Crassadoma

Lore & Background

Adults of Crassadoma gigantea have one valve (the right valve) fixed to a base, often a vertical rock face. This valve often becomes deformed to fit the contours of the rock. The left valve is roughly circular but irregular in outline, thick and heavy, with deep radial ribs. Every third or fourth rib is clad with blunt spines that overhang each other, which often get abraded and worn smooth as the scallop ages. There are flaps of shell known as auricles on either side of the straight hinge. The general colour is brown with the inside of the shell being glossy white with a purple patch at the hinge and a large, central adductor muscle scar. Between the valves, the margin of the orange mantle can be seen, with a row of tiny blue eyes and a sparse fringe of short tentacles.

Crassadoma gigantea is found on the Pacific Coast of North America, from British Columbia south to Baja California and Mexico. It does not move around as an adult but lives on the sea floor. Its favoured habitats include inside crevices and under boulders, or cemented to rock surfaces, corals or man-made structures such as harbour pilings. It is found at depths down to about 80 metres (260 ft). The scallop is cryptic and difficult to detect because of the sponges, sea anemones, hydroids, barnacles, bryozoans, worms and algae which tend to grow on the shell. The parasitic boring sponge Cliona celata damages the shell by making hollows in it, and additional shell material is laid down by the mantle tissue to compensate. Other sponges, such as the yellow boring sponge (Cliona californiana) that grow on the shell may be considered mutualistic, granting some protection against predators such as the sunflower starfish (Pycnopodia helianthoides) and purple sea star (Pisaster ochraceus).

Reader's Guide

Crassadoma is notable as a monotypic genus of rock scallops that undergo a dramatic shift from free-swimming juveniles to permanently cemented adults. Its single species, Crassadoma gigantea, is distinguished by its purple-hinge and heavy, irregular shell that conforms to the substrate. The species is ecologically significant as a filter feeder in eastern Pacific waters, and its cryptic nature—camouflaged by fouling organisms—provides a unique example of mutualism and parasitism. The parasitic boring sponge Cliona celata damages the shell, while the yellow boring sponge Cliona californiana may offer protection against predators like the sunflower starfish and purple sea star. Experimental evidence suggests that rock scallops grow faster in deeper water, possibly due to reduced fouling. The species spawns in summer, with veliger larvae spending about 40 days in the zooplankton before settling. Juveniles are free-living and can swim, but by the time they reach 4.5 cm in diameter they typically cement themselves permanently. Rock scallops can live at least 20 years, shifting energy from growth to gamete production around mid-life. This life history and its interactions with fouling organisms make Crassadoma a subject of interest in marine biology and ecology.

Did You Know?

• Crassadoma gigantea is the only species in the genus Crassadoma.
• Juveniles are free-swimming but become permanently cemented to the substrate by about 4.5 cm in diameter.
• The shell often has sponges, sea anemones, hydroids, barnacles, bryozoans, worms and algae growing on it, making the scallop cryptic.
• The parasitic boring sponge Cliona celata damages the shell, but the yellow boring sponge Cliona californiana may protect against predators.