Diversity of Exumbrellar Surface in Jellyfish with Remarks on the Optical Property of a Microvillar Array

To understand the adaptation of body surface structures, the ultrastructure and optical properties of exumbrellar tissue were examined in cnidarian jellyfish, in which the epidermis is directly exposed to the external environment. The microscopic morphology of the exumbrellar epidermis varies from species to species. It can be flat or uneven, with/without cellular bulges, cilia, or an array of microvilli. Rigorous coupled wave analysis indicates that the light reflectance on the exumbrellar surface is expected to be very low owing to the very small differences in the refractive indices between exumbrella and seawater. The nanostructures modeled from the microvillar array of Spirocodon saltatrix reduced the reflection of light with large incident angles, which potentially affected the visibility of the contour of the umbrella. Therefore, the microvillar array may serve functions comparable to those of the nano-scale nipple array in some other metazoans. Moreover, an uneven pitch (distance between the apexes of neighboring microvilli) in the microvillar array potentially enhances the anti-reflection property. Although the difference in reflection reduction was only slight, this may be an example of unevenness in real biological structures that enhances the functional properties of the nanostructure.