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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Phototaxis | 5/5 | https://en.wikipedia.org/wiki/Phototaxis | reference | science, encyclopedia | 2026-05-05T07:16:22.043277+00:00 | kb-cron |
== Light based vertical movement == A depth can also be selected based on light levels: The brightness decreases with depth, but depends on the weather (e.g. whether it is sunny or cloudy) and the time of the day. Also the color depends on the water depth and dissolved and suspended matter. The only consistent factor is that at a given place, deeper water is darker. In water, light attenuates differently for each wavelength. The UV, violet (> 420 nm), and red (< 500 nm) wavelengths disappear before blue light (470 nm), which penetrates clear water the deepest. The wavelength composition is constant for each depth and is almost independent of time of the day and the weather. To gauge depth, an animal would need two photopigments sensitive to different wavelengths to compare different ranges of the spectrum. Such pigments may be expressed in different structures. Such different structures are found in the polychaete Torrea candida. Its eyes have a main and two accessory retinae. The accessory retinae sense UV-light (λmax = 400 nm) and the main retina senses blue-green light (λmax = 560 nm). If the light sensed from all retinae is compared, the depth can be estimated, and so for Torrea candida such a ratio-chromatic depth gauge has been proposed. A ratio chromatic depth gauge has been found in larvae of the polychaete Platynereis dumerilii. The larvae have two structures: The rhabdomeric photoreceptor cells of the eyes and in the deep brain the ciliary photoreceptor cells. The ciliary photoreceptor cells express a ciliary opsin, which is a photopigment maximally sensitive to UV-light (λmax = 383 nm). Thus, the ciliary photoreceptor cells react on UV-light and make the larvae swimming down gravitactically. The gravitaxis here is countered by phototaxis, which makes the larvae swimming up to the light coming from the surface. Phototaxis is mediated by the rhabdomeric eyes. The eyes express at least three opsins (at least in the older larvae), and one of them is maximally sensitive to cyan light (λmax = 483 nm) so that the eyes cover a broad wavelength range with phototaxis. When phototaxis and gravitaxis have leveled out, the larvae have found their preferred depth.
== See also ==
Photokinesis Phototropism (more relevant to plants and fungi)
== References ==
== Further reading ==