Natural History

Armadillos of the Sea

Chitons possess a range of amazing qualities.

By Brian Heagney, B.Sc Marine Biology ~ Photos By Marta Morton, www.harbourclubvillas.com

 

Tucked away in intertidal rock pools on the southwest point of Gibbs Cay in the Turks Islands, there are clusters of tiny dinosaurs called chitons. These ancient mariners are easily overlooked by the untrained eye, but they do deserve a second glance if you have the chance to visit “Stingray Island.” With a fossil record stretching back to the Devonion period 400 million years ago, these surprising little critters have a design that has successfully stood the test of time.

Chitons have 8 overlapping armor plates held by a girdle.

Chitons, commonly referred to as Coat of Mail shells or Sea Cradles, are a relatively small marine mollusc easily recognized by eight overlapping armor plates (valves) surrounded and held together by a leathery girdle or mantle. Small nodules of the mineral aragonite embedded in the shell provide a lens through which the aesthetes (unique light sensitive cells) lying below can detect light, movement and possibly even discern shapes. The chiton essentially “sees” through these opaque rocks in its shell, visual equipment unlike that of almost any other creature.

The girdle is often ornamented with hairy tufts, bristles, spikes or scales that provide camouflage and may also aid in defence. In some species including the largest (the Gumboot Chiton or Wandering Meatloaf), the mantle actually covers the entire shell.

The armor plates themselves are articulated and can flex and move over each other, offering both protection and freedom of movement over the jagged intertidal rocks on which they choose to make their home. When a chiton dies, the girdle decomposes and the individual plates fall apart. These may be discovered by keen-eyed beachcombers and are referred to as Butterfly Shells.

Most chitons are herbivorous grazers, roaming the rocks under cover of darkness, feeding on encrusting algae by scraping it into their mouth with a tooth-covered tongue called a radula (from the Latin radere “to scrape”). There are, however, some carniverous chitons, competing with all the suspense and horror of a good Ridley Scott movie. The predatory species Placiphorella velata waits patiently in ambush, its body held aloft. Smaller animals seeking shelter and shade under this murderous cave are crushed to death and consumed should they inadvertently touch the sensitive tentacles below and spring the deadly trap above.

The chiton’s teeth are of significant interest to science as their microscopic structure and composition—a matrix of organic tissue and inorganic minerals—makes the teeth incredibly wear-resistant, allowing the chiton to nonchalantly chew through rock. A chiton literally makes its home (scar) in the rock by eating the rock away. The teeth contain magnetite or iron (II, III) oxide, a crystal compound that is also found in the beaks of homing pigeons and is the most magnetic of all the natural minerals. These highly magnetic inorganic teeth are found nowhere else in the animal kingdom and may explain the chiton’s remarkable homing ability, after a night of foraging, to use the Earth’s magnetic field to navigate back to precisely the same home scar in the rock.

Their taxonomic class name is Polyplachophora (many plated). Unlike most molluscs, conch for example, chitons cannot withdraw back into their shell. Instead they use their very powerful, muscular foot to cling to the rocks like a limpet and are almost impossible to prise off. When dislodged from the substrate, the chiton can roll up into a protective ball, like a tiny marine armadillo.

The chitons’ main predators are man (naturally), seagulls, starfish, crabs, lobsters and fish. Chitons are eaten in several parts of the world including the Caribbean islands of Trinidad, Tobago, The Bahamas, Aruba, Anguilla, Bonaire, St. Maarten and Barbados. The foot of the chiton is prepared in a manner similar to abalone. They don’t seem to be on the menu in the Turks & Caicos though, conch being the much-preferred option.

Next time you look into a rock pool you may see a little armored snail, present from the dawn of time with magnetic teeth that can pulverize rock with its tongue and see through eyes of made of stone—an amazing little animal that you probably didn’t even know was there.

A native of Ireland, Brian moved to the Turks & Caicos with his wife Sabine in 2016 where they opened The Humpback Dive Shack on Grand Turk.



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