Story and photos by Darelle Snyman
The Caribbean Sea is known for its diverse marine life, and among the countless creatures that inhabit this underwater paradise are the bivalve mollusks. These mostly overlooked creatures live their lives hidden between two hinged shells or valves. In the grand scheme of marine life they do not attract a lot of attention unless they end up on your dinner plate, yet they have established themselves as crucial role players within the marine ecosystem, having an enormous impact as water filters in the habitats they frequent. Bivalve mollusks have graced our planet with their presence for millions of years and boast an astonishing level of diversity. From the familiar oysters, clams, scallops, and mussels to the more obscure, these creatures are found in a wide range of habitats which has led to a variety of specialized lifestyle adaptations. So let’s meet some of these two-shelled organisms that call the Caribbean home.
I will start off with the resident most likely to attract attention, the rough file clam (Ctenoides scaber), also known as the flame scallop (although, as a member of the family Limidae, it is not closely related to true scallops). Its vivid red mantle, fringed with a mesmerizing display of red or white tentacles, captures attention and makes them attractive photography subjects. The vibrancy of their mantle is the result of high levels of carotenoids in their body. These relatively abundant bivalves can be found wedged into crevices or resting in their own nest made from small coral pieces and rocks. They only eat phytoplankton, which they painstakingly sort through, selecting only the best morsels.
Unlike many of their kin, these bivalves are able to move around by clapping their valves together; and they can, when the need arises, propel themselves out of harm’s way from predators such as crabs and shrimps.
Another file clam you might swim into is the Antillean file clam (Limaria pellucida), a small file clam with a salmon pink mantle fringed with long pink to reddish tentacles, often bearing faint white bands. Its pale coloring differs starkly from the vibrancy of the flame scallop, so confusion between the species is unlikely.
An interesting bivalve I encountered in Bocas del Toro, Panama, hiding among turtle grass is the amber pen shell (Pinna carnea), a delicate beauty with a fragile shell that comes in shades of amber or light orange. What makes these bivalves unique is that they burrow into the sandy substrate as they grow with only the wide, posterior ends of their triangular shells exposed to facilitate filter feeding. Like many of their kin they also rely on protein filaments called byssal threads to anchor themselves firmly. They can reach 30 centimeters (12 inches) in length and in their exposed position, their delicate shells are easily damaged, often showing breaks and scars. They do, however, possess an incredible ability to repair their shells. Studies have shown that the mantle, which secretes the shell, can repair over 3 millimeters in 24 hours. Their sedentary position means that their exposed valves soon become colonized by algae and other sessile invertebrates, which provide additional protection and camouflage. They not only act as a growth substrate for other organisms but may also host a number of symbionts within their mantle cavity. Critters such as anemones, cardinal fish, pea crabs and shrimps have been found sheltering inside these amber valves.
The Atlantic thorny oyster (Spondylus americanus) is another bivalve resident whose shell hosts a variety of epibionts such as barnacles, sponges, and hydroids. You will find these solitary bivalves firmly cemented to rocky substrates where they will stay unless dislodged by a storm. Their most striking feature is the profusion of spiny projections covering their upper valve, creating the ideal microhabitat for other species. In spite of their common name, these spiky individuals are not closely related to true oysters but rather to scallops. Their valves are joined with a ball-and-socket type of hinge, whereas most bivalves have a toothed hinge joining their shells. Like scallops their white blotched, brownish mantle is edged with eyes. These are not true eyes, but rather light-sensitive spots that detect changes in movement and light intensity, allowing the oyster to close its shell quickly. These eyes might not give them a view of the world around them but are essential to their survival.
Another master of camouflage is the frond oyster (Dendostrea frons). Because their sturdy shells provide a hard substrate for other organisms to grow on, their shells are often covered in algae and other organisms. This keeps them safe from predators while also contributing to a greater biodiversity in the life of a reef. They can mold their shells to conform to the shape of their surroundings, further allowing them to blend into the environment seamlessly. They seldom exceed two inches in size and the key to their identity is the distinctive saw-toothed edges of their valves. Frond oysters are true oysters, belonging to the family Ostreidae, which include all the commonly consumed oyster species. Frond oysters, however, are not widely consumed, even though they are considered a delicacy, having an apparently milder and sweeter flavor than their popular counterparts. Their role within the reef ecosystem makes them valuable reef residents, so strict harvesting regulations afford them protection.
Atlantic wing oysters (Pteria colymbus) in turn belong to the Pteriidae family, the bivalves renowned for producing economically important pearls as a result of their natural defense mechanism against irritants. They secrete a calcium carbonate “pearl sac” to isolate irritants that lodge in their mantle folds. These distinctive bivalves get their name from the wing-like extensions on its shell called auricles. The auricles on adult shells tend to be less pronounced than those of younger individuals. Although found attached to a variety of substrates by their strong byssus threads, they are commonly found living in association with sea whips and other gorgonians to maximize access to the currents. Their shells are typically brownish, with rays of lighter colors, and often covered with a matted brown, fur-like covering called the periostracum. The interior of their shells tends to be a shiny pearly gray. During spawning they obscure the water with thousands of eggs and sperm and a female can produce more than a hundred million eggs yearly. The young oyster that develops from a fertilized egg glues itself onto a suitable hard surface where the now-called spat matures.
To find our last bivalve, the flat tree oyster (Isognomon alatus), you have to venture closer to shore as these individuals prefer to attach themselves in clusters to the stilt roots of red mangrove trees. Their flat profile, as indicated by their common name, is an adaptation to their tree-dwelling lifestyle where they can cluster together in great numbers. The exterior of the valves are rough and sculpted by concentric circles which give information about the oyster’s growth and age, much like tree rings. The interior contrasts greatly with their rather dull exterior, being a lustrous, cream-colored nacre. These unassuming bivalves make an important contribution to the overall health and balance of mangrove ecosystems, which are among our most vital coastal habitats. Serving as a food source for various predators, they are also an important component of the marine food chain.
It is clear there is more to these underestimated architects of the marine ecosystem and their significance is underscored by their role as filter feeders, habitat providers, and food sources. I hope you enjoyed this glimpse into the lives of these uniquely adapted marine critters.
