Chitons: Primitive Inhabitants of Today’s Seashores

The intertidal zone is a rich habitat, teeming with marine life ranging from the beautiful to the peculiar and fascinating. Upon careful observation, one can find an animal believed to have persisted in this environment for over 400 million years — the mighty little chiton (pronounced ky-tuhn, where ky rhymes with eye). According to fossil records, chitons predate the first dinosaurs by over 150 million years and have survived multiple mass extinction events, including the one that wiped out most dinosaurs.

Chitons are relatives of marine snails, found primarily in intertidal and shallow sub-tidal zones. However, a restricted few species have also been observed near deep-sea hydrothermal vents at depths greater than 5,000 m.

Chitons belong to a class called Polyplacophora, which roughly translates to “bearing multiple plates”. The name is apt, as chitons are classified as having eight plates on the back (top for the viewer), made up of a tough calcium carbonate-based mineral called aragonite. These plates overlap slightly on both sides, providing an armour that protects the soft body and is flexible to accommodate movement. These plates are surrounded by a flexible, fleshy skirt-like part called the girdle, which is part of the main body of the chiton. Like other marine snails, chitons crawl along on a muscular foot that occupies the entire ventral (lower) surface and adheres to the hard substrate strongly.

There are more than 900 surviving species of this primitive animal, and an individual’s life span typically ranges from a few years to 20 years. Their size ranges widely from less than two centimetres to 30 cm long. The largest chiton, commonly called the gumboot chiton, measures over 30 cm, can weigh as much as 2 kg, and is believed to live up to 40 years.

Chitons vary in appearance and can be found in shades of grey and brown, red, green, and pink. Depending on their species, the girdle may be completely smooth or have scales, spicules, and granular protrusions, which may serve as tools for defence and camouflage. Some individuals take this one step further and exhibit biofluorescence on the girdle that shows up in a glowing, neon green colour under UV light.

As nocturnal animals, chitons feed at night and hide in crevices or under rocks during the day. While they primarily feed on algae and encrusting organisms like sponges and bryozoans some species also feed on plankton, , crustaceans, and sometimes even small fish. The mouth is located on the underside and is equipped with a nailfile-like tongue called the radula. The radula has multiple rows of tiny sharp teeth coated with hard magnetite, a ferric mineral (iron). Chitons use this radula to scrape algae off rocks or catch small plankton in shallow water.

Common predators of these armoured creatures include sea stars, crabs, birds, fish, and larger gastropods (marine snails). A number of people across the world eat chitons, prominently those settled along coastlines and on islands. Evidence suggests these animals have historically been part of these peoples’ regular diets.

Chitons have evolved in small but very significant ways over the last few centuries. While they lack functional eyes, some species have hundreds to thousands of tiny eyespots called ocelli. Ocelli are basically aragonite (calcium carbonate) lenses derived from the shell plates. They are like photoreceptor cells, capable of sensing changes in light. While incapable of true vision, these ocelli can sense shadows and differences in light intensity, allowing the chiton to perceive an approaching predator. Interestingly, the literature suggests that chitons have always had extremely rudimentary precursors of these ocelli, but the more advanced photosensitive versions they have today only date back a maximum of ten million years in fossil records, which is a mere fraction of the time chitons have been around.

Even more intriguing than the ocelli is that chitons exhibit a strong homing instinct, navigating to multiple places to feed and return “home”. While this navigation capacity itself is not peculiar, observing it in animals as primitive as chitons merits more observation. One theory suggests the chitons simply follow a trail of chemicals they secrete behind them, much like other snails. Another theory suggests that they can memorise the area’s topography (presumably like a negative film), and navigate their return based purely on the images from their ocelli and the physical attributes of the substrate. After a few experiments, yet another theory hints at the possibility that chitons, using the iron crystals in their radula, can sense the Earth’s magnetic field and polarity through magnetoreception and find their way home.

One may wonder what ecological function such small creatures play in a vast ecosystem. “Strength in numbers” holds true in this case, as chitons and other intertidal grazers keep the algal growth in check, contributing to an overall balance. Besides threats from predators, indirect factors like pollution, and climate and human-driven changes along coastlines threaten chiton habitats.

Chitons have persisted for millions of years, and one does wonder if they will still be around for another million. Will they remain as unchanged and unaffected as they have been so far, or will they evolve eyes more complex than their current ocelli? Whatever changes time may bring, these small, comparatively less complex creatures will still represent a chunk of this planet’s long history.