In
a Realm of Blind Fish, Mr. Right and Mr. Big Are One and the Same
By CAROL KAESUK YOON
An article taken from the New York Times -
April 20, 2004
http://www.nytimes.com/2004/04/20/science/20FISH.html
AMBURG, Germany, April 19 - In a long, crowded
room stuffed full of small aquariums at the Zoological Institute of the
University of Hamburg, oddities of the fish world swim back and forth behind
glass to the background music of bubbling water. Plucked from the pitch-black
depths of caves around the world _ in Oman, Croatia, Brazil, Mexico _ many of
these fish are blind, their unseeing eyes staring out, oblivious to the humans
that stare in at them or the daylight that pours in the windows. Others are not
only blind, but also eyeless, cruising along with their eerily blank heads
alongside other bizarre-looking swimmers. Watching these queer creatures, anyone
can understand why biologists have long studied the most striking of the
physical changes _ like eyelessness and loss of coloration to near translucence
_ that typically accompany the shift from daylight into eternal night. But such
a focus has left much of life in caves a biological mystery. Now some scientists,
including researchers in Hamburg, have begun looking beyond the gross physical
changes to the subtleties of life in caves, in particular mating. How, for
example, can a female fish wisely choose a mate she cannot see? In new studies
of the Atlantic molly, these scientists are finding that mate choice is so
critical and that size is so important that cave-dwelling females have developed
a way to assess potential mates without seeing or contacting them, consistently
going for the bigger, better-fed males, even in complete darkness. "It was
a surprise," said Martin Plath, a graduate student who has carried out the
work with fellow behavioral biologists at Hamburg, Dr. Jakob Parzefall, Dr. Kay
E. Kvrner and Dr. Ingo Schlupp. Noting that the fish, mollies, had probably
colonized the cave recently, he said, "Sexual selection for size must be
very strong." But the new studies, some recently published in Behavioral
Ecology and Sociobiology, are not merely investigations of cave curiosities.
Researchers say the question is how animals adapt to major environmental change:
the switch to cave living serving as a proxy for any number of changes, like
warming environment or deteriorating habitat. Nearly all Atlantic mollies, or
Poecilia mexicana, live in well-lighted streams and rivers. But in 1962, near
Tapijulapa in southern Mexico, scientists discovered what remains the only known
population of the species living in a cave. And it is a bizarre cave at that.
The river flowing through it is a milky white from heavy concentrations of
hydrogen sulfide. The sulfur causes the darkened cave and much of the nearby
forest to reek of rotten eggs, and makes the slippery walls drip with sulfuric
acid. The milky pools where these inch-long fish live stretch far back into the
cave. They are slimy with unusual bacteria that require sulfur, not sunlight, to
live. In fact, the air is so heavy with sulfur odors that researchers do not
stay in the cave for more than an hour or two at a time. "It's not that
bad, though," Mr. Plath insisted, noting that a bit of contact with
sulfuric acid could be quite good for the skin. "You just have to have
someone waiting for you at the cave entrance, in case you don't come out."
The fish, their cave and the smelly, milky river flowing from it are so striking
that they have become central to an annual rite of the people of the area: they
drop a poisonous plant into the waters just inside the cave, Mr. Plath said,
killing mollies that flow out of the cave. They then collect and eat the fish in
a fertility-related rite, he said. Back in Hamburg, the scientists tested the
preferences of the cave females in darkness by watching them by video camera in
infrared light. In some species of fish, the females make obvious signals that
they prefer or disdain particular males, but the Atlantic mollies show their
preferences by swimming longer near larger males. Scientists suspect the cave
mollies are using a sensory organ known as the lateral line _ a line of pores
along the side that like eardrums can sense pressure changes _ to find the
bigger males. Fish normally use the lateral line to detect objects, and all
mollies have such a lateral line. But scientists found that females from sunny
streams were able to use their eyes only to assess which males were bigger and
that those females were unable to use their lateral lines to detect the bigger
males when given a choice in darkness. While the cave females have had to evolve
a novel way of detecting large mates, they enjoy an advantage in mating with the
lights out. Standing in front of a tank full of mollies from a surface stream
population, Mr. Plath describes how these zippy males never stop trying to mate
with females. Pointing to a large male darting madly after a female that is
dashing away, he says, "That's a nip and another nip," referring to
the repeated movement of the male's head toward the female in an effort to taste
the female's chemical signals. "There!" he says, pointing out a brief
instant when a male swimming alongside a female tried to mate, an action so
rapid that it was visible only to well-trained eyes. "If you don't separate
males and females, the females just die," Mr. Plath said. "Males
constantly harass them. It's copulating, copulating, nothing but copulating and
the females can't feed." But life in caves has calmed the molly males,
which do not harass females and are over all a more sluggish bunch than their
light-dwelling counterparts. The reason, researchers say, is probably that in
the dark, oxygen-poor waters of the cave, simply finding a female is an
energy-intensive task, making harassment on top of that too costly.