Meet the Comb Bittersweet!
Tucetona pectinata (Gmelin, 1791) is the most common out of two locally found species of the bittersweet family, the Glycymerididae. Its shell may reach 30 mm (about 1.2 inches), and is circular, with a sculpture of 20–35 relatively wide ribs that are flat in cross-section.
As the image above shows, the ribs are overlaid with a microscopic sculpture of regularly spaced, commarginal (“concentric”) threads. The shell hinge (below) has about 20–24 small “teeth” that ensure that the shell valves close just so. The color is variable, usually white to grayish-white with irregular brown streaks of variable hues.
Bittersweet clams have numerous, simple eyes that resemble the ommatidia, or individual units, of insect eyes. The eyes can be seen as tiny dark dots along the mantle edges of the clam in the photo below (arrows), Studies on the eyes of bittersweet bivalves (family Glycymerididae) suggest that they can be used to sense approaching predators by sudden changes in light intensity (the “shadow reflex”) and/or by detecting their movement.