What Are Bivalves? 5 Examples of These Amazing Creatures - AZ Animals
Bivalves are a subgroup of mollusks and are eaten as a delicacy around the world.
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Bivalves make up a large group of animals. It's estimated that there are well over 20,000 known species of bivalves in the world. Because of how little people know about the oceans, scientists estimate that there are likely thousands more species not yet known to science.
They are not only interesting species unlike most other animals in the world, but bivalves play an important role in the environment and the lives of humans. Most are ocean species, but there are still quite a few species you'll find swimming in lakes and rivers.
If you aren't quite sure what bivalves are and want to learn more about these amazing animals, then you'll want to keep reading.
What Are Bivalves?
Bivalves are a type of mollusk. They are part of the class Bivalvia inside the phylum Mollusca. These animals are classified by a two-part hinged shell that is hard. This shell covers an invertebrate, usually one that has a very soft body. The shells are often round or oval.
Like most aquatic animals, bivalves have gills that they use to breathe through.
Bivalves don't have something that people would recognize as a head. However, quite a few do have a 'foot'. This foot allows bivalves to move. They can use them to attach to a surface, crawl along the sea floor, or even burrow under the sand.
They are aquatic animals. Most of them are filter feeders. They clean the water by picking out and eating particles that come through their shell.
There are thought to be about 9,200 different bivalves in the world. They live in all parts of the world, including very cold areas, tide pools, and vents deep underground. There are even freshwater bivalves from the Unionidae and Corbiculidae families.
The only place that these amazing creatures can't be found, is deep on land. Some can handle drought conditions and live in areas that are dry for a long while, but they cannot solely live on land.
Examples of Bivalves
There are five main groups of bivalves; scallops, cockles, mussels, oysters, and clams. While they are all bivalves, they each have unique properties and characteristics.
Scallops
You will find scallops swimming around in the ocean.
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Scallops are unique animals. Unlike some other bivalves, they're actually a free-swimming mollusk. The abductor muscle inside the scallop helps them to open and close their shell. This helps move water and allows the scallops to "swim" through the ocean. This is the same muscle that people end up eating.
Scallops are easily identifiable from other bivalves due to the shape of their shell. Like the popular gas company, Shell, scallops have a rounded, almost saucer shape for their shells. The shells are often ribbed or fluted. At the bottom, where the shape becomes fluted, there are two wings on either side. They are most often red or brown but can be fully white or other colors.
There are over 400 species of scallops in the world. All of which are part of the family Pectinidae. They are an ancient group of animals. Some studies have shown that they have existed in some form for over 240 million years.
Though scallops don't have a head, they do have eyes. There are an estimated 200 eyes on the edge of their body. These eyes don't allow them to see as humans see, but they do help them detect light, movement, and predators in the area.
Like some other bivalves, scallops can produce pearls. However, they aren't as shiny as other pearls and aren't as much in demand.
Cockle
Though they might not be as well known as other bivalves, cockles are another major group of the group.
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When it comes to bivalves, the least heard of are often the cockles. They are edible, just like most other bivalves, but they aren't widespread in the US as a food source. However, you'll find them in many other countries across the world as a well-loved meal. They aren't always great for commercial fishing. While some live at low tide levels, some are found 1,500 feet under the water, and some or even found in the intertidal zone, which makes it hard to farm for these animals.
They are very distinctive animals once you get to know what they look like, thanks to their shells. Their shells are very symmetrical and round. They are also fairly heart-shaped from certain angles. Because of this, they are sometimes called heart clams.
Cockles create their own shell, and it grows every year to fit their needs. However, they slow their growth in winter. This creates growing bands on their shell. If you look carefully, you can determine the age of a cockle by how many growing bands they have.
There are over 250 species throughout the world. Cockles tend to live in dense populations, with some of them reaching up to 10,000 individuals per square meter. All cockles fall under the Cardiidae family.
Cockles vs. Clams
Cockles are often confused with clams, but there are a few differences between them. The biggest is that while clams can live in saltwater or freshwater, cockles are solely saltwater animals. Another difference is that clams come in all sorts of shapes and sizes, while cockles tend to be fairly small and rather consistent in size. They range from somewhere between 0.5 inches and 6 inches in size.
Mussels
Some mussels live in very specific habitats and are found in only one or two places in the whole world.
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Mussels are an interesting group of bivalves. Many mussels stay in the same place their whole life, but some move with the help of their foot.
Then there are some unique species that, though they have a foot to help them move around, they tend to use fish as a way to travel. When these mussels are small, they will attach themselves to the gills of fish. Once the fish reaches a stream or river and are a good size, the mussel will detach.
This process doesn't actually hurt the fish in any way. To get fish close enough to attach, mussels have developed lures that look like prey fish.
Mussels are also often used to determine how healthy water is. In areas where there is a lot of sediment, pollution, or bacteria, mussels will quickly die off. Since this often indicates the quality of water is poor, many people have come to learn that if mussels are happy and growing in an area, the water is usually healthy and clean as well.
Mussels are found in both freshwater and saltwater. The mussels that live in different bodies of water are broken up into groups known as marine mussels and freshwater mussels. Both groups fall under the Mytilidae family.
They are considered to be some of the longest-lived invertebrates in the world, making it over 100 years. Thanks to predators and fishing, however, most only live between 10 and 50 years. Like cockles, mussels have growth lines on the shell. However, these lines don't always correlate to a year of growth.
Oysters
It is oysters you have to thank for many of your pearls.
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Most oysters are part of the superfamily Ostreoidea. However, not every species falls under that category.
Despite being filter feeders like other bivalves, oysters are fairly picky eaters. They enjoy very specific algae and plants, and their very favorite is plankton.
Because oysters don't move much, their shells are shaped by the habitat they live in. Other mussels, the rocks they are on, and any other object in the area will determine the shape the oyster shell grows in. Depending on their region, they also grow differently, a trait known as phenotypic plasticity. This makes it difficult for scientists to determine what species oysters belong to, as they all look a little different.
Interestingly, all the oysters around the US are the same species, Crassotrea virginica. This includes the oysters around the Gulf of Mexico, Chesapeake Bay, and Virginia. Though they are the same species, they may not taste the same.
This is because oysters get their flavors from their environments. Depending on the salt levels in the water, and the nutrients, the oysters from each location taste a little different. Some of the flavors you may expect to taste from an oyster include sweet, metallic, mild, buttery, and briny.
The growth of oysters is determined by several factors, but salt levels play a huge role. Oysters in salty environments will often grow at a faster rate than the average inch per year.
Clams
There are many different kinds of clams in the world.
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Clams are the most diverse of all bivalves. There are over 15,000 different species of clams. While some clam species look very similar, there's a lot of diversity overall. For example, while most clams spend their lives burrowed into the usual sand and mud, some species burrow into things like rock and wood.
Some clams, like the giant clam and the geoduck, live to be over 100 years old. Geoducks have been found to live for at least 165 years. The smallest, largest, and most unique bivalves in the world are all clams.
The smallest clam in the world is the saltwater clam (Condylonocula maya). They are less than 0.020 inches in length. Meanwhile, another clam, the giant clam (Tridacna gigas), is the largest bivalve in the world. They often weigh more than 440 pounds and stretch out over four-and-a-half feet.
Why Are Bivalves Amazing?
The reason that these animals are so amazing is for several reasons. The main reasons are that they are important to the ocean's ecosystem. They act as essential prey to a lot of local wildlife and help keep the water clean.
Just in the US alone, the economic value of mollusk hunting, specifically bivalve mollusk harvesting, is over $1 billion a year. The number of mollusks caught for food each year is double that of crustaceans. While not all mollusks caught are bivalves, a good majority of them are.
Besides all that, they are just really cool animals. Unlike hermit crabs, which have to constantly find a new home as they grow bigger, bivalves can make their own shells. They also can make their shells grow as they grow, so they only need the one shell for the entirety of their lives.
They make their shells via an organ called the mantle. This organ produces calcium carbonate. This is what makes up the external shell of the bivalve. As the invertebrate grows and gets crowded in its shell, the mantle produces more calcium that allows the shell to grow and create more space.
The photo featured at the top of this post is © Lam Van Linh/Shutterstock.com
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