There are other researchers investigating notched seed-shrimp bioluminescence in the USA. In the early 1980s, Jim Morin, then of the University of California, Los Angeles, went in search of bioluminescence on the reefs of the Caribbean Sea. What he found was unexpected. There were the usual starfish and worms glowing as they roamed sloth-like over the seafloor. But in the open sea above them were luminous flashes that rivalled those of fireflies on land for their spectacular exhibitions, appearing like a firework display. The fireflies of the sea were in fact notched seed-shrimps. Later, Jim Morin was joined by Anne Cohen of the Los Angeles County Museum of Natural History, who had been rearing notched seed-shrimps in her lab. Considerable documentation and analysis of the Caribbean bioluminescence followed.
It became evident that different patterns of flashes were being produced in the Caribbean waters. Soon after sunset, blue lights would be flashed in the water column, one swiftly following another, to create specific patterns like constellations in the sky. About fifty different patterns were identified in total. A sequence of about ten flashes would take a few seconds to complete, and the eye would always be drawn in the direction of the pattern. Sometimes the flashes would move vertically upwards in the water, sometimes directly downwards. Some flashes would move horizontally, others at an angle, while sometimes single flashes would be replaced by groups of flashes, all moving in unison to create a new pattern. Within these sequences, individual flashes could be evenly spaced or become increasingly closer to their neighbours. All quite spectacular.
The notched seed-shrimp maestros were captured in nets in mid-performance. They were all males, but were being tailed by female notched seed-shrimps. The Caribbean males would emerge from the sand, swim into the open water and flash their lights. These seductive dances would catch the eyes of females, luring them too into the water column. From here on they would be uncontrollably attracted towards the males, and presumably all would be in the mood for mating. Although mating could not be observed with the low magnification cameras employed underwater, evidence was found to suggest that these flash patterns really were courtship rituals, like the iridescent display of the Skogsbergia species in Australia.
The males producing the horizontal pattern, and the females attracted by this pattern, all belonged to the same species. Similarly, the males and females associated with the angled pattern all belonged to the same species, a different one from that of horizontal persuasion. And so the story continued, until some fifty different species were found to match around fifty different patterns. In the Caribbean, it seemed that notched seed-shrimps had evolved a nice strategy for mate recognition and courtship - it had to be really efficient to outweigh the disadvantages inherent in making oneself so conspicuous to predators. This was a strategy where many species could be packed into a restricted environment and still easily recognise and mate with their own kind. Mistakes, potentially as costly to a species’ hopes of survival in the long term as they are embarrassing in the short term, were minimised. This brings us to the subject of evolution.
Lou Kornicker of the Smithsonian Institution in Washington, DC, had produced taxonomic publications the size of telephone directories on lightweight seed-shrimps, including notched seed-shrimps. His work provided a reliable database of body parts and the variety of forms of notched seed-shrimps. And an evolutionary tree was inferred at last.
The global view - evolution of all notched seed-shrimps
The evolution of the Caribbean species was analysed in further detail. It emerged that similar looking flash patterns of bioluminescence belonged to species that were closely related. So the evolution of flash patterns was not haphazard, but rather orderly, in a stepwise manner. A disordered evolution would have implied the patterns were adaptive: adapted to the specific environment of a species. But a gradual evolution inferred the flash patterns were evolving in synchronisation with the species themselves. So what can be learnt from all of this? Before advancing further with this line of thought, notched seed-shrimp iridescence should be reconsidered.
The evolutionary tree of notched seed-shrimps revealed a trend - bioluminescence appeared only and always in one half of the tree. All bioluminescent species were related - bioluminescence had evolved just once in notched seed-shrimps, and was retained in all descendants of the forebear. At another level, the bioluminescent half of the tree could be further divided into those species that produced patterns of flashes, and those that flashed only to avoid predation. At the beginning of the complete tree stood the baked bean, and bioluminescence evolved a few branches later. A broader investigation of diffraction gratings revealed that the bioluminescent flashing species all possessed fairly similar and rather rudimentary halophores like those of the baked bean. So halophores, and consequently iridescence, had not been evolving within the bioluminescent flashing branches of the tree. Meanwhile, the remainder of the notched seed-shrimp tree of life was telling a different story.
We have learnt that the diffraction gratings of notched seed-shrimps can be ordered into a neat sequence. This sequence becomes increasingly clear when bioluminescent species are disregarded - the bioluminescence species were clumped together at the start of the sequence. It so happens that the order of species within the sequence of iridescence matches precisely the order of the species inferred from the evolutionary tree, from those that derived earliest from the seed-shrimp ancestors, to the most recently derived. So the members of the non-bioluminescent half of the evolutionary tree have been gaining increasingly efficient diffraction gratings and, consequently, light displays. At the very top of this iridescent half of the tree was Skogsbergia, the movie star.
Considering that bioluminescent flash patterns and iridescent displays are employed for mating purposes, they surely now have implications for evolution. If genetic mutations occur when an individual is conceived, the diffraction gratings of an offspring may be different from those of its parents. If the mutation is somehow advantageous, such as being a more efficient signal for mating, it can be retained within the future evolutionary line. A more efficient signal for mating, in the case of the notched seed-shrimps, would be a more complex pattern of bioluminescent light or a brighter, or bluer, iridescence. Blue light travels best or furthest through sea water, with green not far behind. If the new design of signal mutates further throughout the future evolutionary line, the signal of the future can become unrecognisable from the original, ancestral signal. Eventually a point is reached where the ancestral forms, which have continued to reproduce without signal mutation, can no longer recognise the ‘future’ signal. Considering we are talking about a code for courtship, the ancestral forms can no longer mate with the contemporary signallers. A new species has evolved. The new species would appear as the most derived on the evolutionary tree, at the tip of the branches.
An analogy to this story could be found among human beings. Humans adorn themselves with clothes, scent, jewellery or body art to attract the opposite sex. Different races of humans decorate themselves to different extremes, so much so that a female of one race will not necessarily attract the male of another, or vice versa. Consider those Amazonian men with plates inserted in their lower lips. European races, for instance, probably would not find this particularly alluring, and so Europeans and Amazonians do not interbreed. This keeps the races separate, and thus is analogous to the different species of seed-shrimps in our story. But imagine a new trend emerging in the Amazon where, in one village, it was no longer considered attractive to possess a plate in one’s lip, but rather a tattoo on one’s face. Before long a new race would have emerged following the incompatibility of plate-bearing and tattoo-wearing individuals, based on courtship display. The two races in the Amazon are now as divorced from each other as they are from Europeans, although still more closely related to each other on the tree of races. It should be made clear that this is not a case of evolution, but human invention. Returning to evolution, the notched seed-shrimp story can continue from here, with new species ev
olving that bear more attractive or flamboyant costumes.
The point of this whole story, and this chapter so far, is that notched seed-shrimps appear to have been evolving to become increasingly well adapted to light. The very first notched seed-shrimps of 350 million years ago are represented today by the living fossil, the baked bean, with its primitive form of diffraction gratings. In subsequent evolution, light became something to which the notched seed-shrimps adapted strongly. Light has imposed a momentous selection pressure throughout their evolution. In fact their adaptation to light may even explain the evolution of the notched seed-shrimps, via the changes that took place in their courtship displays. This is nice to know. It is one thing to determine an evolutionary tree, but something else altogether to explain it. Here we can explain why different species of notched seed-shrimps were evolving. But the important message for this book is that light can have a powerful influence on evolution. And this does not apply only to notched seed-shrimps, as will be demonstrated after an epilogue to the seed-shrimp.
The strong adaptation to light has been a hugely successful strategy for the notched seed-shrimps. The fossil record suggests that 350 million years ago notched seed-shrimps were rare. The SEAS project revealed that today they are the commonest multicelled animal group on the Australian continental shelf at least. An evolutionary success story for these seed-shrimps, with a happy ending . . . so far anyway. Evolution continues.
Natural diffraction gratings
Another important conclusion to emerge from this study of seed-shrimps was that diffraction gratings do exist in nature. This finding itself became the foundation for another project - to unearth any other diffraction gratings that lay hidden within the animal kingdom. Confidence in a positive result was high because now it was known what to look for. And indeed further cases emerged. But more unexpectedly, another link between diffractive structures and evolution appeared, in the case of the upside-down fly.
Diffraction gratings were found within a range of invertebrate animals, from the hairs of worms to the wings of flies. In fact the bristle worms are particularly well endowed with diffraction gratings, and reveal a variety of diffractive forms. This finding will become important later in the following chapter.
In addition to strict diffraction gratings, similar structures were discovered which also cause sunlight to diffract, but this time the light reflected would appear a metallic white, or silver, in colour. This resulted from diffraction gratings running in a variety of directions, where their reflected spectra would overlap. Sunlight would be split into its spectrum, which would then be reconstructed. This was comparable to Newton’s famous ‘two-prism’ experiment, where one prism cleaved sunlight into its component colours, while another was positioned to recombine the colours. After passing through the two prisms, normal sunlight resumed. The mechanism of reflection in the newly discovered diffractive structures was essentially the same as for scattering, where microscopic particles reflect all wavelengths in sunlight equally in all directions. The fibres in the paper of this book perform this task. There was, however, an angular attribute to the newly discovered structures - white light could be reflected in just one direction. This equated to a very strong reflection if one happened to be looking from this direction. And the most impressive effect of all belonged to the upside-down flies.
Figure 5.5 Electron micrograph of a hair from Lobochesis longiseta, a bristle worm. The ridges are spaced about one micron apart, forming a diffraction grating that causes a spectral effect.
Australia’s upside-down flies
The upside-down flies fall within a group of small flies described by David McAlpine of the Australian Museum. David McAlpine also noticed similarities between and peculiarities in the behaviour of species from this group. Some plants have long, vertically upright leaves. These leaves provide a home for the ‘McAlpine flies’, which often rest on the leaves in a group, bodies oriented vertically. The gathering of the flies is an act of safety - there is, after all, much to be said for safety in numbers. Also the possibility of reproduction is enhanced if potential mates are close by and easy to find.
The flies involved in this story belong to many species, and collectively live in Africa, Madagascar, South-East Asia and Australia; the ancestral species became divided as the continents separated millions of years ago. In fact the ancestral fly is known: one specimen has been found preserved in amber in wonderful condition.
I borrowed the amber specimen from a museum in Göttingen, Germany. The amber has been fashioned into a neat block, about a centimetre square and a few millimetres deep, and is mounted on a glass microscope slide. Inside the amber are two flies, one the size of a large mosquito with big, perfectly preserved eyes, and a smaller specimen, which is the one of interest here. The smaller fly was described by the German biologist Willi Hennig, a man rather more famous for his development of a phylogenetic method - the main tool used to study evolution today. Unfortunately, the fly is orientated in a most inconvenient way. Along with inconsistencies in the amber, it can be seen only in a limited and distorted view, so it is not easy to say whether or not this ancestral specimen possesses reflective patches. To make matters worse, amber would affect the optical properties of many reflector types, such as diffraction gratings. We would need to see this fly in air, not in amber. And the rarity of this specimen has resulted in a ban on any potentially destructive handling, so an informative dissection is out of the question.
The living relatives of this amber specimen, however, do possess light reflectors - diffractive structures based on a system of hairs that appear silver. The hairs come in different shapes and sizes, and can be aligned differently, although always spaced evenly. The specific, microscopic characters determine the optical properties of the complete reflector, which vary from species to species. And a pattern emerged from a study of this variation.
The fly in amber evolved along two separate paths. Like the history of notched seed-shrimps, the evolutionary tree originating from the fly in amber can be divided into two halves. On one side we have the right-way-up flies, and on the other side the upside-downs. But all have one thing in common - they reflect silver light upwards, towards the sky. This reflection probably acts as a signpost to other flies in the vicinity, to invite them to a gathering.
The right-way-up flies, orientated vertically on their host leaves, go about their business with heads facing the sky. They live in South-East Asia and Australia. Those (‘primitive’) species of right-way-up flies with the oldest ancestors possess very inefficient reflectors, positioned between the eyes so that sunlight can be reflected back towards the sky. This reflective patch must have proved rather useful to the fly. It was not only passed on to the next species in the evolutionary line, but it was also improved upon. The physics of the reflector became more efficient, and, consequently, its visual effect became more striking. This trend can be traced through the entire evolutionary line of the right-way-up flies. The next species to evolve not only improved upon the physics of the reflector again, it also sprouted more reflectors over its body. The additional reflectors appeared only on other sky-facing parts of the fly, such as the front parts of the first pair of legs. A greatly speeded up film of evolution through geological time would reveal reflectors blooming from increasingly more parts of the upward-facing body. Furthermore, the reflections would appear increasingly brighter as the optical properties kept on improving via evolution. And exactly the same was happening, independently, in the other half of the evolutionary tree.
The upside-down-flies live in Africa, Madagascar and Australia. They are so called because a strange thing happened at the beginning of their history. As the ancestor represented in amber evolved in this half of the evolutionary tree, it turned upside-down. Still living on vertical leaves, and still with its body orientated vertically, it turned through 180° - and never looked back. The upside-down flies all face the ground, so that their rear ends point towards the sky. This could be explained by their occ
upation of slightly different plant species - plants with predatory spiders lurking near their leaf bases. So an upside-down fly could keep a lookout for dangers from below. The upside-down flies continued to aggregate, and probably also employed reflectors to call to their friends. So how could they signal towards the sky when they are facing downwards? They simply ‘moved’ their reflectors so that they faced the other way.
The upside-down flies have reflectors on the backward-facing parts of their bodies. And they evolved almost in tandem with their right-way-up counterparts. Again the reflective patches increased both in abundance and efficiency throughout the evolution of the upside-down group. There were, however, differences in the designs of reflectors between the right-way-up and upside-down flies. In fact it is the most recently evolved upside-down fly that owns the most efficient reflector. This champion upside-down fly lives, of course, in Australia, and from rudimentary beginnings it has evolved a type of reflector never before seen in the world of optics, let alone biology. This could even have been applied to human optical devices. But it is the evolutionary tale that is relevant to this book.
In The Blink Of An Eye Page 19
