Handy Pocket Guide to Tropical Seashells. Pauline Fiene-Severns
varia, Variable ab alone, 21 mm
Haliotis asinina, Donkey's ear abalone, 45 mm
Haliotis ovina, Oval abalone, 32 mm
Most tropical abalones are small and thin-shelled, unlike the thick-shelled temperate abalones which are regularly harvested for food and for their opalescent shell interior. Abalones are usually found in shallow water beneath rocks on the shoreline and on reef flats from which they emerge at night to graze on algae.
They can crawl amazingly quickly. Close examination of the low, flat shell will reveal the characteristic spiral of a snail, with the aperture greatly expanded to accommodate the large, muscular, adhesive foot. Hundreds of tentacles protrude from the edge of the shell, presumably as sensory organs. During respiration, water is drawn in over the foot, passing beneath the lip of the shell over the gills, and is finally exhaled through a series of holes on the top of the shell.
Like the limpets, abalones are an early group in the evolution of mollusks, possessing a primitive type of gill structure and method of water circulation.
Top Shells
Family Trochidae
Trochus niloticus, Commercial top, 73 mm
Tectus triserialis, Tiered top 40 mm
Trochus conus, Cone top, 62 mm
Trochus maculatus, Maculated top, 35 mm
Top shells comprise a very large, diverse group of shells found from the shallow water of tide pools down to great depths. They generally have a shell that is thick and strong and, like the limpets, have evolved a high-spired, straight-sided shape that helps them hold on in surgy water. Most graze on seaweed and can be seen crawling about on the reef in shallow water at night.
When found by divers, most are heavily encrusted with calcareous (shell-like) deposits that hide the sculpture and color beneath. We recently encountered a top shell with the somewhat unusual habit of parasitizing black coral. Individuals appeared to be living on the black coral and stripping away the coral tissue.
Those species possessing an iridescent interior, especially the large Trochus niloticus, are harvested for commercial uses, including the manufacture of curios and pearl buttons that were very popular 100 years ago. These can still be found on some clothing as the fashion demands.
Turban Shells
Family Turbinidae
Operculum ("cat's eye") from Turbo chrysostomus, Gold-mouth turban, 20 mm
Turbo bruneus, Brown Pacific turban, 50 mm
Turbo petholatus, Tapestry turban, 52 mm
Turbo chrysostomus, Gold-mouth turban, 53 mm
Turban shells are a large family with solid, globose shells that live in calm, shallow waters. They can be distinguished from the top shells by the operculum which is heavy and calcareous as opposed to thin and horny as in the top shells. Their rounded sides also distinguish them from the straight-sided top shells.
Many seashells have a "trap door" called an operculum, which protects the soft animal when it withdraws into its shell. In the turbans this operculum is heavily calcified and is sometimes called a "cat's eye." Unique and beautiful in its own right, this operculum alone can weigh as much as half a kilogram in the giant Turbo marmoratus.
Turban shells feed on microscopic algae by rasping them from hard surfaces, leaving a trail of clean substrate behind them. Accordingly they are most common on rocky bottoms where algae are profuse. Because of their substantial shell, empty turbans are likely to be found washed up, unbroken, on beaches.
Nerites
Family Neritidae
Nerita squamulata, Scaled nerite, 20 mm
Nerita undata, Waved nerite, 36 mm
Nerita plicata, Plicate nerite, 21 mm
Nerites are mainly intertidal species, living along the shoreline attached to rocks, mangrove roots or man-made structures from the splash zone on down into intertidal waters. Their distinguishing characteristic—a semicircular aperture, straight across one end—can be seen when viewed from beneath. This unusual aperture shape led to their common name of "slipper shells." The aperture is also characterized by strong tooth-like structures that are often splashed with color. Slipper shells are flattened, oval and thick-shelled, characteristics that allow them to survive the rugged conditions of the splash zone while remaining near their food source, the algae that thrive where sunlight and water exchange are greatest.
Some species are eaten by people after being heated and plucked from the shell with a sharp stick. Others are eaten raw as an intertidal hors d'oeuvre, but first one must get past a very tight-fitting calcareous operculum which protects the animal when it is withdrawn.
Periwinkles
Family 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