Photosensory Organs Assemble at Low Temperatures

Biological Strategy

Red Ophiocoma

AskNature Team

Sense Light (Visible Spectrum) From the Environment

Living systems constantly receive signals from their environment that help them survive. Light (in the visible spectrum) can come from other living systems (such as fireflies) or from non-living sources (such as the sun). Survival often depends on sensing and responding to challenges like low light conditions or light that has been altered in some way. Because basic survival is at stake, living systems must excel at meeting those challenges. A well-known phenomenon is how water bends light. A stork trying to catch a fish underwater can compensate for this bending effect so that when it strikes at the fish, it has a good chance of catching it.

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Echinoderms (Sea Stars, Sea Urchins)

Phylum Echinodermata (“spiny skin”): Sea stars, brittle stars, sea cucumbers, sea urchins

Echinoderms get their name from their tough, spiny outer skin, which is clearly visible in some species, like sea urchins. They are known for their five-point radial symmetry and specialized water vascular system, which depends on changes in water pressure to extend or contract tiny tube feet. Pumping water through this system allows echinoderms to eat, move, or cling to surfaces, but it doesn’t generally enable very rapid movement. Some even have the ability to regrow body parts, which comes in handy if a limb is nibbled off by one of the ocean’s very fast-moving predators.

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Arms of brittlestars have crystal lenses that focus light efficiently, yet are assembled from natural materials at sea temperature.

“Certain single calcite crystals used by brittlestars for skeletal construction are also a component of specialized photosensory organs, conceivably with the function of a compound eye. The analysis of arm ossicles in Ophiocoma showed that in light-sensitive species, the periphery of the labyrinthic calcitic skeleton extends into a regular array of spherical microstructures that have a characteristic double-lens design(…)The microlenses are optical elements that guide and focus the light inside the tissue(…)The optical performance is further optimized by phototropic chromatophores that regulate the dose of illumination reaching the receptors. These structures represent an example of a multi-functional biomaterial that fulfills both mechanical and optical functions.” (Aizenberg et al. 2001:819)

“Brittlestars form a nearly perfect optical device with micron-scale,
lightweight, mechanically strong, aberration-free, birefringence-free,
individually-addressed lenses, which offer a unique focusing effect,
signal enhancement, intensity adjustment, angular selectivity, and
photochromic activity.” (Aizenberg and Hendler 2004:2066)

Aizenberg J; Tkachenko A; Weiner S; Addadi L; Hendler G. Calcitic microlenses as part of the photoreceptor system in brittlestars. 2001. Nature 412:819-822.

Aizenberg J; Hendler G. 2004. Designing efficient microlens arrays: lessons from Nature. Journal of Materials Chemistry 14:2066-2072.