Researchers at the University of Maryland have found that larval blind cavefish (Astyanax mexicanus) retain a functioning pineal eye that is capable of detecting light.
In the study published by Masato Yoshizawa and William Jeffery in the most recent issue of the Journal of Experimental Biology, the authors studied the response of larval cavefish reared from eggs kept in total darkness to the shading of light.
It was found that the larval fish responded to the dimming of light by shading the pineal eye and swimming vertically upwards, a response that ...resembles that of amphibian tadpoles rather than other teleost larvae, which react to shadows by swimming downward....
Shadow responseThis shadow response was found to be the strongest about one and a half days after fertilization, and disappeared after seven and a half days after fertilization.
The researchers found the shadow response to be substantially reduced in both cavefish larvae and larvae of the surface, sighted form after the removal of the pineal eye, indicating that the pineal eye is responsible for this repsonse.
This is supported by the fact that removal of one or both bilateral eye primordia did not affect the shadow response.
The role of a functioning pineal eye in blind cavefish larvae remains unclear.
Role of pineal eyeThe authors speculate that this may be due to the necessity of the neurosecretory role of the pineal gland for survival in the cavefish, or to the fact that ...the cascade of regulatory gene expression leading to photoreception and melatonin synthesis may be integrated to an extent that it cannot be easily uncoupled during relatively short evolutionary intervals....
The authors conclude that ...light detection by the larval pineal gland may be conserved because it is beneficial for survival in the cave environment. The cave systems inhabited by Astyanax cavefish can contain karst `windows', areas of ceiling collapse that allow dim light penetration, and are subject to periodic episodes of extensive flooding...
Floods could propel cavefish from the light-less cave interior to semi-lighted locations, such as near cave entrances or spring resurgences. Both scenarios could expose cavefish larvae to predation in lighted habitats. Conservation of the pineal eye could be used to avoid the potential threat of exposure to light and predation.
For more information, see the paper: 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.