Blind Cavefish Still Sense Light
A fluke observation revealed that blind cave fish might not be so blind after all, but can respond to light as do surface fish.
Masato Yoshizawa of University of Maryland was freshening up the tank water when he noticed some curious behavior; the fish seemed to be following the shadow cast by the pipette.
"I thought it was my imagination," Yoshizawa said, "but they synchronized coming up to the surface. This is just really showing the light response."
The impromptu pipette shadow test was conducted on the larvae of blind cavefish Astyanax mexicanus, which do not have the completely functional bilateral eyes surface fish use to detect shape and color. The "blind" fish responded to the shadow as well.
Researchers Yoshizawa and William R. Jeffery tested the shadow response reaction again after removing the eyes of surface and cave fish larvae. Still, the fish followed the shadow.
The pineal eye was removed in the next study. That time, Yoshizawa said, the shadow response disappeared in both fish species, suggesting that the pineal gland is responsible for light detection.
"We don't know if the cavefish eyes still have a function," Yoshizawa said. "I'm interested in why pineal eyes still function in cavefish."
The researchers have a couple speculations.
The first, Yoshizawa said, concerns a relationship with the environment.
Only the cavefish larvae have shown the shadow response, not the adults, which might suggest that the light detection is a defense mechanism. Yoshizawa said that the response was noticed in larvae until they were about a week old. Before this time before the swim bladder and thus swimming capability is fully developed the larvae are vulnerable to predators and will seek shelter. As a possible defense mechanism, the ability to avoid the spotlight and hide amongst shadows would be beneficial.
The second speculation Yoshizawa proposed suggests that the shadow response mechanism is a byproduct of the pineal gland that serves more important biological functions.
This gland, Yoshizawa said, secretes a hormone related to seasonal growth and reproduction, both necessary for the animal's survival. Thus, according to Yoshizawa, if the shadow response is functionless, its expression could be due to its relationship with the pineal gland, which has other observed functions to account for its presence.
In terms of evolution, traits that are not necessarily harmful to an individual might continue to be expressed in the species even if the trait no longer serves a function. Yoshizawa referenced older lineages of cavefish, which show a more degenerated pineal gland.
"We assume that Astyanax mexicanus will lose all light sensing organs eventually," Yoshizawa said.
In the meantime, however, the young cavefish continue to follow the shadow of the researchers' pipettes while the researchers try to discover the purpose.
Written by Kate Liebers
Reviewed by Jeffrey Kost
Published by Pooja Ghatalia