Key to BC Chitons Glossary and Species List
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Key to BC Chitons Glossary and species list Introduction A chiton in life holds fast to the substrate, either on rocks, or docks and pilings. Therefore most people only see the back (dorsal) side of a chiton, which has eight articulating shell plates surrounded by a muscular girdle. Chitons are often covered by epibiota, algae and encrusting animals, which camouflage their surface. This makes it difficult to find them on the shore, and once found, it makes it difficult to find the specific characters needed for identification. Epibiota can be removed by gentle scraping with a fingernail or the blunt side of a small knife without hurting the animal. And with practice and careful observation the differences that separate species quickly become apparent. The ventral ʻunderneathʼ side of a chiton is its vulnerable soft foot, which it uses for creeping around on the surface. The space between the foot and the girdle (the outer ring of muscle) is called the pallial cavity and contains the gills. The gills hang down from the roof of the pallial cavity when the animal is in its natural position. A chiton can use muscular action to change the pressure inside the pallial cavity, and this creates suction to stick it to the surface. The suction of a chiton is extremely strong and attempts to dislodge a chiton from its surface can kill it, if it is not done carefully. Also on the underside of a chiton is the mouth, which contains a rasping radula tongue for scraping algae and other food off rocks. The radula contains many rows of complex teeth that are literally capped with iron. Having metal teeth is assumed to be an adaptation to grazing on rocks, which would rapidly erode or break weaker teeth. Chitons have no eyes or senses on their heads; although they have microscopic sensory pores that penetrate through their shells. These pores are called ʻaesthetesʼ and in some species contain light-sensitive pigments, which makes them ʻshell eyesʼ. Chitons are fascinating creatures with an extremely long fossil record, and they are sometimes called ʻliving fossilsʼ because they seem to have changed so little in their body plan over evolutionary time. However they are not famous for complex behaviour, and most move very slowly especially when they are exposed at low tide. Some species, particularly Lepidozona spp. are relatively more active and will crawl at speed if you are patient. This is some motivation toward securing a correct identification of your finds. There are two major evolutionary groups or clades of chitons: Lepidopleurida and Chitonida. The Lepidopleurida is the older evolutionary lineage and is represented in B.C. by Leptochiton rugatus and some similar species in very deep water, which all have small bodies and pale, thin shells. All other chitons are in the Chitonida. This group includes the familiar Mopalia (mossy chiton) and Katharina (black Katy chiton), the giant Cryptochiton (gumboot chiton), and a surprising number of other species in all shapes and sizes. Altogether there are 48 species of chitons reported from British Columbia, 35 of which are included in this key, and there are more waiting to be found. J.D. Sigwart & R. Dickson. Key to BC Chitons. Published online, Royal BC Museum (2010). 1 http://taxonomy.royalbcmuseum.bc.ca/Taxonomy/viewer/BC_Chitons.aspx Key to BC Chitons Glossary and species list Authors Dr Julia Sigwart is a research associate of the RBCM and the Laboratory Director of the Queenʼs University Marine Laboratory in Northern Ireland. Ms Rian Dickson is currently a graduate student at SFU, and prepared an earlier version of this key and glossary when she was a student at UVic. If you have any questions about this key, please contact <[email protected]> Further Notes . In this key, images of chiton are always displayed with their head (anterior) either at the top or to the right of the frame. Terminology used in the key follows the meanings explained in this glossary. Records and illustrations of ranges, both geography and depth, are not precise but intended for general guidance. Definitive ranges are based on specimen records, identified by experts, that are held by the Royal BC Museum or other museums. However, the species list, at the end of this document, is definitive and represents the total known and validated species of chitons recorded in British Columbia. Anatomical Terms See figures, page 6, for illustration of some terms. antemucronal area The front triangular area on the tail valve (see fig. 9) which is located (figure 1) in front of the mucro. anterior The front (head) end, or relatively nearer to the front. The head valve of a chiton is semicircular or crescent-shaped and does not have a central peak. apex (pl. ʻapicesʼ) Central point of the posterior (overlapping) edge of an intermediate (figure 1) valve; generally this point is somewhat projected, but the posterior valve margin may be straight (without an apex). apophyses Anterior projections of the articulamentum; the forward ʻwingsʼ of a (figure 5) valve that stick underneath the next valve in front (the head valve does not have apophyses). Singular (one side), apophysis. articulamentum The underlying layer of the valves, generally smooth and pale- (figure 5) coloured. Lateral projections of the articulamentum form insertion plates. (The articulamentuum is not visible in living chitons.) beak A distinct projecting apex on the valve margin. Generally this point is (figure 1) somewhat projected, but the posterior valve margin may be straight (without an apex) bifurcating Dividing into two branches; referring to patterns of sculpture on some (figure 2) chitons. J.D. Sigwart & R. Dickson. Key to BC Chitons. Published online, Royal BC Museum (2010). 2 http://taxonomy.royalbcmuseum.bc.ca/Taxonomy/viewer/BC_Chitons.aspx Key to BC Chitons Glossary and species list bristle Branched setae; referring to specialised girdle covering. (figure 3) carinate Shaped liked a keel or ridge; referring to a raised edge down the central axis of the intermediate valves, creating a distinct ridge down the middle of a chiton. caudal sinus An indented notch on the posterior margin of the tail valve, usually (figure 1) associated with a posterior notch in the girdle. central area On each of the intermediate valves, the triangular area formed (figure 1) between the two lateral areas. diagonal On the intermediate valves, a chevron extending from the apex to the two corners of the posterior (overlapping) margin of the valve. If visible, the diagonal may be a raised rib or may be clearly by the different patterns of sculpture in the central area (anterior and 'inside' of the diagonal) and pleural areas (posterior and 'outside' of the diagonal). The diagonal is also visible on the articulamentum of most species. dorsal Relating to or situated on the back; the upper surface of a chiton. The (figure 1) dorsal surface is the only surface normally visible in life, when chitons are attached to rocks. flammule A spot of colour resembling a small flame (may be reddish, or orange; or blue in e.g. Tonicella venusta). foot The strong muscular region occupying most of the ventral area. (figure 4) girdle Flexible, leathery, muscular ring holding the chiton valves in place; (figure 1) often ornamented with scales or spicules (see fig. 4); also called the 'perinotum' in other literature. granule Very small or minute elevation; referring to the sculpture on the dorsal (figure 2) surface of a valve. growth lines Concentric lines or ridges, visible on the dorsal surface of a valve, (figure 2) which correspond to the outward pattern of valve (shell) growth. holobranchial Having gills that extend the whole length of the foot. insertion plate Narrow lateral extension of the articulamentum of a valve that projects (figure 5) into the girdle. intersegmental Between segments (between valves). jugal area Surface of the tegmentum in the middle of a valve; in some chitons, (figure 1) the central area of the valve is divided into a distinct jugal area surrounded by the pleural areas. J.D. Sigwart & R. Dickson. Key to BC Chitons. Published online, Royal BC Museum (2010). 3 http://taxonomy.royalbcmuseum.bc.ca/Taxonomy/viewer/BC_Chitons.aspx Key to BC Chitons Glossary and species list jugum / jugal area Central longitudinal ridge on intermediate valve, or a distinct area (figure 1) around the middle axis within the 'central' area of an intermediate valve. lateral area Side slope of a valve, posterior to the diagonal rib on an intermediate (figure 1) valve. longitudinal The 'length' of the shell from anterior to posterior; the direction of the longest axis on the whole animal. longitudinal rib Ribs that run parallel to the longitudinal axis of the body, pointing in the anterior-posterior direction. marginal fringe Spicules or spines arising from the outer edge of the girdle, in contact (figure 3) with the substrate that an animal is on, in life. mucro The distinctive small projection on the tail valve, which is usually close (figure 1) to the centre of the valve. pallial groove The space between the girdle and the foot on the ventral surface of a (figure 4) chiton, which houses the gills. pleural area On the intermediate valves, the area situated between the central (figure 1) jugum and the valve diagonal. posterior The back (tail) end, or relatively nearer to the back. postmucronal area The area of the tail valve posterior to (behind) the mucro, toward the (figure 1) outer valve margin. quincunx, An arrangement of five things in a square, with one at each corner, quincunal and one in the center, referring to valve sculpture with off-set rows of granules. radiating ribs Ribs that extend from the apex of a valve towards the anterior margin; often present on the head valve and intermediate valves.