Identification resources
Below are the characteristics of each taxonomic rank of Acanthochitona sp. to assist in making a correct identification in the field.
Phylum Mollusca:
A generalised mollusc is bilaterally symmetrical and has a mantle, which is the dorsal body wall over the visceral mass (below the shell). The mantle is sensory and secretes proteins, calcium salt and mucus. Calcareous material excreted from the animal is used to form a calcareous shell on its dorsal side. A muscular foot with a flat, ciliated sole is found on the ventral surface of the animal and is used for locomotion. A radula is unique to molluscs and consisting of rows of tiny chitinous teeth found inside the mouth for feeding (Figure 1) (Ruppert et al 2004).
Figure 1: Lateral view of a generalised mollusc, adapted from Ruppert et al. 2004.
Class Polyplacophora:
Polyplacophorans are dorso-ventrally flattened and have an elongated oval body with a poorly developed head that is hidden from sight on the dorsal side. They have eight distinct overlapping shell plates known as valves, which are the defining characteristic of the class. These valves have many different sculptures and colours and the morphology of chiton valves is useful in taxonomy to describe a species (Figure 3). The first valve is the anterior or head valve, which is followed by valves 2-7, known as the intermediate valves. The last valve is known as the posterior or tail valve. The tegmentum of the valves (the exposed area) possess small microscopic holes, aesthetes, that have secretory and sensory abilities and are useful in identification. The mantle extends to form the girdle, which covers the dorsal side of the animal and partially surrounds the valves. The extent to which the valves are covered is different for different chitons and is useful for identification. The girdle secretes a cuticle that has bristles, spicules or scales depending on the species of chiton (Figure 2)(Figure 3)(Jones and Baxter 1987, Kaas et al. 1998).
Figure 2: A lateral view of a common polyplacophoran body plan, represented by the species Lepidochitona cinerea, adapted from Ruppert et al 2004.
Figure 3: Possible plate and girdle characteristics of a polyplacophoran. Plate characteristics: A: radially sulcate. B: finely longitudinally striate. C: quincuncially striate. D: radially sulcate. E: longitudinally sulcate. F and G: concentrically striate. H and I: quincuncially granulose. J: longitudinally pustulose. K: radially pustulose. L: longitudinally sulcate. M and N: tuberculate radial ribs. Girdle characteristics: O: imbricating striated scales of girdle. P: leathery girdle. Q: granulose girdle. R: spiculose girdle. S: spinulose sutural tuft. Adapted from Gowlett-Holmes 1998.
Order Neoloricata:
This order consists of all living chitons, that posses an articulamentum on their valves. The articulamentum is a layer of the valve that is hidden under the girdle and forms the insertion plates (Figure 4). The other order in the class polyplacophora is Paleoloricata and this order consists of all extinct chitons that did not have an articulamentum on their valves and therefore lacked insertion plates (Kaas et al. 1998).
Figure 4: Example of the 4th intermediate valve of a polyplacophoran from the order Neoloricata, with an articulamentum present, which forms the insertion plates. Adapted from Ruppert et al 2004.
Suborder Acanthochitonia:
Within this suborder, all chiton's have insertion plates present on all valves and they are usually toothed or slitted with the last valve having large insertion plates. The tegmentum is reduced or can be completely absent and the girdle is spiculose (Kaas et al. 1998). Chitons in this suborder also have abanal gill arrangement and cupsule structures on the hull of their eggs (Okusu 2003).
Family Acanthochitonidae:
Chitons within this family are usually small to medium sized with a body that is oval to elongate in shape. The tegmentum of the valves is usually reduced due to a fleshy girdle that can be smooth to spiculose and sutural tufts of spicules (found at the joining of two valves) are usually present. The valves are small to large in size and also posses an articulamentum with well pronounced insertion plates. The head valve usually has up to 5 slits on the insertion plate (Figure 4). The major teeth of the radula have tridentate caps in this family (Gowlett-Holmes 1998).
Figure 4: Valves of polyplacophoran from the family Acanthochitonidae (Acanthochitona bednalli). A: Head valve. B: intermediate valve. C: Tail valve. The head valve demonstrates 5 slits, common in this family. The articulamentum is in the form of insertion plates and sutural laminae. Insertion plates are found laterally on valves and the sutural laminae are extensions of the anterior side of the intermediate and tail valves. Note that the sutural laminae is divided by a jugal sinus whereas in other polyplacophorans the sutural laminae is not divided. Adapted from Gowlett-Holmes 1998.
Genus Acanthochitona:
Acanthochitona species have a broad girdle that surrounds the valves and reduces the tegmentum, to a greater or lesser extent, depending on the species. The head valve will have 5 slits in the insertion plates and the rest of the valves have 1 pair of slits in the insertion plates. The sculpturing on the top of the valves (tegmentum) is course and consists of papillae (elevated granules), usually arranged in radiating rows that are round to teardrop shaped. The girdle can be spiny or nude but tufts of spicules around the head valve and at the suture of each valve are usually present, with most species typically having 18 of these sutural tufts (Jones and Baxter 1987).
Acanthochitona sp.
The family Acanthochitonidae and genus Acanthochitona are difficult to identify to species level as there is lack of a worldwide monograph for this family. Therefore, the specific species name for Acanthochitona sp. is currently unknown. However, the characteristics of Acanthochitona sp. can be found on the following pages: Description, External Morphology and Internal Anatomy.
Keys are also useful to correctly identify a species. This key below is taken from Kaas et al. 1998 and was useful in identifying this chiton down to the family level.
Key to the suborders of the order Neoloricata
1(a) Valves without or with unslitted insertion plates.....................................................2
(b) At least one of the valves with a slitted insertion plate............................................3
2(a) Valves with well developed tegmentum, without or with unslitted insertion plates.............................................................................................................Lepidopleurina
(b) Valves with reduced tegmentum and large, unslitted insertion plates.............................................................................................................Choriplacina
3(a) Tegmentum always present, value one with six or more slits in insertion plate...............................................................................................................Ischnochitonina
(b) Tegmentum absent, or number of slits in valve one at most five.................................................................................................................Acanthochitonina
Key to the families of the suborder Acanthochitonina
1(a) Body elongate oval; insertion plate of head valve mostly with five slits................................................................................................................Acanthochitonidae
(b) Body veriform; insertion plate of head valve with three slits, other valves having none..............................................................................................................Crytoplacidae
The Marine Species Identification Portal is useful to assist in identifying a chiton to genus or species. This website can be visited by clicking Here. |