NOTHING can catch the eye of a male horseshoe crab more easily than a female.
Now neurologists in New York who have worked out how the male crab鈥檚 eye
communicates with its brain say that the animal鈥檚 uncanny ability to spot a mate
lies in its retina.
For the most part, horseshoe crabs (Limulus polyphemus) take little
notice of their surroundings. But behavioural tests have shown that males can
see females in almost any situation, even in the dark or when the female鈥檚 hard
outer shell, or carapace, blends into the background .
This extreme sensitivity to passing sexual quarry has been a mystery. But
Robert Barlow of the State University of New York Health Science Center in
Syracuse suspected that it might have something to do with the way the retina
encodes information. 鈥淎ll retinas are little brains,鈥� says Barlow. 鈥淭hey process
information before they send it on.鈥�
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To test this idea, Barlow and his colleagues developed a computer simulation
of a crab retina, based on the physiology of photoreceptors and how they are
arranged in a crab鈥檚 eye. Then they showed the computer retina a crab鈥檚-eye view
of the world: a digitised video shot from a camera mounted on a crab鈥檚 back,
aligned with the animal鈥檚 visual field.
While the researchers were taping, they dragged objects of various shapes and
sizes in front of the crab at different speeds. As expected, the crab was
unfazed by most of these, but started crawling toward any object of female
proportions moving at crab speed.
Surprisingly, the output signal of the virtual retina became stronger when
the objects moved like female crabs. Barlow says it鈥檚 as if the retina were
telling the brain which bits of information to pay more attention to. 鈥淭he eye
is tuned to be specific to mates moving across its visual field,鈥� he says.
This specific tuning comes from the way the visual receptors are arranged.
Receptors are sensitive to their neighbours and will fire more strongly when
surrounded by other active receptors. The signal is dramatically stronger when
an object is just big enough to stimulate a cluster of receptors, rather than
just one or two. The optimum size is that of a female crab a few feet away, as
Barlow鈥檚 team reports in Proceedings of the National Academy of
Sciences (vol 194, p 12 649).
Maureen Powers, director of vision research at Vanderbilt University in
Nashville, says the crab work is a big step forward. 鈥淥ne of the enduring
puzzles has been trying to figure out how those neurons work together.鈥� Although
human vision is vastly more complicated, Powers says the research may help us to
understand why some things always catch our attention.