A Window to the Seasons
Some people speak of having a 'third eye', which they believe enables them to see things that the other two cannot. Such a structure may well exist, though it is highly doubtful that this 'extra' eye grants any of the mystical insight often attributed to it. But what this third eye actually does is no less wonderful.
The structure in question is part of the brain called the pineal organ (or epiphysis cerebri). In humans, the pineal organ is buried deep within the brain and - until recently - no one was sure what, if anything, it did. In sharks and other 'lower' vertebrates, however, the pineal organ is located on the upper surface of a part of the forebrain known as the epithalamus. In some terrestrial vertebrates, such as the tuataras (Sphenodon spp.) of New Zealand, this organ is covered by a clear, eye-like blister on top of the head between the eyes. Over the pineal organ of many pelagic (open sea) tunas and sharks, there is a thin translucent 'window' in the skin and underlying skull. This 'window' was first and vividly demonstrated by shark biologist Samuel Gruber and his co-workers in an elegantly simple experiment: the glow of a flashlight placed in the mouth of a Lemon Shark (Negaprion brevirostris) could be seen clearly through the top of the shark's head. The 'pineal window' thus allows diffuse sunlight to filter through the skin and skull of these fishes to stimulate the brain directly.
The pineal organ responds to ambient light levels by producing a pair of hormones, melatonin and serotonin. Melatonin (which has recently been promoted as a 'natural' sleep aid) is produced primarily during periods of darkness, while serotonin (which has been linked to depression) is produced primarily during periods of light. The relative concentration of these two hormones vary in inverse proportion to one another, depending upon the average intensity and duration of daylight. Since average ambient light levels fluctuate gradually throughout the year, in pelagic sharks - such as the Great White - the pineal organ may enable tracking seasonal changes in day-length. This ability may be particularly important in co-ordinating significant events in a shark's life cycle, such as migration, mating, and pupping. Thus, the pineal organ in sharks may function as a kind of built-in calendar.
In the early 1980's, a new psychiatric illness appeared in which sufferers underwent severe depression during fall and winter months, followed by mild mania in the spring. This illness was dubbed Seasonally Affective Disorder or SAD. In 1986, psychiatrist R.A. Wehr and his co-workers published a study that revealed that SAD could be treated with very bright fluorescent lighting during crepuscular (dawn and dusk) periods. It is now believed that sufferers of SAD are particularly sensitive to changes in the relative proportions of melatonin and serotonin, which stimulates their undesirable mood swings The details of how ambient light levels affect the human pineal organ, tucked away deep in the brain, are unclear at present*. But it seems likely that the co-ordination of important life cycle events is a major adaptive value of this built-in calendar.
Perhaps the brains of humans and White Sharks are not nearly so different as had long been supposed.
* = A 1999 popular article by Mark Caldwell reported on recent research about SAD and biological clocks. Melatonin and serotonin are not the direct cause of daily biological rhythms, although these compounds do seem to play a role in 'setting' internal clocks depending upon seasonal changes in day-length. The actual clock mechanism depends upon a roughly day-long cycle of manufacture and destruction of two proteins (dCLOCK and dBMAL1). This process occurs in paired structures of the brain called suprachaismatic nuclei, composed of a cluster of about 10, 000 neurons located behind and beneath each eye. The precise relationship between the cellular activity of the suprachiasmatic nuclei and melatonin-serotonin levels remains elusive.
