Heat is partly responsible for this. It is warmer during the day than at night, and many animals are adapted to warmth. But animals from most phyla can be adapted to the cold. This is not an evolutionary impossibility. So we can consider at least part of the day-night biodiversity difference as evidence towards the power of light as a stimulus affecting life on Earth. Begin to remove this stimulus and evolution becomes much less complicated. I say ‘begin’ because night-time on land is only a step towards total darkness.
At night, other senses are employed. But this is where the big difference between light and the other major stimuli becomes clearly evident. I refer to the difference in presence. Light strikes the Earth and oozes through the canopies of trees, between rocks and blades of grass, and into the waters - it cannot be avoided. Light infiltrates an environment whereas the other major stimuli do not. This explains why owls, equipped with extremely sensitive hearing and the potential to further develop other senses, do not relinquish their use of light. In fact vision has evolved further in owls. A mouse that has detected the flight of an owl may freeze and become inaudible - the equivalent of invisible to light. But where invisibility demands great evolutionary effort, inaudibility requires only temporary stillness.
Up to this point I have considered the major senses - senses that are common in nature. These are smell and taste (which are quite similar), sight, hearing and touch. But at night, one of the minor stimuli becomes important. This stimulus carries the advantage of light in being unavoidable. As described in Chapter 3, bats hunt using radar.
Radar is a minor stimulus/sense as a result of requiring considerable evolutionary expense and chemical and mechanical effort just to infuse the stimulus into an environment in the first place. Light, on the other hand, is a pre-infused stimulus. Only then, when radar has been launched into the air, can its detection be compared to vision. And even so, a bat’s radar invites little evolutionary change in the animals not directly affected by this stimulus. Light, on the other hand, affects everything in the environment where it exists.
The owl is completely unaffected by the bat hunting moths around it. During the daytime, however, apparently isolated predator-prey relationships begin to interact with each other. The food web and animal behaviour become increasingly complex. So in addition to the direct reduction in niches at night, through the degeneration of light and shade partitions for instance, evolution is stimulated much less at night. Again, in this chapter I place emphasis on the predator-prey scenario because the first rule of survival is to avoid becoming a meal. So this interaction is as important as it gets.
On land, the transition from light to almost dark happens quickly, during sunset or at dusk. So few animals on land are adapted to anything other than light or almost dark conditions. But in the sea there is another transition from light to dark - a transition in space. Marine animals can be compared from different depth ranges, living under different light levels.
The biggest clues towards solving the Cambrian enigma from night-time on land are the reductions in both biodiversity and complexity of behaviour that accompany a reduction in light. We will develop this understanding throughout this chapter, but further clues can be found in the deep sea, where evolution within a tiny branch of the animal tree can be tracked through time.
The deep sea
The Scavengers of East Australian Seas, or ‘SEAS’, expedition was established to scientifically document the entire community of scavenging crustaceans - the group of arthropods that include the crabs, shrimps and lobsters - along the east coast of Australia. Before 1990, traps were set for these animals, but these were poorly designed and caught only individuals bigger than a few millimetres. In fact a twelfth-century fish/crayfish trap was recovered from the River Thames where it passes the Tower of London, and its design turned out to be superior to twentieth-century traps. Its overall form was that of a wickerwork cone, with a funnel-like entrance. Beyond the entrance lay an additional but narrower funnel-like entrance, creating two chambers inside the cone that could hold catches of different sizes. The victims would have been lured into the cone by bait in the smaller chamber. The whole trap was weighed down on the river-bed by two large flints, and connected to the surface by a rope.
Not only had scientific scavenger traps fallen below twelfth-century standards, but they had been set sporadically - on a random basis within small areas, and without the bigger picture in mind. Jim Lowry had been thinking about this lax approach for some time, and decided he would paint the bigger picture, and in turn lay the foundations for scavenging crustacean conservation.
Scavenging crustacean communities are exceptionally important because they clean the sea floor of dead organic matter such as fish carcasses, which would otherwise consume valuable oxygen in the water as they decayed. And throughout the course of a normal day there is quite a fall of bodies to the sea floor. Also scavengers are a noteworthy part of the marine food web - they in turn provide food for other inhabitants of the sea, and complete the cycle of organic nutrients.
Figure 4.1 A twelfth-century fishing trap recovered from the River Thames.
Jim Lowry had moved from Virginia in the USA to the Australian Museum in Sydney via a lengthy spell in New Zealand. He chose his back garden as a study site - the east Australian coast, in fact, and no small undertaking.
Jim Lowry lives on a small island within a marine inlet to the north of Sydney. He travels to work by motorboat and motorbike. His bike is a beautiful, black and chrome 750cc machine. His boat is rather less impressive, but is affectionately known as ‘The Flying Scud’. Scud is the American slang, though not quite a household name, for amphipod - a type of crustacean. Amphipods are commonly encountered on beaches, near rock pools, in the form of ‘beach fleas’. Often they have shrimp-like bodies that are flattened from side to side. Jim Lowry studies amphipods. He produces (along with his co-worker, Helen Stoddart) some of the finest taxonomic work to be found anywhere.
Taxonomy is arguably the oldest scientific profession. It involves documenting and describing new (to science) species using consistent methods, and is one of the most essential of all scientific disciplines. Scientific classification began with the Swedish botanist Carl Linnaeus in the eighteenth century. We still use his system, but Darwin and Wallace’s theory of evolution has allowed scientists to see diversity as the result of a dynamic process rather than a static picture. Considering the extinction rate induced by humans, and that only about 10 per cent of the Earth’s species have so far been described, we should really be in a hurry to get on with taxonomy. Taxonomy is also important from an evolutionary perspective. We must describe and collect nucleic acids from the species alive today in order to perform evolutionary and genetic diversity analyses. Better to collect DNA from species while they are alive rather than extinct. Remember the drama of collecting ancient DNA from just a single extinct species such as the mammoth? Unfortunately we have been a little slow off the blocks, to say the least. Today species are disappearing faster than they are being described.
Jim Lowry’s interest in scavengers stems from the amphipod connection - amphipods are among the chief scavengers. The other principal scavenging group was thought to be isopods. Isopods are also shrimp-like animals but their bodies are typically flattened from top to bottom, rather than side to side. Isopods include woodlice - the only members of the group with any notoriety, although probably bad examples since most isopods are marine.
Jim Lowry designed a scavenger trap not too far removed from the twelfth-century model. Plastic drainpipes were sectioned into short tubes to form the frame of the trap and to provide a robust structure; his traps were destined for deeper waters. Plastic funnels were cut accordingly to provide two different apertures, and they were glued into the ‘drainpipe’ tubes to form the two chambers. A mesh was fixed at the end of each tube to allow water to flow through the trap rather than sweep it away. The size of the mesh was important - holes half a millimetre in size were selected, allowin
g anything smaller to escape, but anything larger to be caught.
The traps were tested near Sydney. At the Australian Museum thick rope was sectioned into 50-metre lengths. This was coiled carefully - a poorly coiled rope can quickly resemble a plate of spaghetti and be just as useful - and baled on to ‘The Flying Scud’. House bricks were also loaded on to the boat, along with orange plastic buoys. ‘The Flying Scud’ was towed out to the coast, stopping at a petrol station along the way to buy frozen pilchards. Frozen pilchards sell well in Australia, where fishing is popular and pilchards make suitable bait.
Figure 4.2 A typical scavenging isopod, amphipod and ostracod (seed-shrimp).
On board ‘The Flying Scud’, a pilchard was placed inside the smallest chamber of each trap. The traps were individually tied to two house bricks and one end of the 50-metre length of rope. The other end of this rope was tied to a buoy, and the whole apparatus was then hurled overboard to a depth of 25 metres. The length of the ropes had to be greater than the depth of the water in which they trailed, so that, as the traps rested on the seabed, the rope provided some ‘give’ in strong currents. Notes were made of the positions of each trap in relation to objects on the shoreline.
The following morning, ‘The Flying Scud’ returned to the study site to recover the traps. Finding the buoys was not so easy, and some traps were lost. But the traps retrieved were opened on board - and everyone was happy. Jim had caught his amphipods and isopods. The test run was a success, although it was clear that improvements to the protocol were necessary if the more turbulent seas off Australia were to be prospected. The marine snail community introduced one unanticipated problem, however. Sometimes they too were attracted to the smell of fish, and in a frenzied bid to dine on the pilchard, they became jammed in the entrance hole, thus spoiling the traps. News also arrived from the fisheries industry of some gigantic isopods living in deep waters off the north-east coast of Australia - and they were feeding on dead fish. All of this called for adjustments to the trap design.
Jim Lowry opened his map of the south Pacific and pinpointed his targets. Several towns were marked at different latitudes, from New Guinea in the north, traversing the eastern Australian coast to Tasmania in the south. From each town, traps would be set along a line of latitude, beginning at 50 metres deep and ending at 1,000 metres. The expedition was starting to get serious.
Behind the scenes, the SEAS project was taking shape. Jim managed to recruit several students and technicians at the Australian Museum to work on his new traps - the deadline for his first boat launch was approaching. A production line unfolded and the finished traps were piled on to a huge trailer at great speed. The new traps were all covered with metal grids to keep out the snails. And to counter the giant deep-sea isopods anticipated, the traps were placed inside much larger structures, which were actually modified lobster traps. All the equipment could be stacked, so a single trailer, albeit fully laden, was adequate.
The deeper sampling sites called for a bigger boat, and ‘The Flying Scud’ was retired. Commercial fishing vessels were chartered from each town, and these were equipped with a global positioning system, or GPS. This system employs satellites to locate precisely any coordinates, even at sea, and so traps could theoretically be found easily. But to fight the stronger currents in these deeper seas, and keep the traps in their original positions, anchors and heavier lead weights entered the equation. A certain amount of drift was still predicted, so the markers at the surface were upgraded too to prevent them being dragged under. Huge buoys and flags were tied on to the cage-like trailer, which was beginning to look like a travelling circus wagon. And the great bundles of rope, now up to a kilometre and a half long, only confirmed the resemblance.
The SEAS bandwagon rolled into Cairns in north-eastern Australia in 1990, and the expedition was launched. Everything went smoothly. The traps were set one afternoon, and most were collected successfully the following morning. Amphipods and isopods were recovered, and nearly all were new species. The Australian Museum jeep towed the gear to the next site and the sampling continued . . . and so on. At each port of call a different fishing vessel awaited, each equipped with a different captain and crew.
The SEAS project was a great success in that hundreds of new species were recovered during the original sampling expedition and in the repeats. Interestingly, as will become evident, the species tended to get larger as the depth increased.
The ecological results of the SEAS project are in the throes of being published. All I can say here is that they reveal, for the first time, the fate of the better known fish and other marine animals in one of the largest environments on Earth. For the first time we will understand the biology of the crustacean scavenging community, which will have all sorts of implications in fisheries practices and management. We won’t be able to produce a management plan for the seas, and ultimately preserve our fisheries industry and marine biodiversity, if we don’t know what’s down there. The SEAS project is a wonderful success story, but it was the results of the isopod part of the research that are relevant to this chapter.
Steve Keable, a member of the SEAS team interested specifically in the isopod catches, set some traps by hand in shallow water on the New Guinea leg of the trip. He did catch isopods, but decided to cut his plans short when, surfacing from a dive one day, he spotted a local tribesman standing astride a large rock, bow in hand with an arrow strung and pointing in his direction. Steve continued with his shallow-water sampling in safer waters off Australia, and with considerable success. Faced with so many new species of shallow-water isopods, he left the deep-water species to Jim Lowry, who could not resist these amazing forms.
It was the shallow waters that revealed the greatest diversity of scavenger species. As the trap localities became deeper, the number of species caught became fewer. The total number of individuals became fewer too, but not so the total weight of the catches - the animals were getting bigger. And they were dominated by those giant isopods that fishermen had warned of, known as Bathynomus. Bathynomus was no longer a myth to the SEAS team.
The deep-sea traps were hauled to the surface by a winch. Each trap came into view in the water as it was lifted closer to the ship, at which point members of the crew leant over the hull to heave it on board. It was immediately obvious there was a living animal in the trap. Large crab-like legs began to poke out through the holes in the large outer trap, and scraping sounds were heard as sharply pointed feet crawled over the rigid plastic sides of the trap. The whole trap moved around as it lay on the deck, surrounded by the onlooking crew. Then the trap was opened.
Everybody gasped. What appeared beggared belief, best described as something out of science fiction. One is invariably taken aback by an encounter with the unknown, and here the crew were witnessing something they had never seen before - not on TV, not in books and not in aquaria. By science fiction I refer to movies about aliens or, more appropriately, those 1960s cult classics where giant tarantulas or ants chased helpless humans some ten times smaller than them.
Out of the deep had risen an isopod that looked like a woodlouse. But this creature could never be mistaken for a woodlouse - it was fifty times bigger. This was Bathynomus. The fishermen’s legend had come to life, and giant, robust isopods were now roaming the deck. At fifty times their normal size, the jaws of a woodlouse look quite fiercesome, and their steps seem almost mechanical. Their heads, face on, look like stormtroopers from Star Wars, and their bodies resemble small but significant tanks, some half a metre long. Bathynomus indeed appears more machine than animal (see Plate 13).
It took a while for the unfamiliar to become the familiar, and the sight of a Bathynomus scurrying across the deck like an armoured vehicle, with jaws chomping, continued to be breathtaking. Those fortunate enough to see elephants in Africa, tigers in Nepal and bears in Canada should try adding Bathynomus to their list.
Something Bathynomus shared in common with these animals was its eyes, but Bathynomus lives in w
aters up to a kilometre deep, so what use are eyes here? Well, some sunlight exists, even at these depths, although only the blue component remains. And like the eagle owl, which also lives under dim light conditions, the eyes of Bathynomus are big. So in parts of our planet that remain too dim for us to see, but are reached by sunlight all the same, there live other animals exercising vision.
At a kilometre in depth, the sea is comparable to night-time on land in that light as a stimulus to behaviour and as a selection pressure to evolution is greatly reduced. But it is still present. This is not the completely dark scenario towards which I am aiming in this chapter, but it is a step in the right direction. Again we can learn that, where light is greatly reduced, biodiversity diminishes in unison. As the SEAS traps were set deeper, the number of species in their catches was reduced.
The deep sea is extremely interesting because there are many more amazing and unknown creatures to be discovered. New finds continue to enthral us every year. And the trend towards gigantism seems to hold, along with the low diversity levels as compared with shallower, brighter environments. Taxonomists studying sea spiders - marine members of the arthropod phylum most closely related to true spiders - also confirm that deep-sea faunas are discernible for their low species diversity while sometimes displaying an amazingly high abundance for a single species. The considerable size and weight of animals in the deep sea suggest that resources are not always limiting. But the reduction in light is a major factor in the reduction of evolution in the deep sea, implied by the depleted variety of species.
In The Blink Of An Eye Page 15
