Calcitic microlens arrays in Archaster typicus: microstructural evidence for an advanced photoreception system in modern starfish
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Recent studies have shown that in some light-sensitive species of an brittlestar (Asteroidea, Echinodermata), the upper surface of the dorsal arm plate bears arrays of hemispherical microstructures which in combination with underlying neural bundles and intraskeletal chromatophores probably function as a compound eye. These calcitic lenses possess superior properties such as light weight, mechanical strength, and very low aberration and birefringence; they display a unique focusing effect, signal enhancement, intensity adjustment, angular selectivity, and photochromic activity. The discovery of these unique optical structures revealed that brittlestar visual system is more sophisticated than initially thought and has inspired active interest toward designing of biomimetic highly tunable optical elements for a wide variety of cutting-edge technological applications. Up to this moment, analogous spherical calcitic lenses have been only reported in a few species of modern brittlestars and starfish. Similar calcitic microlenses have been also observed in the Late Cretaceous fossil echinoderms. Here, we report the structural evidence for the presence of calcitic microlenses in an extant species of starfish Archaster typicus. The close resemblance in microstructure and location between the transparent regions of compact stereom described above and microlenses in the photosensitive brittlestar Ophiocoma wendtii suggests that these regions may be involved with the photoreceptor system in A. typicus.