In The Blink Of An Eye (26 page)

Notched seed-shrimp bioluminescence evolved independently of eyeless seed-shrimp bioluminescence. The bioluminescence of notched seed-shrimps originates from organs in their lips. Katsumi Abe, a Japanese biologist from Shizuoka University, was a serendipitous
explorer of notched seed-shrimp bioluminescence. He concluded that the chemicals responsible for this light evolved from digestive enzymes. This makes sense - the bioluminescent chemicals do share exit valves with digestive enzymes. And this could be a significant finding since the foundation of bioluminescent chemicals is a contentious issue of evolution. Sadly, Katsumi Abe died before the full extent of his research, or his thoughts, could be known. Fortunately his students, and his colleague Jean Vannier from the University of Claude Bernard in Lyons, France, are continuing along Katsumi's path.

The independent origin of notched seed-shrimp bioluminescence is echoed in its rudimentary function. Like the eyeless seed-shrimps that luminesce from their shells, the Japanese notched seed-shrimps also employ their light to counter predators. But the notched seed-shrimps endeavour to confuse rather than deter their predators. When a fish gets too close for comfort, and the seed-shrimp assumes it has been spotted, a blinding flash is created. The intense light briefly stuns the fish (just as we are often momentarily blinded by a glimpse of the sun), giving the seed-shrimp an opportunity to run for its life. And like a magician's assistant, when the smokescreen has disappeared, so has the seed-shrimp. That this feat is performed at all means it must be effective, because disadvantages are inherent in this strategy. A flash of light may curb the aggression of a prospective predator, but will also attract the attention of a more distant enemy - a flashing light is more conspicuous than a steady one. This antipredator response does appear to have been the original purpose for notched seed-shrimp bioluminescence, when it first evolved. But it has also provided a base for an evolutionary campaign on the notched seed-shrimps in the Caribbean.

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 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 evolving 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.

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.