Freshwater pearl mussel

The freshwater pearl mussel (Margaritifera margaritifera) is an endangered species of freshwater mussel, an aquatic bivalve mollusc in the family Margaritiferidae. This special mussel lives in clean, fast-flowing rivers and streams, where it burrows into the sandy or gravelly bottoms.

Although called the freshwater pearl mussel, it is not the only mussel that can make pearls. Other freshwater mussels can also create pearls and mother of pearl. Today, most cultured pearls come from different mussel species in Asia and North America, but the freshwater pearl mussel was once important for people searching for natural pearls.

The inside of its shell has a thick layer called nacre, which gives the shell a beautiful shine. Scientists have learned that these mussels can live for many years, with some living up to 210–250 years, making them one of the longest-living animals in freshwater environments.

Subspecies

The freshwater pearl mussel, Margaritifera margaritifera, has several subspecies. One well-known subspecies is Margaritifera margaritifera margaritifera. Another is Margaritifera margaritifera parvula. In Ireland, there is a critically endangered subspecies called Margaritifera margaritifera durrovensis, also known as Margaritifera durrovensis. This special subspecies is protected under laws that help keep nature safe.

Main article: Habitats Directive

Description

The freshwater pearl mussel is one of the longest-living invertebrates. One specimen found in Estonia in 1993 was 134 years old.

Like all bivalve molluscs, it has a shell made of two parts that hinge together to protect its body. The shell is large, heavy, and often yellowish-brown, darkening with age. Inside, the shell has a pearl-white surface that can show beautiful iridescent colours. The mussel also has a large white foot that helps it move slowly and bury itself in the river or stream bottom.

Distribution

The freshwater pearl mussel lives in many parts of the world, especially in cold and temperate areas. You can find them in eastern Canada, New England in the United States, and across Europe. They live in rivers and other freshwater habitats from western Russia all the way to northeastern North America.

Some countries have larger populations than others. For example, Scotland has more than half of the world’s breeding populations, though they face threats from illegal harvesting. Other places like Ireland, Denmark, and parts of the Czech Republic also have smaller groups of these mussels, while in some areas like Lithuania and Poland, they are no longer found.

Habitat

The freshwater pearl mussel lives in clean, fast-flowing streams and rivers. It buries itself in fine gravel and coarse sand, usually in water that is about half a meter to two meters deep, though sometimes it can be found deeper. The mussel needs clean gravel and sand to breathe, especially when it is young, because if the stream bottom gets too muddy, it cannot get enough oxygen.

These mussels also depend on healthy populations of certain fish, like salmon and trout, because they need these fish for part of their life cycle.

Lifecycle

The freshwater pearl mussel can live up to 130 years. It starts life as a tiny larva, about 0.6 to 0.7 millimetres long, released into the water from an adult mussel along with millions of other larvae. These larvae, called glochidia, need to attach to a juvenile fish from the salmonid family, such as an Atlantic salmon or sea trout, to survive.

Once attached to a fish's gills, the glochidia grow until they drop off the next year. The young mussels then settle into clean gravel or sand, where they burrow in and grow very slowly. They reach maturity in 10 to 15 years and can produce up to 200 million larvae over their long lives.

Threats and conservation

The freshwater pearl mussel used to be very common in rivers around the world, but now its numbers are falling in every country. This decline is mainly due to human activities that harm rivers and streams, such as building dams, pollution, and introducing non-native species. For example, the zebra mussel has caused problems for freshwater pearl mussels wherever it has spread.

To help protect this endangered species, many efforts have been made. These include moving adult mussels to new areas, growing baby mussels in special facilities, and releasing young trout that can help the mussels survive. Habitat restoration projects have also been important, especially since salmon and trout are crucial for the mussel's life cycle. One notable project is the LIFE R4ever Kent, which aims to restore the population of freshwater pearl mussels in the River Kent by improving habitats and breeding more mussels for release.