In The Blink Of An Eye (22 page)

Scavenging isopods were once caught during some early random trapping in Indian and Mexican waters. Steve Keable compared his shallow-water Australian isopods with these species. Just as there were considerable differences between each species within Australia, the scavenging isopods from India and Mexico were very different again. They were all related, in that they belonged to the same small branch of the evolutionary tree, but they had diverged considerably, to adapt to different niches in different light environments. So what can we learn from this?

The global picture informs of considerable evolution over 160 million years in an environment with substantial sunlight. One hundred and sixty million years ago, a population of ancestral isopods was divided geographically, travelling in different directions on board the continental shelves of three different plates. The ancestral species continued to evolve, but in three different environments. The result is that evolution yielded copious species in each case, but was different each time. Two environments are never the same, and evolution is reflected in this. But remember that here we are dealing with environments where light is present. In contrast, Steve's clearly defined mission was not to be echoed in Jim Lowry's task.

Jim was left with
Bathynomus
to tackle. At first, this appeared to be a prize project -
Bathynomus
was a magnificent animal. Then problems started to arise. The
Bathynomus
collected from each depth range on the Australian plate, beginning at 200 metres, all appeared similar. Those marked differences belonging to the shallow water isopods, obvious even to the inexperienced eye, were simply not there. There were slight differences - some individuals had four spines on a leg where others had five - but were these enough to designate more than one species of
Bathynomus
in the Australian fauna, and indeed, were there
any
new species here at all? These questions were at the foundation of Jim's taxonomic task, and the answers lay with the
Bathynomus
of India and Mexico.

Figure 4.3
Simplified schematic section through two of Earth's plates, showing the submarine landscape between, including their line of separation.

A ‘species' can be considered a group of similar individuals that reproduce in their natural environment. The word natural is important - related but different species can sometimes reproduce in an artificial environment, but would not do so under natural conditions. Of course, we could not observe the mating behaviour of
Bathynomus
at 1,000 metres. But when enough physical characteristics are recognised to reinforce a particular relationship, this can provide evidence towards classification. The characteristics of the other legs of the Australian
Bathynomus
were consistent with those of the first leg considered. Maybe this was grounds for designating two separate species from Australia. Maybe evolution had been slow in the case of the Australian
Bathynomus
over the last 160 million years - genetic variation was obviously very limited. It was time now to turn our attention to India and Mexico.

From fossils, we know that
Bathynomus
also existed earlier than 160 million years ago. It had travelled on separate plates, diverging from an original supercontinent to the regions that are now Australia, India and Mexico. In other words, it had not evolved from shallow-water isopods independently in all three locations during the past 160 million years. An examination of the
Bathynomus
caught by fishermen from India and Mexico would inform us what had happened to the ancestor over that 160 million years of living in different, isolated environments.

The pattern of the shallow-water isopods was not replicated in the deep. The
Bathynomus
of India and Mexico did not differ greatly from those of Australia - they were almost identical. Almost, but not quite. There was a size difference - although all were huge by isopod standards,
exclusive size ranges were identified. These echoed the slight differences in shape such as spine numeration. But quite categorically,
Bathynomus
showed little variation in shape between species. It lived in deep water with little sunlight . . . and it had hardly evolved at all over 160 million years.
Voilà!
The evidence we have been looking for, and the point of the whole SEAS story.

This story paints a picture of what happens in environments with little light compared to those environments with considerable light. But the otherwise
x
,
y
, or two-dimensional spatial picture, has a third axis -
z
. The
z
axis represents time. And the complete picture is of restricted evolution where light is reduced. Genetic mutations have been diminutive as a result of modest selective pressures - pressures where light is not dominant.

Just to confirm that light is a major limiting factor here, we can compare the fauna living
within
the sediment of the sea floor of shallow and deep regions. Below the surface of sediment there is no sunlight. So a very different ecosystem exists there, a system not adapted to light. We have always known that the fauna of shallow water sediment is reasonably diverse, where most species derive from ancestors in the exposed waters above, but ecologists had predicted the opposite for deep-sea sediment. Then, in the 1960s, scientists from the Woods Hole Oceanographic Institution in Massachusetts collected deep-sea sediment samples using newly developed equipment. This technology was capable of collecting more specimens from a given area than ever before. And what it collected was beyond all expectations.

Although there were fewer individuals in the deep-sea sediment compared with its shallow-water counterpart, the number of species was similar. The diversity of life in deep-sea sediments was equal to that in the shallows. So a diversity of animals
can
potentially survive in the deep sea, and evolution
can
be as prolific as in the shallows - temperature and pressure, for instance, are not necessarily limiting to speciation. But where animals are adapted to sunlight, and the light levels fall, then the evolutionary brakes are applied and diversification slows down. The potential niches available diminish drastically. Armed with this clue towards solving the Cambrian enigma, we can leave the deep sea.

Now that we have adapted our vision and thinking to the dark, we are ready to examine an environment that is in total darkness. Rather than choosing the Abyssal Plain, I will select an environment that is slightly more accessible, and consequently one whose inhabitants are better known. Can we strengthen the message taken from the continental shelf and slope as light is removed from the equation completely? The answer to that question follows.

In his book
Colours of Animals
, Sir Edward Poulton devoted a certain amount of space to cave animals. He stated unequivocally that animals living in darkness were pale because pigment would not be visible in these situations and so would no longer be of any use to the animals. Poulton strongly favoured what became known as the Darwinian view of colour - that ‘wherever colour is seen, it is due to the favouring influence of natural or sexual selection'. That Darwin carefully chose the words ‘
Whenever colour has been modified for some special purpose
' seemed to have been overlooked. So it is not surprising that Poulton extended his argument into environments without light. He suggested that in caves ‘it [pigment] is, therefore, no longer maintained by natural selection, and
therefore
it disappears'. The second
therefore
became the subject of great dispute.

Another biologist of the time, J. T. Cunningham, believed that pigment was produced directly by the action of light on the skin. So he thought cave-dwelling animals were pale coloured because there was no light to stimulate the development of pigment. According to Cunningham, light and pigment were directly related. According to others, light is not the cause of pigmentation; it only puts in motion the machinery produced in the animal by natural selection.

Today, armed with genetic theory, we understand that Cunningham was wrong. But does the pigment machinery, or rather the process of genetic mutation and new gene deployment, stop working when light is removed? Are the cogs in this colour machine literally solar powered, in that they cease to turn without sunlight? Maybe the pigment
machine has a reverse gear, one that is engaged in the absence of light. To uncover the complete story, we too should look into the caves.

It has been worthwhile examining environments with gradually decreasing levels of light, from the dim night-time of land to the almost complete darkness of the deep sea, if only to compare the communities which inhabit them with those of caves. In caves a similar transition in light levels exists. But here the transition occurs much more rapidly. Light fades away in caves over metres, rather than hundreds of metres as in the sea. And at the end of the journey into many caves, we reach a true, undeviating condition of total darkness on Earth.

I first became interested in caves when Mike Gray, an arachnologist at the Australian Museum, allowed me to examine his latest find. Mike had recently been underground in the Nularbor Plain of South Australia. Soon after entering the cave, he found himself in total darkness. And the fauna, visible under torchlight, rapidly became less diverse as his journey continued. But Mike found what he was looking for - a spider. More than that, he found a new species of spider. It's not unusual to find a new species of spider in Australia - Mike's previous discovery was made in his own garage. But this cave dweller appeared different to his garage specimen, or indeed any other spider living outside caves. It was related to the infamous ‘Sydney funnel web' which meant it was supposed to have either six or eight ‘eyes'. But with the aid of a microscope it became clear this cave species, just 15 millimetres long, had
no
eyes.

The deep-sea animals I had examined were adapted to even the most minuscule quantities of light present in their environment - they had big eyes. The cave spider was denied
any
light and had given up the evolutionary struggle to see. Its lack of ‘eyes', nonetheless,
was
an adaptation to light. But did this ‘eye' loss take place quickly through time? And how powerful was the selection pressure to lose ‘eyes' a negative evolutionary response to light? It is difficult to answer these questions taking the cave spider as a model - we know too little about its relatives. But cave fish have been studied more intensively, and we have enough pieces of their puzzle to trace their journey through time, from the open ocean and into caves.

Sometimes bioluminescence exists in caves as it does in the deep
sea - cave animals can produce their own light, like living torches. This, again, makes matters complicated - to begin with, we no longer know the exact light conditions. Bioluminescence may create an effectively continuous light field, or it may be intermittent. The light field may be relatively bright, dim or varied to any extent. Although bioluminescence probably causes a fairly faint light field within the big picture, it is best at this stage to consider just those caves where bioluminescence is absent, where the condition of total darkness is satisfied. Such a situation exists within the marine caves of Mexico.

Most inhabitants of marine caves today originate directly from their ancestors in the open sea. Either these ancestors now no longer exist, or they have moved into some other extreme environment. For instance, one group of small crustaceans, called remipedes, is virtually confined to cave habitats today, even though their evolutionary origin was in the open sea. They are known as a relict fauna - species derived from groups that were formerly widespread and diverse but now survive exclusively in a cave, possibly, according to Bermudan cave biologist Thomas Iliffe, because of reduced competition or predation. Iliffe found that some remipedes in eastern Atlantic caves look very similar to those in caves of the western Atlantic, and this similarity was not the result of convergence - the evolution of similar bodies to adapt to similar environments. Instead the similarity signalled almost zero evolutionary activity. The caves have been separated geographically for over 100 million years and, as for
Bathynomus
, very little evolution had taken place in the dark environments. Even closer to the
Bathynomus
story, isopod crustaceans found living in caves, isolated for over 100 million years, also bore a remarkably close resemblance to each other. In fact this story is echoed in many types of animals. And in most cases the explanation given for their current cave living is the same - that their ancestors once inhabited shallow, open seas but were driven out by competitors and predators among the new faunas that appeared throughout geological time. But the Mexican cave fish can provide more information. They have a very close relative living outside their caves today.