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.
5
Light, Time and Evolution
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 receiv
ed. 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.
The seed-shrimps that happened to like frozen pilchards and wandered into the traps were the lightweights, the group that had left little behind in the fossil record. And in particular it was just one family of lightweights - Cypridinidae, seed-shrimps that generally have a small but well-defined notch at the front of their shells. I will refer to the Cypridinidae as the ‘notched’ group. Notched seed-shrimps are usually the size and shape of tomato seeds, and typically spend much of their time buried in the sand on the sea floor. The tomato seed lookalikes occurred in the traps set in shallower waters. They were common also in traps set in deeper waters, but at depths of 200 and 300 metres they were occasionally accompanied by an oddball among notched seed-shrimps - the ‘baked bean’.
Figure 5.1 A notched lightweight seed-shrimp with one half of its shell removed to reveal its body and limbs inside (from Cannon, 1933, Discovery Reports). The arrow points to the halophores of the left first antenna.
The very first trap set in a depth of 200 metres off the coast of Sydney was hauled up and opened on board the fishing vessel hired for the job. The sight was as amusing as it was unnatural - the trap was full of what appeared to be baked beans. ‘Baked bean’ is the official nickname given by local fishermen to ‘giant’ orange/red seed-shrimps called Azygocypridina . Sometimes they are brought up in the catches of fishermen, who have no idea what they are dealing with, merely that they ‘don’t make good eating’. Baked beans appear to be confined to the edges of the continental shelf. Like real baked beans, these seed-shrimps are about a centimetre long, oval and slightly flattened from side to side. They are orange/red because at a depth of 200 metres and beyond sunlight is almost exclusively blue. You can be sure, at least, that at such depths orange and red will be absent from the sun’s spectrum. And with nothing left to light up the baked beans, they appear to be invisible. In a totally dark room, for instance, an orange cannot be found with a blue torch. But there is a difference in the appearance of the deep-water seed-shrimps and baked beans - the seed-shrimps possess their characteristic notch at one end. And so did the Scottish fossils.
Living fossils
Every once in a while a ‘living fossil’ is discovered somewhere on Earth. Living fossils are species alive today that closely resemble forms otherwise found only as fossils, species that lived during ancient times. The nautilus could be considered a living fossil because it shares its looks, behaviour and, more importantly, its place in the evolutionary tree with the extinct nautiloids and ammonoids. But the nautilus lives on.
One of the most recent living fossils to emerge is the Wollemi pine. Fossils of this type of conifer were once known only from rocks containing dinosaur bones, and it was thought to have been extinct for millions of years. It was an important subject in palaeontology. Then all the hard work put into extracting and extrapolating the fine details of its anatomy to bring it to virtual life was undone by a single event - the discovery of a living specimen. Virtual life was suddenly replaced by real life - an occupational hazard for palaeo-artists (although a rare one).
In a remote part of New South Wales, Australia, forty adult Wollemi pines were found very much alive in a deep, sheltered gorge. The gorge supported a warm, temperate rainforest. It may seem amazing that the massive pines had not been discovered before, but much of inland Australia is actually unknown to science. There is considerable biology to be done in Australia. Spiders, for instance, are among the more populous animals on Earth, yet around two-thirds of Australia’s spider species probably remain undiscovered and unnamed. So it is not so surprising that if a new species of tree is discovered today, it will turn up in Australia. Of course, as the world’s rarest tree, the Wollemi pine must be closely monitored and protected, so much so that its precise location has been kept a secret. Even cultured specimens growing in Australian botanical gardens are kept under lock and key, and beyond the reach of the horticultural black market.
There is an obvious link between the Wollemi pine and the SEAS project. In the deeper localities, hagfish were caught in the large scavenger traps. Protected within a scabbard of slime, hagfish appear like eels. They have primitive mouthparts, and indeed are today’s representatives of a primitive form of fish. Their mouthparts are quite an issue because hagfish have a strong fossil record, dating back some 500 million years, but the fossils show no sign of jaws. And the living hagfish confirm this - they really are jawless. The jaw is a feature of more derived forms of fishes - sharks and bony fish. But hagfish are scavengers, and can really get by without a jaw in certain environments, those in which they are preserved today. The relevance of the Wollemi pine and hagfish to this chapter is that the baked bean is a living fossil too. Although less apparent to begin with, morphometric analyses were employed to expose this truth.
Baked beans showed similarities in form to the 350-million-year-old, oval fossils discovered by David Siveter, who by this time had classified them as lightweight seed-shrimps. Morphometrics can give mathematical values to shapes. A morphometric value, in the form of relative coordinates on a grid,
was given to the Siveter fossils, and it seemed appropriate to put the baked beans to a similar test. The match was perfect. David Siveter was right - he had found lightweight seed-shrimps that lived 350 million years ago. To be more specific, these fossils belonged to the notched group of lightweight seed-shrimps. Before long, David Siveter and his team discovered further fossil lightweight seed-shrimps, now that they knew what to look for.
Different forms of lightweight seed-shrimps were uncovered from older rocks, but here the baked bean forms were absent. The lightweight group as a whole could be dated back 500 million years to just after the Cambrian. But it seemed that the baked bean form, and the notched seed-shrimps in general, evolved about 350 million years ago. Now, for the first time, we were beginning to trace the geological history of the much forgotten lightweight seed-shrimps. But it was useful, also, to have a date on the evolution of baked beans for another reason.
Diffraction gratings - a subject of physics
The study of structural colours in animals has a long and distinguished history. Robert Hooke possibly pioneered the subject in the seventeenth century with his interpretation of the metallic appearance of silverfish insects, just pipping Newton to the post. And ever since, this subject has been famously represented, up until the work of Sir Andrew Huxley, Sir Eric Denton, Michael Land and Peter Herring in the latter half of the twentieth century. Consequently, animal forms of multilayer reflectors and structures that cause the scattering of sunlight, with all their variations, had been well documented and interpreted by biologists. But these were also subjects of optical physics. Physicists have been experimenting with optical materials for centuries, and had converged on the same structures that occurred in nature. Yet the two fields of biology and optical physics never seriously crossed paths.
In The Blink Of An Eye Page 17
