Bivalvia

Bivalvia, or bivalves, are a group of aquatic molluscs found in both saltwater and freshwater. They have soft bodies enclosed in a shell made of calcium carbonate, which is made of two parts called valves that hinge together. Unlike many other molluscs, bivalves do not have a head or certain organs like the radula.

Common examples of bivalves include clams, oysters, cockles, mussels, and scallops. Most bivalves live on the sea floor or in sediment, where they filter feed, eating tiny particles from the water. Some, like scallops, can swim short distances, while others, such as shipworms, bore into wood or stone to live inside.

Bivalves have been an important food source for humans for thousands of years. Oysters were farmed by the Romans, and today, farming bivalves, known as mariculture, is a big industry. Bivalves also provide pearls, and their shells are used to make jewelry, buttons, and crafts. They even help control pollution in water. Bivalves have been around since the early Cambrian period, more than 500 million years ago, and there are about 9,200 known living species.

Etymology

The word Bivalvia was first used by Linnaeus in 1758. It describes animals with shells made of two parts, called valves. The name comes from Latin words meaning "two" and "door leaves".

Other animals, like some small sea snails, also have paired shells, but they are not true bivalves.

Anatomy

Bivalves have soft bodies enclosed in two hinged shell pieces called valves. They lack a head and some usual body parts found in other sea creatures. Their bodies are flat and symmetrical on both sides.

The shell is made of two hard pieces joined by a ligament. These pieces can be made of different materials, such as calcite or aragonite. Inside the shell, the mantle covers the body and helps create the shell and its hinge. Some bivalves have tubes called siphons that let water flow in and out.

Bivalves move their shell pieces with strong muscles. They also have a simple nervous system without a brain, made of small groups of nerve cells. Their senses include feeling and tasting the water, and some can even see simple shapes with basic eyes.

Bivalves breathe through their gills, which also help them eat by filtering tiny food from the water. Most eat by filtering, but a few eat larger food. Their digestive system processes this food, and they get rid of waste through special tubes.

Most bivalves release eggs and sperm into the water to reproduce, and their young develop into adults over time. Some freshwater bivalves have young that attach to fish before becoming adults.

Comparison with brachiopods

Brachiopods are marine animals with shells that look similar to bivalves because they are about the same size and both have hinged shells made of two parts. However, brachiopods and bivalves evolved from different ancestors, so their similarities happened because they live in similar environments and developed similar solutions to the same challenges. This is called convergent evolution.

Both brachiopods and bivalves have shells made of two parts, but these parts are arranged differently. In brachiopods, the shell parts are on the top and bottom of the body, while in bivalves, the shell parts are on the left and right sides and are usually mirror images of each other. Brachiopods have a special feeding structure called a lophophore, which they share with some other sea animals. Their shells are often made of calcium carbonate, while bivalve shells are always made of calcium carbonate, usually in a form called aragonite.

Evolutionary history

The Cambrian explosion happened around 540 to 520 million years ago. During this short but important time, many major animal groups, including the first creatures with hard shells, appeared. Bivalves and similar animals showed up then and left fossils in rocks.

Early possible bivalves include Pojetaia and Fordilla. Only a few kinds of these early bivalves exist today. During the Early Ordovician, bivalves became much more diverse. By the Early Silurian, their gills were changing to help them feed. In the Devonian and Carboniferous periods, they developed siphons and a muscular foot, letting them bury deep in the sand.

By the middle of the Paleozoic, around 400 million years ago, brachiopods were very common filter feeders. But during the Permian–Triassic extinction event 250 million years ago, bivalves grew much more diverse. Even though many bivalves were affected by this event, they recovered and thrived in the following Triassic period. Brachiopods, however, lost most of their species. Bivalves may have succeeded partly because they could burrow to hide from predators. New adaptations helped bivalves live in new ways, like developing shell spines to stay afloat in soft sediment, gaining the ability to swim, and in a few cases, beginning to hunt other animals.

Diversity of extant bivalves

Bivalves come in many different sizes. The smallest living bivalve is Condylonucula maya, which is only about 0.52 mm long. The largest living bivalve is often considered to be the giant clam Tridacna gigas, which can grow up to 1,200 mm long and weigh more than 200 kg. There are about 9,200 different kinds of living bivalves grouped into 106 families.

Distribution

Bivalves are very successful animals that live in water all around the world. Most of them live buried in sand or mud on the ocean floor or in freshwater places. Many can be found where the ocean meets the land, called the intertidal zone, and in deeper water areas.

These animals live in warm tropical waters as well as colder areas. Some bivalves can even live in very harsh places. For example, one type lives under thick ice in the cold Arctic, and others live near very hot underwater vents in the deep ocean. Some bivalves can survive in very deep water, while others live on land near the water, attached to leaves and trees in warm places.

Freshwater bivalves also have different homes. Some only live in one small area, while others have spread far away from where they started, sometimes causing trouble for other plants and animals in new places.

Behaviour

Most bivalves stay in one place their whole lives, often living buried in sand, silt, or mud under the sea. This helps protect them from waves, drying out, and some animals that might eat them. They stretch out special tubes called siphons to get food and air when the tide is high and pull them back in or close their shells when the tide goes out.

Some bivalves, like mussels, stick to rocks or other hard surfaces using strong threads. Oysters can form thick layers on rocks and are also good at filtering water for food. Even though they filter lots of water, bivalves often close their shells to rest, following patterns set by the moon and sun. Some small freshwater clams can climb on water plants using their foot to find the best spots for feeding.

The thick shells of bivalves make them hard for predators to eat, but some animals still try. Fish, birds, sea otters, and walruses all eat bivalves. Crabs and starfish can also open bivalve shells to eat them. Some bivalves, like razor clams, can dig quickly into sand to hide. Others, like scallops, can swim short distances by opening and closing their shells fast. When threatened, some bivalves can lose parts of their siphons or release nasty-smelling fluids to scare off attackers.

Mariculture

Main article: Oyster farming

Main article: Scallop aquaculture

Oysters, mussels, clams, scallops, and other bivalve species are grown in the sea and lagoons using natural food from their environment. In 2010, one-third of the world's farmed food fish came from bivalves and filter-feeding carps without needing special feed. The Romans were the first to farm European flat oysters in shallow ponds, and similar methods are still used today. Baby oysters, called seed oysters, can be raised in special facilities or collected from the wild.

Juvenile oysters can be grown in trays or on ropes, away from predators like starfish and crabs. Some are placed directly on the seabed and grow for about two years before being harvested. The Pacific oyster, originally from Japan, is farmed in many parts of the world. It grows quickly and can be ready for harvest in 18 to 30 months. Other species like the Sydney rock oyster and various mussels are also farmed using similar methods. In 2010, the total production of bivalve molluscs from mariculture was over 12 million tons, showing significant growth since 2000.

Use as food

Bivalves like oysters, scallops, clams, ark clams, mussels, and cockles have been eaten by people for a very long time. People cook or eat them raw. The amount of bivalves traded around the world has grown a lot over the years.

Eating raw or not well-cooked shellfish can sometimes make people sick. This can happen because bivalves can collect bad things from the water, like tiny germs or harmful plants. These can make people ill if they eat the shellfish.

Ecosystem services

Marine bivalves help keep coastal waters clean by eating tiny plants and particles. They can take nutrients from the water and put them back on land or change them so they float away. This helps when too many nutrients get into the water from farms and sewage.

Bivalves can also collect harmful substances from the water, like heavy metals, because they cannot break these down. This can be dangerous for people who eat them, but it also helps scientists learn about pollution in the water. Some types of bivalves are good at showing how much pollution is around them. Crushed shells from bivalves can also help clean water by taking away some heavy metals.

Other uses

Conchology is the study of mollusc shells, and many people enjoy collecting them. Shells can be found on beaches or bought to display in homes. The largest collection is at the Smithsonian Institution, with over 20 million shells.

Shells are used in many creative ways. They can be pressed into concrete for paths or walls, or used to decorate items like picture frames. Shells can also be made into jewelry, such as necklaces, or used as tools and utensils. Some shells have been used for thousands of years by different cultures.

Buttons have been made from shells for a very long time. Shells can also be used to make a special fabric called sea silk. Crushed shells are sometimes added to animal food to help them stay healthy.

Mother-of-pearl, a shiny layer found inside some shells, is used to make beautiful decorations and jewelry. Pearls are formed when a tiny irritant gets inside a shell and layers of this shiny material build up around it. Most pearls come from special farms where the irritant is placed inside the shell on purpose.

The scallop shell is a symbol for many things. It is linked to journeys and travel, and also to ideas about growth and new beginnings. It appears in art, architecture, and even as a company logo.

Bivalvian taxonomies

For many years, scientists have disagreed on how to group bivalves. In the past, they used just one feature, like the shape of the shell or the type of hinge, to sort them. This led to many different naming systems.

In 2010, scientists created a new way to group bivalves using many features, like shell shape, tiny structures, and modern genetic studies. This new system is now used by experts around the world. Some scientists still debate where to place one group, but ongoing research helps to clarify these relationships.

Practical taxonomy of R.C. Moore

R.C. Moore created a useful way to group bivalves based on shell structure, gill type, and hinge teeth. He described two main groups:

Subclass:Prionodesmacea

Order

Paleoconcha

Taxodonta: Many teeth (e.g. order Nuculida)

Schizodonta: Big bifurcating teeth (e.g. Trigonia spp.)

Isodonta: Equal teeth (e.g. Spondylus spp.)

Dysodonta: Absent teeth and ligaments joins the valves.

Subclass:Teleodesmacea

Order

Heterodonta: Different teeth (e.g. family Cardiidae). [ Lower Ordovician – Recent]

Pachydonta: Large, different, deformed teeth (e.g. rudist spp.). [ Late Jurassic – Upper Cretaceous]

Desmodonta: Hinge-teeth absent or irregular with ligaments (e.g. family Anatinidae).

Prionodesmacea have a special shell structure, separated mantle lobes, and simple hinge teeth. Their gills vary. Teleodesmacea have a different shell structure, connected mantle lobes, well-developed siphons, and specialized hinge teeth. Most have a certain type of gill.

1935 taxonomy

In 1935, Johannes Thiele created a system that divided bivalves into three groups based on teeth and muscles. One group had many small teeth, another had uneven muscles, and the last had special gills. This group with special gills was further split into four smaller groups.

Taxonomy based upon hinge tooth morphology

The layout here follows Newell's 1965 classification based on hinge tooth morphology (all taxa marked † are extinct) :

The idea that one group of bivalves is separate is debated. Most now think it belongs within another group.

Taxonomy based upon gill morphology

Another way to group bivalves uses gill types. This separates them into three groups: one with simple gills, another with filamentous gills, and the last with all others. One group was separated because of differences in their gills. These belong to the superfamily Poromyoidea and have a muscular septum instead of filamentous gills.

2010 taxonomy

Main article: Taxonomy of the Bivalvia (Bouchet, Rocroi, Bieler, Carter & Coan, 2010)

In May 2010, a new way to group bivalves was published. The scientists used many types of information, like genetic studies, anatomy, shell shape, and where they are found. This system recognizes 324 families, some only known from fossils and others still living today. This classification is now used by the World Register of Marine Species.

Proposed classification of Class Bivalvia (under the redaction of Rüdiger Bieler, Joseph G. Carter and Eugene V. Coan) (all taxa marked † are extinct) :