Meeting Abstract

89.2  Friday, Jan. 7  Seeing through rocks: Chitons use calcium carbonate lenses to form images SPEISER, D.I.*; JOHNSEN, S.; Duke University; Duke University dis4@duke.edu

Certain chitons, including the Caribbean Acanthopleura granulata, have hundreds of small (80-100 µm), lens-bearing eyes embedded in their dorsal shell plates. Using energy-dispersive x-ray spectroscopy (EDS) and x-ray diffraction, we found that A. granulata’s lenses are made of calcium carbonate. Chitons thus join trilobites and brittlestars as the only animals known to have lenses primarily built from a material other than protein. Behavioral evidence suggests that chiton lenses are involved in image formation: A. granulata displayed a visual response to objects with an angular size of 9°, but not to equivalent, uniform changes in down-welling irradiance (integrated from 400 to 700 nm). An angular resolution of 9° was consistent with eye morphology in A. granulata, provided that this animal’s biconvex lenses act as focusing mechanisms for an array of small, camera-type eyes and not as light-guides for the individual facets of a larger compound eye. Behavioral trials also indicated that A. granulata was, in some ways, less visually responsive than the eyeless chiton Chaetopleura apiculata. While C. apiculata did not distinguish between objects and uniform changes in lighting conditions, it reacted to targets moving as fast as 120°/s and changes in irradiance as small as 1%. In contrast, A. granulata only reacted to objects moving at 46°/s or slower and changes in irradiance of 5% or greater. The transition between extraocular photoreception and eyes in chitons may, therefore, involve a functionally consequential drop in the number of photons gathered by individual photoreceptors.