was the transformation of fins into limbs.

Around 375 million years ago comes the first evidence of a courageous fish species taking its first steps towards terrestrial living. This species is called Tiktaalik rosaea. Its first fossil was discovered in 2004 (which you can read about in this book and this paper). It has a unique physiology, and is thought to represent an example of an evolutionary transitional species between fish and amphibians. It is therefore the direct ancestor of both you and me, as well as every other amphibian, reptile, mammal and bird on earth. Tiktaalik has a mixture of physiological features similar to tetrapods (four-legged animals) and to fish. It has fins and scales like all fish, but unlike fish it has a flat head and a neck. It also has holes on the top of its skull called spiracles. These indicate that the fish had primitive lungs, as well as gills, allowing it to breathe both on land and underwater.

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Skull of Tiktaalik showing spiracles (holes above eyes)

Perhaps most importantly the skeleton inside Tiktaalik’s fins has a similar bone structure to that of the tetrapod limb. Neil Shubin, author of Your Inner Fish, describes how the tetrapod limb structure follows the same pattern in all tetrapods, from horses to bats to humans. This structure you can see in the diagram of the human arm below, simply put there is one big bone (humerus) followed by two bones (the radius and the ulna) followed by a bunch of little bones.  You can see in the picture below that Tiktaalik has a bone structure that shows this pattern.

 

It is also evident that it used these specialized fins to move around on land, as it had a robust enlarged shoulder which would have been able to support the weight of the animal.This shows that fish ventured onto land before they got legs. You can watch Neil Shubin, one of the discoverers of Tiktaalik talk about the discovery below.

So why did this species spend some of its time on land? Scientists believe that the environmental conditions that this species was living in encouraged the species to venture onto land. Tiktaalik lived in shallow waters which were oxygen poor. The ability to use its fins to move on the land allowed it to access more areas of food, and to move from one aquatic area to another. Its ability to breathe oxygen also gave it an edge against other fish species, as it did not rely on the oxygen dissolved in the water to breathe.

I have focused this blog post on Tiktaalik because it represents a crucial step in the evolution of tetrapods. They are the oldest fossils discovered which have the novel traits associated with tetropods and therefore represent the earliest examples of at least partial terrestrial living. The cocktail of fish-like and tetrapod-like characteristics of Tiktaalik have paved the way for a better understanding of how fish ventured onto land, and ultimately got their legs.

As ever thanks for reading, and if you would like to read more about tetropod fossils from later in the fossil record, this paper has some great information.

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Photo credit Tiktaalik 1:

<a href=”http://www.flickr.com/photos/62281335@N07/6662865611″>Field Museum: Tiktaalik</a> via <a href=”http://photopin.com”>photopin</a&gt; <a href=”https://creativecommons.org/licenses/by-nc-nd/2.0/”>(license)</a&gt;

Photo credit Tiktaalik skull: By photographed by Richard G. Clegg, tweaked by dave souza [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0/)%5D, via Wikimedia Commons

Photo credit Tiktaalik’s limb: photographed by Richard G. Clegg, tweaked by dave souza (http://en.wikipedia.org/wiki/Image:Tik_limb_1a.jpg) [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0/)%5D, via Wikimedia Commons

How the tiger got her stripes

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What do a tigers stripes, your fingers and and a crocodiles evenly spaced teeth have in common?

In my last blog post, I talked about the evolutionary explanations for the different coat patterns of wild cat species, which focused more on why these patterns arose rather than how. How these patterns are actually formed in a growing embryo is a question which requires us to look to developmental biology for answers.

We must first turn to a mathematician in the 1950s, who had just published a set of equations which he proposed could explain morphogenesis, the development of regular repeating patterns seen in biological systems. This includes everything from the stripes on a tigers fur, to the whorls on plant leaves, to the formation of digits on a human hand. This mathematician was Alan Turing, most famous for cracking the Enigma code during the Second World War.

He proposed that these patterns were formed by a reaction-diffusion system. The system involves the reaction between chemicals, and the movement of these chemicals through a system, much how a drop of food colouring will spread through water. One important thing to note is that the chemicals would move through the system at different rates. The chemicals responsible for the formation of patterns in biological systems are called morphogens. Turing thought that interactions between a pair of morphogens are responsible for pattern formation. One morphogen would result in the activation of genes needed for pigmentation in a cell, whereas the other would inhibit this process.

One of the defining features of the reaction-diffusion system is that it is self-organising and self-regulating. The only thing needed to generate a pattern is some activator morphogen. This is because the activator morphogen stimulates further production of itself as well as stimulating the production of the inhibitor morphogen. As I mentioned earlier, the chemicals do not spread through a system at the same rate. The inhibitor morphogen spreads quicker.

Mathematical biologist James Murray explained Turing’s idea in this way:

“Imagine a field of dry grass dotted with grasshoppers. If the grass were set on fire at several random points and no moisture were present to inhibit the flames, the fires would char the entire field. If this scenario played out like a Turing mechanism, however, the heat from the encroaching flames would cause some of the fleeing grasshoppers to sweat, dampening the grass around them and thereby creating periodic unburned spots in the otherwise burnt field.”

Evidence for the self-organising characteristic of Turing pattern formation is the observation that each animal has a unique pattern, with offspring of patterned animals not having the same pattern of their parents.

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You can see the subtle differences in coat patterning between this little guy and his mum

Therefore the formation of a stripe can occur as follows:

  1. As the activator spreads through a system it causes pigmentation, the production of more activator and the production of inhibitor.
  2. The activator has a head start at first, but as the inhibitor moves faster through the system it will catch up with the activator in space
  3. This will cause the activator to stop inducing pigmentation resulting in the end of the stripe

A whole array of different patterns, from spots to stripes to swirls can be produced just by changing certain characteristics of the activator and inhibitor morphogens, such as the rate at which the activator spreads through a system.

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Just a small selection of the different patterns seen in nature

The main problem with Turing’s theory was the elusive morphogens involved in pattern formation. Up until recently, there had been no chemicals which had been identified as morphogens. However in 2012 a study on the formation of the ridges on the human palate (press your tongue against the roof of your mouth and you can feel them) finally identified the morphogens that were responsible for their formation. This is the first time that there has been experimental evidence for Turing’s reaction-diffusion system. Below you can watch Dr Jeremy Green from Kings College London talking about the research which led to this evidence.

Thanks for reading!!

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Photo credit tiger: <a href=”http://www.flickr.com/photos/94178903@N03/24527170969″>LOOK</a&gt; via <a href=”http://photopin.com”>photopin</a&gt; <a href=”https://creativecommons.org/licenses/by-sa/2.0/”>(license)</a&gt;

Photo credit tiger cub and mother: <a href=”http://www.flickr.com/photos/8070463@N03/6812337407″>Luva confronting her mother</a> via <a href=”http://photopin.com”>photopin</a&gt; <a href=”https://creativecommons.org/licenses/by-nd/2.0/”>(license)</a&gt;

Photo credit zebra fur:

<a href=”http://www.flickr.com/photos/20428799@N06/4253949714″>Black&White</a&gt; via <a href=”http://photopin.com”>photopin</a&gt; <a href=”https://creativecommons.org/licenses/by/2.0/”>(license)</a&gt;

Photo credit leopard fur:

<a href=”http://www.flickr.com/photos/8070463@N03/8402942475″>Leopard rosettes</a> via <a href=”http://photopin.com”>photopin</a&gt; <a href=”https://creativecommons.org/licenses/by-nd/2.0/”>(license)</a&gt;

Photo credit leaf patterning:

<a href=”http://www.flickr.com/photos/91431940@N00/2570812232″></a&gt; via <a href=”http://photopin.com”>photopin</a&gt; <a href=”https://creativecommons.org/licenses/by-nd/2.0/”>(license)</a&gt;

Photo credit giraffe fur:

<a href=”http://www.flickr.com/photos/49462908@N00/1855198930″>Skin</a&gt; via <a href=”http://photopin.com”>photopin</a&gt; <a href=”https://creativecommons.org/licenses/by-sa/2.0/”>(license)</a&gt;