The thorny oyster, also known as spondylus, is a genus of clams, bivalve mollusks that are quite common in the waters of the Caribbean, especially between Brazil and the Carolinas. Spondylus species can also be found in the Indo-Pacific region, especially around Australia, China, Japan and the Philippines. They prefer coral reefs and rocky areas or places where there are wrecks with crevices to hide in and shallow waters not deeper than 40 meters.
The spondylus genus includes many species that differ considerably in size – some species are only a few centimeters across, while others measure almost 30 cm in diameter –, but they all share a few features: they all have shells that are white, cream, brown, yellow or reddish-orange on the outside and they all have thorns and ribs and smooth, light-brown or orange on the inside. The two halves of the shell are joined with a hinge that resembles a ball-and-socket mechanism – a feature that distinguishes these oysters from other clams that usually have toothed hinges. The lower half of the shell is usually more convex, while the upper half is flat, with thorns protruding on the surface and around the edges as well. Thorny oysters have a well-developed nervous system as well as several eyes and many short tentacles distributed around the edge of the shell. They are shy animals that withdraw into their shell at the smallest sign of danger and they are sessile, too, but they don’t anchor themselves to the surface they want to stick to – they cement themselves to the substrate or to the corals. They are also able to move by flapping their shells, but they prefer to stay settled.
All spondylus species are filter-feeders, living on the food particles delivered to them by the water currents. They are omnivorous, but their favorite food is phytoplankton and food particles that contain calcium, absolutely necessary for shell growth. While eating, the animals opens its shell a little and draws in water through the gills that extract the nutrients, then the water is pumped out.
Very little is known about the reproduction of thorny oysters; the only thing that seems to be sure is that the larvae are free-swimming and after birth they instantly start seeking for a suitable spot on the substrate to settle on. Young thorny oyster individuals look for areas where there is plenty of calcareous matter available to facilitate the development of the calcium-carbonate shell.