01 Leptochiton scabridus

Length 4.1 mm. Dorset, England. February 2018. Leg. S. Trewhella.

Anterior on right of frame.

In dorsal view, when extended, the width is about 50% of the length. The sides of the compound shell are almost parallel, forming an oblong with rounded ends, but the varying extension of the peripheral girdle affects the animal's outline. In this case it is retracted at the head end, giving an apparent anterior taper to the outline.

The eight thin, fragile, whitish, slightly translucent valves have pinkish tint due to the red body showing through. Some of the valves are stained yellowish or light brown.

Full SPECIES DESCRIPTION BELOW

Sets of OTHER SPECIES at: www.flickr.com/photos/56388191@N08/collections/

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

 

Leptochiton scabridus (Jeffreys, 1880)

Synonyms: Lepidopleurus scabridus (Jeffreys, 1880);

 

GLOSSARY BELOW. This account uses 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

Extreme maximum length 8mm, but usually smaller; smallest chiton species in British waters; unlikely to be found by casual observation. In dorsal view, when extended, width about 50% of length, sides of shell almost parallel, forming oblong with rounded ends 1Ls flic.kr/p/255RPLq . Girdle width variable on same individual; from 32% of chiton's total width when active and in good condition 2Ls flic.kr/p/FurHFx to 15% when preserved or inactive/withdrawn, Kaas & Belle (1985) and Jones & Baxter (1987). Variation in girdle affects outline of entire animal; may differ from oblong formed by shell. Eight thin, fragile, whitish, slightly translucent valves, often stained yellowish or light brown and sometimes with pinkish tint due to red body showing through 1Ls flic.kr/p/255RPLq . Valves principally made of 1) tegmentum: dorsal layer of aragonite permeated and weakened by canals, and 2) articulamentum: ventral layer of aragonite. Head valve semicircular, posterior margin widely V-shaped 3Ls flic.kr/p/244RWAy ; approximately crescentic on live animal 2Ls flic.kr/p/FurHFx . Intermediate valves (ii to vii) are all almost equal in length, so virtually no anterior or posterior taper on the complete shell. Individual valves ii to vii have straight or almost straight, parallel anterior and posterior edges and lack any sign of a beak 3Ls flic.kr/p/244RWAy . When an intermediate valve is viewed from the posterior, it has a low profile, H/W c. 30%, with a very slight keel 4Ls flic.kr/p/FurFJM & 5Ls flic.kr/p/H1XySJ . The tail valve viii 6Ls flic.kr/p/FurFzP is large and semicircular with a swollen, central mucro and a concave postmucronal slope that terminates in a low flare.

Canals permeate the tegmentum and terminate on its dorsal surface in large granules. Intermediate valves ii to vii each have two widely separated, small, triangular, translucent, white apophyses on their anterior edge that extend under the next valve forwards 3Ls flic.kr/p/244RWAy & 4Ls flic.kr/p/FurFJM . Tail valve viii has a rounded trapezoidal apophyses 6Ls flic.kr/p/FurFzP . L. scabridus, like others in family Leptochitonidae, lacks insertion plates that most chitons have at the ends of intermediate valves, at the anterior of the head valve and posterior of the tail valve.

The dorsal surface of the tegmentum 2Ls flic.kr/p/FurHFx & 3Ls flic.kr/p/244RWAy has large granules that vary from rounded bosses to rough angular sculpture. They are arranged 2Ls flic.kr/p/FurHFx radially on the head valve i, postmucronal area of tail valve viii and the lateral triangles of intermediate valve ii to vii. They are arranged in longitudinal lines on the central areas (= jugal and pleural areas) and antemucronal area of tail valve viii. On intermediate valves ii to vii, the granules are largest 3Ls flic.kr/p/244RWAy near the anterior and lateral edges of each valve. They are largest near the entire periphery of tail valve viii 6Ls flic.kr/p/FurFzP , and largest near the curved edge of head valve i 3Ls flic.kr/p/244RWAy . Lines of granules may be indistinct if the shell is worn or has adhering secretions or deposits. Lateral and pleural areas are slightly differentiated by the slight elevation of the lateral triangle 3Ls flic.kr/p/244RWAy & 5Ls flic.kr/p/H1XySJ and the difference in orientation of granules. Growth lines may be visible on the valves 5Ls flic.kr/p/H1XySJ .

 

Body Description

The head and foot rarely, if ever, protrude into view naturally on a live animal 2Ls flic.kr/p/FurHFx , and can only be examined if the animal is removed from substrate or placed on glass. The head consists of a mouth surrounded by a large pinkish-white hood with a red-pink longitudinal band at each side 7Ls flic.kr/p/244RWqy ; it has no eyes or sensory tentacles.

Aesthetes (sensory tissue) fill canals that permeate the tegmentum and parts of articulamentum. They terminate in granules as sense organs on the dorsal surface of valves, but are not visible without high magnification. The perinotum (dorsal surface) of the girdleis yellowish-white. Sometimes it is stained brown, but there is no recognisable colour-pattern. It has densely packed, squarish scales with rounded corners 9Ls flic.kr/p/244RVN1 . The hyponotum (ventral surface of girdle) has about five distinct rows running around it of white, imbricated, elongate, obtusely pointed scales 8Ls flic.kr/p/FurF9P . The peripheral margin of the girdle has a dense fringe of yellowish spicules, and a row of more widely spaced, longer white spicules 9Ls flic.kr/p/244RVN1 .

An open, narrow mantle cavity runs around the whole animal. It contains about six red gills on either side close to the posterior (merobranch arrangement) 10Ls flic.kr/p/H1XxUG . Between the foot and hyponotum, the unobtrusive mantle fold helps the foot seal the mantle cavity into a closed tube containing the gills 10Ls flic.kr/p/H1XxUG & 11Ls flic.kr/p/244RVzW . The anus is on a large whitish papilla 12Ls flic.kr/p/H1XxQU that opens into the mantle cavity at the posterior where the girdle deflects dorsally for expulsion of faeces. The nephridiopores and gonopores open laterally into the posterior quarter of the cavity. Chitons have no penis as fertilization is external. The expanded foot is an elongate ellipse, broader at the front than at the posterior 11Ls flic.kr/p/244RVzW . When live and in good condition, the sole is a striking carmine red 7Ls flic.kr/p/244RWqy , but paler when fully expanded 11Ls flic.kr/p/244RVzW . The foot fades on death 13Ls flic.kr/p/H1XxL5 or when it is in poor condition. The sides of the foot and periphery of the sole are usually pinkish white, and the foot may be paler medially.

 

Key identification features

*or ** indicate principal distinguishing features.

Leptochiton scabridus (Jeffreys, 1880).

1*: Small; extreme maximum length 8mm; eliminate any larger specimen from consideration. Valves whitish sometimes tinted pink and/or with yellowish or light brown staining. No lateral insertion plates or slit 4Ls flic.kr/p/FurFJM .

2*: Only at LWS and sublittorally. Brittany, Channel Islands, south coast England & west coast Ireland. Probably overlooked because small; be alert for it in S.W.Wales & W. Scotland.

3: Girdle yellowish-white sometimes with light brown staining, but no recognisable colour-pattern 9Ls flic.kr/p/244RVN1 .

4*: Arch of valves has a low profile, H/W c.31% and a very slight keel.

5: No posterior beak on intermediate valves.

6*: Valves have large, raised granules 2Ls flic.kr/p/FurHFx in rows running radially on valves i & viii and on lateral areas of valves ii to vii. Rows longitudinal on central areas of valves ii to vii. (Oriented as on L. cancellatus, but coarser.)

7**: Sole strikingly bright red 7Ls flic.kr/p/244RWqy , but fades when dead 13Ls flic.kr/p/H1XxL5 or in poor condition and may then resemble sole of L. cancellatus.

8: About six gills each side, close to anus at posterior (merobranch) 10Ls flic.kr/p/H1XxUG .

 

Similar species

Leptochiton cancellatus

1: Small; maximum length 9mm. Valves whitish or cream, often stained in patches or streaks with black manganese and/or rust coloured ferrous deposits. No lateral insertion plates or slits 15Ls flic.kr/p/H1XxEJ .

2: Only at LWS and sublittorally. All round Britain.

3: Narrow whitish girdle has no pattern, may be stained with rust coloured ferrous, and/or blackish manganese, deposits. Dorsal girdle scales squarish with rolled tip.

4*: Arch of valves is moderately high smooth curve with no sign of a keel. Elevation of arch of valves iv & v; H/W c.46% 14Ls flic.kr/p/244RVsG .

5: No posterior beak on intermediate valves.

6*: Cancellated by closely packed, almost touching, small, uniformly sized ,oval granules in chainlike lines (Oriented as on L. scabridus) 16Ls flic.kr/p/244RVbj .

7*: Sole whitish pink centrally, becoming reddish pink towards periphery. 17Ls flic.kr/p/22oDHKW .

8: Gills close to anus at posterior (merobranch) 17Ls flic.kr/p/22oDHKW .

 

Lepidochitona cinerea (Linnaeus, 1767)

L. scabridus might be superficially mistaken for a juvenile pale L. cinerea.

1*: Maximum length 28mm. Valves diversely coloured, including whitish 18Ls flic.kr/p/244RV8y . Lateral insertion plates separated by single slit on each end of valves ii to vii.

2*: Midshore level and below. All round Britain. Commonest littoral chiton.

3**: Girdle has a unique (in N.W. Europe) lozenge pattern 19Ls flic.kr/p/23MwvgF , but sometimes indistinct, especially on whitish specimens, juveniles of which might be confused with Leptochiton scabridus.

4*: Arch of valves is keeled 14Ls flic.kr/p/244RVsG .

5*: Distinct posterior beak on intermediate valves.

6: Dorsal surface of valves has densely packed rounded granules, not in straight lines.

7: Sole pinkish white to orange pink, usually with grey viscera showing centrally 20Ls flic.kr/p/244RTF5 .

8*: Usually 16 to19 gills each side for whole length of foot (holobranch) 20Ls flic.kr/p/244RTF5 .

 

Leptochiton asellus (Gmelin, 1791)

1*: Maximum size 18mm X 10mm. Valves whitish, often with distinct black longitudinal lines 21Ls flic.kr/p/255RDKd or general staining by black or rust coloured mineral deposits 22Ls flic.kr/p/H1XuoU . No lateral insertion plates or slits.

2: Only at LWS and sublittorally. All round Britain; probably commonest sublittoral chiton.

3: Whitish girdle has no pattern, may be stained with rust coloured ferrous, and/or blackish manganese, deposits. Dorsal girdle scales elongate, bluntly pointed 23Ls flic.kr/p/FurCkn .

4**: Arch of valves is quite high with distinct keel and straight* side slopes 23Ls flic.kr/p/FurCkn . (Elevation of arch valves iv & v; height/width 36%)

5: Often a slight beak on intermediate valves, especially valves ii & iii.

6: Roughly oval granules on valves in slightly disjointed lines, oriented as on L. scabridus.

7: Sole whitish pink centrally, becoming reddish pink towards periphery 22Ls flic.kr/p/H1XuoU .

8: Eight to thirteen ctenidia in posterior half of pallial groove (merobranch).

 

Habits and ecology

Lives on stones resting on sand or gravel at LWS and sublittorally. On shores, many finds are under large rocks embedded up to 20cm into clean sand or gravel, sometimes in pools. If displaced from the substrate, it can roll up. Respiration: cilia on its gills and mantle create an inhalent water-current entering the pallial cavity wherever the girdle is raised at the anterior. The water current passes through the gills and then along the cavity as an exhalent current to exit at the posterior under the raised girdle.

In the absence of eyes or sensory tentacles on its head, it senses the environment through aesthetes exposed on the surface of the valves.

It feeds by scraping micro algae and associated organisms from the rock surface using its hard radula of chitin mineralized with magnetite. The water current in the pallial cavity carries excreta from the lateral nephridiopores to the posterior where faecal pellets from anus join the flow; all are expelled at the posterior under the raised girdle. It travels by monotaxic retrograde compression waves on the sole of the foot.

Breeding is dioecious. The water current in the mantle cavity carries sperm or ova from lateral gonopores to the posterior and out under the deflected girdle. As fertilization is external, synchronised emission of sperm and ova is needed to ensure success; the trigger in many chiton species is moon-phase/ state of tides. Planktonic trochophore larvae hatch and metamorphose into small adult-form young without an intervening veliger stage.

 

Distribution and status

Canary Islands, Mediterranean, Iberia, Brittany, south coast England and west Ireland. GBIF map www.gbif.org/species/5193784 Supposedly rare in Britain, but probably overlooked because of small size and living under large rocks embedded into sand or gravel. NBN distribution map:

species.nbnatlas.org/species/NBNSYS0000183422#tab_mapView

Acknowledgements

I am most grateful to Steve Trewhella for images, a specimen and information. Without his help the account would have not been possible.

 

Links and references

 

Botelho, A. 2013. Zoologger: mollusc grows hardest teeth in the world New Scientist 3rd October 2013 www.newscientist.com/article/dn24329-zoologger-mollusc-gr...

 

Fernandez, C.Z., Vendrasco, M.J. & Runnegar,B. 2007. Aesthete canal morphology in twelve species of chiton (Polyplacophora). The Veliger 49(2) 51 – 69 www.researchgate.net/publication/246548141_Aesthete_Canal...

 

Fox, R. 2007. Invertebrate Anatomy On Line. Katharina tunicata lanwebs.lander.edu/faculty/rsfox/invertebrates/katharina....

 

Jones, A.M. & Baxter, J.M. 1987. Molluscs: Caudofoveata, Solenogastres, Polyplacophora and Scaphopoda London, Linnean Society, and Estuarine and Brackish-water Sciences Association.

 

Kaas, P. & van Belle, R.A. 1985. Monograph of living chitons Vol 1: 49 to 51. Leiden-London-Köln-København: E. J. Brill/W. Backuys. Preview at

books.google.co.uk/books?id=CKautf2EUPkC&pg=PA7&s...

 

Light, J.M. & Baxter, J.M. 1990. Leptochiton scabridus (Jeffreys, 1880) – a live record from County Donegal, Ireland. J. Conch. Lond. 33: 318.

 

Light, J.M. & Killeen, I.J., 1992. Further notes on Leptochiton scabridus (Jeffreys). J. Conch. Lond. 34: 117.

 

Matthews, G. circa1954. The identification of British chitons. Papers for Students No.9. London, Conchological Society of Great Britain and Ireland.

 

Nature22 website, chiton page. (Good images of several chiton species.) nature22.com/estran22/mollusques/polyplacodentale/chitons...

 

Schwabe, E. 2010. Illustrated summary of chiton terminology. Spixiana 33(2):171-194.

Free pdf at verlag-pfeil.de/04biol/pdf/spix33_2_02.pdf

 

Current taxonomy: World Register of Marine Species (WoRMS)

www.marinespecies.org/aphia.php?p=taxdetails&id=140211

 

Glossary

3Ls = image 3 in Leptochiton scabridus album.

aesthete (in chitons) = one of complex of canals filled with sensory tissue that permeate tegmentum and parts of articulamentum. Occur in bundles of a large megalaesthete surrounded by several smaller radiating micraesthetes that open as sensory macropores and micropores on dorsal surface of valves. Some are photoreceptors; other function(s) uncertain, may include chemoreception, mechanoreception, properiostracum replenishment and/or secretion of protective substances.

 

a.k.a. = also known as.

antemucronal area = area situated to anterior of mucro.

apophysis (pl. apophyses) = natural protruberance within a shell for attachment of muscles. On chitons, anterior extension of articulamentum which underlies preceding valve; on all valves except head valve.

 

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 = ventral shell-layer of chiton valves, usually hard, white, porcelaneous aragonite and often differently coloured in central part. (Partially overlain ventrally by inconspicuous myostracum layer.)

 

cancellated = lattice like pattern.

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

chitin = semitransparent flexible horny protein.

chitinous = (adj.) made of chitin.

 

dioecious = having separate male and female individuals, not hermaphrodite.

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

 

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

 

gonopore = opening through which eggs or sperm are released.

haemoglobin = oxygen-carrying substance in blood; scarlet when oxygenated.

 

holobranch (of chitons) = arrangement of gills in pallial groove that extends full length of foot.

 

hyponotum = ventral surface of chiton’s girdle.

imbricated = (of scales, plates, etc.) arranged to overlap and alternate like roof tiles.

insertion plate = (on most chitons) extension of articulamentum 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 to vii), except head valve (i) and tail valve (viii).

 

jugal area = triangular middle section of central area of intermediate valves, with apex pointing to posterior; discernible when defined by differences of colour and/or sculpture (dorsal surface).

 

jugal tract = triangular middle section of central area of intermediate valves, with apex pointing to posterior; discernible when defined by densely arranged aesthete pores (ventral surface).

 

jugum = triangular middle section of central area of intermediate valves. (See jugal area and jugal tract.)

 

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.

 

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

 

magnetite = mineral of iron oxide, hardest material made by any living organism (Botelho, 2013).

 

mantle = sheet of tissue covering visceral mass of molluscs. Secretes shell of shelled species, and forms part or all of dorsal body surface (notum) of those without shells. On chitons, forms mantle/pallial cavity and is toughened to form the girdle surrounding the shell valves.

 

megalaesthete = (see aesthete).

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

 

micraesthete = (see aesthete).

monotaxic = (of locomotion waves on foot) single series of waves across complete width of foot.

 

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

 

myoglobin = red oxygen-binding protein in muscle tissue; often in buccal-mass muscles of gastropods. Similar to red haemoglobin in vertebrate blood, but green haemocyanin is usual oxygen-carrier in mollusc blood. See www.researchgate.net/publication/251227038_Radular_myoglo...

 

myostracum = microscopically thin discontinuous innermost layer of chiton valve.

 

nephridium (pl. nephridia) = cilia-lined excretory/osmoregulatory tubule (kidney).

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

 

odontophore = firm, approximately ellipsoid, structure of cartilage supporting radula. Protruded like a tongue to operate radula. Often reddish from myoglobin, and medially grooved.

 

papilla (pl. papillae) = small nipplelike protuberance.

perinotum = dorsal surface of chiton’s girdle.

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

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.

 

porcelaneous = resembling vitreous glazed ceramic material.

postmucronal = situated to posterior of mucro on tail valve.

properiostracum (on chitons) = proteinaceous material covering the shell. Different composition from periostracum of most other molluscs.

 

quincunx = pattern of five as on playing card.

radula = ribbon of chitin bearing chitinous teeth that is extruded on a tongue-like odontophore of cartilage to rasp food. On chitons and limpets, teeth are usually impregnated with magnetite, a hard magnetic mineral of iron.

en.wikipedia.org/wiki/Radula

 

radular sac = tube, ending in a caecum, where radula is created and stored.

retrograde (of locomotion waves on foot) = waves travel from anterior to posterior.

SEM = scanning electron microscope.

sublittoral = below level of low water spring tide

trochophore = spherical or pear-shaped larvae that move 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.

 

 

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

 

valves (on chitons) = the eight dorsal, articulated shell plates.

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

 

Done