Donax trapesialis, Guerra & Castro & Martinez & Passos & Marques & Siqueira & Dornellas, 2025

Guerra, Juliano, Castro, Guerra de, Martinez, Pablo Ariel, Passos, Flávio Dias, Marques, Rodrigo Cesar, Siqueira, Ana Paula & Dornellas, Siqueira, 2025, Geometric morphometrics of donacids from Brazil reveals a new species, Donax trapesialis n. sp. from Northeastern coast, Zoosystema 47 (19), pp. 399-412 : 406-408

Donax trapesialis n. sp.

( Figs 8 ; 9 )

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TYPE MATERIAL. — Holotype. Brazil • 1 specimen; Sergipe, Aracaju, Robalo beach ; 11°01’47”S, 37°04’39”W; depth 0.5 m; II.1996; Carmen Guimarães leg.; intertidal sand; CZUFS BIV-00123 .

Paratypes. Brazil • 50 specimens; same data as for holotype; CZUFS BIV-00124 , CMPHRM 7519B , MZSP167895 , MNRJ 37065 , ZUEC-BIV 8626 .

ETYMOLOGY. — This species is named after its trapezoid shape, a result of the more ventrally positioned anterior and posterior extreme points of the shell.

OTHER MATERIAL EXAMINED. — Brazil • 60 of 15.664 specimens; same data as for holotype; CZUFS BIV 00120 • 60 of 12.865 specimens; Sergipe, Aracaju, Atalaia beach ; 10°58’43”S, 37°2’11”W; CZUFS BIV 00121 • 60 of 27.066 specimens; Sergipe, Aracaju, Mosqueiro beach ; 11°06’19”S, 37°07’45”W; CZUFS BIV 00122 .

TYPE LOCALITY. — Brazil, Robalo beach, Aracaju, Sergipe state (11°01’47.4”S, 37°04’39.0”W).

DIAGNOSIS. — This species differs from all other Brazilian species of Donax by the following combination of characteristics: evenly rounded posterior ridge, height/length ratio of 1/2 to 3/5, trapezoid shape with anterior and posterior extreme points close to the ventral margin, and concave or flat ventral margin.

DESCRIPTION

Characteristic “donaciform”, wedge shape of the genus, laterally compressed with a truncated posterior side and elongated anterior side ( Morton 2016). Shell minute for the genus (8.5 mm width × 4.5 mm height, holotype), whitish or light grayish, often with triradial purple zones. Valve somewhat trapezoid due to the more ventrally positioned anterior and posterior extreme points. Posterior and anterior external surfaces of the shell ornated by radial incised lines and divided by an evenly rounded posterior ridge. Ventral margin flat or concave around its middle, with thinner and more numerous crenulations at the central region of the shell, larger and less numerous at the anterior and posterior sides, disappearing near the anterior end. Prominent umbones. Narrow hinge plate, with lateral teeth near the umbones, thicker in the left valve than in the right valve. Symmetrical posterior and anterior cardinal teeth in the right valve, uneven in the left valve. Large pallial sinus, exceeding half the height of the shell chamber and more than half the length between both adductor muscle scars. Thin, deciduous periostracum, visible only near the ventral margin of the shells. Soft parts could not be studied due to poor fixation and preservation.

REMARKS

Despite the sympatric occurrence with D. gemmula along the Sergipe coast, this species exhibits several key morphological differences. Notably, it is larger overall and wider relative to its height (height/width ratio of 1/2 to 3/5 for D. trapesialis n. sp. and> 2/3 for D. gemmula ). Additionally, D. trapesialis n. sp. possesses a more elongated anterior side, due to a more posterior position of the umbo, a flatter or more concave ventral margin (compared to the convex margin of D. gemmula ), and a more ventrally positioned anterior and posterior extreme points of the shell. Overall, its shape is more trapezoidal and less subglobose. In contrast to D. striatus , D. hanleyanus , and D. denticulatus , this species exhibits an evenly rounded posterior ridge. These latter species possess a distinctive sharp posterior ridge with a carina running from the dorsal to the ventral margin. This carinate feature is even present in juveniles of D. hanleyanus and D. striatus , highlighting a consistent difference ( Morrison 1971; Barroso et al. 2013). Morrison (1971) previously suggested that specimens from a lot deposited in the ANSP collection, obtained from Alagoas should be juveniles of D. vellicatus (ANSP 244133, Fig. 10 ). However, the consistent absence of adult D. vellicatus specimens among the extensive collection of D. trapesialis n. sp. housed in the Invertebrates Collection (CZUFS BIV 00120, 15.519 specimens; CZUFS BIV 00121, 12.835 specimens) makes this hypothesis unlikely. This lack of adult D. vellicatus specimens further strengthens the case for recognizing D. trapesialis n. sp. as a distinct species.

Figures

FIG. 8. — Donax trapesialis n. sp. holotype, CZUFS BIV-00123, Sergipe, Aracaju - Robalo beach. Scale bar: 5 mm.

FIG. 9. — Donax trapesialis n. sp. paratypes, CZUFS BIV-00124, Sergipe, Aracaju - Robalo beach. Scale bar: 5 mm.

FIG. 10. — Donax trapesialis n. sp., ANSP 244133.Maceió, Ponta Verde,identified by Morrison (1971) as possible young specimens of D. vellicatus Reeve,1855. Scale bar: 5 mm.