Polyplacophora

Polyplacophora is a group of mollusks commonly referred to as chiton. Chiton typically have a flattened oval shaped body with 8 shell plates running along their back. Most chiton are herbivores that graze on algae. Their teeth, called a radula, is hardened with metal allowing them to scrape algae right off hard rocks! They are found world-wide in marine habitats from the rocky intertidal zone to the deep ocean.

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Taxonomy

Polyplacophora together with the Solenogastres and Caudofoveata, makes up the group Aculifera which is sister to all other mollusks (Conchifera). The common ancestor to all mollusks is hypothesized to have looked most like a polyplacophoran. Polyplacophora is divided into 4 orders. These include the surviving orders Lepidopleurida and Chitonida and the extinct orders Chelodida and Multiplacophorida. It is estimated that 920 species of chiton exist today with 120 of them falling into the Lepidopleurida order and 800 in Chitonida.

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Lepidopleurida

About 120 living species

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Chitonida

About 800 living species

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Chelodida

Extinct

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Multiplacophorida

Extinct

More than 8 shell plates.

Polyplacophora Orders

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Anatomy

All living chitons have 8 shell plates that are connected to form a flexible sheet of armor. This allows the chiton to remain protected even as it crawls across uneven surfaces. When chiton is detached from the rocks they curl into a ball like and armadillo with their shell plates protecting their soft underside. The shell plates are made of aragonite, a form of calcium carbonate that is harder than more commonly occurring calcite. The chiton’s shell plates are surrounded by a fleshy looking disc called a girdle. The girdle is often covered in scales and spicules that are also made of aragonite. The girdle is lined with a set of small “eyes” called aesthetes. The aesthetes give the chiton a 360-degree view of their surroundings, albeit a very crude one only able to sense light and sometimes rough shapes.

Main features of the chiton underside are the foot, mouth, and ctenidia. The foot is like a large suction cup sticking the chiton firmly to rock surfaces. The chiton moves by undulating this foot in a wave like motion. This style of movement allows them to move both forward and backward.  Chiton all move relatively slowly with some moving much slower than others. 
 

In order to breath the chiton pulls water under the girdle near its mouth into a cavity called the mantle groove that exists between the girdle and foot. The water is pushed through this groove past a set of gills called ctenidium and exhaled at the rear of the chiton. 

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Ecology

Most chiton are herbivorous grazers feeding almost exclusively on algae like coralline algae, cyanobacteria, algal mats, and seagrasses. However, other unique feeding strategies can be found within this group including carnivores and detritivores. Prey for carnivorous chiton include crustaceans, worms, starfish, and sponges.

Chiton are eaten by a wide variety of animals including fish, birds, starfish, crabs and other mollusks. They are even eaten by people in many places around the world.

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Reproduction

Chitons reproduce through broadcast spawning which means large amounts of eggs and sperm are released into the water column. Broadcast spawning events typically occur at night and all members of species are coordinated to release at the same time by environmental cues like the phase of the moon or tide. However, there are a few unique species that brood their eggs in the mantle groove until they are developed larvae or even small juveniles.

Citations

Kocot, K. M., Poustka, A. J., Stöger, I., Halanych, K. M., & Schrödl, M. (2020). New data from Monoplacophora and a carefully-curated dataset resolve molluscan relationships. Scientific Reports, 10(1), 1–8. https://doi.org/10.1038/s41598-019-56728-w

Ponder, W. F., Lindberg, D. R., & Ponder, J. M. (2020). Polyplacophora, Monoplacophora, and Aplacophorans. In Biology and evolution of the mollusca (Vol. 2, pp. 67-107). Boca Raton: Taylor & Francis.