Despite numerous studies on animals known to show metallic-like reflections, such as many beetles, butterflies, fishes and hummingbirds, there remained physical, optical structures that were known to physicists but not to biologists. Prisms, for instance, had never been found as light reflectors in animals. Perhaps their precise shapes or copious volumes made prisms an evolutionary impracticality. ‘Prisms’ can be found occurring naturally, nonetheless, in raindrops that refract and reflect sunlight to create a rainbow.
In 1818, another type of physical structure with reflective properties was invented in a physics laboratory - the diffraction grating. Fine copper wire was wound tightly around a screw, and the acutely grooved surface created by the wire caused sunlight to be split into its component colours: a spectrum was reflected. A different colour could be seen from different directions. Diffraction gratings could be considered as tiny corrugated sheets, where the spacing of the grooves are fairly constant and approximate to the wavelength of light. At their most efficient they are microscopic. Diffraction gratings became major players in the scientific and commercial worlds of optics, and have become refined and varied to produce an array of optical effects. They are responsible for the metallic-like, coloured holograms found on credit cards or foil-type wrapping paper, and now they are also being used on stamps and bank-notes since they are difficult to forge. But they were unknown in nature and the subject of animal structural colours until 1993.
Figure 5.2 A diffraction grating splitting white light into a spectrum.
A sudden flash of green light
My role in the SEAS project was to describe the new species of seed-shrimps collected. Sixty unknown species of notched seed-shrimps emerged from an area where only a couple of species were thought to exist in total. But it was not simply their diversity that made the notched seed-shrimps such important scavengers; it was their abundance. A single trap, basically a foot-long section of drainpipe baited with a couple of dead pilchards, would attract up to 150,000 individuals. Considering the short distances seed-shrimps are prepared to travel for food, the SEAS findings indicated that notched seed-shrimps were probably the commonest multicelled animals on the Australian continental shelf. Yet until then they were virtually unknown. This typifies how little we know of the smaller, but probably more common, life forms on Earth. But thanks to the SEAS project, the secret of the notched seed-shrimps had been revealed. Well, at least the secret of their Australian affluence. A further secret lay waiting to be discovered, one that could only be revealed using microscopes.
To examine the body parts of preserved seed-shrimps, their shells must be removed. This operation involves manipulating a specimen under a microscope and attempting to sever the muscles that hold the shell closed. The tiny size of most seed-shrimps makes this job difficult, and often several attempts are needed. The seed-shrimps tend to roll around and fall in exactly the positions that are not required of them. One exceptionally long day in the Australian Museum I had been battling with seed-shrimps for this very reason. It was time to go home but I was delayed. Then something happened that would change the course of my research - I saw a flash of light.
As one preserved seed-shrimp rolled over in the glass dish under my microscope, it caught the microscope’s light and sent an extremely brief blaze of green light towards my eyes. Unsure of what it was, or indeed if I was seeing things, I rolled the specimen over again, in an attempt to repeat the performance. Once again it shimmered with green light. Holding it in the appropriate position, the green reflection shone continuously. The shell of the animal appeared rather dull and its background was decidedly black, but the green light was blazing like a neon sign in the night. I asked my nearest companions, the amphipod specialists Jim Lowry and Helen Stoddart, to double-check that this was really happening. It was, but it shouldn’t have been. There was a big literature on seed-shrimps, and green flashes were not part of it.
The green part of the seed-shrimp belonged to its first pair of antennae. These antennae are equipped with long hairs, and each long hair is the bearer of smaller hairs, called halophores. Halophores are flexible because they are made of minute rings, stacked side by side. They are held together by a thin, elastic outer skin. But like the fine wire wrapped around a screw, they cause tiny ridges and grooves to appear on the outside of the halophores. The light microscope indicated that the green flash came precisely from the halophores. The electron microscope revealed the spacing of the very regular grooves - it approximated the wavelength of light. The surface of a halophore was a diffraction grating. Again, it shouldn’t have been. There was also a considerable literature on structural colours in animals, and diffraction gratings, like seed-shrimps themselves, were absent from it.
Figure 5.3 Scanning electron micrograph of a diffraction grating of the ‘baked bean’ (Azygocypridina lowryi). Spacing between grooves is 0.6 microns. (Plate 15 in the colour section shows the iridescent effect of this structure.)
Next, the halophores from a range of notched seed-shrimp species were examined. All possessed the iridescent character but to varying degrees. Some species reflected a spectrum of colours, others just green, just blue, or just blue and green. The electron microscope provided the source of these variations - the diffraction gratings were different. This was becoming interesting, but before investing further time and money in notched seed-shrimp iridescence, a great barrier had to be crossed. The question hanging over this work was, ‘Does iridescence play a role in the lives of seed-shrimps?’ This question was fundamental. If the answer was ‘no’, it was time to forget that the original green flash had ever happened. A colour that has no function must be purely incidental (I say incidental rather than accidental because everything that has evolved, even those things with a function, are accidental). And an incidental colour has no place in the literature of either seed-shrimps or animal structural colours. But if the answer to this big question was ‘yes’, it would be time to call in the optical physicists. So how does one find the answer to such a question, especially since there is so little background information on notched seed-shrimp behaviour from which we can start? Well, sometimes one needs some luck.
The feeding mechanism of notched seed-shrimps was unknown, but the SEAS project had elevated feeding to the top of the ‘things to study’ list. To be entitled to wear the crown of scavenger, notched seed-shrimps must have an efficient feeding mechanism. So when an opportunity arose to film notched seed-shrimps in action, it was grasped with both hands.
In 1994 a film crew came to town to record the marine life of Sydney. On a wharf just within the harbour, they constructed an impressive aquarium through which fresh seawater flowed continuously to create a deceptively natural environment. The run-of-the-mill anemones, starfish and crabs were introduced and conducted their business as usual, which was monitored in detail via a camera so large it must have been good. Certainly the highly magnified pictures on the monitor were impressive, as was the control system - the camera could be steered on tracks in all directions. And somehow the film crew came to believe, or were tricked into believing, that seed-shrimps would make compelling viewing, and seed-shrimps were hired as extras on the final day of filming.
Wasting no time, a scavenger trap was rushed from the Australian Museum to the local beach - Watson’s Bay, within Sydney Harbour. The beach was 100 metres or so long, and there was time to target only one spot - what was hoped would be a seed-shrimp hotspot. The rocks that bordered the beach were the initial choice, until a fish and chip shop was spotted at the end of a wharf. Their degradable waste often ended up in the water: what better place to find scavenging seed-shrimps than on a pile of discarded fish carcasses? Notched seed-shrimp heaven had been found - the recovered trap was full of them.
The seed-shrimps in the trap were transferred to a large bucket of seawater and chauffeur-driven to the film set. They began performing well. Some were swimming at full speed while others were stripping a pilchard to the bones. According to the script, it w
as the eaters that would star. And it was good to find that one part of the seed-shrimp’s body had evolved into a relatively large, saw-like tool that could slice efficiently through fish skin. But the show was stolen by two individuals on the surface of the bucket of water - they appeared to be mating. This was certainly not in the script. Notched seed-shrimps, or any lightweight seed-shrimps for that matter, had never been accurately observed mating. All that was about to change.
The pair were transferred to the big stage during the final hour of filming . . . and they mated, shells juxtaposed, lower surface to lower surface. It was nice to discover this, but the real cause for celebration happened just seconds earlier when the male seed-shrimp performed a courtship ritual. He circled the female then . . . he released a flash of blue light! His iridescent halophores had been withheld within his shell. Then, when he was in full view of the female, his halophores emerged from his shell in all their iridescent glory. And, like a peahen with the tail of a peacock, the female seed-shrimp was suitably impressed - they mated. It was extremely fortunate that a single pair of notched seed-shrimps had chosen that particular moment to mate, and with only an hour’s worth of film remaining. This was just lucky.
The discovery that iridescence was employed by notched seed-shrimps changed everything. Rather than ending up as a footnote in some obscure publication, notched seed-shrimp iridescence could now be taken seriously. It was time to alert the physicists. The species captured on film was Skogsbergia, named after an early seed-shrimp specialist. This notched seed-shrimp displays exceptionally spectacular iridescence, but in the males only. The females are quite dull in comparison. And this difference between sexes became clear in the electron microscope.
The antennae of male and female Skogsbergia were coated in a thin layer of gold and then bombarded with electrons, rather than light. The images formed from reflected electrons revealed male antennae swamped in the iridescent halophores, and sparse halophores on female antennae. At higher magnifications, differences between individual halophores emerged. Male halophores have the profiles to cause an optimal reflection of blue light at both macro- and microlevels. In terms of optics, they form extremely efficient diffraction gratings. The diffraction gratings of female halophores, on the other hand, are decidedly crude. This conclusion was reached following collaboration with physicists. Optical physicists employed their rigorous electromagnetic scattering theory on the iridescence of Skogsbergia, followed by that of other notched seed-shrimps. A pattern gradually began to emerge.
Figure 5.4 Frame from a video recording of a pair of the notched seed-shrimp Skogsbergia species mating. The iridescent flash of the male is arrowed.
Different species of notched seed-shrimps possessed different iridescent properties. Optical efficiency values were given to all of them and the possibility arose that they could be placed in a sequence, in order of iridescent effectiveness. Efficiency values were calculated using both the physics of the diffraction gratings and the design of the halophores on a larger scale. The values derived from many components - so many that a more sophisticated sequence could be constructed using cladistic methods.
Cladistics is a mathematical method for calculating relationships between species based on a character set - each species on Earth has an individual set of characteristics, both structural and genetic. The relationships are illustrated in the form of a family tree, and the family tree can be used to suggest an evolutionary tree. Cladistics is a common tool in the study of evolution, and in this case it did indeed generate a neat sequence of species based on their iridescence. And the increasing sophistication in structure of halophores was mirrored by their visual effects. The observed effect of iridescence also appeared to be transforming in spectral content and intensity. Those species at the beginning of the sequence reflected all colours equally, each colour projected in a different direction, while those at the end of the sequence were reflecting only blue light, and more intensely than ever before. Green and blue-green reflections lay somewhere in between. But what did this sequence infer? Did it in fact mean anything? Could there be implications for evolution here? The question of evolution was confronted first, and the work of specialists in bioluminescent seed-shrimps became appropriate to the case.
Bioluminescent seed-shrimps
Chapter 3 introduced the remarkable array of mechanisms that have evolved to provide colour in animals living under sunlight. Sunlight is reflected, transmitted and absorbed in all sorts of ways to produce different visual effects. The iridescence of the notched seed-shrimps considered so far is an example of one reflective effect. But what about those animals living without sunlight? This case was examined in the previous chapter, but only in part. Actually there remain animals living without sunlight, such as some deep-sea or nocturnal species, that are extremely visual. The animals referred to in Chapter 4, that were evolving modestly, did not employ light to any notable degree. So how do you operate with light in the absence of a light source? Quite simply, you make your own.
Seed-shrimps cannot generate electricity to power miniature light bulbs. Instead they adopt a more efficient method of yielding light - they bioluminesce. Two chemicals - a luciferin and a luciferase - react with the oxygen in water, and light is emitted as a byproduct. The light is referred to as bioluminescence. Only about 20 per cent of the energy fed into a light bulb fuels light; the rest is lost as heat. Bioluminescence is less wasteful - almost all of the energy investment becomes light, and so it is known as ‘cold light’. Luminescence can be seen at fairgrounds at night. Plastic tubes containing luminescent chemicals, separated by a thin glass wall, are sold as necklaces for children. When the plastic tube is bent, the inner glass wall is broken. The chemicals mix and the necklace glows in the dark like a neon sign (although neon signs themselves employ a different mechanism). Similar plastic tubes are sold to Scuba divers and fishermen for conducting their business in the dark. It is easy to find a luminescing diving buddy in the water at night, and a fishing float that luminesces is just as conspicuous in the dark. Sometimes the plastic tubes are unnecessary for these marine activities; natural bioluminescence can suffice.
Waters abundant with bioluminescent dinoflagellates - single-celled organisms - can make swimming or diving at night an extraterrestrial experience. As the movement of arms and legs agitates the dinoflagellates, they react by mixing their luminescent chemicals. And the effect is so powerful that a sharp human silhouette is clearly visible as a blue or green glow in the dark. In fact the Australian Navy are clearly concerned because they closely monitor the geographical movements of their bioluminescent natives. No matter how cleverly you design your ship, if it sails into a crowd of bioluminescent dinoflagellates it lights a beacon to let everyone know where you are. And lightweight seed-shrimps have evolved bioluminescence with similar aspirations.
One group of lightweight seed-shrimps, the Halocyprida, produce bioluminescence from organs in their shells. I will refer to the Halocyprida as the ‘eyeless’ group, since all their representatives lack eyes. The two bioluminescent chemicals are pumped from eyeless seed-shrimps into the water, where they react to form a luminescent cloud. These eyeless species gather at the ocean surface at night and bioluminesce for all they’re worth. The result is a mass ‘light bomb’; a patch of bioluminescence so bright it can be detected by satellites in space. And the reason for this is so as to generate a burglar alarm. The seed-shrimps are eaten by small fish, and small fish are eaten by bigger fish. Any small fish entering the light zone becomes a most conspicuous silhouette, and the alarm bells sound for the bigger fish. Not surprisingly, the eyeless seed-shrimps remain undisturbed at night.
There is another group of lightweight seed-shrimps that bioluminesce, and these, as it happens, are the notched seed-shrimps. But only some of them are luminescent, perhaps half of all notched seed-shrimp species known. I found my first bioluminescent notched seed-shrimp on a beach in Australia. Although they were known to live there, all I could find was a lumines
cent crab, impressive as it was. A crab glowing intensely in the dark is an extraordinary sight, but it was not the crab itself that was glowing: it was its food. The crab, a transparent species, had eaten a notched seed-shrimp and the bioluminescent chemicals were mixing in its stomach. There have been similar reports of this happening in other parts of the world, so maybe bioluminescence is not such a problem for the crab.
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.
In The Blink Of An Eye Page 18
