A man sitting on front of a microscope.
It was in a sample from the annual environmental monitoring that Arne Nygren found the mussel that was thought to be extinct in our waters.
Photo: Susanne Liljenström

Extinct mussel found after 30 years


A mussel that is believed to be extinct in Swedish waters has been found in the Singlefjord north of Strömstad. The discovery was made in a sample from the annual environmental monitoring of soft bottoms along Sweden's coasts.
"I immediately thought it was something out of the ordinary. And when I saw the ridges, I was absolutely sure, even though this was a very small specimen", says Arne Nygren about his discovery.

Arne Nygren is a researcher at the Department of Marine Sciences and works with environmental monitoring of Swedish coastal waters. Polychaete worms, brittle stars, small crustaceans, and other organisms retrieved from the seabed are identified, measured, and weighed In a lab upstairs in Tjärnö Marine Laboratory.

Most findings in the samples are familiar, but the other week Arne Nygren was startled. He had found a mussel in the genus Cardiomya, more specifically a cross-ribbed cardiomya, Cardiomya costellata.

A whitish small mussel.
The Cross-ribbed cardiomya has a distinctive shape and characteristic ridges. The mussel in the photo is 2 millimetres long and 1 millimetre high.
Photo: Fredrik Pleijel

Used to live in the area

"The mussel has been considered extinct in Swedish waters. Previously, it has been found in suitable locations in the Koster area, but despite a number of inventories in the area during the 2000s, it has not been possible to find it until now, more than 30 years since it was last seen," says Arne Nygren.

The small Cardiomya can grow to about a centimetre in size and have an appearance that is usually compared to a gravy snipe. It is found on current-washed soft bottoms throughout the North Atlantic, where it feeds on small crustaceans and other things that live among sand and gravel grains.

"I found the mussel in a bottom sample from a depth of 56 metres in the Singlefjord, not far from Tjärnö. This is the first time we have taken samples in the area. Here is exactly the habitat that the mussel wants.

Prefers unusual bottom environment

One explanation why the Cardiomya has not been seen for a long time is that there is a lack of suitable habitats. Areas with bottom currents that keep sand and gravel free of sediment are uncommon on the Swedish west coast. Eutrophication and trawling also lead to an increasing number of seabeds being covered with dead phytoplankton and other particles that fall down. Many benthic organisms cannot tolerate being embedded in this way.

The cross-ribbed Cardiomya was found in a bottom sample taken in May last year, during the annual environmental sampling along the Swedish west coast.

"It's really a fun little mussel to find after so many years! Without our monitoring of the seabed, it is very possible that the mussel would not have been found and that we still believed that it had disappeared from our coast," says Arne Nygren.

Text: Susanne Liljenström

Environmental monitoring of coastal waters

In Swedish environmental monitoring, regular surveys of the environment are carried out, for example of benthic animals in our coastal waters. The aim is to map the state of the environment and detect changes, but also to follow up on the effects of various measures.

There are about 300 sampling sites along the west coast of Sweden, half of which are visited and sampled every two years. The monitoring is funded by the Swedish Agency for Marine and Water Management, the county administrative boards, Kosterhavet National Park and the Bohuskustens vattenvårdsförbund.