Bivalves

 

WHAT IS A BIVALVE?

A bivalve consists on three parts; a bottom shell, a top shell and a valve. When you chunk it out into two parts (‘bi’ and ‘valve’) it really actually means Two shells that open and close. Some typical types of bivalve include :  oysters, mussels, scallops and clams. Most Bivalves move around by opening and closing its body rapidly using its valves. This then causes a propulsion helps move through the water. There are more 15,000 species of bivalves from the Bivalvia phylum. The largest bivalves are the giant clams found near the Koror Island in Micronesia. The gargantuan mollusks can weigh up to 227 kilos. The smallest living bivalve however, measures only half a millimetre, the Nut clam. Some other types of bivalves that I have not mentioned are octopuses, squids and snails.

CHARACTERISTICS OF BIVALVES

The shell of a bivalve is made up of calcium carbonate and is secreted by the mantle (a soft body wall). Shells grow from organisms extending out from the hinge, hardening as it gets older.

Lots of species of bivalves are filter feeders meaning that they filter water through their gill while the food gets stuck in mucus. The flow of the water then cycles through them. Other than filter feeders, some bivalves, like some types of scallops are predatory.

Lots of different bivalves have different lifestyles. Species such as mussels and oysters spend their lives attached to a substrate. Some others burry themselves into the ground like what a clam does. Scallops however, swim around the sea floor and hunt, since they are predatory. Bivalves have highly reduced heads and basic nervous and sensory systems. Most species of bivalve have a chemosensory cells at the edge of their mantle. They are used to detect chemical signals in thee water.

COMMERCIAL VALUE

Lots of bivalve species have commercial value, including scallops, clams and pretty much everything else  we have mentioned so far. Bivalves are valued as food sources, and have been used in a variety of different way through history. Most clothes buttons were made from bivalve shells before the invention of plastic. Also, shell fragments have been used for a range of things, from roads to fertilizers. But we can’t forget the fact that sometimes you find pearls in oysters that you can trade in for dosh.

 

At times some bivalves can become toxic to any animals and humans who eat them. Toxins produced by a certain algae bio-accumulate in the tissue of mollusks, and then they may be eaten by consumers.

CLAMS

Clams live underground, using their one muscular foot to dig into the sand or mud. The take in water for filter feeding and gas exchange from something called a siphon. For some species of clam the siphon is really long allowing the clam to stay safe quite deep under ground. The clam is the common dinner of species such as eels and starfish. A clams defensive mechanism is to shut itself as tightly as it can so the predators can’t get in.

The giant clam can reach a diameter of 1.2 meters and it is unusual because it harbours algae within its tissue.

SCALLOPS

Scallops are different from the rest of the bivalve phylum in the fact that they swim through using jet propulsion.  A sudden contraction in the muscle closes the shells and the water ejects from its body. So, scallops swim in small spurts.

Scallops mostly live on sand and mud bottoms. They may attach to rocks, but they can detach whenever they want, leaving them to feel free to swim away whenever they want. Jet propulsion is also used an escape method with scallops. Some potential predators of the scallop are starfish, snails and fish. The large, well developed muscle of the scallop is the edible bit that people eat.

Scallops are also weird compared to other bivalves because they actually have well developed eyes. Scallops have a group of eyes around the edges of the shells to help them spot predators before its too late. So really, scallops are quite good at NOT getting  eaten, which is good for them.

Scallops are normally found in massive groups, so if you find one your probably going to find more. These groups are sometimes only temporary, but permanent beds occur when temperature and food is optimal. Commercial fishing of scallops has diminished natural scallop population. Scallop farms have been established for eating purposes.

MUSSELS

Mussel are sedentary. They attach themselves to a substrate using secret threads called byssal threads, they are produced by the byssal organ on the muscular foot. Mussels frequently occur in big colonies, forming massive mussel beds. Like lots of other bivalves the mussel filter feeds.

Fresh water species of mussel can be found in freshwater rivers and streams. Unlike marine larvae, which swim free, the larvae of mussels are parasitic (really small and are born on other creatures), most often on fish.Numerous species of fresh water mussels are endangered, including around half of all the species that occur in North America.

The zebra mussel, an invasive freshwater species of mussel, was introduced into North America in 1980’s from original European habitat. It populated rapidly through to Eastern Northern America. The zebra mussel has had an impact on water supplies in America by clogging up the pipelines and other structures such. Lots of effort goes in controlling the spread. Zebra mussels can be spread into other waterbodies by fishing activity and boats, in which they often attach to.

OYSTERS

Oysters spend their lives attach to a substrate, such as a rock. Like many other bivalves, oysters are found in beds and are filter feeders. Oysters are usually tough and lumpy on the outside and shiny and smooth on the inside.

Oysters are weird because of the fact that they are male when they are younger and are female when older. The largest oysters reach to sizes of centimetres. Oysters don’t stop growing until they die, which has been known until 100 years.

Several species of oysters are considered delicacies in countries and are farmed for food. Oysters are sometimes best known for their pearls as well. These can be sold for $500 to $10000, Some dosh there. When something gets lodged inside its shell, an oyster will deposit of pearly material that layers the inside of the shell.

By Will Farquhar

 

References:
http://www.encyclopedia.com/plants-and-animals/animals/zoology-invertebrates/bivalve

• Google Images