Masato Yoshizawa describes his discovery that blind cavefish larvae retain a shadow response as pure luck. 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. Intrigued, Yoshizawa and William Jeffery began investigating the apparently impossible(p. 292).
But how had the team discovered this intriguing phenomenon? Cleaning out the bowls where Mexican tetra embryos develop into larvae, Yoshizawa had noticed that the larvae of surface-dwelling tetras reacted as he moved his pipette above them; they began swimming to the surface. Yoshizawa explains that Xenpous larvae seek shelter in shadows by swimming towards them. Could blind tetra larvae from cave-dwelling colonies be reacting in the same way? Yoshizawa explains that `if they had the same response it would be interesting, and if there was no response, it would be reasonable'. So he held an object above the blind cavefish larvae's bowl; the tiny swimmers began to ascend. Despite having little or no reason to sense light and dark, the larvae moved towards the shadow. So how were they sensing the shadow?
Yoshizawa and Jeffery reasoned that it could be one of two ways. Although the adult fish lack functioning eyes the embryos begin developing eye structures early in their development, before they are lost again later. Could the larvae's eyes pick out the shadow? Or maybe the cavefish larvae had retained another light sensitive organ, the pineal gland. Yoshizawa decided to test if he could detect the photosensitive pigment rhodopsin using an antibody sensitive to forms of the protein found in the pineal gland and eye. Choosing cave and surface-dwelling larvae that reacted to shadows, Yoshizawa treated them with the antibody and checked where it detected rhodopsin: the pineal gland, and not the eyes, harboured the photosensitive pigment. And when he tested the shadow responses of surface and cavefish larvae whose eyes had been removed, they retained their shadow response only if they retained the pineal gland too: the pineal gland was the light sensitive organ that sent the larvae towards cover.
But why have the cavefish preserved a light sensitive organ and a shadow response after a million years of natural selection? Yoshizawa and Jeffery aren't sure, but they have plenty of ideas. For a start, caves aren't always dark; it depends how deep into the system you are, and sometimes there are cave-ins and light floods in. But Yoshizawa points out another possibility;the pineal gland supplies the body with melatonin, a key reproductive and seasonal growth hormone. Yoshizawa suspects that the selective pressure to retain the body's melatonin supply was greater than the passive accumulation of errors which could have led to the pineal gland's loss. So the larvae's light sensitivity could just be a case of serendipity. Whatever the reason,the cavefish larvae don't seem concerned; whenever Yoshizawa casts a shadow,they head for it.