The Book of Shells

A Life-Size Guide to Identifying and Classifying Six Hundred Seashells

The University of Chicago Press

WHAT IS A MOLLUSK?

Mollusks are among the oldest and most diverse groups of animals on the planet. Like all taxa, they are defined by their genealogy. That is to say, they have a common ancestor from which all members of the phylum Mollusca, living and extinct, are descended.

EARLY MOLLUSKS

The earliest mollusks were small (1/25–1/12 in/1–2 mm), marine, bilaterally symmetrical animals, with an anterior head, a ventral foot, and a posterior mantle cavity that contained paired gills, sensory organs called osphradia, openings of the genital and excretory organs, and the anus. The head contained a mouth with a radula, a ribbonlike feeding structure unique to mollusks that is like a flexible rasp. The foot was an elongated structure used for locomotion, and the visceral mass, situated above the foot, contained the major organ systems, including the heart, kidneys, digestive glands, and gonads. The nervous system consisted of three pairs of ganglia, one for each body region (the head, foot, and viscera). A cuticle covering the body secreted calcareous spicules or scales.

Over the course of geological time, the descendants of this common ancestor diversified and differentiated, giving rise to multiple branches, each with distinctive features and adaptations. Many of the most basal of these branches, the classes within the phylum Mollusca, diverged during the Cambrian period. Some, such as the Gastropoda, Bivalvia, and Cephalopoda, underwent significant anatomatical changes, producing combinations of features that enabled rapid exploitation of new environments. Other classes (among them the Polyplacophora, Monoplacophora, Scaphopoda) retained their basic anatomical organization; they persist to the present day, little modified and with comparatively low diversity. Mollusks are so ancient and diverse that there are few diagnostic characters that are both unique to Mollusca and ubiquitous to all its classes.

CHITONS

The chitons (Class Polyplacophora) have elongated, flattened, bilaterally symmetrical bodies covered by a shell of eight overlapping transverse plates that are surrounded by a cuticularized girdle (muscular band). The foot is long and muscular, and flanked on both sides and by a long mantle cavity that contains multiple pairs of gills (from 6 to 88). The head is reduced, lacking eyes and tentacles. Light-sensing cells that are unique to chitons pass through tiny canals in the shell plates. All chitons live in the ocean, most on rocky bottoms in fairly shallow water where they graze on algae and sponges.

GASTROVERMS

Gastroverms (Class Monoplacophora) are relatively small (1/36–1 1/2 in/0.7–37 mm), ovate, bilaterally symmetrical mollusks that have a single, conical, limpetlike shell with eight pairs of serially repeated muscle scars. They were thought to be extinct, but thirty living species have been discovered since 1957, nearly all from deepsea habitats (571–21, 289 ft/174–6,489 m), where they inhabit muddy, rocky, or gravelly bottoms. All feed on organic matter and on small animals in the sediment.

BIVALVES

Bivalves (Class Bivalvia) are the second largest class of mollusks. They have a bilaterally symmetrical body that is completely enclosed in a shell consisting of two valves (left and right) that are connected by an elastic ligament. The head is reduced, and the radula is absent. Most bivalves have a capacious mantle cavity that accommodates large gills. In addition to being a respiratory organs, they filter food particles from the water. Some primitive forms feed directly on the organic matter in fine sediments, a few specialized groups derive nutrition from symbiotic algae or bacteria, while others capture and consume small crustaceans and worms in the deep sea. Most bivalves burrow in sand or mud, some in wood, clay, or coral. Some attach to hard substrates with threadlike strands (byssus), others by cementing one of their valves. Several different groups have adapted to freshwater habitats.

SCAPHOPODS

Scaphopods or tusk shells (Class Scaphopoda) comprise a small group of about 600 living species. They have tall, bilaterally symmetrical bodies completely contained in a long, curved, tapering tubular shell that is open at both ends. Scaphopods lack eyes and gills. They burrow in soft bottoms using a foot that emerges from the larger opening. The smaller opening remains near the surface of the sediment. Scaphopods feed on microscopic organisms in the sediment, which they capture with thin, threadlike tentacles called captacula.

GASTROPODS

Gastropods or snails (Class Gastropoda) comprise the largest class of mollusks. During their larval stage, all gastropods undergo torsion, a process that twists the animal until the formerly posterior mantle cavity is rotated to a position over the head, resulting in an asymmetrical animal with a single coiled shell. Snail shells assume a variety of forms, ranging from microscopic (1/75 in/0.3 mm) to enormous (39 in/1 m). The shell of a snail may be external, internal, or entirely absent. Like bivalves, snails inhabit all marine and freshwater habitats. Unlike any other mollusks, snails developed lungs and have also colonized land environments ranging from forests to mountains to deserts. Snails may be herbivores, carnivores, parasites, filter feeders, detritivores, or even chemoautotrophs.

CEPHALOPODS

The earliest cephalopods (Class Cephalopoda) had external shells, with chambers that were interconnected by a tube that allowed them to become gas-filled and buoyant. During the course of their evolution, the vast majority of cephalopods have lost an external shell. Some, including sepia, cuttlefish, and squid have internal shells that have been reduced to various degrees; octopuses lack any shell at all. Some cephalopod lineages developed the ability to swim by undulating their fins, as well as by jet propulsion.

Cephalopods inhabit all oceans at all depths. Many live in shallow coastal areas, while others are pelagic, spending their lives swimming or drifting through the open ocean at great distances from surface, shore or bottom. Cephalopods range from 1 in (25 mm) to more than 46 ft (14 m) in length, and include both the Giant Squid and the even larger Colossal Squid, the largest known invertebrate. All are predatory, with the head and mouth surrounded by muscular, sucker-bearing tentacles that capture prey, which is then eaten with a parrotlike beak and radular teeth.

WHAT IS A SHELL?

As broadly defined, a shell is a hard outer covering that encases certain organisms, usually for the purpose of protecting them from the environment. Many organisms, ranging from microscopic foraminifera to turtles, produce shells using a variety of materials.

HOW A SHELL FORMS

External shells composed of calcium carbonate are secreted by many invertebrate phyla, among them Cnidaria (corals), Arthropoda (crabs and barnacles), Echinodermata (sea urchins), Brachiopoda (lamp shells), and Bryozoa (moss animals), yet the term "shell" or, more specifically "seashell" almost inevitably conjures the image of the calcified external skeleton of a mollusk. These molluscan shells are the subject of this book.

The shell is secreted by the mantle (or pallium), a specialized tissue that is present in every mollusk. One section produces a thin layer of a protein called conchiolin. Other cells secrete a fluid into the narrow space between the animal's tissues and the conchiolin layer. Calcium carbonate crystallizes from the fluid onto the inner surface of the conchiolin, producing a continuously mineralized shell. The shells of all mollusks are secreted outside the animal's tissues. Unlike the bones of vertebrates, shells do not contain cells or DNA.

In all mollusks, shell growth occurs through the addition of new bands of conchiolin along the existing edges of the shell, followed by crystallization of calcium carbonate onto this matrix. Shells can be made thicker by the successive secretion of conchiolin matrix and calcium carbonate to produce additional internal layers.

BIVALVES

The shells of bivalves consist of two separate valves. The tiny larva of a bivalve produces a single, uncalcified, caplike shell, called a pellicle. As the larva grows, it is gradually enveloped by two mantle lobes, each developing a separate center of calcification—the dissoconchs, the parts of the shell produced after the larva metamorphoses, assume the proportions and features of the adult bivalve. Most bivalves are composed of two valves that are mirror images of each other. The shell usually consists of three layers: an outer periostracum, which may be quite thick in some species, and outer and inner shell layers. The outer layer forms surface details such as scales or spines. In some bivalves, the shells have become reduced; in others, they have become incorporated into large, cylindrical tubes.

SCAPHOPODS

The tubular shell of scaphopods originates as a small, caplike shell in the larva. During development, the edges expand to surround the larva and fuse along the opposite side to form a tube. After metamorphosis, growth occurs through the addition of shell to the circular edge of the anterior opening, producing a shell consisting of a periostracum and two to four layers of aragonite, a crystalline form of calcium carbonate. As the shell grows, the length and anterior diameter increase and the inner walls thicken. The posterior opening is maintained at an appropriate diameter by the mantle, which dissolves, constricting portions of the shell.

CHITONS

The shells of chitons are secreted as eight separate plates, and include a head valve, six intermediate valves, and a tail valve. Each valve is composed of four separate layers. The outermost layer is the periostracum; beneath it is the tegmentum; then comes the articulamentum, the thickest and hardest of the layers; the innermost layer is the hypostracum, which is composed of columnar crystals. The valves are held together by muscles and a cuticular girdle that fits between the tegmentum and articulamentum. Depending on the species, the girdle may be covered by proteinaceous hairs, or calcareous spines, granules, or scales.

GASTROPODS AND CEPHALOPODS

The shells of gastropods and cephalopods all begin as simple, cap-shaped shells formed by the larvae of these mollusks. Growth occurs by incremental addition to the roughly circular rim to produce a conical shell. In extinct ancestral cephalopods, the caplike larval shells continued to grow into long, narrow, conical tubes. The animals remained near the base of the cone, and periodically sealed off the upper portions of the cone with partitions (septa). The few surviving species of Nautilus are the only cephalopods living today that still have external shells. In all other living cephalopods, the shell has become internal, greatly reduced, or is absent.

Gastropods also begin life with caplike larval shells. However, their larvae undergo torsion, a 180-degree twisting of the body that produces anatomical asymmetry. This, in turn, leads to helical coiling of their shells, almost invariably in a right-handed spiral. The shape of the resulting spiral can assume a staggering variety of forms, many of which are adaptations to particular environments. It is not uncommon for similar shell forms to be exhibited by distantly related snails, as a result of convergent evolutionary adaptations to particular habitats. As in the cephalopods, the shell has become reduced, internal, or lost within several lineages of gastropods.

SHELL COLLECTING

Since prehistoric times, humans have acquired shells and accorded them a treasured status. Seashells have been used as tools and currency; they have been incorporated or depicted in ornamental and ritualistic objects; they have held significance for many cultures, even those far from the sea.

HISTORY OF SHELL COLLECTING

The business of gathering shells as specimens goes back to Roman times at least. Collections of shells are among the artifacts found in the ruins of Pompeii. The expansion of the known world in the late Middle Ages led to a fascination in Europe with curiosities brought from distant lands. Merchants and aristocrats assembled collections of rare objects, including shells, that became symbols of their wealth and prestige. Scholars were employed to arrange, organize, and publish on these vast collections, many of which became the foundations of major museums.

TYPES OF SHELL COLLECTION

Most shell collections are general collections that strive to assemble a broad sampling from the enormous variety of shells. Some collectors focus on more specialized collections. Some may be restricted to the varieties of shells that occur in a particular area or habitat. Others confine their interests to a specific group of closely related shells. Cowries, cones, murexes, volutes, olives, and scallops are among the most popular groups for such collectors. Still others prefer to collect micromollusks, specimens with adults that do not exceed ½ inch (10 mm) in length.

FINDING SPECIMENS

Adding specimens to one's collection has its own pleasures. It can involve rewarding personal effort: strolling purposefully on a beach after a storm; searching rocky shorelines at low tide; snorkeling or scuba diving; or other, more specialized means such as rock scrubbing, dredging, or trapping. As well as enabling the collector to select specimens, these activities offer the opportunity to observe shells in their element, and to develop an appreciation for the way each one has become adapted to its environment. Novice collectors should be aware, however, that some regions require permits or a fishing license to collect mollusks, while others either limit or prohibit the collection of living specimens. Collectors should respect the natural habitat, and minimize their impact on the areas in which they collect. After examining the underside of rocks, for example, they should be returned to their original position and orientation.

ESSENTIAL EQUIPMENT

Equipment varies with the type of collecting, and each collector soon develops a personal set of gear optimized for specific habitats. The basics usually include a plastic bucket, a few plastic bags, forceps for retrieving small specimens from crevices, a knife or spatula for dislodging chitons, limpets, or bivalves from the rocks to which they are attached, and perhaps a garden spade and a small sieve for burrowing mollusks. Other items of equipment may include a digital camera and a portable GPS device, but the most important tools are a pencil, a few labels, and a notebook in which to record important details about each specimen collected.

RECORDING DATA

The value of a specimen lies not only in its rarity and perfection, but also in the quality of the collecting information that accompanies it. This should include, at a minimum, the precise location, depth, date, and even time it was collected, as well as ecological information, such as "on rocks exposed at low tide" or "buried in fine sand at edge of eel grass bed." Anyone reading the label should be able to return to the place where the specimen was collected.

PREPARING A SPECIMEN

Unless the collector has a saltwater aquarium, the tissues must be promptly removed from the shell before it is added to a collection. Placing the shell in warm water and bringing it to a boil for a few minutes usually loosens the attachment of the animal to the shell. When the shell is cool enough to touch, the animal can be carefully unwound from the snail or extracted from the bivalve, using forceps or dental tools. Occasionally, a second boiling is required.

Alternatively, the specimens may be frozen by placing the collecting bags in the freezer. After a day or so, the shell is thawed, and the tissues can be carefully removed. Often the specimen has to be frozen and thawed repeatedly to loosen the animal so it can be removed in one piece. Some collectors (and most museums) prefer to have specimens in their natural state. Others choose to remove encrusting organisms and the periostracum by soaking the specimen in diluted bleach, then picking and cleaning with dental tools and and toothbrushes.

ORGANIZING A COLLECTION

In a collection, all the specimens of the same species that were collected together at the same place and time should be grouped together, with a label that includes the detailed collection data. When the specimens are identified, the genus and species should also be added to the label. Many collectors maintain a catalog of their collections, either a handwritten ledger, or more frequently as a spreadsheet on their computers. This allows them to keep track of large and growing collections.