Thorny oysters are a genus of bivalve molluscs and are the only genus within the family Spondylidae. The term ‘thorny oyster’ is a misnomer since these organisms aren’t closely related to true oysters and are actually much more closely related to scallops.
Unlike scallops, however, Spondylus cement themselves to rocks or another suitable substrate rather than attaching by means of byssus (a bundle of filaments that function to secure the scallop to a solid surface). They are also overtly differentiated from scallops by the fact that the two halves of their shells are connected through a ball-and-socket type hinge instead of the toothed hinges found in most other bivalves.
Spondylus are sessile and subsist by filter feeding. Like their scallop cousins, they have numerous eyes lining their mantles and a well-developed sensory nervous system. Assuming the eyes function similarly to those found in scallops, they help the animal detect changes in light and motion. That allows Spondylus to react to environmental changes like the shadows cast by an approaching predator.
(Incidentally, if you’re interested in learning more about how those eyes work, I recommend The Optics of Life)
The exterior shells of Spondylus species can be highly ornate and archaeological evidence suggests that they have been prized for their attractive appearance since the Paleolithic era.
The mantle of live Spondylus is perhaps even more striking. Spondylus varius, for instance, boasts a brightly colored and bespectacled mantle that gives the animal a toothy grin. You might have to do some traveling to see it in person, though. The species is found in the Indo-Pacific Ocean and off Australia, China, the Philippines, Japan, and Taiwan.