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Home › Sea Life Facts › Marine Species › Invertebrate › Mollusks › Bivalves

Fascinating Facts about Bivalve Mollusks

[Kingdom: Animalia] [Bivalves Phylum: Mollusca] [Classification: Bivalvia]

There are at least 15,000 different types of living bivalves, one of the most widespread and adaptable groups of all aquatic mollusks.

Having already colonised almost every aquatic environment, members of the Class Bivalvia exist in most bodies of water, including tiny streams and deep trenches.

Global Distribution and Habitats of Bivalves

In general, bivalves can live almost anywhere there is water, including:

• Arctic and Antarctic Oceans (even beneath frozen ice)
• Freshwater ecosystems (e.g., mussels in the order Unionidae)
• Hadopelagic zone (the deepest region of the ocean)
• Hydrothermal vents (e.g., in the Pacific Ocean)
• Tropical Indo-Pacific regions (flat coral reefs and mangrove forests)

In fact, the Class Bivalvia is a group of two-shelled mollusks that are present in marine, brackish, and freshwater environments.

The vast majority are marine bivalve species that inhabit coastal ecosystems, where there's an abundance of sand, mud, gravel, or rocks.

A good example is the giant clam (Tridacna gigas) that lives in the Indian Ocean and the South Pacific. It is widely accepted as being the largest living of all bivalve mollusc species.

But wait - there's more:

Some of the common bivalves that live in brack water are clams, cockles, mussels, and oysters. This high bivalve diversity means many can tolerate the harsh conditions found in some tidal flats and estuaries.

The freshwater bivalve families (such as freshwater mussels) are found in rivers, streams, and lakes. Many species are essential for helping sustain healthy freshwater ecosystems because they filter water and they host fish larvae (Unionidae for example).

Interesting Fact: The primary reason bivalves exist in water bodies that range from tiny streams to the deepest trenches is because they can adapt their shell structure, feeding strategies, and behavioural traits (e.g., attached vs. burrowing) to their natural habitats.

Bivalve Characteristics and Behaviour

The main lifestyle niches vary between the shallow burrowers, such as clams that live buried in the sand or mud, and the attached forms, such as mussels, jewel box clams, and thorny oysters (Spondylus).

Most of the free-living bivalve mollusks live on the ocean floor, such as scallops. They can also 'swim' by clapping their shells.

There are also a few symbiotic species, like the giant clam species that host photosynthetic algae. Whereas, some of the deepest bivalves host chemosynthetic bacteria at abyssal depths around 10,000 metres deep.

Key features of bivalve mollusks:

• Bivalves are creatures with two (2) shells (valves): The main body and internal organs of a bivalve are enclosed inside a hinged, two-part shell made of calcium carbonate. Even though the valves can open, they are kept together by the hinge ligament and strong adductor muscles.
• Unlike most aquatic mollusks, bivalves lack any form of 'distinct' head and they do not use a toothed tongue (radula structure) to eat.
• The body shape of the Class Bivalvia is mostly lateral and compressed. The flattened soft body fits snugly inside the tough shell.
• Some bivalve species have a mantle that also can adapt into siphons (incurrent and excurrent) that help with feeding and respiration.
• Besides gas exchange, the gills are also used for food collection.
• The burrowing species have a muscular foot that can dig into, and excavate, different kinds of soft sediment.
• Even though most species have a simple nervous system, some have eyes located on the edge of the mantle that are light-sensitive (e.g., scallops).

Pro Tip: The main section contains a list of mollusks (the largest marine phylum), with fun and interesting facts about their behavioural patterns in the ocean.

What Do Bivalves Eat?

By and large, bivalve mollusks are filter feeders that use ctenidia (gill structures) to filter plankton and other types of tiny organic particles from the water, including:

• Bacteria (microorganisms in mucus that covers the gills)
• Detritus (decaying plant and animal matter)
• Microscopic algae (phytoplankton)
• Zooplankton (microscopic animals)

In suspension feeders, water flows in through the 'incurrent siphon' and then it gets drawn over the gills. The food particles get trapped in mucus and the ciliated filaments (tiny hair-like extensions) move any food-laden mucus to the mouth and the stomach for digestion. Thus, clean water will exit through the 'excurrent siphon'.

Fun Facts: Even though scallops are filter feeders, they're capable of swimming away from perceived threats. One of the saltwater clams (shipworms) can bore holes into wood (and destroy it) and digest cellulose - with some help from intracellular symbiotic bacteria.

Different Types of Bivalve Mollusks

How Do Bivalve Mollusks Reproduce?

The reproductive strategies of bivalves vary according to the species. Even so, most of them share some general characteristics and breeding factors.

For example, the vast majority have separate sexes (male and female), even though some are hermaphroditic - such as oysters and scallops.

Here's the thing:

Fertilisation is usually an external process, meaning the males and females release their eggs and sperm into the water column at the same time.

However, some of the freshwater bivalves use their siphons to inhale sperm and achieve internal fertilisation. Thus, females brood larvae either inside their gills or their mantle cavity.

Life Cycle of Bivalves

As a rule of thumb, all marine invertebrates use environmental cues to trigger the release of gametes. In general, this will be lunar cycles or a rise in water temperature.

Fertilised eggs first develop into free-swimming trochophore larvae, followed by veliger larvae (using small ciliated lobes to swim and feed).

Bivalve larvae eventually settle in a suitable location on the seafloor. Then they metamorphose into juveniles that have their distinctive bivalve shells.

Interesting Fact: Unionidae (freshwater mussels) have an unusual parasitic larval stage known as glochidia. This is a temporary phase when they need to attach themselves to the fins or gills of fish before they finally transform into juveniles.

Bivalves Predators and Threats

It's fair to say that bivalves face multiple predators and threats in the wild. Some of the creatures that naturally prey on most of the species (especially clams and oysters) include:

• Birds (e.g., ducks, oystercatchers, and seagulls)
• Marine mammals (especially sea otters and walruses)
• Fish (e.g., drum, flounder, rays, skates)
• Marine invertebrates (crabs, sea stars, octopuses, and sea snails)
• Humans harvest bivalves for food (especially clams, mussels, oysters, and scallops)

Some of the environmental threats that are also threatening the long term survival of some species in vulnerable locations, include:

• Destruction of their natural habitat (mostly due to dredging, coastal development, and trawling)
• Sea pollution (chemicals, heavy metals, and sewage)
• Ocean acidification (climate change and warming seas weaken bivalve shells due to a reduction in the availability of calcium carbonate)
• Parasites and disease (particularly in farmed mussels and oysters)
• Invasive species (e.g., the accidental introduction of zebra mussels (Dreissena polymorpha) from the lakes of southern Russia are threatening native freshwater bivalves)

Important: In 2024, the IUCN Red List of Threatened Species assessed the conservation status of many bivalve mollusks as being of "Least Concern" (LC). However, they list the giant clam (Tridacna gigas) as being "Critically Endangered" (CR).

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