News about the rock eyes of the marine chiton Acanthopleura granulata

Two well-camouflaged specimens of Acanthopleura granulata, recognizable by the shell and girdle underneath. Photo: Hans Hillewaert / <a href=""target="_blank">Wikimedia Commons</a>, <a href=""target="_blank">Creative Commons License</a>

The small animals have lenses made of aragonite and can probably detect the appearance of predators through changes in brightness

The lens is a central component of the eye of humans and animals because it focuses the light and makes the environment recognizable. Until briefly, only lenses of organic material were known in nature- now, eyes in simple marine animals have been studied in more detail, in which the lens is made of transparent stone. It concerns Aragonit, a not-at-all-rare mineral of calcium carbonate (CaCo3).

The animals have the Latin name Acanthopleura granulata. They live in the warm sea in the Gulf of Mexico or off India down to a depth of 1 m, where they graze dead material from the bottom and stones. Unlike land snails, whose shells consist of a solid structure, their body protection consist of several strips arranged side by side. Under this solid but flexible shell lies the body and the creeping foot. A distinctive feature is a kind of soft girdle that forms the lower edge of the shell.

The animals grow to a maximum size of a few cm.

Below: Chiton eyes, with their translucent lenses and pigment layers clearly visible. The scale applies to (C) only and represents 200 μm.

The eyes have to be imagined as tiny depressions in the shell. In the studied species, their number amounts to several 100 or even several 1000 per specimen, distributed over the outer skin. They consist of a pigment layer, a retina, and the lens made of aragonite.

Aragonite eyes are rigid and therefore cannot adjust their focus or viewing directions in the way the soft eyes of many creatures can. But the chiton‘s ability to process visual information goes beyond individual eyes: The rigid optical elements are interconnected through a complex microscopic channel network that comprises photosensitive cells and neural tissues.

The animal gets visual feedback from this network, but each eye isn’t processing much data. Because a single eye is roughly the width of a human hair, single-eye chiton vision is far from high-definition.

It is known that the animals perceive changes in brightness above them and can thus sense the appearance of enemies. Since the aragonite lens has two focal lengths, this is possible underwater and above water.

Eye cells of Acanthopleura granulata.

However, numerous questions remain unclear: Do all the tiny images come back together in the chiton’s nervous system? Is it able to take those fragments and form a full picture? Does the visual information acquired from individual eyes reconstitute as a more high-definition image?

To clarify them, a leading scientist in the field has now been given generous funding: Ling Li, associate professor in the Department of Mechanical Engineering, has been awarded $1.05 million over three years.

The project reunites Li with a former collaborator, University of South Carolina Associate Professor Daniel Speiser. They also enlisted the expertise of an internationally recognized applied mathematician specializing in image processing, Daniel Baum of the Zuse Institute in Berlin.

Virginia Tech

Science Direct

(02.11.2023, USA: 11.02.2023)