Tuatara: Volume 4, Issue 1, July 1951
A Key to the Common Chitons of New Zealand
A Key to the Common Chitons of New Zealand
Linnaeus used the one generic name, Chiton for all coat-of-mail shells and though this generic name is now restricted to a West Indian species, ‘chiton’ has now become an almost universally accepted vernacular name for this group of the Mollusca. There is no such unanimity as regards the scientific name for the class, the following names having been applied:— Amphineura, Polyplacophora, Loricata, Crepipoda and Polyplaxiphora with a number of variations in spelling.
Sixty-one species and subspecies are recognised from New Zealand and the outlying islands. Chitons are all marine forms, the majority living near or below low water mark. A few species occur between tidemarks, and in New Zealand some of the commonest forms have adapted themselves to this habitat. The usual station is on, or under rocks, a few attaching themselves to algae or living in holes in the holdfasts, and one species in New Zealand is found buried in the sand.
The key that follows is an attempt to provide a means of identifying the common New Zealand species. The characters used are in some cases superficial and of little actual significance in systematic work in the group. For this reason the key will not necessarily apply on the generic level to any but New Zealand forms. A number of local groups appear to be very variable, the characters used to differentiate species being very fluid. In these cases it has not been possible to key as far as species or subspecies (e.g. some of the species of Notoplax and Maorichiton and subspecies of Acanthochiton zelandicus). Some species from deep water or shells known only from a few specimens have been omitted as have those not recorded from the three main islands. Suter in the Manual of New Zealand Mollusca gave an account of the chiton fauna but intensive work by Ashby and Mestayer, the results of which were published in the Transactions of the New Zealand Institute made many alterations and additions necessary. In a series of papers published in the Australian Zoologist from 1929 onwards, T. Iredale and A. F. B. Hull provided a monograph of the New Zealand chiton fauna, correcting many errors and evaluating previous work. This account is not always very clear and there are a number of groups which require much more attention before the systematics are thoroughly understood. Since 1930 little systematic work has been carried out on the group in this country.
The shell consists of eight overlapping plates or valves kept in position by an encircling muscular girdle. The valves overlap so that the posterior edge of the anterior valve projects over the anterior edge of the succeeding valve and so on throughout the length. The valves comprise an anterior valve, six median valves and a posterior or tail valve. Only under exceptional circumstances are there more or less than eight valves, and such an occurrence is usually caused by an injury at an early stage. The valves are often indicated by the letters, i to viii, the anterior valve being numbered, i, and the posterior, viii. Each valve is composed of two layers, the outer layer or tegmentum and an inner layer or articulamentum. The tegmentum may be sculptured in a variety of ways, the actual method being used as a differentiating character. The surface of each median valve is divided for descriptive purposes into five areas, the central area or jugum surrounding the central ridge, two pleural areas and two lateral areas. (Fig. 2.) In one large group of chitons the distinction between the pleural and lateral areas is not well marked. (Fig. 3.) The posterior valve is divided into two areas by the position of the mucro. The tegmentum is perforated by numerous pores of two sizes, megalopores and micropores. The megalopores lead into knobs of refractory tissue called megalaesthetes. In some genera the megalaesthetes function as eyes.
The articulamentum is larger than the tegmentum and the marginal portion is covered by the girdle. It projects in front as two sutural Jaminao with a sinus between. (Figs. 2 and 3.) Along the sides of the median valves and around the free edges of the anterior and posterior valves, insertion plates are usually developed. These serve to attach the valves to the girdle. The insertion plates may be cut into teeth by slits. In some genera these teeth are finely cut (or pectinate). (Fig. 4.) The girdle may be clothed with small scales, corneous or hairy processes, spicules or a combination of these. Coloration is somewhat variable in the group though some species appear to have a specific colour pattern which remains constant.
Collection and Preservation
Chitons present some difficulties as regards collection because of their power of adhesion and their usual habit of curling up when removed from the rock. A thin-bladed knife is the best implement for removing the animal from the substratum, care being taken not to damage the girdle. Before the animal has time to curl up it should be pressed tightly against a slip of wood or glass and tied down with tape. The slip may be used for a number of specimens if required. When full, the slip and specimens may be immersed in alcohol to kill the animal. If the animals are to be preserved they may be removed from the slip and stored in alcohol. Otherwise the animal may be removed and the specimen stored dry. Systematic work requires some of the specimens to be disarticulated so that details of the articulamentum may be studied. This may be done by soaking page 6 fresh specimens in water for a few hours when the girdle may be stripped off easily, or by boiling dried specimens.
In the key that follows, the letters A, C, F. R., and M. refer to the marine provinces of New Zealand: A, Aupourian, i.e. the northern half of the North Auckland peninsula; C, Cookian, i.e. the remainder of the North Island and the northern two-thirds of the South Island; F, Forsterian, i.e., the southern third of the South Island and Stewart Island; R, Rossian, i.e., the Auckland, Bounty, Antipodes and Campbell Islands; M, Moriorian, i.e., the Chathams group. The size given is the approximate limit of the species in length and is an indication of relative size. The key is of the alternative type; two numbers are given, e.g. 1 (4). If the animal does not fall into category 1, then the number in brackets gives the alternative possibility.
Key to Common Chitons
|1 (4)||Valves lacking insertion plates but with small, distinct, triangular sutural laminae, girdle scaly.||G. Terenochiton|
|2 (3)||50 radials on anterior valve, maximum length 7 m.m.||(F.) T. otagoensis|
|3 (2)||Many more than 50 radials on anterior valve, maximum length 14 m.m.||(A.C.) T. iniquinatus (Fig. 16)|
|4 (1)||Valves with insertion plates.|
|5 (38)||Valves i to vii or i to viii with slits on insertion plates, teeth smooth or but slightly roughened, never finely denticulate.|
|6 (21)||Surface of median valves divided into five areas, central jugum, two pleural areas and two lateral areas. (Fig. 2.)|
|7 (12)||Posterior valves with crescentic series of well developed teeth.|
|8 (9)||Valves porous at the edges. Sutural plates connected across the sinus. Girdle scales of elongate glassy spicules packed latitudinally so that only the ends show. Pleural areas bearing grooves or ridges and distinctly separated.||(C.) Icoplax kapitiensis|
|Some specimens from N.Z. have been identified as I. puniceus, a South American species. This specific identity is most uncertain. Three other species have been described from N.Z., all from deep water and all poorly known.|
|9 (8)||Valves solid at the edges, girdle covered with flat imbricating scales. Single slits in median valves, more than 8 slits in anterior valve.||G. Ischnochiton|
|10 (11)||Up to 45 m.m. in length. Girdle scales faintly striated, with some smooth scales. Posterior valve twice the size of other valves. Colouring very variable. Lives on the underside of stones and moves very rapidly when disturbed. Common.||(A.C.F.M.) I. maorianuspage 7|
|11 (10)||Up to 20 m.m. in length. Girdle scales deeply grooved. In size, outline and colour very close to Terenochiton iniquinatus which may be distinguished externally by having longitudinal striations on the intermediate valves.||(F.R.) I. circumvallatus|
|12 (7)||Posterior valve with a median tail sinus but no slits. Girdle hairy or bare or with scales. Anterior valve with 8 slits.||F. Plaxiphoridae|
|13 (14)||Girdle covering of minute triangular scales. A few scattered hairs may be present. External sculpture of very fine wrinkles, radials obsolete. 17 m.m. (C.)||Vaferochiton murdochi|
|14 (13)||Girdle bare or bearing corneous processes.|
|15 (16)||No external sculpture, shell large and glossy, girdle very wide and fleshy beset with long hairs. Rocky shores on exposed rocks. 87 m.m. (A.C.F.) Fig. 6.||Guildingia obtecta|
|16 (15)||Exterior markedly sculptured.|
|17 (18)||Mucro of posterior valve upturned. Girdle with prominent sutural tufts. Shell surface heavily sculptured. Lives in hollows in the holdfasts of Durvillea. 15 m.m. (C.F.)||Frembleya egregia|
|18 (17)||Mucro normal, terminal. External sculpture of nodulous radials and wavy lines.|
|19 (20)||Sutural laminae continuous. 45 m.m. (A.C.M.)||Diaphoroplax biramosa|
|20 (19)||Sutural laminae separate. 40 m.m. (A.C.F.)||Maorichiton spp.|
|Five species have been named but only two, caelatus and metonomazus are recognized from the mainland. The distinctions are not very clear, as sculpture and shape are very variable in the genus, and no attempt is made to key the species further.|
|21 (6)||Surface of intermediate valves divided into 3 areas, a narrow dorsal, central area and two latero-pleural areas. (Fig. 3.)|
|22 (23)||Girdle scales of glassy spicules as in Icoplax. Sculpture entirely, minutely granulose forming a matt surface. Lateral areas slightly raised and scarcely distinguished. (A.C.F.) 22 m.m.||Paricoplax crocina|
|23 (22)||Girdle naked with pores bearing tufts of bristles. Valves usually encroached on by the girdle. F.||Cryptoconchidae|
|24 (31)||Anterior valve without radiating ribs.|
|25 (26)||Shell large (up to 60 m.m.), almost covered by the thick, fleshy girdle. Uncovered portion of valves practically linear. (A.C.F.M.)||Cryptoconchus porosus (Figs. 12 and 13)|
|26 (25)||Shell of medium size (up to 30 m.m.). Girdle leathery, set with large, prominent bunches of tall glassy spicules at the sutures.||G. Acanthochiton|
|27 (28)||Dorsal area smooth. 30 m.m. (A.C.F.)||A. zelandicus|
|Three subspecies have been named but the characters used appear very variable.||page 8|
|28 (27)||Dorsal area longitudinally grooved.|
|29 (30)||Posterior valve small, tegmentum almost as long as broad. (A.)||A. brookesi|
|30 (29)||Posterior valve large, tegmentum much broader than long. 18 m.m. (C.)||A. thileniusi|
|31 (24)||Anterior valve with 5 radiating ribs.|
|32 (37)||Girdle leathery, naked except for pore tufts. Insertion plates of posterior valve large.||G. Notoplax|
|33 (34)||Shell surface smooth and shining. Apparently lives buried in sand. 30 m.m. (A.C.)||N. cuneata|
|34 (33)||Latero-pleural surfaces sculptured with raised flat-topped grains.|
|35 (36)||Dorsal area coloured bright purple. Girdle encroaching over about half of each valve. 60 m.m. (A.C.F.M.)||N. violaceus (Fig. 11)|
|36 (35)||Dorsal area same colour as rest of shell. Five other species of Notoplax have been named from N.Z. The distinguishing characters are not well marked and the shells appear to vary geographically and bathymetrically. These species are not further diagnosed here.||Notoplax spp.|
|37 (32)||Girdle thick, fleshy, beset with microscopic white spicules, with sutural tufts of larger spicules. Insertion plate of posterior valve short and multi-slit. 17 m.m. (A.C.F.)||Craspedochiton rubiginosus|
|38 (5)||All valves, or valves i to vii possessing insertion plates cut into teeth by slits. Teeth sharply sculptured or ‘pectinated’ outside by fine vertical grooves.|
(The figures are not drawn to scale)
Fig. 1 Composite diagram of a chiton showing the parts of the shell and types of ornament.
Fig. 2 Median valve of Ischnochiton (after Suter).
Fig. 3 Median valve of Acanthochiton (after Suter).
Fig. 4 Exterior view of anterior, median and posterior valves to show main features.
Fig. 5 Interior view of anterior, median and posterior valves to show main features.
Fig. 6 Anterior and posterior valves of Guildingia obtecta (after Iredale and Hull).
Fig. 7 Lateral view of anterior valve of Aulacochiton haurakiensis.
Abbreviations: A. articulamentum; AV. anterior valve; CP. corneous processes; IP. insertion plates; J. jugum; LA. lateral area; M. mucro; PA. pleural area; PL. pleuro-lateral area; PV. posterior valve; S. spicules; SC. scales; SI. sinus; SL. sutural lamina; T. teeth; TE. tegmentum.
|39 (42)||Girdle leathery, beset with hair-like corneous processes.|
|40 (41)||Valves appearing smooth to the naked eye, but under a lens sculptured all over with minute diamond-shaped scales. Shell large (up to 123 m.m.). Brown in colour though often covered with encrusting organisms when adult. Girdle brown, corneous processes very obvious. Low tide on rocks among algae. (A.C.F.)||Eudoxochiton nobilis (Figs. 14 and 15)|
|41 (40)||Valves with well marked radials on lateral areas. Shell of medium size (up to 30 m.m.). Broad concentric bands of green on all valves. Girdle appears velvety due to the presence of small hairs. Eyes present on anterior valve and in a line on each lateral area. The surface is almost always clean and polished. (A.C.F.M.)||Onithochiton neglectus|
|42 (39)||Girdle scaly.|
|43 (44)||Lateral outline of anterior valve concave (Fig. 7). Anterior valve with 14 radial ribs bearing small nodules near the girdle. Central area of first median valve sculptured with V-shaped radial ribs. In the following valves, the longitudinal ribbing continues across the jugal tract. A yellow strip across the centre of valves iii to viii, surrounded on each side by a triangular red patch which occupies most of the pleural areas. Lateral areas and anterior valve orange. A rare shell, usually taken in deep water. Previously known as Lorica haurakiensis. [B. C. Cotton (Rec. S. Austr.Mus., vol. 6, p. 437, 1941) noted that Lorica had been used by Bronn in 1848 for a genus of Crustacea and is therefore not available for chitons. The name accepted is Aulacochiton Shuttleworth 1853, genotype Chiton volvox Reeve 1847.] 50 m.m. (A.C.F.)||Aulacochiton haurakiensis|
|44 (43)||Lateral outline of anterior valve convex or straight.|
|45 (48)||Central areas with longitudinal riblets, extending down side of ridge.|
Fig. 8 Median valve of Anthochiton aereus.
Fig. 9 Median valve of Anthochiton suteri (after Iredale and Hull).
Fig. 10 Median valve of Sypharochiton pelliserpentis.
Fig. 11 Median valve of Notoplax violaceus.
Fig. 12 Dorsal view of Cryptoconchus porosus.
Fig. 13 Dorsal view of anterior, median and posterior valves of Cryptoconchus porosus.
Fig. 14 Shell of Eudoxochiton nobilis.
Fig. 15 Under side of anterior and median valves of Eudoxochiton nobilis.
Fig. 16 Median valve of Terenochiton iniquinatus (after Suter).
|46 (47)||Anterior and posterior valve with fine raised radials. Central areas and sides of ridges sculptured with very fine, raised longitudinal ridges. Lateral area with 9-12 fine raised radials. The shell appears almost smooth to the naked eye. 37 m.m. (A.C.F.) Very common.||Amaurochiton glauca|
|47 (46)||Central areas with longitudinal riblets. Sides and ridges of central area both sculptured with nodulose longitudinal ridges. Lateral areas with 3-4 rows of distinct tubercles. Common on rocky coasts about half tide mark. Valves often eroded. 30 m.m. (A.C.F.M.)||Sypharochiton pelliserpentis (Fig. 10)|
|48 (45)||Central areas smooth.|
|49 (52)||Both central area and sides of ridge smooth.|
|50 (51)||Lateral areas with radiate, nodulose ribs. Girdle barred with broad black bands: interspaces brown nearest shell, blue on free margin of girdle. Common on intertidal rocks. 28 m.m. (A.C.F.)||S. sinclairi|
|51 (50)||Lateral areas without radiate ribs. 9 m.m. (F.)||S. torri|
|52 (49)||Central area with a smooth band or triangle on ridge of each valve. Sides of ridges sculptured with prominent longitudinal ribs with comparatively deep grooves between.||G. Anthochiton|
|Six species have been recorded but there is still some confusion as regards distinguishing characters. The six species are keyed below but difficulty will be encountered in identifying all specimens, especially young ones, and reference must then be made to Iredale and Hull. The two commonest species are A. aereus and A. canaliculatus.|
|53 (58)||Anterior valve with ribs nearly smooth.|
|54 (57)||Longitudinal ridges extend right across the pleural area (Fig. 9).|
|55 (56)||Few ridges (6-8 on each side) on pleural areas. 13 m.m.||Lyttelton. A. suteri (Fig. 9)|
|56 (55)||Many ridges on pleural areas (c. 16 on each side, becoming more slender towards centre). Usual colour pinkish-red with a dark streak on each side of central area. Common in oyster debris from Foveaux Strait. 15 m.m. (A.C.F.)||A. canaliculatus|
|57 (54)||Few longitudinal ridges, the upper ones not extending across the pleural areas. Colour usually green with varying quantities of yellow. Quite common on the shores of Cook Strait. 39 m.m. (A.C.F.R.)||A. aereus (Fig. 8)|
|58 (53)||Anterior valve with nodulous rays or with the ribs cut into lozenges.|
|59 (62)||Anterior valve with rounded nodulous rays.|
|60 (61)||Anterior valve with 8 radial ribs. Pleural areas with few ridges (5-6). Known only from one specimen, possibly a young shell from Rangitoto Id. 14 m.m.||A. clavata|
|61 (60)||Anterior valve with 17-24 nodulous radials, pleural areas with 20-25 furrows on each side. Recorded only from the vicinity of Dunedin. 34 m.m.||A. huttoni|
|62 (59)||Anterior valve with ribs cut into squarish lozenges. Otherwise close to canaliculatus in sculpture. 18 m.m. (A.C.F.)||A. stangeri|