Читать книгу Handy Pocket Guide to Tropical Seashells - Pauline Fiene-Severns - Страница 8

Limpets

Family Acmaeidae/Fissurellidae

Patelloida saccharina, Pacific sugar limpet, 27 mm

Scutus unguis, Hoof shield limpet, 24 mm

Diodora quadriradiatus, Four-rayed keyhole limpet, 17 mm

Patelloida saccharina, Pacific sugar limpet, 27 mm

True limpets can be found along exposed rocky shorelines, often above the low tide mark, exposed to air for hours at a time. The conical shell is held against the rock, conserving moisture, but is extended away from the rock when a wave crashes, to allow water to pass over the gills.

At high tide, when moisture loss is least likely, they move around grazing on algae. As the tide falls some species regularly return to a homesite of attachment where they have ground a circular place for themselves in the rock, which fits their shell. Years ago, a few mischievous scientists discovered that by changing the shape or surface features of a particular limpet's homesite, they could limit the ability of that limpet to identify it.

The related keyhole limpets must live at greater depths due to the hole in the top of the shell which would allow too much moisture loss if left exposed. Water exits through this hole after it has entered under the shell's lip and passed over the gills.

Abalones

Family Haliotidae

Haliotis 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 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.

Large Cowries

Family Cypraeidae

Cypraea talpa, Mole cowrie, 55 mm

Cypraea Tigris, Tiger cowrie, 90 mm

Cypraea mappa, Map cowrie, 84 mm

Cypraea argus, Eyed cowrie, 62 mm