Acanthochitona oxum, Jardim & D & Almeida & L. & Simone, 2022

Jardim, Jaime A., De Almeida, Sergio M. & De Simone, Luiz R. L., 2022, Taxonomic study on the Polyplacophora (Chitonida: Ischnochitonidae et Acanthochitonidae) collected by the Marion Dufresne (MD 55) expedition, with description of a new species, Zoosystema 44 (5), pp. 151-157 : 155-156

Acanthochitona oxum n. sp.

( Fig. 2A-I )

urn:lsid:zoobank.org:pub:DCD9295D-1CB3-4192-9144-F54180C9815D

TYPE MATERIAL. — Holotype. Brazil • 1 specimen, 2.3 × 1.6 mm; dry specimen; 1.VI.1987; MD55, sta. DC83, Bouchet, Leal & Métivier leg.; MZSP 112459 .

OTHER MATERIAL EXAMINED. — Brazil • 1 specimen, 1.5 × 0.9 mm; Espirito Santo, Trindade Island ; 20°30’55.6”S, 29°20’21.7”W; MZSP 134390 • 1 specimen, disarticulated valves; Espírito Santo, Trindade Island ; 20°30’55.6”S, 29°20’21.7”W; MZSP 108298 .

TYPE LOCALITY. — Brazil • Espírito Santo, off Guarapari , 18°50’S, 37°57’W, 60 m, continental shelf.

DISTRIBUTION. — Known only from type locality.

DIAGNOSIS. — Animal small (2.3 × 1.6 mm); valves occupying about 30-40% of dorsal surface; colour of tegmentum cream to beige; slit formula 5/1/2; single megalaesthete; presence of sutural laminae; tegmental pustules round to elliptic; mucro submedian; perinotum orange with orange sutural tufts; radula with rectangular central tooth and tricuspidate major lateral tooth.

ETYMOLOGY. — The specific name Oxum, a noun in apposition, refers to the Orixá Oxum, an Afro-Brazilian Candomblé and Umbanda deity known for wearing brightly-coloured yellow clothing.

DESCRIPTION

Animal small (from 1.5 × 0.9 mm to 2.3 × 1.6 mm); shell moderately elevated; tegmentum cream to beige coloured with white spots; slit formula 5/1/2. Valve i elliptic, front slope convex, posterior margin nearly straight, tegmentum sculptured by tegmental pustules (about 20-25 µm long, 20 µm wide – Figs 1R ; 2A ), tegmental pustules presenting megalaesthetes in central portion and micraesthetes on anterior portion of tegmental pustules. Valves ii-vii with convex front margin; hind margin concave near apex becoming straight on outer ends; beaked; pleurolateral areas with identical tegmental pustules to valve i; jugal area wide and smooth; jugal sinus varies from straight to slightly convex; apophysis triangular; insertion plates slightly convex. Valve viii triangular, mucro submedian; pleurolateral and jugal areas like intermediate valves on antemucronal area; postmucronal area concave, becoming straight near mucro. Perinotum covered by spicules (about 30-40 µm long), bearing sutural tufts with about 50 spicules ( Fig. 2E ), orange in colour. Hyponotum covered by spicules (about 10-20 µm long), cristaline, circular ( Fig. 2H ). Gills abanal, merobranchial, with 7-10 ctenidia on each side; radula with rectangular central tooth, major lateral tooth with tricuspidate head ( Fig. 2F, G ).

REMARKS

Acanthochitona oxum n. sp. differs from its Western Atlantic congeners by the presence of elliptic tegmental pustules forming ribs, in valve i, that extend divergently from the apex to the margin of the surface of the valves. It is similar to the pleurolateral area in intermediate valves, but its ribs are transversally arranged ( Fig. 2 ). The head and intermediate valves are light brown, the pleurolateral and postmucronal areas are dark brown, and the perinotum is orange-coloured. Finally, the intermediate and tail valves have a conspicuous diagonal ridge ( Fig. 2A, B ).

The new species differs from A. hemphilli (Pilsbry, 1893) by the wider central area and more elevated valves; the shape and sculpture of the head valve; the colour of the pleural and jugal areas of the intermediate valves; the anterior portion of the jugal area being four to five times wider than the apex; by having a more elliptical head valve with a distinctive sculpture of ribs stretching from the apex to outer edge; pleural and jugal areas are distinguishable by colour; the posterior side of pleurolateral area straight; the tail valve with a prominent central mucro, with a barely visible diagonal ridge and the sculpture of the postmucronal area similar to the head valve. Acanthochitona hemphilli is also found in shallower waters.

From Acanthochtiona pygmaea (Pilsbry, 1893) , A. oxum n. sp. differs by being smaller (up to 2.3 mm long), by the monochromatic orange-coloured perinotum and hyponotum; oval tegmental pustules presenting four micraesthete pores on narrower portion and submedian megalaesthtes; by having a circular valve i; valves ii-vii presenting a wider triangular apophysis and straight to slightly convex jugal sinus, smooth jugal area; valve viii diamond-like with slightly convex jugal sinus, and diamond-like apophysis.

Compared to Acanthochitona rhodea (Pilsbry, 1893) , A. oxum n. sp. differs by having a more prominent apex and a straightmargined pleurolateral area surrounding valves ii-vii; usually slightly concave but sometimes convex jugal sinuses; the diagonal ridges on valves ii-vii barely visible; the postmucronal and jugal areas of valve viii sculptured similarly to valve i, although the jugal sinus is slightly concave.

Acanthochitona oxum differs from A. ciroi Righi, 1971 by its more elliptic valve i, the sculpture of which consists of rows that extend across the entire surface, not just the apex region, and have a smooth area between the rows ( Fig. 2A ), and by valves ii-vii, which have straight margins.

Compared to Acanthochitona brunoi Righi, 1971 , the new species differs by having a head valve that is elliptic, without a prominent apex; intermediate valves with indistinguishable jugal and pleural areas, a straight margined pleurolateral area, and prominent apex; the tail valve with a subcentral mucro, an antemucronal area sculptured similarly to the other valves and a barely visible diagonal ridge; and a more concave postmucronal area. It also occurs at a greater depth than A. brunoi .

The new species differs from Acanthochitona terezae Guerra Junior, 1983 by having solid valves sculptured with rounded tegmental pustules; by having a slightly concave valve i; valves ii-vii with a more concave jugal sinus and valve viii with well-marked antemucronal and postmucronal areas, which are easily distinguishable by a diagonal ridge.

Compared to Acanthochitona astrigera (Reeve, 1847) , Acanthochitona oxum n. sp. can be distinguished by its smaller size (2.3 mm); monochromatic orange coloured perinotum; oval tegmental pustules presenting four micraesthetes on narrower portions; valve i with shorter insertion plates; valve ii-vii presenting posterior margin of pleurolateral area angulated near the apex, apophysis with straight external margin as well as external edge of insertion plates; smooth anterior margin of jugal sinus; and valve viii presenting slightly prominent mucro and straight anterior edge of apophysis.

In comparison with Acanthochitona lineata Lyons,1988 , A.oxum n. sp. differs by its smaller size; orange-coloured monochromatic perinotum; oval tegmental pustules, presenting four micraesthetes on narrower portions; circular valve i; valve ii-vii with straight external edge of apophysis and external edge of insertion plates, straight to slightly convex jugal sinus; valve viii diamond-like, straight external edge, and slightly prominent mucro.

From Acanthochitona worsfoldi Lyons, 1988 , A. oxum n. sp. differs by being smaller; by the monochromatic orange-coloured perinotum and hyponotum; oval tegmental pustules with four micraesthetes on narrower portions; circular valve i; valves ii-vii with apophysis straight with external edge, straight to slightly convex jugal sinus; and valve viii with straight anterior edge of apophysis.

Figures

FIG. 2. — Acanthochitona oxum n. sp.; dorsal view; A, valve i; B, valve v; C, valve viii; D, E: details of the valve v,showing pustules with aesthetes (D) and detail of a pustule (E); F, details of the perinotum;G‒H: detail of the radula, radula, panoramic (G) and dorsal (H) views;I, details of the hyponotum. Scale bars: A-C,100 μm; D, I, 20 μm; E, 3 μm; F, 100 μm; G, 30 μm; H, 10 μm.

FIG. 1. — A‒E, Stenoplax marcusi (Righi, 1971); A, B, lateral view; C, frontal view; D, dorsal view; E, posterior view; F-J. Acanthochitona terezae Guerra-Junior, 1983; F, G, lateral view; H, frontal view; I, dorsal view; J, posterior view; K‒O, Acanthochitona ciroi Righi, 1971; K, L, lateral view; M, frontal view; N, dorsal view; O, posterior view; P‒T, Acanthochitona oxum n. sp.; P‒Q, lateral view; R, frontal view; S, dorsal view; T, posterior view. Scale bars: 1 mm.