The polyplacophorans, or chitons, are also called ‘coat-of-mail shells’ (cióton máille in Irish language), and they are some of the most enigmatic of the familiar marine invertebrates from rocky intertidal ecosystems. Chitons (are a type of mollusc, the group that includes bivalves (clams), gastropods (snails and slugs), cephalopods (octopus and squids) and allies. Like many other molluscs, chitons are known from fossils dating back at least 400 million years, and their general form has changed little in that time. There are 12 species of chitons living in Ireland today (and several fossil species have also been discovered, but they are not included in this Key). Although the species look similar to the untrained eye, you will quickly learn to spot their distinguishing characters.
Worldwide, there are about 600 species of chitons, living in all oceans. All living chitons have eight shell plates (or valves), in a row on their dorsal surface. The shells are surrounded by a fleshy girdle. The shells and girdle are often all that can be seen of a chiton in life as they cling to rocks much like limpets. Underneath, chitons have a strong foot, with gills on either side, and a mouth with a radular tongue used for scraping algae. Chitons' radulae actually contain scraping teeth mineralised with iron pyrite, specialised to withstand constantly scraping on hard rock surfaces.
Like limpets, chitons use their foot to creep along the substrate, but also to clamp onto rocks. They are very strong so it can be difficult to remove them from the rock without breaking their shell. If they become dislodged, most species can roll into a ball to protect the soft foot. All species of chitons in Ireland live in the very lowest parts of the intertidal, or in subtidal waters, and they are only active when the tide is high.
The upper surface of the shell plates (called tegmentum) can be very colourful or sculptured, but because chitons in Ireland are so small this often cannot be seen without magnification. Because chitons live clinging to rocks on the seashore, their beauty is also often obscured by silt or mud. The girdle also is covered with a variety of scales, hairs, or spicules, which vary between species.
Chitons have no eyes in their head, but they can 'see' through their shells with sensory nerves that go through to the top surface of the valves. These 'aesthetes' are an interesting nervous structure which are not found in any other group of animals. Fossil show evidence of aesthete structures, in the form of distinctive shell pores, dating back to at least the Jurassic period. In living animals, the aesthetes are secreted from the mantle and are long cells that protrude through canals in the shell, ending in a cluster at the outside surface. Exactly how these sensory cells function is still largely unknown.
The majority of chiton species have separate sexes and reproduce by external fertilisation in the water column. However, the differences are internal and males and females cannot be identified visually. Juvenile chitons develop with a free-swimming larva which metamorphoses and grows shells after a few weeks. Tiny chitons that have recently settled (<1 mm long) are usually impossible to identify, and are often confused with deep-water species that are tiny for their whole lives. Such species are not known from Ireland, but they have probable simply not yet been discovered.
Using the key
Most of the species that live in Ireland are very small, so they are difficult to identify without considerable practice. Species which live intertidally can be found at extremely low tides, although they will often be hidden under rocks or ledges or in crevices. By far the most common species to be found in Ireland is Lepidochitona cinerea, although it is possible to find most species at low tide or in the shallow subtidal.
Because the Irish species are all relatively small, it is useful to bring along a hand lens, or use a low-powered microscope for preserved specimens. Colour patterns are usually not a reliable identification tool. The more useful features of chitons are their gills, the sculpture on the shells, and the girdle ornamentation. This key has been designed as much as possible to identify specimens without special equipment, dissection, or high magnification.
On each page of the key is a set of columns, with a list of characteristics and illustrations. Clicking on illustrations will open a new window with an enlarged copy. Anatomical terms are linked to the glossary. Clicking on any of these terms will lead you to its definition. The glossary also contains anatomical diagrams for orientation. Any particular chiton should match all of the characteristics in one column or the other. At the bottom of the column is a link to progress to the next stage of the key. When you reach the last stage of identification, the like will lead to a page with a shrot description and distribution map. You can jump directly to these description pages via the species list.
The design of this key is based on a similar key developed for the Royal British Columbia Museum in Canada. Images within the key come from specimens held in the National Museum of Ireland (Natural History) and line drawings are from the Monograph of Living Chitons by Kaas and Van Belle.
References for further reading: