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Invertebrate of the Week #18 – Variable Thorny Oyster (Spondylus varius)

Spondylus varius
Spondylus varius in Fiji. Photo: Nick Hobgood (CC BY-SA 3.0)

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 this organisms aren’t closely related to true oysters and are actually much more closely related to scallops.

Spondylus varius
Spondylus varius in Lembeh Strait, Indonesia.

Unlike scallops, however, Spondylus cement themselves to rocks or other 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 as opposed to the toothed hinges found in most other bivalve molluscs.

Spondylus is a sessile animal and subsists by filter feeding. Like their scallop cousins, they have numerous eyes lining their mantles which feeds into a well developed sensory nervous system. Assuming the eyes function similarly to those found in scallops, they likely serve to help detect changes in light and motion and allow Spondylus to react to environment changes like the shadow cast by an approaching hungry predator.

(Incidentally, if you’re interested in learning more about how those eyes work, I recommend The Optics of Life)

Spondylus regius
Spondylus regius from the Phillipines. Photo: Didier Descouens (CC BY-SA 4.0)

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 image of Spondylus regius (right) is one particularly impressive example.

The mantle of live Spondylus is perhaps even more striking. Spondylus varius (the title subject of this post)for instance, boasts a brightly colored and strikingly bespectacled mantle which almost imparts the illusion of a toothy grin emerging from the rocky substrate. You may 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.

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