Acanthochitona discrepans (Brown, 1827)

Voncina, Katarzyna, Mikkelsen, Nina Therese, Morrow, Christine, Ang, Rory & Sigwart, Julia D., 2023, Clarification of the taxonomic status of Acanthochitona discrepans (Brown, 1827) with new data for the North-East Atlantic Acanthochitona (Polyplacophora, Acanthochitonidae), Biodiversity Data Journal 11, pp. 109554-109554 : 109554

Acanthochitona discrepans (Brown, 1827)

Chiton discrepans : Brown, 1827 - Brown 1827, pl. 35 fig. 20; 1844 - Brown 1844: 65, p. 21 fig. 20.

Chiton discrepans non Chiton discrepans : Sowerby II, 1840a: 2 [in synonymy of C. crinitus (non Pennant)] et mult. auct.

Chiton fascicularis : Brown, 1827 - Brown 1827 (ex parte): pl. 35 fig. 5; 1844 - Brown 1844: 65 (ex parte), pl. 21 fig. 5.

Chiton fascicularis non Chiton fascicularis : Linnaeus, 1767 - Linnaeus 1767.

Chiton gracilis : Jeffreys, 1859 - Jeffreys 1859: 106, pl. 3 figs 9a-c; Sowerby II, 1859 - Sowerby II 1859: pl. 10 fig. 6; Winckworth, 1926 - Winckworth 1926: 15, pl. 1, fig. 1, lb-d.

Chiton fascicularis var. gracilis : Jeffreys, 1865 - Jeffreys 1865: 212; Dean, 1926 - Dean 1926: 21; Pilsbry, 1893 - Pilsbry 1893: 11, pl. 4 fig. 83.

Acanthochitona discrepans : Winckworth, 1926 - Winckworth 1926: 15, pl. 1 fig. 2; Dean, 1926 - Dean 1926: 21; Winckworth, 1932 - Winckworth 1932: 218.

Chiton gracile : Warén, 1980 - Warén 1980: 13.

Acanthochtona crinita : Kaas, 1985 - Kaas 1985: 591, Figs. 28-38.

Acanthochitona discrepans : Kaas, 1985 - Kaas 1985: 598, Figs. 59-75.

Acanthochitona discrepans Type material. Lectotype: TENBM 1983.4588/1, Tenby Museum, Tenby, Pembroke, Wales. Designated by Kaas (1985) (material not seen).

Acanthochitona discrepans Type locality. Wales, Pembroke, Tenby ( Pennant 1777, Kaas 1985).

Materials

Type status: Other material. Occurrence: individualID: SMF 373031; individualCount: 1; lifeStage: adult; otherCatalogNumbers: BOLD:ADZ1926, NIB_CCM_0087; associatedSequences: BOLD:NIB087-21, GenBank: OR526580 ; occurrenceID: F0EE7AE3-9B89-526A-835A-EFD87BE7FF37; Taxon : scientificNameID: Acanthochitona discrepans; Location : country: Northern Ireland; county: Down; locality: Strangford Lough ; verbatimCoordinates: 54°23'38"N 5°34'40"W; Event: eventDate: 14/02/2021; eventRemarks: Christine Morrow, Bernard Picton & Julia Sigwart leg.; Record Level: collectionCode: Malakologie- SMF; basisOfRecord: PreservedSpecimen Type status: Other material. Occurrence: individualID: SMF 373033; individualCount: 1; lifeStage: adult; otherCatalogNumbers: BOLD:ADZ1926, NIB_CCM_0343; associatedSequences: BOLD:CCMMO021-21, GenBank: OR526581 ; occurrenceID: 2F5A0B87-0B67-5DD9-9AB4-D7E2D35306BF; Taxon : scientificNameID: Acanthochitona discrepans; Location : country: Northern Ireland; county: Down; locality: Strangford Lough ; verbatimCoordinates: 54°23'38"N 5°34'40"W; Event: eventDate: 09/04/2021; eventRemarks: Christine Morrow leg.; Record Level: collectionCode: Malakologie- SMF; basisOfRecord: PreservedSpecimen Type status: Other material. Occurrence: individualID: SMF 373032; individualCount: 1; lifeStage: adult; otherCatalogNumbers: BOLD:ADZ1926, NIB_CCM_0334; associatedSequences: BOLD:CCMMO018-21, GenBank: OR526582 ; occurrenceID: F07D922B-2D26-5E53-B753-CD66EB7CC5B1; Taxon : scientificNameID: Acanthochitona discrepans; Location : country: Northern Ireland; county: Down; locality: Strangford Lough ; verbatimCoordinates: 54°23'38"N 5°34'40"W; Event: eventDate: 09/04/2021; eventRemarks: Christine Morrow leg.; Record Level: collectionCode: Malakologie- SMF; basisOfRecord: PreservedSpecimen Type status: Other material. Occurrence: individualID: SMF 373036; individualCount: 1; lifeStage: adult; otherCatalogNumbers: BOLD:ADZ1926, NIB_CCM_0476; associatedSequences: BOLD:CCMMO041-21, GenBank: OR526585 ; occurrenceID: 5E9A5A5A-5676-5772-BC76-253093721D8A; Taxon : scientificNameID: Acanthochitona discrepans; Location : country: Northern Ireland; county: Down; locality: Strangford Lough ; verbatimCoordinates: 54°29'34"N 5°39'03"W; Event: eventDate: 25/05/2021; eventRemarks: Christine Morrow leg.; Record Level: collectionCode: Malakologie- SMF; basisOfRecord: PreservedSpecimen Type status: Other material. Occurrence: individualID: SMF 373024; individualCount: 1; lifeStage: adult; associatedSequences: GenBank: OR145402 ; occurrenceID: 1FFACC15-2880-5B42-A013-76F661C8B18E; Taxon : scientificNameID: Acanthochitona discrepans; Location : country: Northern Ireland; county: Down; locality: Strangford Lough ; verbatimCoordinates: 54°29'23"N 5°32'15"W; Event: eventDate: 19/09/2019; eventRemarks: Julia Sigwart leg.; Record Level: collectionCode: Malakologie- SMF; basisOfRecord: PreservedSpecimen Type status: Other material. Occurrence : individualID: ZMBN140332; individualCount: 1; lifeStage: adult; otherCatalogNumbers: BOLD:ADZ1926; associatedSequences: BOLD:NORCH109-23, GenBank : OR526587 ; occurrenceID: CA9290A1-1E55-5CF3-9F3E-1A8011E24634; Taxon : scientificNameID: Acanthochitona discrepans; Location : country: Norway; stateProvince: Trondelag; locality: Hopavagen ; verbatimCoordinates: 63°35'35"N 9°32'0"E; Event: eventDate: 14/10/2020; eventRemarks: Nina T. Mikkelsen leg.; Record Level: institutionCode: ZMBN; basisOfRecord: PreservedSpecimen Type status: Other material. Occurrence : individualID: ZMBN140293; individualCount: 1; lifeStage: adult; otherCatalogNumbers: BOLD:ADZ1926; associatedSequences: BOLD:NORCH005-21, GenBank : OR526590 ; occurrenceID: 487EB93E-0FDE-5C67-8A8E-A955D0A4495C; Taxon : scientificNameID: Acanthochitona discrepans; Location : country: Norway; stateProvince: Vestland; locality: Bakkasund ; verbatimCoordinates: 60°07'51"N 5°05'31"E; Event: eventDate: 03/08/2021; eventRemarks: Nina T. Mikkelsen leg.; Record Level: institutionCode: ZMBN; basisOfRecord: PreservedSpecimen Type status: Other material. Occurrence : individualID: ZMBN140294; individualCount: 1; lifeStage: adult; otherCatalogNumbers: BOLD:ADZ1926; associatedSequences: BOLD:NORCH006-21, GenBank : OR526591 ; occurrenceID: C5272212-FC93-5596-9127-090EB552C728; Taxon : scientificNameID: Acanthochitona discrepans; Location : country: Norway; stateProvince: Vestland; locality: Bakkasund ; verbatimCoordinates: 60°07'51"N 5°05'31"E; Event: eventDate: 03/08/2021; eventRemarks: Nina T. Mikkelsen leg.; Record Level: institutionCode: ZMBN; basisOfRecord: PreservedSpecimen Type status: Other material. Occurrence : individualID: ZMBN140295; individualCount: 1; lifeStage: adult; otherCatalogNumbers: BOLD:ADZ1926; associatedSequences: BOLD:NORCH007-21, GenBank : OR526592 ; occurrenceID: D519A7ED-0999-53BA-9539-819BC258DA1C; Taxon : scientificNameID: Acanthochitona discrepans; Location : country: Norway; stateProvince: Trondelag; locality: Hopavagen ; verbatimCoordinates: 63°35'34"N 9°32'02"E; Event: eventDate: 15/10/2020; eventRemarks: Nina T. Mikkelsen leg.; Record Level: institutionCode: ZMBN; basisOfRecord: PreservedSpecimen Type status: Other material. Occurrence : individualID: ZMBN46755; individualCount: 1; lifeStage: adult; otherCatalogNumbers: BOLD:ADZ1926; associatedSequences: BOLD:NORCH059-22, GenBank : OR526594 ; occurrenceID: 6E34B9BD-25B0-55B0-B0D3-3E7A4AE7E5B4; Taxon : scientificNameID: Acanthochitona discrepans; Location : country: Norway; stateProvince: Vestland; locality: Saevroysund ; verbatimCoordinates: 60°48'14"N 4°48'29"E; Event: eventDate: 21/06/2021; eventRemarks: Nina T. Mikkelsen leg.; Record Level: institutionCode: ZMBN; basisOfRecord: PreservedSpecimen Type status: Other material. Occurrence: individualID: SMF 373034; individualCount: 1; lifeStage: adult; otherCatalogNumbers: BOLD:ADZ1926, NIB_CCM_0471; associatedSequences: BOLD:CCMMO037-21, GenBank: OR526583 ; occurrenceID: 9F36D135-538C-5B4A-8322-E4135C813101; Taxon : scientificNameID: Acanthochitona discrepans; Location : country: Northern Ireland; county: Down; locality: Strangford Lough ; verbatimCoordinates: 54°29'34"N 5°39'03"W; Event: eventDate: 25/05/2021; eventRemarks: Christine Morrow leg.; Record Level: collectionCode: Malakologie- SMF; basisOfRecord: PreservedSpecimen Type status: Other material. Occurrence: individualID: SMF 373035; individualCount: 1; lifeStage: adult; otherCatalogNumbers: BOLD:ADZ1926, NIB_CCM_0475; associatedSequences: BOLD:CCMMO040-21, GenBank: OR526584 ; occurrenceID: C8E53288-0BE9-53D7-AD6E-1CCBA45BE000; Taxon : scientificNameID: Acanthochitona discrepans; Location : country: Northern Ireland; county: Down; locality: Strangford Lough ; verbatimCoordinates: 54°29'34"N 5°39'03"W; Event: eventDate: 25/05/2021; eventRemarks: Christine Morrow leg.; Record Level: collectionCode: Malakologie- SMF; basisOfRecord: PreservedSpecimen Type status: Other material. Occurrence : individualID: ZMBN140330; individualCount: 1; lifeStage: adult; otherCatalogNumbers: BOLD:ADZ1926; associatedSequences: BOLD:NORCH108-23, GenBank : OR526586 ; occurrenceID: 1B7EC7A4-D596-5C8D-8154-A3017FE66F2A; Taxon : scientificNameID: Acanthochitona discrepans; Location : country: Norway; stateProvince: Trondelag; locality: Hopavagen ; verbatimCoordinates: 63°35'35"N 9°32"E; Event: eventDate: 14/10/2020; eventRemarks: Nina T. Mikkelsen leg.; Record Level: institutionCode: ZMBN; basisOfRecord: PreservedSpecimen Type status: Other material. Occurrence : individualID: ZMBN140333; individualCount: 1; lifeStage: adult; otherCatalogNumbers: BOLD:ADZ1926; associatedSequences: BOLD:NORCH110-23, GenBank : OR526588 ; occurrenceID: E888D7B1-95EF-5C98-9E1E-B109714BB316; Taxon : scientificNameID: Acanthochitona discrepans; Location : country: Norway; stateProvince: Vestland; locality: Espegrend ; verbatimCoordinates: 60°16'11"N 5°13'19"E; Event: eventDate: 25/05/2021; eventRemarks: Nina T. Mikkelsen leg.; Record Level: institutionCode: ZMBN; basisOfRecord: PreservedSpecimen Type status: Other material. Occurrence : individualID: ZMBN140331; individualCount: 1; lifeStage: adult; otherCatalogNumbers: BOLD:ADZ1926; associatedSequences: BOLD:NORCH111-23, GenBank : OR526589 ; occurrenceID: 342B65CF-9A4E-5005-A996-C5ABB08B0C88; Taxon : scientificNameID: Acanthochitona discrepans; Location : country: Norway; stateProvince: Vestland; locality: Puddefjorden ; verbatimCoordinates: 60°22'51"N 5°19'30"E; Event: eventDate: 24/08/2021; eventRemarks: Nina T. Mikkelsen leg.; Record Level: institutionCode: ZMBN; basisOfRecord: PreservedSpecimen Type status: Other material. Occurrence: individualID: SMF 373025; individualCount: 1; lifeStage: adult; associatedSequences: GenBank: OR145401 ; occurrenceID: 025E0A78-B0C7-5BCA-BA86-7D6549DA52A4; Taxon : scientificNameID: Acanthochitona discrepans; Location : country: Northern Ireland; county: Down; locality: Strangford Lough ; verbatimCoordinates: 54°29'23"N 5°32'15"W; Event: eventDate: 09/19/2019; eventRemarks: Julia Sigwart leg.; Record Level: collectionCode: Malakologie- SMF; basisOfRecord: PreservedSpecimen Type status: Other material. Occurrence : individualID: ZMBN140296; individualCount: 1; lifeStage: adult; otherCatalogNumbers: BOLD:ADZ1926; associatedSequences: NORCH008-21, GenBank : OR526593 ; occurrenceID: 4FFE8F85-37DE-54CF-8353-2586579A9A8D; Taxon : scientificNameID: Acanthochitona discrepans; Location : country: Norway; stateProvince: Trondelag; locality: Hopavagen ; verbatimCoordinates: 63°35'34"N 9°32'02"E; Event: eventDate: 15/10/2020; eventRemarks: Nina T. Mikkelsen leg.; Record Level: institutionCode: ZMBN; basisOfRecord: PreservedSpecimen

Description

Animal small to medium size (in examined material, BL: 11-22 mm, BW: 5-11 mm, BL/BW ratio: 2.1), outline oval, moderately elevated (elevation ratio 0.41), semi-carinated, side slopes flat to slightly raised, apices not prominent; girdle wide, spiculose (Fig. 1 C). Dorsal colouration not uniform, often mottled, variable; tegmentum brownish to dark red, lateropleural and antemucronal areas with blue, dark brown or beige maculation, jugum usually dark brown to black; colour of perinotum beige to pale yellow mottled with olive green or brown; normally 18 sutural tufts (Fig. 1 C).

Head valve slightly wider than long, almost semicircular, anterior slope slightly convex, posteriormost margin straight without a notch (Fig. 1 C, Fig. 5 A). Intermediate valves ellipsoidal, wide, anterior margins slightly rounded at both sides of straight or little concave jugum, side margins rounded, very slightly beaked with hind margin slightly concave at both sides of the apex; smoothly rounded or semi-carinate; jugal areas striated, wide, wedge-shaped, little elevated and hardly separated from the lateropleural areas (Fig. 1 C, Fig. 5 B, C). Tail valve roughly semicircular with central, moderately-elevated mucro and posterior slope steep and straight (Fig. 5 D).

Tegmentum uniformly covered with rather densely distributed (density from 24 to 50 granules per 1 mm2, mean = 37, n = 8 specimens), oval to elongated granules, arranged in quincunx order, except for the jugal area of intermediate valves; the granules raised, flat topped, to slightly concave (Fig. 1 C, Fig. 4 B, Fig. 5 , Fig. 6 B). Proximal granules more elongate and merging into jugum; in lateral areas, granules spaces less than granule-width apart. Each granule with 1-2 macroaesthetes (single macroaesthete located in posterior third; second, if present, located centrally) surrounded by 26-40 very small posteriomedially located microaesthetes (Fig. 4 B, Fig. 6 B).

Articulamentum well developed, solid, slightly pinkish ventrally beneath tegmentum. Apophyses large, separated at jugum, broadly rectangular to wing-shaped, trapezoidal in tail valve (direct dorsal view can create appearance of triangular outline); insertion plates wide, continuous with apophyses, slits deep, extending proximally into shallow dorsal channels (slit formula: 5/1/2) (Fig. 5 ).

Girdle wide, fleshy, leathery looking, brighter than tegmentum, deeply encroaching sutural areas. Colour from beige or light yellow to pale brown with brown and olive-green streaks and blotches, in some specimens prominent dark green bands with fuzzy boundaries located near valves between the sutural tufts; sutural tufts with translucent or brownish spicules; base of sutural tufts creamy-yellow or brown (Fig. 1 C). Dorsal surface densely covered by minute, thin brownish spicules (up to 68 x 7.5 µm) sculptured with fine riblets, pointed, but often broken (Fig. 6 D, F). In some specimens, randomly interspersed longer and thicker spicules, located only in outer half of the girdle identical to spicules of marginal fringe (Fig. 6 F). Sutural tufts prominent, with around 15-40 thick, sharply pointed, smooth needles, measuring up to 1000 µm x 60 µm, surrounded by much shorter and thinner, but also smooth bristles (Fig. 6 C). Marginal fringe with rounded, solid spicules, ribbed on proximal half, distal half ribbed or smooth, up to 300 x 35 µm. (Fig. 6 E-F). Hyponotum densely covered with imbricating flattened, elongated spicules, finely striated near the top with 3-4 ribs, much larger than dorsal spicules, up to 120 x 20 µm (Fig. 6 F).

Radula central tooth elongated, without cusp, almost straight at the top, keeled near base; first lateral tooth wing-shaped and wrapping around central; second (major) lateral massive with accessory plate tricuspidate with outer denticle shorter than the others; cusps pointed and triangular in outline (Fig. 6 A).

Gills merobranchial, composed of 9-13 ctenidia on each side reaching around half body length.

Diagnosis

This species can be distinguished from other Acanthochitona species from NE Atlantic on the basis of a set of morphological characters. It differs from A. crinita (Pennant, 1777) by the number, size and arrangement of aesthetes on tegmental granules (1-2 macroaesthetes - a single macroaesthete located in posterior third, second, if present, located centrally, surrounded by very small, posteriomedially located 26-40 microaesthetes in A. discrepans vs. one macroaesthete subcentral surrounded by larger, posteromedially located 8-16 microaesthetes in A. crinita ), dorsal spicules (smaller, thinner spicules up to 68 x 7.5 µm in A. discrepans vs. longer, thicker spicules up to 100 x 15 µm in A. crinita ), tuft needles (longer spicules up to 1000 µm x 60 µm, surrounded by a large number of somewhat shorter and much thinner needles in A. discrepans vs. longer spicules up to 340 x 20 µm, surrounded by a smaller number of very short and thin needles in A. crinita ). Acanthochitona discrepans can be distinguished from A. fascicularis (Linnaeus, 1767) by the shape of the intermediate valves (ellipsoidal in A. discrepans vs. triangular in A. fascicularis ), jugal area (hardly raised, not sharply separated from the latero-pleural areas in A. discrepans vs. raised, sharply separated from the latero-pleural areas in A. fascicularis ), shape of dorsal granules (decidedly apart, oval to elongated oval in A. discrepans vs small, round, densely packed granules with the incision in the middle in A. fascicularis ), density of tegmental granules (24-50 granules per 1 mm2, mean = 37 in A. discrepans vs. 42-70 granules per 1 mm2, mean = 52 in A. fascicularis ), number of microaesthetes (26-40 microaesthetes in A. discrepans vs. 0-6 microaesthetes in A. fascicularis ), number of bristles in the sutural tufts (15-40 in A. discrepans vs. 55-120 in A. fascicularis ). Acanthochitona discrepans differs from A. oblonga Leloup, 1981 by the shape of dorsal granules (rather densely distributed, oval to elongated granules, arranged in quincunx order in A. discrepans vs. widely apart and very much elongated in A. oblonga ), number and arrangement of microaesthetes (26-40 microaesthetes located posteromedially in A. discrepans vs. 6-9 microaesthetes located mainly in the central area in A. oblonga ). It can be distinguished from A. pilosa Schmidt-Petersen, Schwabe et Haszprunar, 2015 on the basis of the shape of the IV valve (very slightly-beaked valve with hind margin slightly concave at both sides of the apex in A. discrepans vs. valve with triangular posterior margin with no apex in A. pilosa ), number of microaesthetes on the tegmental granules (26-40 microaesthetes in A. discrepans vs. around 14 microaesthetes in A. pilosa ), large dorsal spicules (only sometimes present and very sparse large dorsal spicules in A. discrepans vs. always present, dense and straight spicules in A. pilosa ).

Distribution

This species seems to have a more northern distribution than Acanthochitona crinita . The range of species is from Norway (molecular data and SEM photos from this study), through the north coast of Ireland (molecular data and SEM photos from this study) to the south of Wales (locus typicus). The literature records from more southern regions must be treated with caution as they probably represent other misidentified Acanthochitona species.

Taxon discussion

The original locus typicus of Acanthochitona discrepans (Brown, 1827) is in Tenby, Pembroke in Wales. Kaas (1985) chose the lectotype from the syntype series (Lyons collection, Tenby Museum). In our study, we re-describe this species, based on the specimens from the Strangford Lough from the Northern Ireland (ca. 320 km north from the locus typicus). However, our specimens in all aspects correspond to the description provided by Kaas (1985); Kaas also examined the specimens from Strangford Lough, confirmed that they are A. discrepans and stated that this species is fairly common there.

Figures

Figure 1. Images of the adult specimens of Acanthochitona, dorsal view. A A. crinita from England, BL not measured; B A. crinita from the Azores, BL not measured; C A. discrepans from Norway, BL 17 mm; D A. fascicularis from Ireland, BL 23 mm; E A. fascicularis from the Azores, BL 13 mm.

Figure 4. Morphology of the granules of tegmentum with the macro- and microaesthetes. A Acanthochitona crinita; B A. discrepans; C A. fascicularis.

Figure 5. Acanthochitona discrepans from Northern Ireland (A, D) and Norway (B-C). All in dorsal view. A Valve I; B Valve V; C Valve VII; D Valve VIII.

Figure 6. Acanthochitona discrepans from Northern Ireland (A-E) and Norway (F). A Radula; B Valve VII, detail of the tegmentum in the lateropleural area; C One of the 18 tufts of spicules on the girdle; D Dorsal spicules; E Dorsal and marginal spicules; F Dorsal, marginal and ventral spicules.