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Mussels and Clams (Bivalvia)

Bivalvia Linnaeus 1758

(Acephala Cuvier 1798, Pelecypoda Goldfuss 1820, Lamellibranchia Blainville 1824)

 

 
Class Name Number of Species Percent %
Gastropoda ca. 65,000 - 80,000 ~ 76%
Bivalvia ca. 20,000 ~ 21%
Cephalopoda ca. 900 ~ 1%
Scaphopoda ca. 900 ~ 1%
Monoplacophora ca. 25 < 1%
Polyplacophora ca. 1,000 ~ 1%
Solenogastres ca. 300 < 1%
Caudofoveata ca. 150 < 1%
Mollusca ca. 85,000 - 100,000  
 


New Window: Enlarge Diagram!
  Number of species in Mollusca, displayed by classes, including percentage. Sources: WoRMS: MolluscaBase eds. (2025): Mollusca LINNAEUS, 1758.
Contents

Introduction

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When walking along the beaches of Brittany in France, it is almost impossible not to notice the vast beds of blue mussels. Thousands of these dark bluish-black shells populate the coastal waters. At first glance, these mussel banks appear lifeless—nothing seems to move on their surface. But this impression is deceptive. A closer look reveals molluscs that have adapted in fascinating ways to a very special mode of life and feeding, and that play a crucial role within the coastal ecosystem.

At first sight, mussels appear very unlike other molluscs. Compared to a crawling snail - or even more so, to a squid darting through the water - they seem to have remained on a lower evolutionary level. Yet closer observation of a living mussel shows that this apparent immobility is in fact the result of a long evolutionary adaptation to a unique lifestyle.

Unlike all other molluscs, mussels usually are filter feeders. They draw not only oxygen but also food particles from the surrounding seawater. This method of feeding has proven so successful that mussels, over the course of evolution, have spread into an extraordinary variety of marine habitats, and even into freshwater environments.

YouTube: 5 Most Beautiful Clams In The World (ZoneA).

Summary

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Blue mussels (Mytilus edulis). Photo: Ron Offermans.		  
Mussels live exclusively in water. Over the course of their evolutionary history, however, this group has spread into freshwater multiple times, giving rise to several independently evolved freshwater lineages—one of the best known being the large river mussels (Unionacea).

A characteristic feature of mussels, visible from the outside, is their bivalved shell, which usually encloses the entire animal. The two shell valves are connected by an elastic hinge ligament, which, when relaxed, causes the shell to open. The antagonists of this mechanism are the powerful adductor muscles, with which the mussel can actively keep its shell tightly closed.

 
Experiment with blue mussels: The glass on the left does
not contain any mussels, the glass on the right shows how
mussels filter the water.
Mussels breathe exclusively with gills, whose structure can vary considerably. Based on gill structure, different subgroups of bivalves can be distinguished. In addition to respiration, the gills also serve in feeding: except for a few actively foraging species, most mussels feed by filtering food particles from the respiratory water current. Digestible material is retained, while indigestible particles are expelled and added back to the surrounding sediment. In this way, mussels play an essential role in clarifying the water. Blue mussels, for example, can filter up to 5 litres of water per hour, while oysters are capable of filtering up to 25 litres per hour. Thus, mussels occupy a key position in most aquatic ecosystems.

Most mussel species are dioecious (with separate sexes), though some groups are hermaphroditic. In addition, mussels that live in colonies (such as oysters, Ostrea) may undergo sex reversal, changing into males when too few males are present in the colony to ensure reproduction. While fertilisation and larval development in marine and many freshwater mussels take place in the open water, the large river mussels (Unionacea) carry out fertilisation and the early stages of larval development within the mantle cavity of the female. Their larvae, known as glochidia, develop through a parasitic stage by attaching themselves to a host fish. By contrast, most other mussel species develop through free-swimming planktonic larvae of the trochophore and veliger type.

Soft Body

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Mantle

 
The interior of a mussel, schematic, one shell valve removes. Colour scheme cf.
Body Plan of a Bivalve. Source: Biodidac, Modification: R. Nordsieck.
The soft body of a mussel is covered and protected on both sides by the mantle lobes. The cavity formed by these lobes is known as the mantle cavity. In most species, the mantle edges are fused together, except for two openings at the posterior end of the mussel, through which respiratory water and food enter the mantle cavity, as well as an opening for the foot. Apart from the foot, the rest of the mussel's soft body lies within the mantle cavity.


Model of a scallop (Pecten jacobaeus) from the Vienna Natural
History Museum. Photo: Robert Nordsieck.		  
The mantle edge consists of three folds with different functions:

The outer fold secretes the shell and the periostracum (the shell skin), the middle fold serves sensory functions, the inner fold regulates the water flow within the mantle cavity.

Due to the predominantly sedentary (sessile) lifestyle of most mussels, the head has regressed almost completely, except for the mouth region (for that reason, Cuvier in 1798 called the bivalves Acephala - headless molluscs).

In swimming bivalves such as scallops (Pecten) and file clams (Lima), which require more detailed information about their environment, the mantle edge is lined with simple eyes (ocelli). In giant clams (Tridacna), symbiotic algae (zooxanthellae) live within the mantle tissue. The mussel provides protection for the algae, while in return benefiting from the photosynthetic products they produce.

In burrowing or boring mussel species, the mantle openings are often extended into tube-like structures so that the animal can continue to draw in water and food while remaining buried in the substrate. These tubular extensions of the mantle are called siphons: an inhalant siphon for intake and an exhalant siphon for expulsion. Both may be fused into a retractable double tube, which in its extended state can even exceed the mussel's own length. The soft-shell clam (Mya arenaria), for example, lives buried in the sediments of the Wadden Sea and relies on its siphons to feed; if washed out of the sediment, it will die. By contrast, the blue mussel (Mytilus edulis) lives attached on the substrate and has no siphons: if it becomes covered by sediment, it too will perish.

The Gills

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A freshly opened oyster (Ostrea edulis).
The gills located in the mantle cavity of the mussel serve not only for respiration but also, in most species, for feeding. Their structure varies and allows the distinction of several subclasses:

The circulatory system of mussels, as in most molluscs, is open. The heart consists of two auricles and a single ventricle.

Nutrition and Respiration

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Respiration and nutrition in a blue mussel. Source: Aquascope.		  
The primitive bivalves of the subclass Protobranchia collect edible particles such as protozoa, eggs, larvae, and digestible detritus from the surrounding substrate with their elongated labial palps. The food is then transported along a ciliary groove to the mouth. In contrast, most of the more highly developed bivalves feed exclusively by filtering their respiratory water. Cilia in the mantle cavity create a directed water current that enters through one siphon and exits through the other. Digestible particles suspended in the water are trapped by the gills and transported as a mucus-bound package towards the mouth.


A clam digs itself into the ground using its foot. (see text).		  
Because they feed by filtration, mussels come into contact with very large volumes of water, which makes them especially vulnerable to pollutants dissolved in it. Industrial activity along coasts and in low-lying inland regions poses a particular problem for marine mussels near the shore as well as for freshwater mussels in rivers and lakes of the lowlands.

For humans, it is of particular concern that mussels tend to accumulate toxins in their tissues. The pollutants released by industry into the water ultimately end up in the blue mussels and oysters that we want to consume.

The Foot

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The mussel's foot can take on different shapes depending on lifestyle and locomotion; for example, beam-like, tongue-shaped, or worm-like. In swimming and sessile mussel species, the foot is often greatly reduced. In some species (such as the blue mussel Mytilus, the ark shell Arca, the scallop Pecten, or the pen shell Pinna), a byssus gland is located at the end of the foot. This gland secretes a proteinaceous substance that hardens in water into threads, allowing the mussel to anchor itself to the substrate. The byssal attachment can later be released when the mussel cuts off the threads (Mytilus) or sheds them entirely (pearl oyster Pinctada). Blue mussels also use their byssus threads defensively, entangling smaller snails such as dog whelks (Nassarius / Hinia).

Locomotion

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Swimming flame shell (Lima hians). Picture: Erling Svensen.
Although mussels are generally known as sedentary (sessile) animals in the adult stage, there are in fact many different forms of locomotion within this group. Many species that appear sessile as adults are still capable of moving short distances across the substrate. The foot is pushed into the sediment, anchored by hydrostatic pressure of body fluids, and then used to pull the entire animal forward. In this way, mussels can not only move along the bottom but also burrow into the sediment. Blue mussels (Mytilus) also use their byssus threads for locomotion: a thread is cast out until it attaches to the substrate, after which the mussel shortens it step by step, pulling itself slowly forward.

Some bivalve groups (such as nut clams Nucula, bittersweet clams Glycymeris, tellins Tellina, and venus clams Venus) possess a true crawling foot, similar to that of snails.

Certain species (such as scallops Pecten and file clams Lima, see photo on the right) can even swim freely in the water. By snapping their two shell valves together rapidly, water is expelled from the mantle cavity, and the mussel is propelled in the opposite direction. Along the mantle edge of such mussels there are usually numerous simple eyes (ocelli), which provide information about light conditions in the environment, as well as tentacle-like appendages with which the mussel can sense its surroundings.

Reproduction and Development

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A thick shelled river mussel's (Unio crassus)
glochidia in a minnow's gills (Phoxinus phoxi-
nus). Picture: Susanne Hochwald [2].		  
Most bivalve species are dioecious, meaning there are both male and female individuals. Fertilisation and subsequent larval development usually take place externally in the water. In giant clams (Tridacna), for example, the release of eggs and sperm can be hormonally synchronised. After developing through a trochophore or veliger larval stage, the larva undergoes metamorphosis into a juvenile mussel, which then seeks out a suitable place to settle for its adult life. In blue mussels (Mytilus), oysters (Ostrea), and other colonial species, juveniles usually remain close to the colony and attach themselves not only to the substrate but also to other mussels. This is how the extensive mussel beds of blue mussels (Mytilus) come into existence, like they are familiar from the Wadden Sea.

 
Glochidium of a river mussel (U. crassus).
Picture: Susanne Hochwald [1]
Freshwater mussel species, however, show very different methods of reproduction and development: Like freshwater and terrestrial snails, these mussel groups have adapted over the course of evolution to the fluctuating environmental conditions that distinguish freshwater from the relatively constant conditions of the sea. The members of the group Unionacea, which includes the native large freshwater mussels (painter's mussel, swan mussel, duck mussel, and freshwater pearl mussel), develops through a parasitic larval stage known as the glochidium, which must successfully attach to a passing fish of the right species in order to continue development.

Glochidia: Larvae of Freshwater Mussels.

By contrast, most small freshwater mussels (pea clams Pisidium and fingernail clams Sphaerium) are hermaphrodites and give birth to living larvae (ovoviviparity). The zebra mussel (Dreissena polymorpha), which also belongs to the Dreissenidae, develops through a planktonic larval stage similar to the veliger, just like its marine relatives.

The Shell

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The systematic name of the bivalves (Bivalvia - the two-shelled molluscs) refers to their most characteristic feature: their shell, consisting of two separate valves. Adapted to the mussel's way of life, the shell varies greatly in form and serves as the most important characteristic for identification. It can be oval, elliptical, wedge-shaped, or sheath-shaped. The two valves may be very similar, especially in upright-living mussels such as river mussels, or very different in shape, especially in sideways-living species such as oysters (Ostrea) or scallops (Pecten).

The two valves are joined posteriorly by an elastic ligament, which opens the shell when relaxed. The mussel must actively close the valves against the ligament's resistance using one or two adductor muscles, which can be clearly seen when the shell is open. The inner dorsal margin of the valves is often thickened and may bear interlocking teeth that provide lateral stability and prevent slipping. The form of this hinge apparatus varies among mussel groups and is an important distinguishing feature.

 
Hinge and ligament of a river mussel (Unio tumidus).
Photo: M. Kohl.

Shell halves of the swan mussel (Anodonta cygnea).
Photo: M. Kohl.		  
The valves of most mussels completely enclose the soft body of the animal. Additional shell material is secreted around the umbones (the shell beaks) by calcifying cells at the mantle edge, so that the shell grows continuously together with the animal.

Most of the shell is composed of layers of prismatic crystals of the mineral aragonite. This shell layer, the ostracum, may be covered externally by an organic layer known as the periostracum. In addition, many mussels produce an inner shell layer of plate-like aragonite crystals. This thin layer reflects light in many colours (iridescence). If the mussel coats a foreign body trapped between mantle and shell with this material, a pearl may form. For this reason, the inner layer is also known as mother-of-pearl (hypostracum). Pearl mussels exist both in the sea (e.g. Pinctada) and in freshwater (e.g. Margaritifera in central Europe and North America, Hyriopsis and Cristaria in East Asia). Human use of pearls in jewellery has a long history, with marine pearls being far more common than the much rarer freshwater pearls.

Where naturally formed pearls are insufficient to meet demand, pearls are now cultivated by artificially implanting foreign bodies into mussels. Pearl mussels, like blue mussels, are farmed in hydrocultures. Mussel farming, especially of oysters and blue mussels, is now practised on almost all coasts of the world.

The harvesting of pearls from the freshwater pearl mussel (Margaritifera margaritifera) has proven ecologically disastrous and, in the case of European populations, almost fatal. Today, the freshwater pearl mussel has become virtually extinct in rivers and streams, not only because of increasing water pollution but as a consequence of rampant uncontrolled pearl poaching over centuries. Moreover, its reproduction via glochidia depends exclusively on the brown trout (Salmo trutta fario), which has increasingly been displaced by the introduced rainbow trout (Oncorhynchus mykiss), an unsuitable host.

Not only because of their pearls, mussels are among the most economically important molluscs for humans. Prehistoric shell middens (Køkkenmøddinger, Danish for "kitchen waste heaps") have been found at human settlement sites and bear witness to the importance of mussels (mainly cockles and blue mussels) as a food source since the Palaeolithic.

Sources

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Literature

Links

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Latest Change: 02.10.2025 (Robert Nordsieck).