Littorinidae
Littorina scabra, Rough periwinkle, 19 mm
Littorina pintado, Dotted periwinkle, 17 mm
Periwinkles are the most common shells of the high shoreline where they graze on fine algae. Although capable of living above the high tide mark for long periods of time, they are tied to the ocean by the need to keep their gills moist and by their mode of reproduction. After mating, females either bear live young, lay a gelatinous egg mass, or shed fertilized eggs into the ocean where development takes place.
Because they live exposed, they are rather small and drab so as not to attract the attention of predators. Some species can be found living on the leaves of mangrove trees, surprisingly high out of the water. The name "periwinkle" comes from the Elizabethan words "penny winkle," meaning small whelks that used to cost a penny per handful. Since they live in large colonies, often out of the water, they have been an abundant, easily collected food for many peoples for generations. They have also been commonly used for adornment.
Horn Shells
Family Cerithiidae
Clypeomorus coralium, Coral horn shell, 25 mm
Cerithium echinatum, Spiny horn shell, 42 mm
Pseudovertagus aluco, Aluco horn shell, 57 mm
Rhinoclavis sinensis, Obelisk horn shell, 46 mm
Horn shells are a large group characterized by an upturned canal in their shell that protects the siphon and allows them to live just beneath the surface of the sand. They are some of the most commonly-seen and abundant seashells in shallow water. At shallow depths the smaller species prefer to live in sand pockets and calm, protected bays, while the larger species are normally found in open sand beyond the reef
These shallow-water dwellers can best be seen in the early morning before wind-driven waves and tides obliterate the trails left in the sand from the evening's foraging. The trails of deeper-water dwellers can be found throughout the day.
All are algae and detritus feeders and are common prey for predatory mollusks, especially moon shells which drill a hole through the shell and extract the animal. Hundreds of drilled horn shells, occupied by hermit crabs, can sometimes be seen on the reef flat at low tide.
True Conchs
Family Strombidae
Strombus aurisdianae, Diana conch, 61 mm
Strombus urceus, Little bear conch, 54 mm
Strombus lentiginosus, Silver conch, 64 mm
Strombus luhuanus, Strawberry conch, 52 mm
Stromboids are named for the U-shaped (stromboid) notch at the front (bottom in the photo) end of the outer lip. One of the eyes is held upright, protruding through the notch in the shell, while the other is kept low, beneath the lip of the shell. The eyes are well developed and the animal responds quickly to movement and light changes. True conchs are herbivorous and are usually shallow-water dwellers. Because of their weight they are able to live on the reef flat without being tumbled as easily by water movement. Many of the larger common species are edible.
The true conchs have several unusual means of locomotion. One species, Strombus maculatus, has been known to leap farther than 1 m by digging its large operculum into the bottom and catapulting itself off the bottom. Another very unusual member of this family, Tere-bellum terebellum, is shaped like a bullet and can propel itself rapidly away from the bottom for up to 3 m, by flapping its fleshy foot.
Spider Conchs
Family Strombidae
Lambis crocata, Orange spider conch, 105 mm
Lambis millepeda, Milleped spider conch, 130 mm
Lambis chiragra, Chiragra spider conch, 180 mm
Lambis scorpius, Scorpio conch, 140 mm
The unusual movement of the true conchs is eye-catching. Instead of gliding smoothly across the bottom as most snails do, they move by means of a strong, curved operculum (the trap door which protects the animal when it is withdrawn). The operculum is dug into the sand and the muscular foot hoists the shell up in a jerky, leaping motion up over the foot.
The spider conchs are characterized by long and thick, yet elegantly curved spinose projections along the lip of the shell. These are reef flat and shallow water dwellers which live out in the open, but algae and encrusting organisms growing on the outside of the shells manage to disguise them and make them difficult to see even though the shells are quite large. The flared edge of the shell protects the proboscis (feeding tube) which sweeps across the bottom in search of food. The sexes can often be distinguished by the form of the spines, with females possessing the longer spinose projections.
Carrier Shells
Family Xenophoridae
Xenophora neozelanica, Carrier shell, 68 mm
Carrier shells are a very old group which has been around since the Cretaceous period 135 million years ago. They have the amazing habit of cementing empty shells, coral or stones to their own shells with a type of nacreous glue secreted by the mantle as the shell forms. This practice may have developed as a protective measure, camouflaging the carrier shell from predators. Some species even attach branching sponges half a meter high!
Several adaptations indicate that carrier shells live on silty ocean floors—among these, a muscular foot which moves the animal around on the bottom with a jerky, leaping motion, and powerful currents produced in the mantle cavity to keep the cavity free of silt. Their silt habitat is further confirmed by the shell species that they have attached to their shells—these also inhabit silty areas. Carrier shells are mostly brought to light in trawls from very deep water and therefore information on their habits and behavior is limited.
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