Bivalvia
Aquatic molluscs with hinged shells, lacking a head.
Bivalvia, also known as bivalves, is a class of aquatic molluscs that have laterally compressed soft bodies enclosed by a hinged pair of calcified half-shells called valves. They lack a head and typical molluscan organs such as the radula and odontophore, and their gills have evolved into ctenidia for feeding and breathing. Common bivalves include clams, oysters, cockles, mussels, scallops, and numerous other families living in saltwater and freshwater. Most are benthic filter feeders that bury themselves in sediment, while others lie on the sea floor, attach to hard surfaces, or swim. Bivalves have long been part of the diet of coastal and riparian human populations, and their shells are used in craftwork, jewelry, and buttons.
- field
- Class of aquatic molluscs
- known_for
- Having a hinged pair of calcified valves; filter feeding; source of pearls and food
- first_appearance
- Early Cambrian, more than 500 million years ago
- number_of_living_species
- About 9,200
Lore & Background
Bivalves have bilaterally symmetrical and laterally flattened bodies, with a blade-shaped foot, vestigial head, and no radula. The shell is composed of two calcareous valves held together by a ligament, and the valves are made of calcite or both calcite and aragonite. The mantle lobes secrete the valves, and the mantle crest secretes the hinge mechanism. The posterior mantle edge may have siphons for inhaling and expelling water. The shell grows larger as material is secreted by the mantle edge, and concentric rings on the exterior are commonly used to age bivalves.
Reader's Guide
Bivalves have long been a part of the diet of coastal and riparian human populations; oysters were cultured in ponds by the Romans, and mariculture has become an important source of bivalves for food. Pearl oysters are the most common source of natural pearls, and bivalve shells are used in craftwork, jewelry, and buttons. They have also been used in the biocontrol of pollution.
Did You Know?
- The name 'bivalve' is derived from Latin bis, meaning 'two', and valvae, meaning 'leaves of a door'.
- Adult shell sizes of bivalves vary from fractions of a millimetre to over a metre in length, but most species do not exceed 10 cm.
- Shipworms, in the family Teredinidae, have greatly elongated bodies with reduced shell valves that function as scraping organs for digging tunnels through wood.
Frequently Asked Questions
What is Bivalvia?
Bivalvia is a class of aquatic molluscs whose soft, laterally flattened bodies are enclosed by two hinged calcium-carbonate valves. The group encompasses well-known animals such as clams, mussels, oysters, cockles, and scallops, found in both saltwater and freshwater environments.
What sets Bivalvia apart from other molluscs?
Bivalves have no distinct head and are missing the radula and odontophore that characterize most other molluscan classes. Their gills have evolved into ctenidia, structures that simultaneously handle gas exchange and food capture.
How do Bivalvia feed?
The vast majority are bottom-dwelling filter feeders that pump water across their ctenidia to trap suspended organic particles. A few lineages, notably scallops, can jet-propel themselves through the water column rather than staying anchored in sediment.
How old is the Bivalvia class?
Bivalve fossils date back to the Early Cambrian, placing the origin of the class at more than 500 million years ago. That makes bivalves one of the longest-continuously represented molluscan lineages in the fossil record.
How many living bivalve species exist?
Approximately 9,200 extant species are currently recognized across the class. They span an enormous range of sizes, from minute freshwater mussels to large reef-building oysters.
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