Light Detection by the Pineal Eye of Blind Cavefish Larvae

[PureInsight.org] Scientists
have found that larval blind cavefish (Astyanax mexicanus) retain a
functioning pineal eye that is capable of detecting light.



Since these blind cavefish do not have complete eye structures,
scientists consider they cannot see anything, but Masato Yoshizawa and
William Jeffery, researchers at the University of Maryland, have found
that is not the case.



One day, while cleaning out the bowls where Mexican tetra embryos were
being developed into larvae, Yoshizawa had noticed that the larvae
reacted as he moved his pipette above them; they began swimming to the
surface. Yoshizawa explains that larvae seek shelter in shadows by
swimming towards them. He held an object above the blind cavefish
larvae's bowl; the tiny blind larvae began to ascend and moved towards
the shadow. So how were they sensing the shadow? Deprived of light for
a million years, the fish had lost many of the characteristics
associated with surface dwellers, including their eyes. Yet the tiny
larvae clearly responded when a shadow passed slowly over them.



How can the blind cavefish larvae "see" things? Although the larvae are
equipped with eye's structures, they degenerate with age. These
structures do not have any light sensing pigment. Even after scientists
removed both bilateral eye primordia, the Mexican blind cavefish larvae
still had the shadow response.



The researchers found the shadow response to be substantially reduced
in cavefish larvae, after the removal of the pineal eye, indicating
that the pineal eye is responsible for this response.



Jim Bowmaker, Professor of Vision Science, Institute of Ophthalmology,
expressed that more and more evidences indicate that the pineal eye in
some mammals can have the similar function of eyes and that this
research proves the point.  



Translated from:

http://www.zhengjian.org/zj/articles/2008/3/12/51602.html



Reference:

Yoshizawa, M and WR Jeffery (2008) Shadow response in the blind
cavefish Astyanax reveals conservation of a functional pineal eye.
Journal of Experimental Biology 211, pp. 292–299.