Juliacorbula aequivalvis ( Philippi, 1836 )

Arruda, Eliane P., 2020, Taxonomic revision of the recent marine Corbulidae (Mollusca, Bivalvia) from Brazil, Zootaxa 4851 (1), pp. 1-59 : 36-39

publication ID	https://doi.org/ 10.11646/zootaxa.4851.1.1
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scientific name	Juliacorbula aequivalvis ( Philippi, 1836 )
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Juliacorbula aequivalvis ( Philippi, 1836)

Figures 10 View FIGURE 10 and 11 View FIGURE 11

Corbula aequivalvis Philippi, 1836: 227–228 ; pl. VII, fig. 4a–4d. Mclean, 1951: 114; pl. 23, fig. 7. Warmke & Abbott, 1961: 207; fig. 31g. Humfrey, 1975: 278; pl. 32, fig. 12. Morris, 1973: 91; pl. 31, fig. 11.

Corbula (Cuneocorbula) aequivalvis . Dall & Simpson, 1900: 473.

Corbula (Tenuicorbula) aequivalvis . Altena, 1971: 77; pl. 8, fig. 4–5.

Corbula (Juliacorbula) aequivalvis . Weisbord, 1964: 393–396; pl. 57, fig. 3–6.

Corbula knoxiana C. B. Adams, 1852: 238–239 . Clench & Turner, 1950: 299; pl.47, fig. 238.

Juliacorbula knoxiana . Olsson & McGinty, 1958: 22.

Corbula cubaniana d’Orbigny, 1853 (in 1841–1853): 283; pl. XXVI, fig. 51-54. Dall, 1881: 114. Dall, 1886: 313; pl. I, fig. 3, 3a–3c. Olsson & McGinty, 1958: 22. Salvador et al., 1998: 1033. fig. 22.

Corbula (Juliacorbula) cubaniana . Abbott, 1974: 539; fig. 6005. Rios, 1985: 270; pl. 94, fig. 1334. Rios, 1994: 292; pl. 99, fig. 1426. Rios, 2009: 586.

Type material examined. Corbula aequivalvis Philippi, 1836 . ZMB 3270/1, lectotype designated herein, open pair, the specimen probably illustrated by Philippi on plate VII, figure 4c; 9.2 mm lenght, 7.5 mm height, Havana, Cuba ( Fig. 10 View FIGURE 10 A–D). ZMB 3270/2, paralectotype, one open pair, 9.2 mm length, 6.5 mm height, Havana, Cuba. It is not certain that these are the specimens mentioned by Philippi in the original description, but these specimens were identified by A. R. Philippi in 1836 and were collected in Havana, Cuba. Philippi wrote that he observed eight complete specimens, but the lot has only three specimens, two of which can be identified as J. aequivalvis , the other one is Caryocorbula swiftiana . Figure 4d View FIGURE 4 of plate VII of Philippi (1836) also appears to be C. swiftiana .

Corbula cubaniana d’Orbigny, 1853 (in 1841–1853). BMNH 1854.10.4.549/1, lectotype designated herein, open pair, the specimen closest to the original length measurement; 10.7 mm length, 7.6 mm height, 5.6 mm width; Havana, Cuba ( Fig. 10 View FIGURE 10 E–H). BMNH 1854.10.4.549/2–12, paralectotypes, 10 pairs, one right valve; Havana, Cuba. The only known text and plate of Corbula cubaniana were published in 1853, in which there is no mention of a type. The lot BMNH 1854.10 .4.549 of the Natural History Museum in London is registered as being the syntype lot purchased of d’Orbigny.

Corbula knoxiana C. B. Adams, 1852 . MCZ 186106, lectotype designated by Clench & Turner (1950, p. 299; pl.47, fig. 238) (validity of designation confirmed herein), left valve; 12.8 mm length, 8.4 mm height; Kingston Harbor , Jamaica, Caribbean Sea ( Fig. 10R View FIGURE 10 ). MCZ 155608 About MCZ , paralectotypes, 2 closed pair, 3 right valves and 5 left valves ; Kingston Harbor, Jamaica, Caribbean Sea .

Additional material. See Appendix. Usually labeled as Corbula cubaniana . USA, Montalva Bay, Puerto Rico. Brazil between Jaraguá, Maceió, Alagoas to Zimbros Bay, Bombinhas, Santa Catarina.

Diagnosis: Shell small to moderate size, thin to thick; subtrapezoidal and inequilateral with strong posterior produced rostrum in thin shells; trigonal to subtrapezoidal, nearly equivalve, subequilateral to equilateral with weak to moderate posterior produced rostrum aligned with anteroposterior shell axis in thick shells. Posterior slope set off by a sharp and strong keel and with a soft radial groove. Sculpture similar in both valves consisting of conspicuous, regular, and sharp commarginal ribs with base width equal to intercostal space. Hinge axis parallel to the anteroposterior shell axis.

Right valve with cardinal tooth pyramidal, stout, with straight apex, not curved dorsally, right-triangle-shaped when viewed laterally. Left valve with a chondrophore projecting obliquely to the sagittal axis and far from free border of the hinge plate. Chondrophore divided by a high ridge, being the anterior area subrectangular and deeply excavated and the posterior area trigonal and flat with rounded tooth-like knob.

Redescription. Shape. Adult shell to moderate size (length: 4–16.6 mm; height: 2.7– 12 mm) trigonal to subtrapezoidal, sometimes elongate, heavy, inflated in thick specimens, quite equivalve, subequilateral to equilateral, with weak to moderate posterior developed rostrum, aligned with antero-posterior shell axis. Posterior slope moderately large and concave, set off by a sharp and strong keel, like a sigmoid line extending from the umbos to the posterior end; keel becomes rather rounded ventrally and intersects ventral margin; posterior slope with a soft radial groove; posterior slope perpendicular with to central slope in trigonal specimens, and forming an obtuse angle in elongate specimens. Valve surface, excluding the posterior slope, regularly convex, right valve slightly larger than left. Umbos prosogyrous with beaks at about 43%–50% of shell length from anterior end. Lunular area concave. Escutcheon lanceolate, flat to slightly convex, ventrally inclined in some specimens, with width similar in both valves and defined by a narrow and sharp radial rib.

Anterior and posterior dorsal margins slightly convex and ventrally inclined, with similar lengths; broadly convex anterior margin below to median antero-posterior shell axis; long and truncated posterior margin, nearly parallel to the dorsal-ventral shell axis; slightly convex ventral margin. Lateral siphonal plate absent.

Ornamentation. External shell surface and periostracum whitish. Sculpture similar in both valves with conspicuous, regular, and sharp commarginal ribs with base width equal to intercostal spaces. Inner surface smooth, white or yellow.

Hinge. Hinge axis parallel to the antero-posterior shell axis. Right valve with cardinal tooth just posterior to beak and a resilial socket moderately sunken below umbo; right cardinal tooth pyramidal, stout, with straight apex, not curved dorsally, right-triangle shaped when viewed laterally. Left valve with trigonal cardinal socket just posterior to beak and a chondrophore projecting obliquely to the sagittal axis and far from free border of the hinge plate; left cardinal socket moderately sunken to the umbo, oblique to the sagittal axis and a right-triangle-shaped, with lateral walls not wrapping around of opening. Chondrophore divided into two areas by a high ridge, being the anterior area subrectangular and deeply excavated and the posterior area trigonal and flat with rounded tooth-like knob. Trough on right valve for reception of left valve continuous with the hinge plate, extending around all margins of the valve.

Muscle scars. Adductor muscle scars well-impressed; anterior adductor scar oval-elongate nearly perpendicular or in high oblique angle in relation to the antero-posterior shell axis; posterior adductor muscle scar rounded, on top of well-elevated callosity perpendicular to the antero-posterior shell axis. Anterior pedal retractor muscle scar elongated; posterior retractor muscle scar rounded; anterior and posterior pedal retractor scar joining adductor scars. Pallial line far from free valve margin, somewhat parallel in relation to the antero-posterior shell axis. Pallial sinus straight, intersecting antero-ventral point of the posterior muscle scar.

Pre-accretion shell. Specimens in pre-accretion stage subtrapezoidal, inequilateral with strong posterior developed rostrum. Posterior slope with a conspicuous radial groove and set off by a stronger keel intersecting ventral margin. Posterior margin longer than in thick shells and forming an obtuse rostrum at the posterior end. Hinge plate thin with inconspicuous knob.

Distribution: From Florida, West Indies, Caribbean Central America and Brazil (Abbot 1974; Rios 1994; 2009; Mikkelsen & Bieler 2007). According Rios (1994; 2009), J. aequivalvis occurs in Brazil from Ceará to São Paulo, from depth of 10 m to 40 m. I have analyzed specimens collected from Maceió Municipality, State of Alagoas to Zimbros Bay, Bombinhas Municipality, State of Santa Catarina, from intertidal zone to 156 m.

Remarks: Juliacorbula aequivalvis exhibits great morphological variation, with some specimens more elongate and a broader posterior slope. The triangular form was described by Philippi (1836) as J. aequivalves ( Fig. 10 View FIGURE 10 A–D) and by d’Orbigny (1853 in 1841–1853) as J. cubaniana ( Fig. 10 View FIGURE 10 E–H) and the subtrapezoidal-elongate form was named J. knoxiana (C. B. Adams, 1852) ( Fig. 10R View FIGURE 10 ). The syntype lot of J. cubaniana has trigonal specimens and others that are somewhat elongate. In the same population from Brazilian coast I have observed trigonal specimens ( Fig. 10 View FIGURE 10 I–L) and others subtrapezoidal-elongate ( Fig. 10 View FIGURE 10 M–P). Trigonal specimens from Brazil seem to be intermediate morphotypes between the trigonal forms described by Philippi and d’Orbigny and the subtrapezoidal forms. Thin and smaller specimens of J. aequivalvis overlap in morphology with Corbula (Juliacorbula) fossilis Pilsbry, 1922 , a fossil species of the Upper Miocene and Pliocene of Central America ( Anderson 1996). According to Anderson (1996), J. aequivalvis may be a descendent of C. fossilis .