|Class Polyplacophora (The Chitons or Coat-of Mail Shells)|
The chitons have long been my main academic interest. When I moved to Scotland, I discovered a large population of the british intertidal species, Lepidochitona cinereus. Although common, it rarely occurs in densities which make ecological and population studies practical but at Easthaven, near Dundee, it occurs in unusually high densities. Thus my early publications were on the ecology of this species. However, the shell valve structures called aesthetes then caught my attention and a period of examination of aesthetes in different species followed. My interests have now widened, particularly with exposure to the Australasian species and my interest now lies in comparative anatomy and morphology of these almost unstudied members of the group.
The Polyplacophora, commonly known as chitons, are slow-moving, bilaterally symmetrical, marine molluscs. They are typically grazers living attached to rocky substrates in the intertidal and shallow sub-littoral coastal regions although some groups are known to occur in deep water down to 5000m.
from New Zealand
Cryptoplax striata from South Australia
They are found throughout the world but are most diverse in Australasia and the tropical Pacific coasts of America. They range in size from only a few mm long to some 10cm although the west American species Cryptochiton stelleri reaches 30cm. World-wide there are some 10 families and about 750 living species: only 12 species occur in Britain (Jones & Baxter, 1987) whilst in Australia some 180 species have been described.
Chitons are dorso-ventrally flattened, oval to elongate molluscs, characterised by 8 dorsal, articulating shell plates (valves) which are frequently obvious and brightly coloured and/or sculptured. They are an ancient group, isolated valves having been found in the upper Cambrian, but the fossil record is generally poor The shell valves contain unique microscopic structures called aesthetes which appear to have both sensory and secretory functions.
SEM of valve surface showing aesthete caps
SEM of fractured valve showing megalaesthete chamber and micraesthete branches
The valves are embedded to varying degrees in a fleshy, muscular girdle which may bear calcareous scales or spicules and is sometimes highly coloured or patterned. The girdle functions like the skirt of a hovercraft, enabling the animal to closely follow the topography of the substrate as it moves. The flexible girdle and articulating valves allow the animal to curl up if it becomes detached.
The ventral surface is dominated by the large creeping foot. The foot is surrounded by a pallial groove containing up to 40 ctenidia (gills). Anterior to the foot is the simple head which bears the mouth but lacks eyes or tentacles. The anus is located on a papilla at the posterior margin of the foot.
The mouth bears a large, well-developed toothed structure, the radula, which contains rows of 17 teeth. The shape of the various teeth are important taxonomic characters.
SEM of radula of Rhyssoplax sp
Light microscope photograph of Rhyssoplax radula showing magnetite-tipped (black) tricuspid teeth
Chitons have separate sexes but lack any sexual dimorphism. They usually lay eggs and these develop via a short-lived larval stage but several species are known to brood their eggs in the pallial groove
For more detailed scientific information on the structure of chitons, visit my technical page (in preparation)