Caryocorbula marmorata ( Hinds, 1843 )

Arruda, Eliane P., 2020, Taxonomic revision of the recent marine Corbulidae (Mollusca, Bivalvia) from Brazil, Zootaxa 4851 (1), pp. 1-59 : 28-31

Caryocorbula marmorata ( Hinds, 1843)

Figure 7 G–N.

Corbula marmorata Hinds, 1843: 58 . Keen, 1966: 268. Coan, 2002: 87–89; figs. 36, 37, 55.

Corbula (Caryocorbula) marmorata . Keen, 1958: 209; fig. 526. Keen, 1971: 264, fig. 676.

Aloidis (Caryocorbula) marmorata . Hertlein & Strong, 1950: 239; pl. II, fig. 17.

Caryocorbula marmorata . Coan & Valentich-Scott, 2012: 1069–1070; pl. 3.

Corbula luteola . Keen, 1958: 209; fig. 525 (not Carpenter, 1864). Keen, 1971: 264, fig. 675 (not Capenter, 1864). Caryocorbula contracta . Mikkelsen & Bieler, 2007: 386–387 (not Say, 1822).

For other synonymies see Coan (2002: 87).

Type material: Not located.

Additional material. See Appendix. Most of specimens labeled as Corbula caribaea . USA between Postsmouth, North Carolina and Palm Beach, Florida. Brazil between Amapá State and Albardão, Rio Grande do Sul State.

Diagnosis. Shell small, thin to thick, oval in specimens in pre-accretion stage, trigonal-ovate and subequivalve in thick ones with a developed, acute, and ventrally gently curved posterior rostrum. Posterior slope flattened, set off by a moderately high and slightly sharp keel extending from the umbos to the posterior ventral margin and intersecting ventral margin. Excluding posterior slope, valves surface irregularly convex with a shallow depression aligned with the dorso-ventral axis and other slight concavity just anterior to keel. Exterior surface white or light brown, mottled with brown or purple, principally in lunular area. Sculpture comprising regularly spaced, high and sharp commarginal ribs and minutes radially aligned pustules, closer to each other on posterior slope.

Redescription. Shape. Shell small (length: 2.7–8.4 mm; height: 1.9–4.6 mm), trigonal-ovate, moderately heavy and inflate, subequivalve and subequilateral, with a developed, acute, and ventrally gently curved posterior rostrum. Posterior slope moderately broad, flattened, set off by a moderately high and slightly sharp keel extending from the umbos to the posterior ventral margin and intersecting ventral margin; posterior slope perpendicular to central slope. Excluding posterior slope, valves surface irregularly convex with a shallow depression aligned with the dorsoventral axis and other slight concavity just anterior to keel; right valve slightly larger and more inflated. Umbos prosogyrous with beaks at about 26%–36% of shell length from anterior end. Lunular area sunken and frequently with a purple patch. Escutcheon elongate, narrow, larger in the right valve, defined by sharp ribs more elevated on the right valve.

Anterior dorsal margin short, slightly concave, and continuous with broadly and convex anterior margin; posterior dorsal margin straight or slightly convex, long, ventrally inclined; posterior margin short, obliquely truncated, forming a short rostrum with posterior end of ventral margin, ventrally curved; ventral margin convex in anterior two thirds and concave near to the posterior margin. Lateral siphonal plate rarely present, when present only in the left valve.

Ornamentation. Exterior surface white or light brown mottled with brown or purple. Periostracum brown, preserved at the margins. Sculpture similar in both valves, comprising commarginal ribs and minutes and radially aligned pustules; commarginal ribs regularly spaced, high and sharp, with bases about two times broader than intercostal spaces; commarginal ribs become moderately elevated lamellae on posterior slope. Radial line of pustules presents in almost all shell surface, closer to each other on posterior slope. Inner surface smooth, white, brown or purple, with brow or white patches.

Hinge. Hinge axis slightly oblique in relation to the anterior-posterior shell axis in both valves. Right valve with a cardinal tooth just posterior to beak, and a resilial socket sunken under umbo; cardinal tooth pyramidal, stout, with curved dorsally apex, equilateral-triangle-shaped when viewed laterally. Left valve plate with a deep cardinal trigonal socket just posterior to beak, and a thick, short chondrophore projecting almost perpendicular to the sagittal plane. Chondrophore shallowly excavated and divided into anterior and posterior area by a radially placed, shallow, narrow groove; anterior region of groove somewhat oval with outer edge convex when viewed dorsally; posterior tooth-like knob low, inconspicuous; anterior cardinal socket with lateral walls slightly wrapping around the opening. Trough on right valve for reception of left valve continuous with the hinge plate, extending around dorsal, posterior and postero-ventral margins.

Muscle scars. Adductor muscle scars slightly impressed in the valve; anterior adductor muscle scar pear-shaped, nearly perpendicular in relation to the antero-posterior shell axis; posterior adductor scar rounded on top of wellelevated callosity that forms an acute angle in relation to the antero-posterior shell axis. Anterior pedal retractor muscle scar elongate; posterior retractor muscle scar rounded; anterior and posterior pedal retractor scar joining adductor scars. Pallial line far from valve margin, oblique in relation to antero-posterior axis (higher anteriorly). Pallial sinus absent, forming a straight or convex line.

Pre-accretion shell. Specimens in pre-accretion stage oval, more equilateral and equivalve. Excluding posterior slope, valves surface regularly convex in thinner shells, becoming irregular as the shells thicken, with concavity aligned with dorso-ventral axis and anterior to keel. Longer posterior margin. Hinge plate thin with inconspicuous knob.

Distribution. Eastern Pacific: Bahía Magdalena, Pacific coast of Baja California Sur, Gulf of California, Gulf to Bahía la Choya, Sonora, México, to Callao, Lima Perú ( Coan & Valentich-Scott 2012). Western Atlantic: Caryocorbula marmorata collected from North Carolina, Gulf of Mexico, Florida ( USA) and Brazil were analyzed, from depths of 2.43 m to 86 m. The specimens registered as having been collected in the intertidal zone can be from dead animals whose shells were swept away. In Brazil, the analyzed specimens were collected from the state of Amapá to Albardão, state of Rio Grande do Sul.

Remarks. Coan (2002) and Coan & Valentich Scott (2012) have stated that C. marmorata occurs in the Atlantic Ocean, where specimens are sometimes labeled as Corbula blandiana C. B. Adams, 1852 (= C. dietziana C. B. Adams, 1852 ), Corbula barratiana C. B. Adams (= C. swiftiana ) or Corbula contracta Say, 1822 . I am hesitant to say that the C. marmorata found in the Atlantic Ocean is the same species that occurs in the Pacific Ocean, as these two populations may be separated by at least 3.1–3.5 million years, the average time allocated to the closing of the Isthmus of Panama ( O’Dea et al. 2016). However, until there is more evidence to separate these two populations, I am considering them to be the same species. I have found C. marmorata in lots identified, principally, as C. caribaea (= C. swiftiana ).

Caryocorbula marmorata differs greatly from C. swiftiana in its ventrally curving rostrum and straight posterior margin; while in C. swiftiana the rostrum is aligned with the antero-posterior shell axis and the posterior margin is sinuous. In addition, C. marmorata has a shell sculptured with strong, high, and acute commarginal ribs while the commarginal ribs of C. swiftiana are low and rounded.

The most similar species to C. marmorata is Caryocorbula lavalleana . Both share a shallow depression just anterior to the posterior radial keel and a short, ventrally curved rostrum, but C. marmorata is differentiated by a more elongate, trigonal-ovate shell sculptured with high and acute commarginal ribs anterior to the keel and the presence of a second shallow depression aligned with the umbo-ventral axis.

It is very difficult to distinguish between thick-shelled and pre-accretion shell of Caryocorbula marmorata and pre-accretion shell of Corbula dietziana as both species possess a shell with a mottled color pattern and a purple patch in front of the umbo (see remarks about C. dietziana ).

Figures

FIGURE 7. A–F. Caryocorbula luteola (Carpenter, 1864). A–B. USNM 14897, lectotype (Coan 2002). A. external view of left valve. B. external view of right valve. C–F. MORG 15899. C. external view of left valve. D. internal view of left valve. E. external view of right valve. F. internal view of right valve. G–N. Caryocorbula marmorata (Hinds, 1843). G–L. José Carlos Tarasconi Private Collection. G. external view of left valve. H. internal view of left valve. I. external view of right valve. J. internal view of right valve. K. hinge of left valve. L. dorsal view. M–N. CMPHRM 2388, specimen in pre-accretion stage. M. external view of left valve. N. external view of right valve. Scale bars: A–J, L–N= 2mm; K= 1 mm. Legend: c= chondrophore; cs= cardinal socket; ct= cardinal tooth; k= knob; ke= keel; ps= pallial sinus; rs= resilial socket.