03 Leptochiton asellus

Whitish valves tinted slightly yellow with characteristic black-brown streaks. Whitish girdle has no pattern and is stained yellow.

Orkney, Scotland, June 2022. © A. Skene.

Full SPECIES DESCRIPTION BELOW

PDF available at www.researchgate.net/publication/387626826_Leptochiton_as...

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

 

Leptochiton asellus (Gmelin, 1791)

 

Synonyms: Chiton asellus Gmelin, 1791; Chiton asellus Chemnitz in Forbes & Hanley 1853; Chiton cinereus Linnaeus, 1767 sensu Montagu, 1803 in Jeffreys, 1865; Lepidopleurus asellus (Gmelin, 1791).

Current Taxonomy WoRMS www.marinespecies.org/aphia.php?p=taxdetails&id=140199

 

GLOSSARY BELOW

 

Shell Description

Leptochiton asellus grows to a maximum length of 18 or 19 mm. It is a broad oval with width 60% to 70% of its length 01La flic.kr/p/2qCvBg4 & 12La flic.kr/p/2qCvypH . Intermediate valves ii to vii have almost straight side slopes. Valves iv and v and are elevated about 36% (Kaas & Van Belle, 1985) of their span 02La flic.kr/p/2qCuqFB creating a low shell profile. They are moderately keeled and, when unworn, each valve usually has a slight beak which is best seen in lateral view 02La flic.kr/p/2qCuqFB & 12La flic.kr/p/2qCvypH .

In clean water, the tegmentum (dorsal surface) is dirty white or light grey, usually with longitudinal dark streaks of black-brown 03La flic.kr/p/2qCw7GL , grey 01La flic.kr/p/2qCvBg4 , brown 04La flic.kr/p/2qCvDPm or olive green. In many places the colour is dominated by deposits of brown 05La flic.kr/p/2qCpLHg or black 06La flic.kr/p/2qCvz9J on the valves and girdle, varying in intensity and extent up to an almost complete cover. When cover is partial, the deposits are usually most dense on the oldest shell material near the posterior margin of each valve, and least intense on recent growth at the ends of each valve and on the anterior margins which are often covered by the adjacent valve 05La flic.kr/p/2qCpLHg .

The head valve (i) is almost semicircular 07La flic.kr/p/2qCvDyG with a straight front slope 08La flic.kr/p/2qCvzLw . The tail valve (viii) is bluntly triangular and narrower than the head valve. Its length is less than half its width 09La flic.kr/p/2qCpGkU and its posterior slope is straight or slightly concave 08La flic.kr/p/2qCvzLw . The mucro is slightly anterior of the centre 09La flic.kr/p/2qCpGkU of the tail valve (viii) but seems more so when the anterior of the valve is hidden under the posterior of valve vii 07 La & 08La flic.kr/p/2qCvzLw . The tegmentum (dorsal surface) of the valves is roughened by many small, low pustules which are arranged in crowded radiating lines on the head valve (70 to 80), postmucronal part of the tail valve, and lateral triangles of intermediate valves (15 to 20) 04La flic.kr/p/2qCvDPm & 07La flic.kr/p/2qCvDyG . They are in longitudinal lines on the central area of intermediate valves (60 to 70) 07La flic.kr/p/2qCvDyG and the antemucronal area of the tail valve 09La flic.kr/p/2qCpGkU . Valves i and viii and the lateral areas of valve ii to vii have concentric growth lines which vary in number and clarity; sometimes being made more discernible by differential staining 01La flic.kr/p/2qCvBg4 . The tegmentum is permeated by aesthete canals containing sensory tissue.

The whitish or greyish blue articulamentum is thin and weak 10La flic.kr/p/2qCwreR . It protrudes from under the tegmentum as a pair of small apophyses which are approximately triangular with a rounded apex, and are separated by a wide sinus 10La flic.kr/p/2qCwreR . As on other Leptochiton species, the ventral surfaces of the valves have no slits forming insertion plates (Matthews, 1967) 10La flic.kr/p/2qCwreR & 11La flic.kr/p/2qCvAXK .

When the narrow girdle is fully spread, each side of it is about 10% of the total body width 12La flic.kr/p/2qCvypH . Dorsally, it is whitish 12La flic.kr/p/2qCvypH with no pattern, but is often stained with brown and black 06La flic.kr/p/2qCvz9J or yellow 03La flic.kr/p/2qCw7GL to rust 13La flic.kr/p/2qCpGgq coloured deposits. It has irregularly spaced, crowded, elongate, bluntly pointed scales which are often stained by deposits 06La flic.kr/p/2qCvz9J , 12La flic.kr/p/2qCvypH & 13La flic.kr/p/2qCpGgq . Ventrally, it has radiating, elongate, rectangular scales with blunt, distal points 14La flic.kr/p/2qCvwCX . The mantle edge has slender, smooth, white needle like spicules and stout spicules (Kaas & Van Belle, 1985) 15La flic.kr/p/2qCvzrZ .

Body Description.

The head and foot rarely protrude into view naturally on live individuals, and are best seen if the chiton is removed from the substrate and attached to glass.

The head consists of a mouth surrounded by the anterior of the foot and a large anterior and lateral rim; it has no eyes or sensory tentacles. When shut the mouth is a transverse slit 16La flic.kr/p/2qCvzdN which opens into circle, sometimes revealing the dull reddish radula 17La flic.kr/p/2qCpG3Q .The median and intermediate plates of the radula widen distally into curved blades. The flesh is whitish or yellowish, often with pink edges on the foot, head and mantle fold 15La flic.kr/p/2qCvzrZ . Dark viscera may show faintly through the slightly translucent sole.

The aragonite valves have a thin weakly developed articulamentum and a tegmentum permeated by aesthete canals. They lack strength and are easily fractured 11La flic.kr/p/2qCvAXK , but strong muscles often hold the broken parts in position and may enable the chiton to continue functioning. The transverse muscles form thick pads in the overlap between adjacent valves 10La flic.kr/p/2qCwreR . They connect the dorsal surface of the anterior insertion plate of one valve to the ventral surface of the posterior edge of the adjacent preceding valve.

The mantle cavity is an open groove around the whole animal 15La flic.kr/p/2qCvzrZ & 18La flic.kr/p/2qCumcG . On each side the groove contains 7 to 13 simply pinnate, fleshy gills 19La flic.kr/p/2qCum3y . They extend alongside the posterior 40% or less of the foot (merobranch arrangement) to the anal papilla from which the anus opens into the mantle cavity at the posterior. Excretory nephridiopores and gonopores open laterally into the posterior quarter of the cavity. There is no penis as fertilization is external.

Key identification features

Leptochiton asellus (Gmelin, 1791)

1) Max. L. c. 18 mm. Broad oval; W/L 60-70% 01La flic.kr/p/2qCvBg4 . Intermediate valves H. c. 35% of their span so a low shell profile 02La flic.kr/p/2qCuqFB .

2) Valves ii to vii have straight side slopes and a keel 02La flic.kr/p/2qCuqFB .

3) Valves dorsally whitish, sometimes dark longitudinal streaks 03La flic.kr/p/2qCw7GL Often widely stained rufous 05La flic.kr/p/2qCpLHg or black 06La flic.kr/p/2qCvz9J . No ventral slits.

4) Girdle whitish with no pattern. Often stained rufous or black. Narrow girdle width on each side about 10% of total width 12La flic.kr/p/2qCvypH .

5) Girdle has crowded, irregularly spaced, elongate, bluntly pointed scales 12La flic.kr/p/2qCvypH & 13La flic.kr/p/2qCpGgq .

6) Dorsally, valves roughened by crowded lines of pustules 07La flic.kr/p/2qCvDyG .

7) 7 to 13 merobranch gills alongside posterior 40% or less of foot; no space separates them from prominent anal papilla 18La flic.kr/p/2qCumcG & 19La flic.kr/p/2qCum3y .

8) Mucro not swollen and slightly to anterior of centre of valve viii. Valves ii-vii all have beak visible from side, unless eroded 02La flic.kr/p/2qCuqFB & 08La flic.kr/p/2qCvzLw .

9) Sublittoral and at LWS on shores. North-west Europe. All round Britain, probably commonest sublittoral chiton.

10) Sole whitish or yellowish often with pink edges on the foot, head and mantle fold 15La flic.kr/p/2qCvzrZ .

 

Similar species

Leptochiton cancellatus (G. B. Sowerby II, 1840)

1) Max. L. c. 8 mm. Intermediate valves H. c. 40% of their span 20La flic.kr/p/2qCpFYb so profile moderately high.

2) Valves ii to vii have smooth, exterior convex curve with no keel 20La flic.kr/p/2qCpFYb .

3) Valves whitish or cream, often widely stained rufous or black. No ventral slits 21La flic.kr/p/2qCvzcf .

4) Girdle whitish with no pattern. Often stained rufous 21La flic.kr/p/2qCvzcf or black.

5) Dorsal girdle scales squarish with rolled tip.

6) Dorsal surface of valves is cancellated by closely packed, almost touching, oval granules in chainlike lines 21La flic.kr/p/2qCvzcf running radially on valves i & viii, and on lateral areas of valves ii to vii (sometimes obsolete on lateral areas) . Lines longitudinal on central areas of valves ii to vii.

7) About 8 gills close to anus at posterior (merobranch) 22La flic.kr/p/2qCumdP .

8) Mucro slightly swollen and to posterior of centre of extracted valve viii (part of anterior concealed on live specimens) No posterior beak on intermediate valves 23La flic.kr/p/2qCwFJH .

9) Only at LWS and sublittorally. All round Britain. Probably overlooked.

10) Sole whitish pink centrally, becoming reddish pink towards periphery 22La flic.kr/p/2qCumdP .

 

Leptochiton scabridus (Jeffreys, 1880).

1) Max. L. c. 8 mm. Intermediate valves H. c. 30% of their span

20La flic.kr/p/2qCpFYb so profile low.

2) Valves ii to vii have slightly convex side slopes and a slight keel 20La flic.kr/p/2qCpFYb .

3) Valves whitish to dull orange 24La flic.kr/p/2qCwJG1 , often widely stained rufous or black. No ventral slits.

4) Girdle whitish with no pattern. Often stained rufous 24La flic.kr/p/2qCwJG1 or black.

5) Dorsal girdle has densely packed squarish scales.

6) Rows of raised, nodules are largest towards anterior and outer margins of valves 25La flic.kr/p/2qCujmn .

7) Gills confined to posterior half (merobranch) 26La flic.kr/p/2qCuz29 .

8) Mucro swollen and near centre of valve viii. Postmucronal slope entirely concave No posterior beak on intermediate valves, but anterior of valve ii tends to be angulated.

9) Only at LWS and sublittorally. Rare, only SW England, Channel Islands and Brittany.

10) Whole sole strikingly bright red 26La flic.kr/p/2qCuz29 .

 

Lepidochitona cinerea (Linnaeus, 1767).

1) Max. L. c. 28 mm. Intermediate valves H. c. 40% of their span 20La flic.kr/p/2qCpFYb so profile moderately high.

2) Valves ii to vii have convex side slopes and a keel 20La flic.kr/p/2qCpFYb .

3) Valves diversely coloured and patterned, including whitish resembling Leptochiton species 27La flic.kr/p/2qCvKp3 . Not often stained black or rufous. Ventral surfaces have 8 slits (i), 1 slit at each end (ii - vii) and 10 to 12 slits (viii).

4) Girdle has a unique (in N.W. Europe) lozenge pattern 28La flic.kr/p/2qCpDgc , but sometimes indistinct, especially on whitish specimens which are easily confused with Leptochiton species.

5) Dorsal surface of girdle has densely packed rounded granules 28La flic.kr/p/2qCpDgc .

6) Dorsal surface of valves has densely packed rounded granules, not in straight lines 29La flic.kr/p/2qCuiw6 .

7) Usually 16 to19 gills each side for whole length of foot (holobranch) 30La flic.kr/p/2qCvH5E .

8) Distinct posterior beak on intermediate valves 29La flic.kr/p/2qCuiw6 .

9) Midshore level and below. All round Britain. Commonest littoral chiton.

10) Sole pinkish white to orange pink, usually with grey viscera showing centrally 30La flic.kr/p/2qCvH5E .

 

Stenosemus albus (Linnaeus, 1767)

1) Max. L. c. 17 mm, exceptionally 22 mm. H. c. 45% to 50% of their span so profile moderately high 31La flic.kr/p/2qCpNpx .

2) Valves ii to vii have slightly convex side slopes and a distinct keel 31La flic.kr/p/2qCpNpx .

3) Valves white 32La flic.kr/p/2qCvCzg , very light buff, tinted pink or golden yellow 31La flic.kr/p/2qCpNpx often stained brown or black 32La flic.kr/p/2qCvCzg . Ventral surfaces have 13 slits (i), 1 slit at each end (ii - vii) and 11 or 12 slits (viii).

4) Girdle coloured as the valves; no pattern. Girdle width on each side about 12% of total width.

5) Girdle has deeply embedded, substantial, rounded, calcareous, evenly spaced, discrete, button-like granules which distinctly contrast with the pale girdle when they are coated with brown or black deposits 32La flic.kr/p/2qCvCzg .

6) Dorsal surface of valves looks smooth to naked eye and on many images but small oval papillae may be visible in good light under magnification.

7) About 20 gills on each side; merobranch but more numerous and extending further forward than on L. asellus. Space between them and the small anal papilla 33La flic.kr/p/2qCwvSo .

8) Valves ii-vii usually lack beaks; occasional on valve vi and/or vii.

9) Arctic , boreal and temperate zones of northern hemisphere. Usually sublittoral, at increasing depths southwards. Rarely intertidal at ELWS 31La flic.kr/p/2qCpNpx .

10) Sole is slightly-translucent white, sometimes showing dark viscera.

Habits and ecology

L. asellus lives on rocky shores at ELWS and below to 250 m (Kaas & Van Belle, 1985). Off Iceland it has been dredged from 65 to 320 m (Delongueville et al. 2024).

The articulated shell enables it to conform closely to uneven rock surfaces which the large foot grips firmly. The aragonite shell is weak and frequently broken, but the valve fragments may be held in position by strong muscles which allow the chiton to continue functioning. It has no eyes on its head but aesthetes in the valves of chitons have sensory functions.

Tufts of cilia on the surfaces of the mantle cavity and gills create aninhalant respiratory water-current entering the mantle cavity wherever the girdle is raised laterally. The current passes along the cavity through the gills to the posterior 18La flic.kr/p/2qCumcG , where faecal pellets from the anus on the anal papilla join the flow to be expelled from under the raised girdle.

L. asellus feeds by scraping microalgae and associated organisms from the rock surface using its radula of chitin hardened with magnetite. It detects suitable food by protruding between each stroke of the radula a subradular ‘tasting’ organ (Yonge & Thompson, 1976). The large anterolateral rim round the mouth 17La flic.kr/p/2qCpG3Q and the anterior of the foot probably help retain loosened food particles.

Reproduction is gonochoric. The water current in the mantle cavity carries sperm or ova from lateral gonopores to exit at the posterior. As fertilization is external, synchronised emission of sperm and ova is needed to ensure success. The ova hatch into planktonic trochophore larvae and later metamorphose into small adult-form young without an intervening veliger stage.

Distribution and status

L. asellus is the most common chiton sublittorally in the north-east Atlantic Ocean from Iceland, Spitzbergen, the Barents Sea and possibly Greenland to Galicia, Spain (Kaas & Van Belle, 1985). GBIF map www.gbif.org/species/5193785 . It is frequent offshore all around Britain with the apparent exception of the north-east of the Irish Sea. NBN map species.nbnatlas.org/species/NBNSYS0000176074

Acknowledgements

I gratefully thank George Brown, David Kipling, Claude Nozères, Ian Paterson, Alastair Skene and Julia Sigwart for use of images, and Paula Lightfoot, David McKay, Simon Taylor and Steve Trewhella for specimens to study and photograph.

Links and references

Delongueville, C., Ólafsdóttir, S.H. & Scaillet, R. 2024. Report of ten years Mollusca collection in Icelandic waters by the Marine and Freshwater Research Institute. Hafnarfjörður, Iceland. www.hafogvatn.is/static/research/files/hv2024_06.pdf

 

Forbes, E. & Hanley S. 1853. A history of the British mollusca and their shells. vol. 2 London, van Voorst. (As Chiton asellus Chemnitz.) archive.org/details/historyofbritish02forbe/page/407/mode...

 

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

 

Jeffreys, J.G. 1865. British conchology. vol. 3. London, van Voorst. (As Chiton cinereus Linnaeus, 1767 sensu Montagu, 1803.) archive.org/details/britishconcholog03jeffr/page/220/mode...

 

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. and Van Belle, R.A. 1985. Monograph of living chitons.

vol 1 Leiden and Boston, Brill.

 

Matthews, G. 1967. The identification of British chitons. Papers for Students No.9. London, Conchological Society of Great Britain and Ireland. (As Lepidopleurus asellus.) conchsoc.org/papers_for_students

 

Schwabe, E. 2010. Illustrated summary of chiton terminology. Spixiana 33(2):171-194. www.researchgate.net/publication/258843995_Illustrated_su...

 

Glossary

aesthetes = complex of canals permeating shell; filled with sensory tissue.

antemucronal = anterior to the mucro on a tail valve (viii).

anterolateral = situated on the side near the anterior.

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

 

aragonite = crystalline mineral-form of calcium carbonate; 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.

articulated = having sections (valves) connected by a flexible joint.

cancellated = has lattice like pattern.

chitin = semitransparent flexible horny protein.

chitinous = (adj.) made of chitin.

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

 

distal = away from centre of body or from point of attachment.

ELWS = extreme low water spring tide, two periods near equinoxes when tide falls lowest.

girdle = peripheral band of thickened, reflexed mantle which encloses ends of valves.

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

 

gonopore = opening through which eggs or sperm are released.

holobranch = gills in mantle cavity extend full length, or almost, of foot.

intermediate valve = valves ii to vii.

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

 

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

 

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

 

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

 

median = at/near centre of body as opposed to the sides.

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

motile = (adj.) able to move spontaneously; applicable to whole animals or to parts of them such as cells, gametes or cilia.

 

mucro = protuberance on tail valve (viii) of chiton demarking posterior from rest of valve; varies in prominence and position.

 

nephridium = tubular excretory/osmoregulatory organ. (a.k.a. kidney).

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

 

pinnate = (simply pinnate) bearing pinnae in one plane like a feather without any branching of the individual pinnae to the bipinnate or tripinnate pattern.

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

planktonic = (adj.) of or pertaining to plankton.

postmucronal = situated to posterior of mucro on tail valve of a chiton.

pustule = pimple

radula = ribbon of chitin bearing chitinous teeth impregnated with magnetite.

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

 

sinus = a cavity, channel or space, such as the gap between apophyses.

sublittoral = below level of low water spring tide.

 

tegmentum = outer shell-layer of chiton valves, usually porous and relatively soft.

 

trochophore = spherical or pear-shaped larvae that move with aid of girdle of cilia. Subsequent veliger stage of most marine molluscs is not found in chitons.

 

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

WoRMS = World Register of Marine Species

 

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