01 Tonicella marmorea

Length 32 mm. LWS. Isle of Lewis, Scotland. April 2018. Leg. D.W. McKay & S.Taylor.

In the N.E. Atlantic, T. marmorea grows to a maximum of 45 mm long and 27mm wide.

To the naked eye, the girdle and valves appear smooth, apart from near-rectangular, distinct, growth lines on the valves.

 

SPECIES DESCRIPTION part A: BELOW

SPECIES DESCRIPTION part B: flic.kr/p/2gzajx3

Key id. features: flic.kr/p/2gzaj8q

Sets of OTHER SPECIES:

www.flickr.com/photos/56388191@N08/collections/

Revised, 2020, PDF version at www.researchgate.net/profile/Ian_Smith19/research

 

Tonicella marmorea (O.Fabricius, 1780)

Images: 1 to 54 T. marmorea; 55 to 59 vacant; 60 to 72 similar species.

Authors: Paula Lightfoot, Simon Taylor and Ian F. Smith.

 

Meaning of scientific name:

marmorea (Latin) = resembling marble

 

Synonyms: Chiton marmoreus O.Fabricius, 1780; Chiton ruber (non Linnaeus, 1767) Spengler, 1797; Chiton laevigatus Fleming, 1813; Chiton latus Lowe, 1825;

Vernacular: Mottled red chiton; Marbled mail-shell; Lleuen fraith y graig (Welsh); Chiton rouge marbré (French); Marmorierte Käferschnecke (German); Marmorleddsnegl (Norwegian); Marmoreret skallus (Danish);

 

GLOSSARY BELOW This account adheres to the standardised terminology for chitons proposed by Schwabe (2010). Some of Jones & Baxter (1987) alternatives are indicated in the glossary as (a.k.a.).

 

Shell Description

Low form in N.E. Atlantic up to 45 mm long and 27mm wide 1Tm flic.kr/p/2gz9Ndp . High form in N.W. Atlantic up to 37 mm long and 20mm wide (Kaas & Belle, 1985). In dorsal view, outline elliptical, larger specimens generally more strongly curved than smaller ones; width about 60% of length. Narrow girdle (total of two sides) occupies between 12% and 35% of animal width, specimens over 20mm length generally having the proportionally wider girdles (Baxter & Jones, 1986).

Eight overlapping valves 2Tm flic.kr/p/2gzajx3 have an upper coloured layer, ‘tegmentum’, of aragonite, and a lower layer, ‘articulamentum’, of white 3Tm flic.kr/p/2gzaj8q or, sometimes on jugal tract, pink 4Tm flic.kr/p/2gzaiL8 or yellow 5Tm flic.kr/p/2gzaipX aragonite. There are also an uppermost layer, ‘properiostracum’, and a discontinuous lowermost layer, ‘myostracum’, that are so thin and transparent as to be unnoticeable.

Dorsal surface (tegmentum) of valves appears smooth to naked eye apart from near-rectangular, distinct, growth lines 1Tm flic.kr/p/2gz9Ndp & 6Tm flic.kr/p/2gzaifJ . Colour variable; dirty white, cream, various shades of brown 7Tm flic.kr/p/2gz9KP1 , red 8Tm flic.kr/p/2gz9Khz , and ochre arranged in blotches or intricate zig-zag lines 9Tm flic.kr/p/2gzahiD . Rarely entirely marmorated. Frequently, but not always, approximately triangular patches on the jugal areas have lighter colours 8Tm flic.kr/p/2gz9Khz . Valves ii, iv and, less consistently, vii often have more dark colour than the other valves 8Tm flic.kr/p/2gz9Khz , 9Tm flic.kr/p/2gzahiD ,10Tm flic.kr/p/2gz9Jnt , 11Tm flic.kr/p/2gzagd2 . Colour partly derives from pigment in properiostracum; brightest on live specimens in good condition and fades rapidly on specimens that are in poor condition or dead 12Tm flic.kr/p/2gzafNQ .

The head valve (i) is almost semicircular 13Tm flic.kr/p/2gz9HfJ , with a shallow V posterior edge when viewed in situ on living specimen 7Tm flic.kr/p/2gz9KP1 . The tegmentum on intermediate valves (ii –vii) has slightly rounded end margins, and nearly straight and parallel anterior and posterior margins 4Tm flic.kr/p/2gzaiL8 , except the anterior of valve ii which projects more at its centre. Valves ii to vii have a keeled jugum 14Tm flic.kr/p/2gzaf1c with small posterior beak 15Tm flic.kr/p/2gzaeRK . Beaks are often eroded away 8Tm flic.kr/p/2gz9Khz .They vary in elevation (H/W% of valve iv) from 20% to 37% or more in NE Atlantic 17Tm and usually higher in NW Atlantic c. 44% (Kaas & Belle, 1985). The side slopes of valves ii to vii are straight 16Tm flic.kr/p/2gz9Gyy or slightly convex, or a combination of straight and convex when viewed from the anterior or posterior 17Tm flic.kr/p/2gzaeBw . Their lateral areas are very slightly raised and separated from the central area by a shallow, unobtrusive, diagonal depression 18Tm flic.kr/p/2gzaeov & 20Tm flic.kr/p/2gzadZ4 .

The tail valve (viii) is small and about twice as wide as its antero-posterior length. The unobtrusive, antemedian mucro is encircled by the concentric growth lines 13Tm flic.kr/p/2gz9HfJ & 21Tm flic.kr/p/2gz9FQz . The steep postmucronal slope is straight in lateral profile 6Tm flic.kr/p/2gzaifJ . The colour of the antemucronal area resembles that of the jugal area of intermediate valves 13Tm flic.kr/p/2gz9HfJ .

Canals permeate the tegmentum 22Tm flic.kr/p/2gzadNh & 23Tm flic.kr/p/2gzadLi and terminate on its dorsal surface as a minute stipple of caps (surface seems smooth to naked eye, at least 3X magnification and cleaned valve needed) 18Tm flic.kr/p/2gzaeov on caps arranged in an offset grid pattern 24Tm flic.kr/p/2gz9FCA . Some canals penetrate the articulamentum to form holes on its ventral surface, especially in the slit rays and jugal tract 23Tm flic.kr/p/2gzadLi & 25Tm flic.kr/p/2gz9Foc , 26Tm flic.kr/p/2gz9Fkw .

Insertion plates on ends of valves ii – vii embedded in the girdle are separated by a single slit 5Tm flic.kr/p/2gzaipX & 23Tm flic.kr/p/2gzadLi ; head valve (i) has 7 to 13 slits 25Tm flic.kr/p/2gz9Foc and tail valve (viii) has 5 to 11 slits 27Tm flic.kr/p/2gz9FcF . On the ventral articulatum a diagonal slit ray runs from each slit to the posterior edge of valves i – vii 25Tm flic.kr/p/2gz9Foc & 26Tm flic.kr/p/2gz9Fkw and from the slits to the mucro position where they meet the open canal ends in the jugum on valve viii 27Tm flic.kr/p/2gz9FcF .

The articulatum on valves ii – viii 3Tm flic.kr/p/2gzaj8q projects forwards as a pair of curved, wide, short (front to rear) apophyses separated by a narrow, straight, jugal sinus (gap) that is less than 25% length of an apophysis 28Tm flic.kr/p/2gzad9b . The apophyses on valve ii are more angular and those on valve viii are smaller and more angular 4Tm flic.kr/p/2gzaiL8 . Apophyses extend under the next valve forwards 2Tm flic.kr/p/2gzajx3 and leave a scar of similar shape on it when the connecting muscle is removed 29Tm flic.kr/p/2gzad5Z . The head valve (i) is the only one that lacks apophyses as there is no valve to its anterior to be attached to 25Tm flic.kr/p/2gz9Foc . [It is difficult to cut away the muscle without breaking the apophyses on freshly dead chitons. They can be disarticulated without damage to the valves by standing them in shallow 10% NaOH overnight. CAUTION; extremely caustic, burns skin and seriously damages eyes. Or, keep in preservative for some time until the muscle weakens and then pull the valves out with strong forceps]

 

Body Description

Head and foot only fractionally protrude into view naturally on live animal 30Tm flic.kr/p/2gzacGE & and can usually only be seen when breeding 31Tm flic.kr/p/2gz9EjD or if the animal is dislodged from the substrate or adhering to glass. When attached to a smooth surface, the extended head is anteriorly convex to fit the curve of the shell, and posteriorly concave fitting round the anterior of the foot and extending laterally as mouth lappets 32Tm flic.kr/p/2gzabYW . The shape varies when the head is retracted or unattached to the substrate 33Tm flic.kr/p/2gzabUC . The head lacks eyes and tentacles; its main feature is a large transverse slit-mouth with wrinkled lips. When preserved, the mouth may gape widely and its fleshy surround may distort to form a hood 32Tm flic.kr/p/2gzabYW . As on chitons generally, the radula is a chitinous ribbon bearing teeth in rows of seventeen 34Tm flic.kr/p/2gz9DMm . In each row, the central (rhachidian) tooth is a reflexed, rectangular channel with a widened chisel-like terminal blade 36Tm flic.kr/p/2gzabrZ [The image and text in Jones & Baxter (1987) differs from this description of the rhachidian tooth which is based on SEM images in this account]. Next to it, the minor (first) lateral tooth is small and bladeless. The major (second) lateral tooth has a long shaft and a large tridentate head ( one small, sharp denticle and two wide, rounded denticles) 35Tm flic.kr/p/2gzabKK . The first and second uncinal teeth are small and unobtrusive between the bases of the major lateral and major (third) uncinal tooth which has a long shaft with head shaped and sculptured like a feather 37 Tm . At the margin of the radula in each row there are three low plates (marginal ‘teeth’). The teeth are colourless when their formation commences at the rear of the translucent radula sac. They darken and harden through rust-red and brown as they move along the sac acquiring magnetite (iron oxide), and the principal cutting teeth are black by the time they emerge from the anterior of the sac 34Tm flic.kr/p/2gz9DMm . Magnetite is the hardest material made by any living organism (Botelho, 2013).

Aesthetes (sensory tissue) fill canals permeating the tegmentum 22Tm flic.kr/p/2gzadNh & 37Tm flic.kr/p/2gz9CVr . They terminate on fine stipple (at least X3 magnification needed) 18Tm flic.kr/p/2gzaeov as sensory organs on the dorsal surface of valves 19Tm flic.kr/p/2gzaed5 , 21Tm flic.kr/p/2gz9FQz , 24Tm flic.kr/p/2gz9FCA .

Canals vertically penetrate the articulamentum in the slit rays and jugal tract as conduits for branches of the body’s lateral nerve cord 22Tm flic.kr/p/2gzadNh . Immediately below the valves, the mantle is tough, thin, translucent epidermis, but it is greatly thickened where reflected around the periphery of the shell to form a fleshy girdle 6Tm flic.kr/p/2gzaifJ into which the articulamentum insertion plates at the ends of the valves are deeply embedded 14Tm flic.kr/p/2gzaf1c & 38Tm flic.kr/p/2gzaaV8 . The girdle has cream or yellowish-white flesh with a tough transparent yellowish cuticle that survives when flesh is removed with NaOH 39Tm flic.kr/p/2gzaaSC . The cuticle on the ventral surface of the girdle (hyponotum) and dorsal surface (perinotum) appears leathery-smooth to the naked eye 6Tm flic.kr/p/2gzaifJ , but strong magnification shows a covering of very small spicules 40Tm flic.kr/p/2gz9CKB & 41Tm flic.kr/p/2gz9CEG . Scanning electron microscope images 42Tm flic.kr/p/2gzaaGT show that the spicules on the (dorsal) perinotum are small, c. 27µ long, widely spaced cones with sharp apices, and that spicules on the hyponotum are longer, c. 35µ , more closely packed ovoids with longitudinal ribs. . The hyponotum is whitish, but usually extensively stained yellowish to brownish by firm contact with the substrate 43Tm flic.kr/p/2gzaakv . The perinotum is yellowish with, usually, a brown or purplish band at each end of valves ii to vii, and several bands associated with valves i and viii 7Tm flic.kr/p/2gz9KP1 . The bands usually have several yellowish blotches on them. Frequently, part or all of the perinotum is suffused with verdigris-green which partially obscures the band pattern 40Tm flic.kr/p/2gz9CKB . The foregoing colours are usual in Britain but Kaas and Belle (1985) state that the girdle is “of an even brownish colour, exceptionally with alternating zones of light and dark brown”. This may be the case in America, though 9 images from Quebec all have banded girdles (DORIS, 2019), and dried or preserved specimens often lose the pattern 12Tm flic.kr/p/2gzafNQ & 38Tm flic.kr/p/2gzaaV8 . The edge of the girdle has a marginal fringe of straight, obtusely pointed, c. 48µ long, spicules that are very difficult to discern with the naked eye 42Tm flic.kr/p/2gzaaGT , 43Tm flic.kr/p/2gzaakv & 45Tm flic.kr/p/2gz9BP8 . The girdle can be flexed upwardly outwards at the anterior 30Tm flic.kr/p/2gzacGE to admit inhalent water to the peripheral mantle cavity, and at the posterior to form a channel for the release of exhalent water, faecal pellets and ova or sperm 31Tm flic.kr/p/2gz9EjD .

An open narrow mantle cavity runs around whole animal; contains 15 to 26 small ctenidia on each side (Baxter & Jones, 1986), often on larger specimens for nearly the whole length of the foot (merobranch, but nearly holobranch, arrangement) 46Tm flic.kr/p/2gza9Xr & 47Tm flic.kr/p/2gza9KN . Number of ctenidia increases with age. Between the mantle cavity and hyponotum the mantle fold is unobtrusive, except where it widens near the posterior into a mantle lappet 30Tm flic.kr/p/2gzacGE and may partly conceal the ctenidia. Anus opens at end of an anal papilla into the mantle-cavity at posterior by a channel to the exterior formed by deflection/ depression of girdle 48Tm flic.kr/p/2gza9pc . The dorso-ventrally flattened pericardium containing the heart is located above the anal papilla, and is sometimes partly visible on live specimens 48Tm flic.kr/p/2gza9pc . Nephridiopores and gonopores open laterally into posterior quarter of cavity. No penis as external fertilization. Foot yellow to orange, elongate ovate, anterior wider than posterior, curvature of ends varies, sometimes truncate 49Tm flic.kr/p/2gza9mG , no medial dividing line. When foot spreads widely, it and the mantle fold/lappet close and conceal much of the pallial cavity 44Tm flic.kr/p/2gzaaj3 . The foot has many strong transverse muscles 50Tm flic.kr/p/2gza92J and the body has a strong lateral longitudinal muscle 38Tm flic.kr/p/2gzaaV8 & 50Tm flic.kr/p/2gza92J that encircles the entire body passing under the outer margins of the valves. The valves have four sets of muscles.:

a) A pair of lateral muscles connect the foot to each end of the valves, one on each side of the slit 38Tm flic.kr/p/2gzaaV8 & 50Tm flic.kr/p/2gza92J , leaving an opaque scar when removed 3Tm flic.kr/p/2gzaj8q .

b) A pair of straight muscles 51Tm flic.kr/p/2gz9AHL pass along the jugum, attaching to the anterior margin of the jugal sinus of each valve and to the body wall under the previous valve.

c) On each side of a valve an oblique muscle 51Tm flic.kr/p/2gz9AHL attaches to the jugal sinus close to the straight muscle and travels anteriorly outwards under the diagonal line that separates the lateral area of the tegmentum from its central area. The anterior end of the muscle attaches to the body wall under the preceding valve on which it leaves a comma shaped scar 3Tm flic.kr/p/2gzaj8q .

d) The transverse muscle 51Tm flic.kr/p/2gz9AHL consists of dorso-ventral fibres connecting the apophyses of one valve to the underside of the preceding valve where it leaves curved scars matching the outlines of the apophyses. The muscle colour is often the same as that of the jugal tract; white, yellow 51Tm flic.kr/p/2gz9AHL or pink 52Tm flic.kr/p/2gza8R8 .

 

Glossary

μm = 0.001 mm, 1000μm = 1 mm

acicular = slender and tapering to a point; needle-like.

aesthetes = complex of canals and cavities filled with sensory tissue that permeate tegmentum and locally penetrate articulamentum. Open as sensory pores on dorsal surface of valves; probably compensate for lack of sensory structures on head. Some sense light; other sensory function(s) uncertain, but various authors have variously proposed chemoreception, mechanoreception, properiostracum replenishment and secretion of protective substances.

 

a.k.a. = also known as.

antemedian = (syn. antemedial) situated to anterior of middle.

antemucronal area = area situated to anterior of mucro.

apophysis = (pl. apophyses) anterior extension of articulamentum which underlies preceding valve; on all valves except head valve (i).

 

aragonite = orthorhombic crystalline mineral-form of calcium carbonate www.minerals.net/mineral/aragonite.aspx . Less common on land than calcite, but, currently, the more frequent mineral-form in oceans and living mollusc shells.

 

articulamentum = inner shell-layer of chiton valves, usually hard, white, porcelaneous aragonite and often differently coloured in central part.

 

calcite = trigonal crystalline mineral-form of calcium carbonate www.minerals.net/mineral/calcite.aspx . More common on land than aragonite, but, currently, the less frequent mineral-form in oceans and living mollusc shells. More resistant than aragonite to acid rain corrosion; forms outer shell layer of shore-dwelling Littorina species in cool climates. (Corrosion of calcium carbonate faster at cold temperatures).

 

cephalic = (adj.) of or on the head.

chemoreception = sensing of chemicals; “smell / taste”.

chitin = semitransparent flexible horny protein. Does not occur in molluscs.

chitinous = (adj.) resembling chitin.

cilia (pl.) = motile linear extensions of membrane used in locomotion, or to create water currents in feeding. (“cilium” singular).

 

coll. = in the collection of (named person or institution) (cf. legit).

ctenidium = comb-like molluscan gill; usually an axis with a row of filaments either side.

 

dioecious = having separate male and female individuals.

dorso-ventrally flattened = as if pressed flat from above.

ELWS = extreme low water spring tide (usually near equinoxes).

epithelium = tissue forming outer layer of body surface, “skin”.

 

girdle = (on chiton) peripheral band of thickened, reflexed mantle that encloses ends of valves.

 

gonopore = genital opening through which eggs or sperm are released.

holobranch = (of chitons) ctenidia in mantle cavity extend full length of foot.

hyponotum = ventral cuticle of chiton’s girdle.

insertion plate = (on most chitons) extension of articulamentum which inserts into mantle on lateral margin of intermediate valves, anterior margin of head valve and posterior margin of tail valve. Inserts into, and anchors valve to, the girdle muscle block.

 

intermediate valve = (of chiton) any valve (ii – vii), except head valve (i) and tail valve (viii).

 

jugal area = summit zone of chiton valves on dorsal surface.

jugal tract = summit zone of chiton valves on ventral surface.

jugum = summit of chiton valves.

lateral area = (on intermediate valve of chiton) triangular area with its base along lateral edge of valve and its apex near the centre of the posterior edge. a.k.a. lateral triangle.

 

legit = (abbreviation; leg. or lgt.) collected/ found by (cf. coll.)

LWS = low water spring tide, two periods of a few days each month when tide falls lowest.

 

mantle = sheet of tissue that secretes the shell and forms a cavity for the gill in most marine molluscs.

mantle cavity = (on chitons) a.k.a. pallial groove; narrow groove around whole foot and head, roofed by mantle and containing ctenidia, nephridiopores and gonopores.

 

magnetite = mineral of iron oxide, hardest material made by any living organism.

marmorated = veined or streaked like marble.

mechanoreception = sensing of touch, sound, pressure change and/or posture.

 

MLW = mean low water mark.

 

merobranch = (of chitons) gills in pallial cavity only in posterior two-thirds of animal.

 

mucro = projection on tail valve (viii) of chiton demarking posterior from rest of valve. Varies in prominence and position.

 

myostracum = thin, inconspicuous, discontinuous, innermost layer of chiton shell.

 

nephridium = tubular glandular excretory/ osmoregulatory organ. a.k.a. kidney.

 

nephridiopore = opening of nephridium for excretion. a.k.a. renal pore.

odontophore = firm, approximately ellipsoid, structure of cartilage supporting radula. Protruded like a tongue to operate radula.

 

pericardium = membranous sac containing heart and start of aorta.

perinotum = dorsal cuticle of chiton’s girdle.

pleural area = (on intermediate valve of chiton) triangular area with its base along anterior edge of valve and its apex near the centre of the posterior edge. a.k.a. median triangle.

 

postmucronal = situated to posterior of mucro.

properiostracum = (on chitons) outermost layer of colour-bearing proteinaceous material, sometimes resembling collagen in texture and differing in composition from periostracum of most other mollusc groups.

 

plankton = animals and plants that drift in pelagic zone (main body of water).

radula = chitinous ribbon of teeth extruded on a tongue-like structure (odontophore) to rasp food.

 

side slope = (on chiton) shape in profile view (from posterior or anterior) of lateral areas of intermediate valves; may be straight, convex, concave or a combination of these.

 

sinus = 1. dilated channel or receptacle containing blood etc. 2. (re. chiton valves) curved bay or gap.

slit ray = row of canal pores running diagonally from lateral slit to posterior edge on ventral surface of chiton valve. a.k.a. notch ray.

 

suture = (of chiton) line where two valves meet.

tegmentum = outer shell-layer of chiton valves, usually porous and relatively soft. (Covered by transparent properiostracum when live.)

 

trochophore = spherical or pear-shaped larvae that swim with aid of girdle of cilia. Stage preceding veliger, passed within gastropod egg in most spp. but free in plankton for limpets, Trochidae, Tricolia pullus and (with no veligers) chitons.

 

uncinate = having a hooked shape.

uncinate blades = largest two teeth in each row of seventeen on chiton radulae; principal scraping blades. Often darkened more than other teeth by high magnetite content.

 

veliger = shelled larva of marine gastropod or bivalve mollusc which swims by beating cilia of a velum (bilobed flap).

 

 

SPECIES DESCRIPTION continues in part B under image 2Tm at : flic.kr/p/2gzajx3 .

 

 

Done