A while back in the Annals, I introduced the mysterious (but not mythical) third eye, made even more peculiar by unexpected findings in the Anolis pineal gland (Moore & Menaker 2011). In a later post, I discussed non-visual photoreception—responses to light that do not require image formation—along with some recent evidence that such responses may be tuned to photic habitat in Anolis (Moore et al. 2012). Now I’m connecting the dots: non-visual photoreception in the Anolis pineal gland appears to be adapted to photic habitat (Moore & Menaker 2012).
The pineal gland can’t be seen externally, but it’s just posterior to the parietal eye (tiny circle in the middle) and right underneath the surface of the skull. Photo credit: TheAlphaWolf, License:Creative Commons Attribution-Share Alike 3.0 Unported
Light regulates the pineal gland's production of the circadian signaling hormone melatonin. Anoles occupy distinct photic niches according to their sun/shade preferences, raising questions about how pineal photoreception adapts to photic habitat.
The idea—that pineal photosensitivity is adapted to habitat light—has been around for a long time. The reasoning is simple. Vertebrate pineal function (reproduction, thermoregulation, circadian rhythms) is important for fitness. The pineal is a sensory organ, translating environmental light and temperature information into systemic hormone signals. Since environmental cues vary among habitats, it is expected that pineal sensory processing would be optimized to a given habitat. Surprisingly (at least to me), no one had ever tested this hypothesis… and a dissertation project (mine) was born.
The most well-conserved function of the pineal gland is the control of circadian rhythms. From hagfishes to humans, light regulates the rhythmic production of the pineal hormone melatonin, which, in a nutshell, tells the rest of the body what time it is. In general, nocturnal animals are more sensitive to the photic regulation of melatonin than diurnal animals. This is taken as indirect evidence in support of the idea that circadian photoreception is adapted to habitat light.
Indirect evidence; there’s the rub. My goal was to find direct evidence for this idea, if it existed, and there were a couple of things on my agenda. 1) Choose a comparative system with a well-supported phylogeny. 2) Avoid nocturnal/diurnal comparisons, which confound temporal niche differences with photic niche differences.
Anoles to the rescue! Exclusively diurnal, good phylogenetics, and variation in photic habitat among closely related species.
Cladogram showing variation in photic habitat among Puerto Rican anoles. The species in bold underline are the ones I tested. (Figure taken from Moore & Menaker 2012)
These plots (from Moore & Menaker 2012) depict the timing of the pineal melatonin rhythm and how light treatment "shifts" this timing. Arrows indicate the time (in "circadian hours") at which melatonin is rising maximally. Arrows indicate the mean time; small dots indicate time for individual pineals. Black=controls; gray=pineals subjected to light-resetting treatment.
The photic regulation of pineal clock function is quantifiable. Light “resets” the clock, and this can be observed as a “shift” in the melatonin rhythm. The magnitude of the shift depends on the timing of the light treatment and the sensitivity of the clock’s photoreceptors. I used a standard photic resetting procedure to look for differences in pineal sensitivity among pairs of species that differ in photic habitat, but not ecomorphology. I found that the shift in the melatonin rhythm was greater in species from relatively more shaded habitats, suggesting that the Anolis circadian system is adapted to habitat light.
- Sensory Ecology of the Third Eye - June 14, 2012
- Anole Visual Ecology, Sans Vision - January 13, 2012
- Surprises from the Anolis “Third Eye” - September 1, 2011
Tikkun olam
Too bad you are far. i can turn on my light receptors in my pineal gland and i see an over lay picture of auric light in my vision around everything as light color bubbles and i see like a tv bleeding in front of unless i close my eyes and it looks better, but my background in my eye lids is super violet and purple. when i stop moving my eyes and anchor them and use certain imagination. it all lights up. i feel a sensation rushing up my body to the top where the pineal is then its flows like water or some liquid into my forehead. i can feel all this from inside even now i see everything. you are on the right path. just dont listen to anyone who sees your work negative 😉