The Sea Creature with a Body of Eyes

Chitons have unique ‘eyes’, unlike any other creature on earth. They have tiny lenses called ocelli are used to distinguish light and shapes. Recent studies revealed how different extant chiton evolved their unique visual systems.⁤

Chitons

Acanthopleura granulata, West Indian fuzzy chiton, tropical species of chiton. Image by PiterKrig via Depositphotos

Chitons are marine mollusks That belong to the class Polyplacophora, which contains about 940 extant and 430 fossil species. Their aragonite shell plates allow them to flex and curl up into a ball for protection. Chitons can be found stuck to rocks, grazing on algae, bryozoans, and diatoms. However, some species are omnivorous or carnivores.

A Body of Eyes

A recent study has revealed that chiton ancestors evolved eyes on four different occasions, which resulted in two distinct eyes used by chitons today. Their light-sensing structures, called aesthetes, are interspersed with either larger shell eyes or smaller, more numerous eyespots connected to optic nerves via openings into shell plates.

Shell Eyes

Chiton shell eyes are embedded within their aragonite shell. Each ‘eye’ contains a lens, pigmented cells, and photoreceptor cells beneath the lens. Shell eyes’ primary function is to detect light, but scientists believe that ocelli can also detect basic shapes.

Eye Spots

YouTube video — “Sea creature makes a thousand eyes from its shell”, Source: “YouTube”, Uploaded: “Science Magazine”

Chitons that have eyespots usually have a large number of slits on their shell segments. Similarly to shell eyes, eye spots consist of a lens, a layer of pigmented cells to reduce light scattering, and photoreceptor cells beneath the lens. Their function is also to detect light.

The Theory of Evolution

Evolution describes how organisms developed from earlier life forms during the earth’s history. Charles Darwin introduced the concept of evolution.

Chiton evolution

Scientists believe that over time, chitons evolved into four different types of visual systems, two of which are used today. Chiton species with fewer slits evolved larger and more complex shell eyes, while those with more slits developed more numerous but simpler eyespots on its shell.

Conclusion

This interesting mollusk is well-adapted for both protection and sensing its environment. Recent scientific studies provide a glimpse into the evolution of interesting marine mollusks.

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Cayla de Souza, M.Sc. Ocean Sciences & Marine Biology

Author at Animals Around The Globe. MSc Ocean Sciences & Marine Biology

Cayla comes from a small coastal town in KwaZulu-Natal, South Africa, near the renowned diving destination Aliwal Shoal, where her deep-seated passion for the ocean first took root. Family trips to the Kruger National Park are some of her favorite memories; there’s nothing quite like being surrounded by wildlife. These experiences inspired her to study the natural world and its creatures, a path she's been on for the past 5 years. She attained her undergraduate and honors degrees in marine biology and recently completed her master's degree in applied ocean sciences, majoring in oceanography.

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