Mineral Eyes
These adjustable plates are from an armour-plated mollusc called a Chiton, in this case an Acanthopleura granulata. Chitons (not to be confused with the material Chitin) are a kind of aquatic woodlouse (pill bug) so-to-speak, and remarkably a Chiton (at least an Acanthopleura granulata) is not only able to roll into a hardened ball, if pried from its intertidal rocks by a hungry predator, but can actually see the threat coming with many hundreds of toughened, protected and replaceable mineral eyes embedded into its dorsal shell plates.
Unlike most eyes we know about, including ours, that are made of protein, Chiton eyes the first to be found that are made of a kind of calcium-carbonate called Aragonite in a crystalline form. It is proposed that given their intertidal habitat these eyes have evolved to resolve a threat equally through water and through air.
The researchers examined the microscopic structure of these aragonite eyes, comparing them with the surrounding armor structure. They also ran experiments and simulations to reveal that the eyes are more than just light-sensitive spots; they actually resolve images. From more than 6 feet away, chitons can see a blur representing a small fish. This gives them time to clamp down hard on the rock below so the potential predator can’t dislodge them, Li said.
Sight has its costs, though. The researchers found that the aragonite eye structures are not as strong as the surrounding armor. Though the two are made of the same mineral, the aragonite in the eyes has a different crystalline structure. That different structure, along with a pore space beneath the eyes, makes them weaker. Thus, they fracture more easily.
More here: https://goo.gl/LhN2xn
Chiton (Wikip): https://goo.gl/yxsNVq
(see more pics)
Hundreds of ocelli are embedded in the dorsal shell plates of certain chitons [ 1 ]. These ocelli each contain a pigment layer, retina, and lens [ 2 ], but it is unknown whether they provide chitons with spatial vision [ 3 ]. It is also unclear whether chiton lenses are made from proteins, like nearly all biological lenses, or from some other material [ 4 ]. Electron probe X-ray microanalysis and X-ray diffraction revealed that the chiton Acanthopleura granulata has the first aragonite lenses ever discovered. We found that these lenses allow A. granulata’s ocelli to function as small camera eyes with an angular resolution of about 9°–12°. Animals responded to the sudden appearance of black, overhead circles with an angular size of 9°, but not to equivalent, uniform decreases in the downwelling irradiance. Our behavioral estimates of angular resolution were consistent with estimates derived from focal length and receptor spacing within the A. granulata eye. Behavioral trials further indicated that A. granulata’s eyes provide the same angular resolution in both air and water. We propose that one of the two refractive indices of the birefringent chiton lens places a focused image on the retina in air, whereas the other does so in water.
Paper (closed): https://goo.gl/H9PV8z
Image: Invertzoo https://goo.gl/pQIVxU
