Are Chitons fossils?

A topic on how chitons develop and live

5 min readAug 15, 2018

Chitons are beautiful marine mollucs that balance tidepools, they have survived multiple extinctions and yet most of the studies on chitons suggest they are experts at survival.

What is a Chiton?

Chitons are beautiful Marine Molluscs that contain 8 plates to make up the shell.The foot is broad and flat, they have a mouth and basic endocrine system and have grills on each side 6 to 88 pairs. Scientist have finally concluded Chitons also hold an amazing secret they have eyes!

There are approximately 1,300 species of chitons, all beautiful in color, complexity and Size.

Kingdom: Animalia

Phylum: Mollusca

Class: Polyplacophora

Family: Mopaliidae

Genus: Mopalia

Species: M. muscosa

https://en.m.wikipedia.org/wiki/Mopalia_muscosa

Southern California Spiny Chiton Surrendered by Pink Volcano Barnacle

Photo by: Alexander_naturalist

What makes Chitons special?

Chitons are special because they resemble a living fossil, I remembered when I first saw a Chiton I had no clue what I was looking at. Through constant study I have learned amazing facts, Chitons not only survived multiple extinction level events they have learned to adapt and evolve eyes!

Chitons are essential to the environment they are filter feeders they clean the ocean water. These creatures are beautiful to observe in the tidepools.

Photo: http://www.sci-news.com/biology/chitons-ceramic-eyes-03453.html

But where exactly are the eyes?

“The shell contains hundreds of small eyes called (Ocelli) each one contains a layer of pigment, a retina and a lens” -Daniel Speiser UC Santa Barbara

Chiton lenses are made of a mineral called aragonite.

Aragonite is a version of Calcium Carbonate. Rocky eyes are special to chitons,they may be the only animal living with these types of eyes.

With time the intertidal zone erodes the eyes. It was estimated the eyes are on the evolutionary time scale to about 10 million years.

Do Chitons have brains?

Chitons lack a clearly demarcated head; their nervous system resembles a dispersed ladder. No true ganglia are present, as in other molluscs, although a ring of dense neural tissue occurs around the oesophagus. From this ring, nerves branch forwards to innervate the mouth and subradula, while two pairs of main nerve cords run back through the body. One pair, the pedal cords, innervate the foot, while the palliovisceral cords innervate the mantle and remaining internal organs.

How do Chitons reproduce?

Most chitons separate sexes and release their gametes in spring time from genital pores located on either side of the body in the pallial (mantle) cavity. Fertilization Is external and development leads to trochophote larva (The second stage).

Photo: Bruce Kerwin

The first Stage: Chiton Egg

Photo: Jenna Valley

During the first stage the egg has been released from a mature Chiton, Chiton eggs are very distinctive because they are surrounded by a thick and often ornate hull (as seen in this picture). The hull has been found to reduce the sinking rate of the egg as well as to focus sperm to specific regions on the egg surface (Buckland-Nicks, 1993) .

The Second Stage: Trochophore Larva

During this stage the body is oval shaped, the apical pile has a tuft of cilia at the center of the lobe. A circle Cilia called prototroch is present at the apical lobe. At the basal part of the body the lobes have developed and this is the most important part until they go into the third stage.

The Third Stage: Development of the shell

This picture shows seven transverse bands on the dorsal side of the larva posterior to the prototroch. These bands delineate the boundaries of the future shell plates; the eighth plate appears later in development (anterior to the prototroch). The dark band is part of the mineral skirt, which can be seen more clearly in the photo below. The foot and mouth of the chiton is developing on the ventral surface.

The Final Stage: Chiton

Instagram: Alexander_naturalist

In the final stage of development chitons spend months in a single scar section creating the perfect home. Chitons spend up to 2 months hunkered down feeding on moss. They are known to be within a foot radius of other chitons but they always stay away from other chitons territories.

Chiton Observations:

Laguna beach, California.

Cape Perpetua, Oregon.

Photo Credit: Girlinwaterphotography

Cape Perpetua, Oregon.

Photo: Girlinwaterphotography

Why are Chiton shells interesting?

Chiton shells form in the final stage, after the (Trochophore larva) develops the shell producing glads, the Chiton goes through a very fast process eating as much moss as possible to start producing (Conchin)

Conchin is secreated by the glands in the mantle and starts forming a shell. The shell of Chiton is Calcium Carbonate (CaCo3) an abundant mineral found on earth.

Chiton shell: