Treasures of the Sea:

shell-54Shells of The Turks and Caicos Islands P3
Story and Photos by Richard Ground

In this, the third and final article in a series on the shells of the Turks & Caicos Islands, I look at the Bivalves. After the Gastropods, Bivalves are the other great division of the Molluscs, and many will be familiar from the dinner table: Oysters, Mussels, Scallops and a wide range of Clams make good eating and are much sought after. While the Gastropods have one shell, which is usually an elaborately coiled tube, the Bivalves have two flat shells, known as “valves,” which are hinged together, and which can open when the animal is feeding, or shut tight when it is threatened.

All Bivalves make a living by filter feeding, which means they suck in water (often through a flexible tube, called a “siphon”) at one end, filter out edible particles, and then squirt it out at the other end. Some spend quiet lives buried in sand or mud. To find them you have to dig, but the expert can tell where to do so from the small mounds of sand they leave when burying themselves, or the holes created by their siphons. Others, like Oysters and Mussels, live in large colonies attached to rocks and other hard substrates, such as pilings and seawalls. But some, like the Scallops and File Clams, can literally fly through the water by the improbable procedure of flapping their valves.

Bivalve shells are common on beaches, and in quiet bays and backwaters they can litter the shore in the thousands. Although many are rather plain and dull, others are quite magical: the outside of some, like the Tellins, are beautifully polished, while for others it is the interior which is glossy smooth and richly coloured.

Ark shells

Arks (family Arcidae) are robust, chunky bivalves, often heavily ribbed, which live under, and sometimes attached to, rocks. In the TCI the largest is the Eared Ark (Anadara notabilis), which can easily reach two inches long, and is white with well-marked ribs. The Zebra Ark, or Turkey Wing (Arca zebra), is only slightly smaller, and is marked with a heavy brown zigzag. The Mossy Ark (Arca imbricata) is named for the appearance of its periostracum (or soft outer covering), which conceals a fairly smooth shell. The Red-Brown Ark (Barbatia cancellaria) lives up to its name, although in life its colour and the delicate ribbing on its surface are often obscured by heavy encrustation and algal growth.

Mussel shells

Mussels (family Mytilidae) live firmly anchored to rocks in the intertidal zone. The large dark outer shell of the Tulip Mussel (Modiolus americanus) resembles the familiar shell which turns up in Moules Mariniere, although some individuals exhibit a pretty striping. The much smaller Yellow Mussel (Brachidontes modiolus) is common on beaches and lives up to its name.

shell-53Arks, File Clams and Mussels

Oysters also live attached to rocks and other submerged things. Lister’s Tree Oyster (Isognomon radiatus) is a strange shell in a family of its own (the impossibly named Isognomonidae). Its valves are thin and leaf-like, but it does not grow on trees: it can be found under rocks. The Atlantic Winged Oyster (Pteria colymbus) belongs to the pearly oysters (family Pteriidae) and is named for its scallop-like “ears.” The Atlantic Thorny Oyster (Spondylus americanus) belongs to the spiny oysters (family Spondylidae). In life it is decorated with long, delicate spines, but these rarely survive on the beach, where the shell presents a rough exterior with a few blunt projections.

Pen shells

Pen Shells (family Pinnidae) are perhaps the strangest of the bivalves. They live upright, permanently fixed in position and half-buried in soft sand or mud. Their translucent shells are shaped more like half-open fans than pens, and are horny rather than hard and stony. The species in the TCI is the Amber Pen Shell (Pinna carnea), and its six inch shells, still often joined together down one side, are often washed up after storms.

Scallop shells

Scallops (family Pectinidae) are distinguished by their rounded shape and the “ears” on either side of the “beak” (the pointed end of the shell). Their classic shape will be familiar to everybody from the logo of the Shell Oil company. The large edible scallops rarely wash up on the TCI beaches, although the occasional shell of the large Zigzag Scallop (Euvola ziczac) suggests that they do occur in deeper waters. On sandy beaches, such as Grace Bay, shells of the Calico Scallop (Argopecten gibbus) are quite common, along with the smaller Thistle Scallop (Aequipecten acanthodes). On rocky shorelines and shallow reefs a different group of small scallops can be found, and these include the Sentis Scallop (Caribachlamys sentis), the Little Knobby Scallop (Caribachlamys imbricata) and the Ornate Scallop (Caribachlamys ornata).

Lima shells

Lima shells, or File Clams (family Limidae), are beloved of underwater photographers, because in life the soft parts are fiery red, and when the shell is open it extrudes a fringe of tentacles. The shell itself is white, although when fresh it usually has a brownish coating of algae. File Clams live on and under rocks, where they like to get wedged tight in a crevice, and then open to expose their gaudy interior, but when disturbed they can move themselves by flapping their shells. There are three types in the TCI: the Rough Lima and the Smooth Lima are different forms of the same species (Lima scabra), while the Spiny Lima (Lima lima) with its more angular shape, is a separate species.

Bittersweet Clam shells

The Bittersweet Clams (family Glycymerididae) are named for their taste. The commonest is the pretty little Comb Bittersweet (Glycymeris pectinata), which is ridged and speckled with brown. It rarely reaches an inch. The other two locally occurring Bittersweets are the Decussate Bittersweet (Glycymeris decussata) and the Atlantic Bittersweet (Glycymeris undata). They are larger, smoother, and covered with brown mottling, and are only distinguishable from each other by the slight backwards curve on the beak of the former.

Lucine shells

Lucines (family Lucinidae) live in the sand. The oddly named Tiger Lucine (Codakia orbicularis) has a robust, ridged shell which can easily grow up to 3 Ð 5 inches, but not a stripe in sight. Chalky white on the outside, the inner valve is glossy and ranges from white edged with pink to a brilliant primrose yellow. The Buttercup Lucine (Anodontia alba) is smaller, at 2 – 3 inches, and is a dull gray outside and a dull yellow inside.

Perhaps the commonest is the strangely named Pennsylvania Lucine (Lucina pensylvanica): although common from North Carolina south, it obviously does not occur in Pennsylvania, which is landlocked. The shell is smooth, white and inflated, with a pronounced crease or ridge on the left hand side. In life the smoothness of the shell is disguised by a ridged periostracum (or semi-soft outer covering), which probably helps it to stay buried. A common small Lucine is the Cross-hatched Lucine (Divaricella quadrisulcata). It, at least, is sensibly named, for its translucent, 3/4-inch shell is covered with fine criss-crossing ridges.

shell-61Jewel Boxes and Oysters

Jewel Boxes (family Chamidae) spend their lives so firmly attached to rocks and coral that they need a crow bar to remove them — they have literally grown onto the surface of their substrate. They are called Jewel Boxes because they have a deep lower valve covered by a flattish and decorated upper one, like a lid. The lid of the Leafy Jewel Box (Chama macerophylla) is decorated with fine fluting, which in fresh specimens is often coloured yellow or orange. The Left-handed Jewel Box (Pseudochama radians) is similar, but viewed from above its decorated “lid” is bent towards the left, hence the name. The Corrugated Jewel Box (Chama congregata) is small, and its surface is covered with brown ripples and the occasional leaf-like projection. The Cherry Jewel Box (Charma sarda) is also small and is distinguished by the deep pink of the lower valve, which is matched by wavy pink patterns among the folds of the “lid.”

Cockle shells

Cockles are heart-shaped shells when viewed end on, hence the family name, Cardiidae. The largest is the Magnum Cockle (Trachycardium magnum), while the handsomest is the Atlantic Strawberry Cockle (Americardia media), although the name is misleading, for it is ridged and marked in purplish brown, not red. The Spiny Paper Cockle (Papyridea soleniformis) has a beautiful, thin shell, lightly speckled with brown. The Egg Cockle (Laevicardium laevigatum), on the other hand, is smooth and shiny, and looks much like its namesake.

Venus (Hard-shell) Clam shells

The Venus or Hard-shell Clams (family Veneridae) are one of the largest bivalve families. Many are ridged, to help them lodge in the sand. They range in size from the huge Southern Quahog (Mercenaria campechiensis) of up to six inches, to the tiny Pointed Venus (Anomalocardia auberiana). In the intermediate size range are the King Venus (Chione paphia) and the Cross-barred Venus (Chione cancellata). The latter is very common in the TCI; when small it is a purplish white, but it develops attractive brown rays when larger.

slide-65Tellin shells

The smoothest and shiniest of the bivalves are the Tellins (family Tellinidae). The best known of these is the Sunrise Tellin (Tellina radiata), which is white with lovely pink rays (hence the name). It is abundant on the beaches of the Turks & Caicos. There is also an all-white form (T. radiata unimaculata), which is slightly less common, but equally lovely. The Speckled Tellin (Tellina listeri) is also very common. Its exterior is rough and ridged, and speckled brown, but its interior is glossy and suffused with yellow.

The Faust Tellin (Tellina fausta) is the largest of the group found in the TCI. It occurs in slightly deeper water and on reefs. Its outside is a coarse gray, but the creamy gloss of its interior ranges from milk white to a glorious buttercup yellow. The smallest is the Pea Strigilla (Strigilla pisiformis), but DeKay’s Dwarf Tellin (Tellina versicolor) is not much larger. The rarest, and the most subtly beautiful, is the Smooth Tellin (Tellina laevigata), whose shell bears fine pinkish rays, and is suffused with yellow at the beaks.

Gaudy Asaphis shell

Finally, in a group of its own, is the Gaudy Asaphis (Asaphis deflorata)(family Garidae). Dull and coarsely ridged on the outside, it gets its name from the rich and variable colours of its interior, which can range from deep purple, through rose pink, to bright yellow, with many mixtures in between. The animal is said to be good eating, and in the French islands of the Caribbean is highly regarded as a delicacy.

Richard Ground has been Chief Justice of the Turks & Caicos Islands since 1998, recently leaving to take up an appointment as Chief Justice of Bermuda. When not at work, his main interest is wildlife photography, with an emphasis on birds. Shells are a new departure; describing himself as an insatiable collector and cataloguer, he says it was not long before he was sorting his finds and searching for rarer and more beautiful specimens.

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