Zygochlamys patagonica ( King, 1832 )

Oviedo, Loreley & Pastorino, Guido, 2023, Description of the early shell morphology of three species of Pectinidae (Mollusca: Bivalvia) from Argentina, Zootaxa 5361 (3), pp. 427-443 : 435-438

Zygochlamys patagonica ( King, 1832)

Figure 4

Prodissoconch

In the single specimen examined by scanning electron microscopy, the microsculpture of the P-1 could not be determined. Prodissoconch-2 presents commarginal growth lines ( Fig. 4F ). The lengths of P-1 and P-2 are 86 µm and 256 µm, respectively (n = 1), the P-1/P-2 ratio is 0.33 ( Table 3 ).

Nepioconch

The microsculpture of the LV shows antimarginal riblets that are widening when these are crossed by thin commarginal growth ( Fig. 4C, E ). This pattern extends onto both the anterior and posterior auricles ( Fig. 4A, B ). The mean height is 2.3 mm (sd = 0.25, n = 6), the mean height of the prismatic stage of the right valve is 1.3 mm (sd = 0.20, n = 6). A radial row of protuberances accompanies the growth of the auricle prismatic stage and extends along the anterior auricle of the right valve ( Fig. 4 D ). The byssal notch is deep and wide.

Post-nepioconch

The nepioconch microstructure continues throughout this entire stage only that it becomes interrupted first by primary and then secondary ribs, together around 21 to 31 (mode = 28, n = 6) ( Table 4 ). Where ribs meet commarginal growth steps, they become nodular; these nodules are best developed on the disc flanks and are more conspicuous on the LV than RV ( Fig. 4A, B ). The riblet-and-gutter microsculpture fills only the interspaces not crossing the ribs; they do become more pronouncedly antimarginal-like and also divergent. On the disc flanks, they meet the shell margin at a rather oblique angle. The sculpture is less conspicuous on the central disc. It is also present on both LV auricles and the posterior one of the RV.

The anterior auricle is slightly longer than the posterior one, giving an AA/PA ratio of 1.33 (n = 6) ( Table 4 ). The free margins of the posterior auricles can be slightly anteriorly inclined and the free margins of the anterior auricle of LV is straight or inclined slightly toward anterior. The byssal notch is as wide as in the previous stage, and the active ctenolium presents two or three free hooked teeth (mode = 3; n = 6) ( Fig. 4D ). The mean umbonal angle is 89° (n = 6). ( Table 4 ). Shell color is orange under natural light but less intense on the RV.

Figures

FIGURE 4. Zygochlamys patagonica. A. Left valve. B. Right valve. C. Post-nepioconch of LV. D. Anterior auricle and byssal notch. E. Microsculpture of Nepioconch of LV. F. Prodissoconch. Scale bars: A= 500 µm; B= 500 µm; C= 200 µm; D= 200 µm; E=100 µm; F=100 µm. All specimens from Fracasso Beach (42.39754S, 64.19199W), Chubut Province, Argentina (CNP-INV 4086).