In The Blink Of An Eye (23 page)

In the previous chapter we saw how the angelfish employs its silvery surface for reflecting light at its opponents, in the style of
Star Wars.
But there is another, more widespread function for the silver colour of fishes - to make them disappear.

In near-surface waters, such as the angelfish's Amazonian habitat, sunlight exists in the form of a beam like a spotlight, as it does on entry through the Earth's atmosphere. But below these waters the beam formation is broken, and sunlight is scattered in every direction. So here objects are illuminated equally from all directions, and no shadows are cast. A mirror in these waters vanishes from sight because in the mirror one sees only a weak reflection of the environment. The mirror becomes an optical illusion - in the direction of the mirror there appears to be only the background environment, with nothing in the way. In the ocean a silver fish is effectively a mirror. A predator looking directly at a silver-sided, or mirrored fish from below sees only a reflection of the surface. So in the direction of the fish there is . . . no fish! But how can a fish's skin act as a mirror? After all, it contains no metal. There is another way of strongly reflecting
all
the colours in sunlight into a beam so that it appears as a very bright white, which we know as silver. We turn to structural colours.

In Chapter 3 we learnt that a thin film causes colour - structural colour. Also, a stack of thin films was found to provide a relatively brighter colour, by reflecting a greater proportion of sunlight. But the reflector caused strong coloured effects rather than white because the thin films were all of the same thickness, and this thickness determined the wavelength, or colour reflected.

Now imagine a stack of thin films of different thicknesses. Imagine some that reflect blue light above others, that reflect green above yet others, that in turn reflect red. As sunlight strikes this structure, its blue rays would be reflected from the top layers, leaving the green and red rays to continue along their original path. As these rays meet with the middle layers, the green rays are reflected, leaving only red rays to continue along their path. And finally the red rays meet with the lower layers and they too are reflected. So the combined effect of all the layers is the reflection of blue, green and red rays in the same direction. And blue, green and red combine to form white, or silver (silver is a strongly directional form of white). With more layers of different thicknesses, more colours in the spectrum can be reflected. And this is how
the fish skin appears silver - it contains a stack of layers of varying thickness.

In the Sierra Madre Oriental mountain range of eastern Mexico lives
Astyanax mexicanus
- a fish some 5 centimetres long commonly kept as pets in domestic aquaria. It is related to South American piranhas. In open waters this fish has average sized eyes, for a fish, and a silver body to provide effective camouflage. I will refer to this form of the species as the eyed cave fish. Its eyes and silver colouration are obvious adaptations to light. The same species of fish also inhabits the extensive cave systems of Mexico, but as a different form . . . or rather forms.

As the eyed cave fish moved deeper into the cave system through geological time, the selective pressures to be adapted to light vanished. And as they did so, the structures and chemicals - the hardware and software - of the animal responded. The eye began to degenerate. The longer the cave fish spent in darkness, equating to the further into the darkness the fish ventured, the more the eye degenerated. The evolutionary machinery had not stood still, but it had engaged reverse gear. ‘Regressive evolution' was the trend as far as light was concerned. The adaptation to light outside the cave had resulted in some expensive hardware and software. Within the cave, the energy of the fish could be put to better use. The visual machinery, which had become obsolete, had to be dismantled. And it was not only the eye that regressed - the silver colouration was affected, too.

At Oxford University, Victoria Welch studied cave fish from within the vast Mexican cave system. She noticed that the fish were becoming less silvery as their habitat moved deeper into the caves. And as the silver disappeared, so their skin became a translucent white colour, with the red of their blood vessels creating an overall pink effect. But the transition from silver to pink was a gradual one, with intermediate forms appearing as an unbalanced collage of both states. This, however, was not the only pattern to emerge.

The eye was absent from all forms of cave fish living in the dark caves. It has undergone regressive evolution rapidly - the eye is a
very
expensive piece of equipment, and one that must be relinquished the moment it becomes obsolete. But the silver colouration turned out to be
a little cheaper in terms of energy investment. In fact the silver colouration may also have been influenced by ‘genetic drift' - mutations that just happen under neutral selective pressures.

Cave fish populations found deeper within the caves had been living in the dark for longer, in geological time, than those populations living nearer the cave entrance, albeit still in complete darkness. And since it took longer for the silver colouration to regress compared with the eye, the cave fish near the entrance of the caves were more silvery than those in the deepest parts of the caves. In fact the fish furthest inside the caves were completely pink.

Victoria questioned what was happening in the skin of these cave fish. How was the silver reflector being effected? She took samples of skin from fish at different depths within the cave . . . and found the cause of the silver decline. Evolution was observed mid-action.

In an electron microscope, the individual thin films, or layers of the silver reflector, can be observed. The eyed cave fish possessed very ordered stacks of layers, which increased gradually in thickness from the blue to the red reflectors. In those fish living near the entrance of the cave, but in the dark, signs of disorder began to show. The layers were beginning to separate, split apart and even become fewer in number. As the fish found from deeper within the cave were examined, these signs of disorder became more pronounced, and the total number of layers gradually reduced. The layers also began to buckle and became randomly distributed within the skin, and the skin became less silver. Eventually, in the fish from the very depths of the cave, the layers had vanished completely from the skin. There was no longer any reflector.

This was a nice find - the different stages of regressive evolution could be observed happening through time. If a silver reflector became obsolete within a sunlit environment, this event would be rapid and impossible to track. The cave finding may also indicate how silver reflectors
evolve
in the first place, possibly by reversing the procedure. But the real moral of this story, for the purposes of this book, is once again that evolution may take place slowly in an environment without light. Indeed, the cave fish had not evolved sufficiently to form a new species during its long history of entering very different environments - all without light.

The lack of light in caves resulted in reduced environmental partitioning into microenvironments - quite the opposite to the case of the West Indian
Anolis
lizards. Consequently, the island-type evolution that is encouraged by microenvironments was absent. The outcome was a lesser variety of species although still a considerable number of individuals in caves. The question I will pose later in this book is: ‘Was the Precambrian environment similar to the modern cave environment? ' We can start to think about this question here, making the clues for solving the Cambrian enigma to be found in the following four chapters appear all the more relevant.

Other experiments have been conducted to show that animals inhabiting dark caves are completely unaffected when light is shone into their surroundings. So they really have become visually neutral. In fact a number of cave animals have been found in illuminated habitats where
no competitors from the surface had access
. They are
never
found in similar habitats that do contain competitors or predators adapted to light, because if they stray into these environments they do not survive for long.

In Chapter 3 I mentioned that many deep-sea animals are red coloured, and that this was an adaptation to light. There is one shrimp that exists either within or at the entrance of deep marine caves. It changes colour from a pigmentless white to red, as it moves from within the cave to the caves' entrance, where light exists. The adaptation to light is significant
everywhere
. Also in Chapter 3, we compared (as we have to some extent in this chapter) the senses of smell and taste, hearing and touch with vision. It was concluded that vision is different because its stimulus, light, was always present in the environment. Every animal in that environment is affected by light. In caves these other senses are extremely well developed, yet evolution labours in first gear. Animals are to some extent in control of how much sound and scent is injected into the environment, but in a sunlit environment the light levels are pre-set.

Darkness is the most obvious characteristic in the caves considered in this chapter. It acts directly on animals by placing blind species at no disadvantage to others. But it also has an indirect action - it excludes photosynthetic organisms, thereby reducing the amount of locally
produced food to zero. This nutritional poverty will affect the cave food web, but it should not affect biodiversity, or the evolution of species, as much as the number of individuals, or density of life. And it is the evolution of species that is most relevant to this book. Indeed, most cave predators have adapted to go without a meal for weeks, even months.

Despite the fact that cave environments are remarkably stable, lacking extremes of anything, and that senses other than vision are remarkably well developed in the dark, diversity in caves is low. Evolution is slow. And this can be attributed to the lack of light to fuel both photosynthetic organisms and vision. Often in this book I have referred to ‘light'
and
‘vision'. Soon I will discriminate judiciously between the two. Light has existed on Earth from its very beginnings. Vision is an adaptation to light. It has not always existed. This is worth thinking about.

Vision will be dealt with exclusively in Chapter 7, but first we will move out of reverse and examine what happens as the
forward
visual gears are engaged in the evolutionary machine, in the case of the luminous seed-shrimps.

Life abounds with little round things

LEWIS THOMAS

 

 

 

Ostracod crustaceans, or seed-shrimps, have travelled through time well. They are abundant today and were equally common throughout the past, right back into the Cambrian period. They are found in all types of water worldwide, and their poor public exposure is not reflected by the extent of scientific attention they have received. Around 40,000 species of seed-shrimp have been described - rather significant, considering we know of only about 8,700 species of birds and 4,100 species of mammals (although this is more in line with some of the other highly diverse invertebrate groups). But when the name ‘seedshrimp' is spoken, the conversation generally refers to just one group of seed-shrimps - Podocopa, species with generally thick, robust shells. I will refer to Podocopa as the ‘heavyweight' group. The bias towards heavyweights has been generated by palaeontologists - heavyweights can be used to indicate the presence or absence of oil reserves - but in this chapter the other side of the story will be heard. It is another group of seed-shrimps that will contribute to the Cambrian enigma. They will introduce the subject of colour to that of animal evolution - a relationship which will be seen to flourish as this book progresses.

Seed-shrimps, like scallops, possess a two-part shell that can enclose the entire body, although typically the shells of heavyweight
seed-shrimps are only a millimetre long. Heavyweights owe their popularity to their shells - the shell chemicals are fossilisation friendly. Consequently they have left an extensive fossil record. Palaeontologists have kept a good eye on the movements and activities of the heavyweights throughout geological time, spurred on by a dangling carrot. There
is
‘gold' at the end of this palaeontological rainbow. Heavyweight seed-shrimps are well-known indicators of oil reserves, and until the recent introduction of more sophisticated oil detection methods, the laboratories of oil companies bulged with heavyweight seed-shrimp specialists. There exists, however, another group of seed-shrimps - Myodocopa, species with generally less robust shells. I will refer to the Myodocopa as the ‘lightweight' group. Lightweight seed-shrimps have a different form of the chemical that constitutes their shells, and this form does not usually give rise to fossils. So for some time we were unsure about the historical whereabouts of the lightweights.

In the early 1980s, David Siveter, a palaeontologist from Leicester University in England and part of Chapter 2's 3D fossil reconstruction team, fractured a rock he had collected from Scotland. The rock was around 350 million years old. Inside it were fossils, oval in shape with a tiny notch at one end, and totalling around 5 to 10 millimetres in length. Could these be seed-shrimps? The shape suggested yes, possibly, but the size no. Not all groups of living seed-shrimps, however, were well understood. And before comparing the Scottish fossils with living species, we need to know exactly what is out there in the water today.

The SEAS expedition did achieve its target - representatives of the scavenging amphipods and isopods were collected successfully. The 'pods had been gathered. But, surprisingly, they were not the most abundant groups of scavenging crustaceans. Another group of crustaceans emerged as the scavenger supremo of eastern Australia - the 'cods. Ostracods - seed-shrimps. This situation was highly irregular - seed-shrimps were not thought to hold a position of any note in the hierarchy of the world's scavengers.