Science Sunday: The Diversity of Eyes
Continuing from last week’s Science Sunday, we’re going to keep looking at some of the ways that species differ in interesting, and sometimes baffling ways. Today’s subject will be a bit less sexy, however, as we’re going to talk about eyes.
For us humans, the vision usually represents 80 percent of the sensory information that we perceive. As far as eyes go, ours aren’t that bad. Normal eyes have a good resolution and colour vision, even though aberrations like short-sightedness and astigmatism are quite common. As with other mammals, our eyes work by gathering light through a lens and forming an image at the back of the eye, called the retina. There are two types of cells on the retina which can register light, called rods and cones. Rods can’t distinguish colours, but they work well at low-level conditions. There are different types of cones which register different wavelengths of light, which give us colour vision. Humans have three types of cones, meaning that our brain gets colour information from three different sources. Most other mammals have only two different cones – as with people who are colour-blind – which in comparison gives a reduced ability to perceive different colours.
Birds have eyes with the same basic structure as the mammalian eye, but with some crucial adaptations for spotting prey from the air and ensuring safe flight. Their eyes are very large relative to their body size, to maximise the intake of light, which means that many birds can’t move the eyes in their sockets. Birds will instead move their heads around to get a full field of view; owls can for example swivel their heads around 270 degrees. Most birds have four cones on their retinas – pigeons even have five – giving them an enhanced colour perception. For some species, one of the cones absorb ultraviolet light, such that the birds can actually see light with UV wavelength.
Both the eyes of birds and mammals are called simple eyes, but not for their lack of complexity or acuity. Indeed, there are “simple” eyes spread through the animal kingdom that showcase true evolutionary ingenuity. Some copepods – a group of crustaceans – even have eyes with two or even three lenses.
Most arthropods have eyes which are quite different from ours, so-called compound eyes. These include among others insects, spiders and crustaceans. Compound eyes consist of many small units which can register light. They are placed on a convex surface, so that each unit is pointing in a slightly different direction, giving a very large view angle. This large view angle is evident for anyone who has ever tried to smack an annoying fly. Unless the movement is coming from directly behind the fly, it will usually have enough forewarning to fly away.
The prize for the currently most complex eyes in the animal kingdom probably goes to the mantis shrimp, coincidentally neither a mantis nor a shrimp. These are over 400 species of crustaceans which are most comfortable hiding under rocks at the bottom of the ocean, waiting for their prey. If you ever see a mantis shrimp. the most striking thing about them might be their eyes. They have two compound eyes mounted onto stalks on their heads, which can continually move and swing around. Each compound eye has about ten thousand units, with 16 different photoreceptors, making the mantis shrimp able to see UV light. In addition to that, each individual eye is divided into three segments, which means that each eye forms an image from three different angles. So, unlike mammals, the mantis shrimp only needs one eye to have depth perception. The complexity of their eyes has probably developed to give a benefit over their prey, or for seeing the fish that in turn hunt them.
Other fascinating compound eyes can be found on the trilobites. Trilobites were a class of sea-living arthropods that, sadly, died out 250 million years ago, after roaming the oceans for over 200 million years. Luckily, they left a big fossil record, which has shown that trilobites developed quite complex eyes very early. Also, the lenses on their eyes were made of calcite, which is a mineral. Now, you might think that having a mineral as a lens is a bad a idea, as they’re usually opaque, but pure calcite is transparent. These calcite lenses had another problem, as they were rigid, unlike our soft lenses which can change shape to focus on different distances. Many trilobites solved this by having eyes with a doublet structure, meaning that each lens had two parts that refracted the light differently, giving an optimised image.
There’s no arguing that being able to see is an advantage for animals, whether it’s for spotting prey, spotting predators before they see you, or just navigating the world. As with many other intricate problems, the question of how to see has been solved in many ways. Each different adaptation gives a view of the needs and trade-offs which helped form the resulting eye in each species.
There will be a few weeks before the next post in this series, and then we’re going to deal with locomotion.