Friday, 21 January 2011

SOS: Save our Science - Seeing Sharks!!
This is the end of my first week in the second lab, it's pretty fun so far, quite enjoying the variety of things I've become involved in and will hopefully get some very exciting data. It's going to be a busy term though. Anyway, more on that later. Here's some fishy news from Australia!

It is a commonly quoted fact that sharks can smell a single drop of blood in over 100 litres of water. As a top predator a shark needs highly developed senses to hunt, so it is surprising that many species are probably completely colour blind! Sharks still have excellent vision, but it is monochromatic i.e. Black and white.

The blacktip reef shark (Carcharhinus melanopterus), one of the species in the study, which lacks cone cells (source)

The evolution of the eye is still highly debated, many suggest that it may have evolved separately at least seven times. In many animals, vision depends upon light detecting cells in the back of our eyes, primarily there are two types, called rods and cones, because of their shape. There are approximately 90 million rods and 4.5 million cones in each human eye. Rods allow us to see in the dark, they are extremely sensitive even to low light levels, but they cannot detect colours very well, which is why everything appears grey-ish in the dark. Cones are less sensitive but they work together to allow for colour vision. Our eyes contain three different types of cones which respond to different colours (wavelengths) of light, these are approximately Red, Green and Blue, both ends and the middle of the spectrum of colours. Our brain combines signals from all three types of cone to identify what colour an object is, much like mixing paints.

False coloured diagram of a rod cell and a cone cell, the cells in the eye that detect light. (source)

Very few animals have three cone types, as we do. Many animals including most fish (bony fish), reptiles and birds have four different types of cone (tetrachromatic), this does not necessarily mean that they can see more colours but they are better able to judge what colour something is, because they have more data. Many other mammals (cats, horses, cows etc.) have two cone types (dichromatic), whilst aquatic mammals (whales, dolphins, seals) are down to one cone. It is believed that early animals all had four cones but two were lost by mammals around the time of the dinosaurs when they were mostly surviving underground in low light conditions. Only relatively recently have our close relatives, the primates, re-evolved a third cone type.

Aquatic mammals provide an example of cheating at colour vision; rods respond more strongly to some colours than others, but are sensitive enough that they will respond to all colours. Certain dolphins, seals and sealions can compare signals between rods and cones to get some idea of colour. Also many colour blind animals can tell the difference between colours based on brightness e.g. yellow will appear as a lighter grey than red or black.

Electron microscope image of rods and cones (source)

 This experiment measured the strength of responses to different colours of light by individual cells in the eyes of 17 different shark species. They were only able to find evidence of cone cells in 7 of these species. All seven species with cones only had one type. Although it appears that the other 10 species may lack cone cells altogether it is possible that they are just very rare. The team was only certain that one species (the Port Jackson shark) had no cones. The cones observed in sharks have similar responses to light as our Green or Red cones, which are very similar.

The responses of rods and cones in the human eye to different colours (wavelengths) of light. Short wavelength cones (S) respond most to blue-ish light, whilst medium cones (M) respond to green-ish, and long cones red-ish. Rods (R) are in the middle of the range. (source)

 The study was unable to determine if sharks can manage rudimentary colour vision as seen in aquatic mammals or if sharks simply use differences in brightness. The part of the shark brain which processes vision is relatively large and complex, so it seems plausible that some interesting image processing is going on. The evolutionary history of shark vision is also poorly understood, sharks have been around since long before the dinosaurs and so are independent of the history of vision in other fish and mammals etc.

It was also found that rods and cones in different shark species were affected by their habitat and behaviour. Sharks living near the sea bed (Wobbegongs) are more sensitive to redder colours than sharks in open water. Also sharks which frequent shallow and/or freshwater are slightly more red sensitive than sharks in deep, salty water. Additionally, nocturnal sharks are more blue sensitive than other species, it is unclear why. None of this means that they can actually process colour just that different sharks will see the same colour as different brightnesses.

The ornate wobbegong (Orectolobus ornatus), is well camoflaged here. It is a bottom dwelling shark which has one type of cone cell which is more sensitive to red-ish light than cones in other species.

Even in land dwelling animals the Blue cone type, which is missing in sharks and other aquatic creatures, is very rare (between 4 and 10% of all cones) implying that it is of minimal importance on land and in the seas. Mysteriously, many species of rays and holocephalans/chimaeras, close relatives of sharks are believed to have two or three cone types. It is also worth remembering that sharks have a wide range of other senses available to them through the lateral line; detecting motion and electric fields in the water, which are much more effective for locating prey.

A chimaera from the species Hydrolagus colliei, a close relative of sharks. (source)

 This study is only small and limited to shark species around Australia, there may yet be a lot to learn from the shark’s eye. Are there any tricks which allow colour processing? Does a shark’s vision change as it matures (some species appear to have different light sensitivity as a juvenile in the shallows compared to an adult in open water)? How did the shark’s vision evolve relative to other animals?

This work has many applications. Knowing that brightness is likely to be more important than colour to sharks allows for the design of swimwear and fishing equipment that will be practically invisible to sharks; helping to protect humans from sharks, and sharks from humans. Much work still needs to be done to clarify why colour is not important to sharks and to understand the shark’s world view.

Might the way sharks see the world be related to the way sharks are seen in the world? The colour of an animal is often important in interactions with others of it’s kind, do sharks identify each other by pattern rather than colour? Is that why many sharks (although not all, see the wobbegong) are grey or is it just camouflage?

Hart NS, Theiss SM, Harahush BK, & Collin SP (2011). Microspectrophotometric evidence for cone monochromacy in sharks. Die Naturwissenschaften PMID: 21212930

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