Caryocorbula chittyana (C. B. Adams, 1852 )

Arruda, Eliane P., 2020, Taxonomic revision of the recent marine Corbulidae (Mollusca, Bivalvia) from Brazil, Zootaxa 4851 (1), pp. 1-59 : 22-25

Caryocorbula chittyana (C. B. Adams, 1852)

Figure 5

Corbula chittyana C. B. Adams, 1852: 238 . Dall, 1886: 316, pl. 2; fig. 6a–6d. Clench & Turner, 1950:: 265–266; pl. 48, fig. 4–5.

Corbula (Caryocorbula) chittyana . Abbott, 1974: 539; fig. 6000.

Caryocorbula chittyana . Mikkelsen & Bieler, 2007: 386–387.

Type material examined. Corbula chittyana C. B. Adams, 1852 . MCZ 177065, lectotype designated by Clench & Turner (1950, p. 265–266; pl. 48, fig. 4–5) (validity of designation confirmed herein), open pair, 8.2 mm length, 5.8 mm height, Caribbean Sea: Jamaica, Kingston Harbor ( Fig. 5 A–D).

Additional material. See Appendix. Most of the specimens labeled as Corbula caribaea . USA, between Sanibel Island, Florida, and the Gulf of Mexico. Brazil, between Jaraguá, Maceió and Ubatuba, São Paulo.

Diagnosis. Shell small, thin to thick, subretangular to subtrigonal and equivalve in pre-accretion stage; trigonal to trigonal-elongate and subequivalve in thick one, with a moderately posterior rostrum. Posterior slope moderately broad, gently concave or plane, set off by a low, rounded to slightly sharp keel. Valves surface, excluding posterior slope, strongly and irregularly convex, showing in most specimens a nepioconch-like shell. Anterior dorsal margin straight, long, continuous with the broad and convex anterior margin; posterior dorsal margin straight, as long as anterior dorsal; posterior margin moderately long, obliquely truncated with an irregular sinuous outline; posterior end frequently extended by lateral siphonal plate; ventral margin straight to slightly convex. Sculpture constituted by low, rounded and sometimes sharp commarginal ribs; commarginal ribs regularly to irregularly spaced with the base width narrower than or equal to interspaces; nepioconch-like shell ornamentation similar to remaining shell, with commarginal ribs and small radially arranged pustules; radial pustules also present in posterior slope.

Redescription. Shape. Adult shell small (length: 4.6– 11 mm; height: 3–7.7 mm) trigonal to trigonal-elongate, heavy, strongly inflated; subequilateral, subequivalve to inequivalve, with a moderately posterior rostrum aligned with the anteroposterior shell axis. Posterior slope moderately broad, gently concave or plane set off by a low, rounded to slightly sharp keel, like a sigmoid line extending from the umbos to the posterior end of ventral margin; keel intersects or dies out before the junction of posterior and ventral margins; posterior slope almost perpendicular with to central slope. Valves surface, excluding posterior slope, strongly and irregularly convex, showing in most specimens a nepioconch-like shell (only change in the growth direction with similar sculpture in all shell surface); right valve larger and more inflated than the left one. Umbos prosogyrous with beaks at about 45%–48% of shell length from anterior end. Escutcheon lanceolate, flattened, ventrally inclined, defined by a conspicuous elevation in the right valve and by a slender radial rib in the left; this slender radial rib formed by confluence and the abrupt decrease in height of the adjacent commarginal ribs.

Anterior dorsal margin straight, long, ventrally inclined and continuous with the broad and convex anterior margin, the latter situated below to the antero-posterior shell axis; posterior dorsal margin straight, as long and ventrally inclined as anterior dorsal; posterior margin moderately long, obliquely truncated; viewed from its inner surface, posterior margin with a irregular sinuous outline; posterior end frequently extended by lateral siphonal plate; ventral margin straight to slightly convex.

Ornamentation. Exterior surface whitish with light brown periostracum. Sculpture similar in both valves constituted by low, rounded and sometimes sharp commarginal ribs; commarginal ribs regularly to irregularly spaced with the base width narrower than or equal to interspaces in both valves; radial pustules present in posterior slope. Inner surface smooth, yellowish.

Hinge. Hinge axis parallel to the antero-posterior shell axis. Right valve with a cardinal tooth below the beak and a resilial socket moderately sunken under the umbo; cardinal tooth pyramidal, with its apex curved dorsally, isosceles-triangle-shaped when viewed laterally. Left valve with deep, trigonal cardinal socket below the beak, with lateral walls slightly wrapping around its opening, and a chondrophore perpendicular to the shell sagittal plane. Chondrophore broad, flattened and divided into anterior rectangular area and posterior trigonal area by a radially placed ridge; posterior margin of chondrophore with a moderate, rounded, tooth-like knob. Trough on right valve for reception of left valve continuous with the hinge plate, extending around all valve margins.

Muscle scars. Adductor muscle scars well-impressed in the valve wall, nearly perpendicular in relation to the antero-posterior shell axis in both valves; anterior adductor scar pear-shaped; posterior adductor rounded. Anterior pedal retractor muscle scar elongate; posterior retractor muscle scar rounded; anterior and posterior pedal retractor scars joining adductor muscle scars. Pallial line far from valve margin mainly in the right valve, oblique (higher anteriorly) or straigth in relation to the antero-posterior shell axis. Left pallial sinus shallow extending to anterior edge of the posterior adductor muscle scar, right pallial sinus not invaginated forming a straight line.

Nepioconch-like and pre-accretion shell. Nepioconch in the sense of Goodwin et al. (2008) not distinguished, but a nepioconch-like structure demarcated by a change in the growth direction; its ornamentation is as of the remaining shell, with commarginal ribs and small radially arranged pustules. Specimens in pre-accretion stage subrectangular to subtrigonal, subequilateral to equilateral, equivalve; posterior margin truncated, comparatively longer than thicker shell, not rostrate; posterior slope keel intersects ventral margin; posterior slope with more radially arranged pustules than thick specimens; hinge plate thin with inconspicuous tooth-like knob; pallial sinus absent.

Distribution. Caryocorbula chittyana has been recorded for the entire Western Atlantic coast, from North Carolina to Florida, the West Indies, the Gulf of Mexico, Colombia ( Mikkelsen & Bieler 2007) and Brazil. In Brazil, specimens were analyzed from Jaraguá in the state of Alagoas to Ubatuba in the state of São Paulo, collected in the subtidal region. The species has been collected in large quantities in the same collection station in the state of Bahia, Brazil, suggesting that this species can be gregarious, as occurs with Varicorbula disparilis ( Mikkelsen & Bieler 2001) and C. patagonica .

Remarks. Among recent Corbulidae , only species of the Corbula genus display growth forms with a distinct nepioconch (see “Shell terminology” and “Corbulid shell growth”). In Caryocorbula species, the growth pattern is characterized by initial deposition of a thin shell followed by relatively uniform valve thickening across the inner valve surface. It is not possible to say with certainty that C. chittyana has a nepioconch as defined by Goodwin et al. (2008), but the change in growth direction seems to be more prominent, as the initial shell is more evident than in other Caryocorbula species, and the presence of radial pustules in the nepioconch-like shell indicates that there is a difference in the shell structure of this region compared to the rest of the dissoconch. Further investigation is needed to understand the accretion process of calcium carbonate layers in this species and test whether there is a difference in the shell structure between nepioconch and mesoconch.

The size at which the shell changes its growth trajectory varies among populations of C. chittyana and can occur in small specimens (<5 mm). As specimens grow older, the ventral margin becomes flattened, forming a platform when viewed from the ventral side, and then changes growth direction, so that it appears that a smaller shell is glued on top of a larger one. In some specimens, the flattening of the ventral margin in the nepioconch-like shell is not as prominent, so that the nepioconch-like shell is not so evident in thick specimens.

When the change in the trajectory of growth occurs late (at approximately 10 mm in length), specimens in pre-accretion stage of C. chittyana are similar to the specimens in pre-accretion stage of C. patagonica . The thickshelled of C. patagonica is distinguished from C. chittyana by a more trigonal outline and the more conspicuous difference in the ornamentation between nepioconch and mesoconch. The main characteristic that distinguishes specimens of C. chittyana in pre-accretion stage from specimens of C. patagonica in the same stage, however, is the chondrophore, which in C. patagonica is more projected from the free border of the hinge plate than in C. chittyana .

Caryocorbula chittyana has often been synonymized with C. swiftiana . However, C. chittyana has a more trigonal shell shape, a short to moderately developed rostrum and a posterior slope almost perpendicular to the central slope. The posterior margin in C. chittyana is moderately long, while it is short in C. swiftiana . Most specimens of C. chittyana have an irregularly convex valve surface with a nepioconch-like shell while that of C. swiftiana is regularly convex, except for a sulcated area just anterior to the keel that frequently forms a soft concavity on the ventral margin of most of the specimens. This concavity is absent in C. chittyana .

Of the Eastern Pacific taxa, C. biradiata (G. B. Sowerby I, 1833) is the most similar to C. chittyana . Both species have irregularly convex shells, with posterior slope almost perpendicular to the central slope. Based on the figures and descriptions given by Coan (2002) and Coan & Valentich-Scott (2012), C. biradiata is distinguished from C. chittyana by its ovate-elongate shell and by the strongly sunken right and left socket under the umbonal region; in C. chittyana the shell is trigonal-elongate, and the right and left socket are not as sunken under the umbonal region.

Figures

FIGURE 5. Caryocorbula chittyana (C.B. Adams, 1852). A–D. MCZ 177065, lectotype (Clench & Turner 1950). A. external view of left valve. B. internal view of left valve. C. external view of right valve. D. internal view of right valve. E–L. Two specimens from MZSP 20164. E, I. external view of left valve. F, J. internal view of left valve. G, K. external view of right valve. H, L. internal view of right valve. M–P. IBUFRJ 16015, not articulated valves of specimens in pre-accretion stage. M, O. external view of left valve. N, P. external view of right valve. Q–R. hinge of shells with different thickening. Q. IBUFRJ 16015, left valve hinge of specimen in pre-accretion stage. R. MZSP 20164, left valve hinge of specimen in post-accretion stage. S–T. Two specimens from MZSP 20164, dorsal view. Scale bars. A–P, S–T =2 mm. Q–R =1 mm. Legends: c= chondrophore. cs= cardinal socket. ct= cardinal tooth; k= knob; ke= keel; lsp= lateral siphonal plate; ps= pallial sinus; pu= pustules; rs= resilial socket.