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Introduction to bivalves

Bivalves can be grouped into a number of functional groups i.e. oysters, boring bivalves, mussels and false mussels. To find more information on the specific groups please follow the links below

The Bivalvia is a class of Mollusca, like Gastropoda (snails and slugs), Cephalopoda (squid, cuttlefish and octopus), Polyplacophora (chitons) and Scaphopoda (tusk shells). The bivalve shell is composed of two valves ("bi-valves"). The valves are joined by a hinge composed of tiny "teeth" and usually also an elastic ligament. The number, size and shape of the teeth as well as the position of the ligament are important characters for the identification of bivalves. In addition bivalves have one or two strong adductor muscles, which are responsible for the closing of the valves. They attach to the inside of both valves and leave more or less distinct "scars" that are also used for identification. The shell is composed of calcium carbonate embedded in an organic matrix. The shell is secreted from the mantle, a thin tissue fold covering the body of the bivalve. Externally the shell usually has a thin organic layer, the periostracum, which is often pigmented. The inner surface is nacreous ("mother-of-pearl"), and scars from attachment of muscles and mantle edge are visible.

Mya arenaria Petricola pholadiformis

How can you tell front and back from up and down and left and right? Hold the bivalve shell so that the hinge faces up; this is the dorsal surface. Then look at the inside of a valve. Does it have one or two adductor muscle scars? Most bivalves have two, and furthermore there will be a line running parallel to the edge of the valve, indicating where the mantle edge has been attached. At one end this line curves inward to form a shallow or deep concavity, the pallial sinus; this is where the siphons can be withdrawn. The siphons are located in the posterior end of the bivalve, so if you hold the valve with the hinge upwards and the inside facing towards you, you have a right valve if the pallial sinus is to the right and a left valve if the sinus is to the left. In some bivalves one adductor muscle, usually the posterior one, is much larger than the other one, usually the anterior one; this is seen in blue mussels. Other bivalves, such as oysters and scallops, only have one adductor muscle. These species do not have a pallial sinus because they do not have siphons.

Inside of Mya arenia shell showing adductor muscle scars and pallial sinus. Photo by Kathe Rose Jensen.

Some bivalves attach to hard substrates. Oysters "cement" one valve to the substrate. Blue mussels attach by byssus threads secreted by a gland at the base of the foot. Scallops lie on the surface of the substrate and can swim by rapidly opening and closing the valves, expelling water from the mantle cavity and moving in the opposite direction. Most bivalves burrow into soft sediment with only the tips of the siphons visible at the surface. The size of the pallial sinus indicates the length of the siphons and therefore the depth at which the bivalve is buried. A few bivalves are capable of boring into hard substrates such as lime-stone, clay or wood.

Several species of bivalves, which have been introduced by human activities, occur in Nordic waters. Some are easy to identify because there are no similar native species. Others are more difficult because they are very similar to native species.

Many bivalve species, in particular oysters, have been cultured for many years. Aquaculture is one of the main reasons why there are so many introduced species. Either a few specimens escape from the culture plots (during transfer of spat or during harvesting) and establish wild populations, or planktonic larvae from cultured specimens disperse to neighbouring areas. Transport of live oysters across borders and continents can also move parasites and epifauna and –flora. Also, transport of "seed" oysters or mussels, sometimes called "spat", can be responsible for introduction of associated fauna and flora.

Almost all bivalves are suspension feeders. They pump water through the inhalent siphon, across the large, complexly folded gills, where food particles, mostly phytoplankton suspended in the water column, is filtered out of the water, which is then pumped out through the exhalent siphon. Bivalves can sort the filtered particles by size and/or weight so that inorganic sand is not ingested. These rejected particles are covered in mucus and expelled as so-called "pseudo-faeces". Bivalves are able to pump large volumes of water and hence to clear the water column of organic particles. On the other hand, the accumulation of faecal particles and pseudo-faeces increases the particulate organic matter in the surface sediment, which may increase oxygen consumption. Clearance and sedimentation are some of the ecosystem impacts of introduced bivalves.