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原文:
http://www.bbc.com/earth/story/20160413-the-fish-that-swims-in-toxins-and-gets-poisoned-by-humans



節錄:

In a dark cave in Tabasco, southern Mexico, near the village of Tapijulapa, lives a community of molly fish. There would be nothing remarkable about this, except that the cave is poisoned with hydrogen sulphide, a toxic gas deadly to life.

http://ichef.bbci.co.uk/wwfeatures/wm/live/1600_900/images/live/p0/3q/tr/p03qtrkt.jpg
Inside the Cueva del Azufre (Credit: Michi Tobler)

The fish have developed a suite of remarkable adaptations to their dark, toxic world. They may even have come up with a way to survive a local religious ritual that involves poisoning them.
In fact, the cave mollies seem to be well on their way to becoming a distinct species. That might seem odd, given that there is no barrier separating them from the light-drenched pools outside the cave where regular mollies live.
But in this case, the reasons might be sticking to the cave for a rather direct reason. Lurking at the cave mouth is a ferocious, gruesome predator.

http://ichef.bbci.co.uk/wwfeatures/wm/live/1600_900/images/live/p0/3q/tr/p03qtrlj.jpg
A cave molly (Poecilia mexicana) (Credit: Michi Tobler)

"The fish are exposed to concentrations of hydrogen sulphide 50 times higher than those generally considered toxic. "

The cave of the mollies is called Cueva del Azufre, literally "Sulphur Cave" – or, confusingly, Cueva de Villa Luz ("Cave of the Lighted House") and Cueva de las Sardinas ("Cave of Sardines"). It is fed by many natural springs and streams, which form several pools inside the cave.

In each pool swim hundreds if not thousands of fish. Natural oil deposits and volcanic activity in the area mean that the water is enriched with hydrogen sulphide. The poisonous gas has also leaked into the air of the cave. In the water, fish are exposed to concentrations of hydrogen sulphide 50 times higher than those generally considered toxic for aquatic species. Oxygen, meanwhile, is much less available than in other aquatic environments.

Most forms of life would be dead within minutes, yet the molly fish remain unharmed. Just how do they do it?


#good2#2    #bad#0  
標籤:
http://ichef.bbci.co.uk/wwfeatures/wm/live/1600_900/images/live/p0/3q/tr/p03qtrn7.jpg
Cave mollies (Poecilia mexicana) do not normally form shoals (Credit: Michi Tobler)

Michi Tobler from Kansas State University in the US has spent decades researching the fish. Tobler believes that, in order to survive, the mollies have had to change both their behaviour and their genes.
"Fish avoid taking in too much hydrogen sulphide by breathing directly at the water's surface," says Tobler. "This compensatory behaviour, which is called aquatic surface respiration, increases their ability to acquire oxygen in the hypoxic [oxygen-depleted] environment, and it likely also minimises hydrogen sulphide uptake by the body."

Tobler's research has shown that aquatic surface respiration is critical for short-term survival. If fish are denied access to the water surface, they quickly die. As well as limiting the amount of toxin that enters their body, the fish are also able to detoxify the hydrogen sulphide once it has entered their system.

Their environment is so extreme that few other animals have followed the mollies into the cave. This means there is less food for the fish to eat and resources are limited. The fish have coped by reducing their energy expenditure. Their bodies have lost their colourful pigmentation, an unnecessary waste of energy and resources in a dark environment where visibility is poor.

They have also developed smaller and less sensitive eyes than their cousins living outside. Instead of eyes the fish rely on a hypersensitive pressure detector that runs down each side of their bodies, known as the 'lateral line' to sense disturbances in the water. They also have a higher density of taste buds to sense their environment. In order to extract the maximum amount of oxygen from the water, the mollies have grown larger heads and gills. They also have smaller brains, possibly to conserve energy. As well as changing their bodies, living in the cave


As well as changing their bodies, living in the cave has changed how the mollies behave.

Cave mollies have a different diet to their surface ancestors. They feed on bacterial films and insects instead of algae. In response, their jaws and intestines have changed.

http://ichef.bbci.co.uk/wwfeatures/wm/live/1600_900/images/live/p0/3q/v9/p03qv915.jpg
There are many species of molly, such as this Yucatan molly (Poecilia verifera) from Arizona (Credit: blickwinkel/Alamy Stock Photo)

Cave mollies are also less aggressive than their freshwater cousins. This could be because aggression is quite a costly behaviour, and is not advisable therefore when resources are limited. Cave mollies also do not hang out together in shoals, and are more solitary in general.

Living in darkness has also changed how the mollies choose their mates. Female mollies prefer larger males, but whilst surface fish can quickly see which males they fancy, in darkness this is more difficult.
Instead the fish rely instead on their lateral line to sense disturbances in the water. The greater the disturbance, the bigger the male.

http://ichef.bbci.co.uk/wwfeatures/wm/live/1600_900/images/live/p0/3q/v9/p03qv92p.jpg
A shoal of mollies (Poecilia sp.) (Credit: Brandon Cole/naturepl.com)


http://ichef.bbci.co.uk/wwfeatures/wm/live/1600_900/images/live/p0/3q/v9/p03qv92p.jpg
A shoal of mollies (Poecilia sp.) (Credit: Brandon Cole/naturepl.com)


"Unlike their surface ancestors, cave mollies have evolved the capability to communicate in complete darkness," says Tobler. "Cave mollies can assess the body size and nutritional condition of a potential mating partner without seeing them."

Another difference between the cave molly and its ancestors is the fact that molly females – which give birth to live young rather than laying eggs – now produce fewer but larger offspring. This makes sense, as larger fish are more protected against the effects of the poison.

As if things were not bad enough for the poor molly – toxic poison in the water, total darkness, slim food pickings – it also has to contend with the local population of humans poisoning it every year as part of a religious ritual.


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