Bivalves

Bivalvia is a class of marine and freshwater mollusks that have a laterally compressed shell composed of to valves (left and right) joined dorsally by an elastic ligament. This ligament in conjunction with interlocking "teeth" on each of the valves, forms the hinge. This arrangement allows the shell to be opened and closed without the two valves becoming disarticulated. Bivalves typically display bilateral symmetry both in shell and anatomy, but there are significant departures from this theme such as scallops and oysters.

This class is the second most diverse group of mollusks behind gastropods. Adult shell sizes vary from one millimeter to over a meter in length, but the majority of species do not exceed 10 cm.

The name "bivalve" is derived from the Latin bis, meaning "two", and valvae, meaning "leaves of a door". Not all animals with shells with two hinged parts are classified under Bivalvia; other animals with paired valves include certain gastropods (small sea snails in the family Juliidae) and some members of the phylum Brachiopoda (image).

California clam shrimp, Cyzicus californicus

Bivalves have long been a part of the diets of coastal human populations. Oysters were cultured by the Romans, and mariculture has more recently become an important source of bivalves for food. Besides their use as food, oysters are the most common source of natural pearls. The shells of bivalves are used in craftwork and the manufacture of jewelery and buttons. Bivalves have also been used in the biocontrol of pollution.

Bivalves appear in the fossil record first in the early Cambrian more than 500 million years ago. The total number of living species is approximately 9,200 that inhabit most aquatics habitats. The marines species range from shallow to the deep waters, from equator to the poles. Bivalves also have colonized brackish-water estuaries and fresh water rivers, streams, and lakes. 

The bivalve shell consists of two valves that are hinged dorsally, usually with interlocking teeth (the hinge), and always with a horny ligament that connects the two valves along their dorsal surfaces and acts to force the valves apart. The interior of the valves contain scars of the various muscles attached to it, in particular the (usually two, sometimes one) adductor muscles that, on contraction, close the valves. The shell can also be internal, reduced or even absent as in shipworms.

Shipworm

Near the hinge of the shell is the umbone or beak, a rounded, knobbly protuberance. This represents the oldest portion of the shell. The hinge area is the dorsal region of the shell and the lower margin is the ventral region. The anterior or front of the shell is where the byssus and foot are located, and the posterior of the shell is where the siphons are located. When the umbone is uppermost, the valve with the anterior end to the left is considered to be the left valve, while the valve with the anterior end to the right is the right valve.

The valves of bivalves are made of either calcite, as is the case in oysters, or both calcite and aragonite. The ligament and byssus, if calcified, are composed of aragonite. The shell usually consists of three layers: an outer periostracum, and outer an inner shell layers.The outermost layer of the shell is the periostracum, a skin-like layer which is composed of a hard organic substance and is usually olive or brown in colour and easily abraded. The outer layer forms surface details such as scales or spines.

In all mollusks, the mantle forms a thin membrane covering the animal's body and extending out from it in flaps or lobes. In bivalves, the mantle lobes secrete the valves, and the mantle crest secretes the whole hinge mechanism consisting of ligament, byssus threads, and teeth.

Most bivalves have a capacious mantle cavity that accommodates large gills, but they don't have head, radula or jaws. In addition to respiration, the gills filter food particles from the water in the majority of bivalves. 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.

The intestine is irregularly looped and opens dorsally into the exhalant area. Also opening into this region are the paired kidneys and, when separate from the kidneys, the gonopores of the paired gonads. The heart typically lies below the center of the valves and consists of two auricles and a single ventricle that supplies both anterior and posterior aorta. The nervous system is made up of three pairs of ganglia.

The bivalve foot is modified as a powerful digging tool in many groups, while in those that live a permanently attached life (e.g., oysters), it is very reduced.

Most bivalves burrow in sand or mud, some in wood, clay or coral. Some attach to hard substrates with threadlike strands of protein (byssus), others by permanently cementing one of their valves.

The tiny larva of a bivalve produces a single, uncalcified, caplike shell,called a pellicle. As the larva growths, 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.

Bivalves are divided into the following subclasses:

Protobranchia
Pteriomorpha

Anomalodesmata

Rostroconchia
Heterodonta

Palaeoheterodonta

Chama limbula (Heterodonta)

Spondylus regius (Pteriomorphia)

Gastropods

The gastropods are the largest (about 80%) and most diverse class of the phylum Mollusca. This class comprises the snails, which have a singe coiled shell into which the animal can generally withdraw, and the slugs, whose shells have been reduced to an internal fragment or completely lost in the course of evolution. Gastropods are not only diverse with respect to the number of species alive today, they are diverse in terms of their size, shape, color, body structure and shell morphology.

The majority of the gastropods are asymmetric and dextraly coiled, and they occupy all marine habitats from equator to the poles, from shall to deeper waters. Multiple groups have colonized freshwater habitats, while others have evolved the ability to breathe air and have colonized terrestrial habitats, including mountains and deserts. Most marine gastropods breathe via their gills, however freshwater and terrestrial species are an exception to this rule and breathe using a rudimentary lung. Those gastropods that breathe using a lung are called pulmonates. Gastropods are among the few groups of animals to have become successful in all three major habitats: the ocean, fresh waters, and land.

This mollusks range in size from 0,33 mm to 1m length, most are mobile, but some cement them selfs to hard substrates while others are external or internal parasites. Gastropods may be herbivores, carnivores, parasites, filter feeders, detritivores or chemoautotrophs (rare). Those that feed on plants and algae use their radula to scrape and shred their food. Gastropods that are predators use a siphon to suction food into the mantle cavity and filter it over its gills. Some predatory gastropods feed on shelled prey by boring a hole through the shell to locate the soft body parts inside.

The coil of the shell usually twists in a clockwise direction, so that when viewed with the apex (top) of the shell pointing upward, the opening of the shell is located on the right. Many gastropods (such as sea snails, terrestrial snails, and freshwater snails) have a hardened structure on the surface of their foot called an operculum that work's as a “trap-door”. The operculum serves as a lid that protects the gastropod when it retracts its body within its shell. The operculum seals the shell opening to prevent desiccation or deter predators. The operculum may be solid and heavy (eg. Turbinidae) or pliant and horny (eg. Muricidae or Trochidae). Sometimes it is very small and obsolete and it isn't big enough to seal the aperture, likes happens in some cones.

      

Chicoreus saulii (Muricidae)

Conus textile (Conidae)

During the larval stage, all gastropods undergo torsion, a very rapid process that results in the rotation of the visceral mass and mantle on the foot. The result is that the mantle cavity (including anus) lies in the anterior body, over the head and mouth, and the gut and nervous system are twisted. . This twisting means that the head is between 90 and 180 degrees offset relative to their foot. Torsion results in asymmetrical growth, with more growth occurring on the left side of the body. Torsion causes the loss of the right side of any paired appendages. Thus, although gastropods are still considered to be bilaterally symmetrical (that's how they start out), by the time they become adults, gastropods that have undergone torsion have lost some elements of their "symmetry". The adult gastropod ends up configured in such a way that its body and internal organs are twisted and the mantle and mantle cavity are above its head. It should be noted that torsion involves the twisting of the gastropod's body, it has nothing to do with the coiling of the shell Some species reverse torsion ("detorsion"), but evidence of having passed through a twisted phase can be seen in the anatomy of these forms. Torsion in gastropods has the unfortunate result that wastes are expelled from the gut and nephridia near the gills. A variety of morphological and physiological adaptations have arisen to separate water used for respiration from water bearing waste products.

Gastropods are dioecious (sexually distinct), and some forms are hermaphroditic. Hermaphroditic forms exchange bundles of sperm to avoid self-fertilization; copulation may be complex and in some species ends with each individual sending a sperm-containing dart into the tissues of the other.

Gastropods are divided into the following basic groups:

• Patellogastropoda
• Vetigastropoda
• Cocculiniformia
• Neritimorpha
• Caenogastropoda
• Heterobranchia

Turbo marmoratus ( Vetigastropoda)

Mitra mitra (Caenogastropoda)

Biological classification

Classificacion of living things is essential to any study of animals and plants, it is usually based on anatomical similarities and differences and is called taxonomy (Taxis means arrangement and nomos means method). The modern taxonomic system was developed by the Swedish botanist Carolus (Carl) Linneaeus (1707-1788). To distinguish different levels of similarity, each classifying group, called taxon (pl. taxa) is subdivided into other groups. The broadest category is the domain, followed by kingdom, phylum, class, order, family, genus and species, the most specific one. The members of a particular species are similar enough to allow interbreeding to result in fertile offspring. To complicate a little bit more we also have intermediate grades such as subclass, suborder, superfamily, subfamily and subgenera.

In 1990 Carl Woese introduced the taxon domain, he divided all the living things in three domains that are organised based on the difference between eukaryotes and prokaryotes. The three domains are as follows:

Archea (Archeabacteria) consists of archeabacteria, bacteria which live in extreme environments. The kingdom Archaea belongs to this domain.

Eubacteria consists of more typical bacteria found in everyday life. The kingdom Eubacteria belongs to this domain.

Eukaryote encompasses most of the world's visible living things. The kingdoms Protista, Fungi, Plantae, and Animalia fall under this category.

Under the three domains we have six kingdoms Plants, Animals, Protista, Fungi, Eubacteria and Archae (or Archabacteria). The first two are commonly understood and will not be expounded here.

Protista, the third kingdom, was introduced by the German biologist Ernst Haeckel in 1866 to classify micro-organisms which are neither animals nor plants. Since protists are quite irregular, this kingdom is the least understood and the genetic similarities between organisms in this kingdom are largely unknown. For example, some protists can exhibit properties of both animals and plants.

Fungi are organisms which obtain food by absorbing materials in their bodies. Mushrooms and moulds belong in this kingdom. Originally, they were part of the plant kingdom but were recategorised when they were discovered not to photosynthesise.

Eubacteria are bacteria, made up of small cells, which differ in appearance from the organisms in the above kingdoms. They lack a nucleus and cell organelles. They have cell walls made of peptidoglycan.

Archae (or Archaebacteria) are bacteria which live in extreme environments, such as salt lakes or hot, acidic springs. These bacteria are in their own category as detailed studies have shown that they have unique properties and features (ex. unusual lipids that are not found in any other organism)which differ them from other bacteria and which allow them to live where they live. Their cell walls lack peptidoglycan. Here is a diagrama of the the taxa in hierarchical order:

Now we can return to our seashells. Seashells belongs to the phylum Mollusca of the animal kingdom that is the larger phylum of invertebrates and has around 85 000 species. This philum is the second phylum with larger number of specimens comprising 23% of all the named marine organisms, but exists mollusks living in fresh water and terrestrial habitats. This animals are highly diverse, not only in size and in anatomical structure, but also in behaviour and in habitat. The phylum is typically divided into 9 or 10 classes, of which two are entirely extinct. Six of this classes are related to seashells: Gastropoda, Bivalvia, Cephalopoda, Scaphopoda, Polyplacophora and Monoplacophora that will be describe in others messages. Cephalopods mollusks, such as squid and octopus, are among the most neurologically advanced of all invertebrates—and the giant squid is one of the largest known invertebrate species. The gastropods (snails and slugs) are by far the most numerous mollusKs in terms of classified species, and account for 80% of the total.

Mollusks have developed such a varied range of body structures, it is difficult to find defining characteristics to apply to all. The most general characteristic of mollusks is that they are unsegmented and bilaterally symmetrical; the presence of a mantle with a significant cavity used for breathing and excrecion, and an organized nervous system.

Here are 2 examples of seashells biological classification :

Domain: Eukaryota

Animal: Kingdom

Phylum: Mullusca

Class: Gastropoda

Subclass: Caenogastropoda

                                               Order: Neogastropoda

Superfamily: Conoidea

Family: Conidae

Genus: Conus

Species: pertusus

Domain: Eukaryota

Animal: Kingdom

Phylum: Mollusca

Class: Bivalvia 

Subclass: Heterodonta

Infraclass: Euheterodonta 

Order: Veneroida 

Superfamily: Cardioidea

Family: Cardiidae 

Subfamily: Fraginae

Genus: Corculum

Species: roseum

Xenophora pallidula

Class: Gastropoda Subclass: Caenogastropoda Order: Littorinimorpha Superfamily: Xenophoroidea Family: Xenophoridae Genus: Xenophora

Author: Reeve, 1842

The seashells that belong to the genus Xenophora are known as a carrier shells because they have the habit of attach empty shells, pieces of coral, stones or grains of sand to their shells. In most species this only occur during their young state and the attachments are only on early whorls. In others such X. pallidula (photos) it continues throughout the growth giving a beautiful and interesting seashell. The Xenophora picks up and clean the selected object, then the animal cements the object to its shell with secretions from the mantle. Bivalve shells are placed with the inner side facing upward, while gastropods are usually attached with the aperture facing up. This process can take up to an hour and a half to complete, then the animal remains motionless for up to ten hours to ensure its new attachment is secure.

It is not known to what extent an artistic sensibility plays a part in this behavior. The seashells who live in shallower water (where there is enough light for them to be seen) probably use their collections for camouflage. The ones that live in the deeper, dark waters, such as X. pallidula (photos), are believed to be motivated more by a desire to not to be sucked into the viscous muck in which they live. Attaching extensions to their shells spreads out the shell's surface area and helps prevent the animal from sinking. The additions may also strengthen the gastropod relatively thin shell.

Not all of the items on a Xenophora's shell are put there by the creature itself. As a solid substrate in a mucky environment, the shell is an enticing landing spot for tube worms, oysters, and, most spectacularly, glass sponges. Their new home even has the advantage of being mobile, transporting a rooted animal to new feeding grounds.