The threatened freshwater pearl mussel is dependent on trout or salmon to become an adult. Which of the two species it chooses as its host is not random, however.
Freshwater pearl mussel. Photo: Bjørn Mejdell Larsen / NINA.
As a larvae, the freshwater pearl mussel spends its time as a parasite on the gills of salmon and trout. A friendly parasite, so to speak, as the hitchhiker does no harm to the fish.
Using the shell to hang on to the gills of the fish, the mussel larvae develops into a tiny mussel before it lets go and falls down into the sand. Photo: Bjørn Mejdell Larsen / NINA.
Scientists from The Norwegian Institute for Nature Research (NINA) recently discovered that in some rivers, there were only mussel larvae on trout, but only on salmon in other rivers, even if both species were present.
This discovery made the scientist design an experiment, putting groups of mussel larvae from different rivers in water tanks with both salmon and trout. The mussel larvae from some rivers only attached to the gills of salmon, others solely on the gills of trout.
- Finding a black and white situation like this convinced me that we were dealing with two different types of mussels, says Sten Karlsson, researcher at NINA.
The DNA-analysis confirmed their suspicion, and the researchers concluded that we have two genetically different groups of freshwater pearl mussels in Norway; trout mussels and salmon mussels.
After studying the mussels in the river, the researchers started to suspect that they were dealing with two different groups of freshwater pearl mussels. The DNA-analysis confirmed their suspicion. Photo: Jon M. Bjerland.
Two species?
The researchers do not yet know whether the two groups of genetically distinct freshwater pearl mussels are really two different species or not, but they seem to have a somewhat different life cycle and growth rate.
- It remains uncertain whether the trout mussel and the salmon mussel can produce viable offspring together. We need to perform more detailed DNA-analysis to answer that question, Bjørn Mejdell Larsen, researcher in NINA, says.
Salmon mussel (left) and trout mussel (right) from a river in Nord-Trøndelag. They look quite similar, but while one group depends on salmon to become an adult, the other group depends on trout. Photo: Bjørn Mejdell Larsen / NINA.
Host species more important than birthplace
Which fish species the salmon trout chooses as its host species, explains more of the genetic variation than its birthplace.
- The genetic difference between a salmon mussel and a trout mussel in the same river is bigger than the difference between a salmon mussel in Southern Norway and a salmon mussel in Northern Norway, Larsen explains.
The DNA-analyses also revealed that there is less genetic variation within a population of trout mussels than within a population of salmon mussels.
In Norway, we have large numbers of both trout mussels and salmon mussels. The salmon mussel populations seems to be doing best, however.
- This might be because the genetic variation in the salmon mussel populations is relatively large, which makes adaptation to environmental changes possible, Larsen explains.
There are also genetic differences between different populations using the same host species.
- Two trout mussels in neighboring rivers in the inland can be very different. This indicates that the mussels are divided into many separate populations, and must be managed independently, Larsen explains.
Salmon louse and the freshwater pearl mussel
Everything that affects the host fish, will also affect the freshwater pearl mussel. Be it salmon louse, Gyrodactylus salaris, hydropower development or acid rain.
For example, the Steinkjer watercourse has recovered after a long battle against Gyrodactylus. Several cohorts of freshwater pearl mussels are missing, however, because of the lack of juvenile salmon in the river.
- As the salmon population recovers, the freshwater pearl mussel will increase as well, Larsen says.
The new knowledge on the genetics of freshwater pearl mussel in Norway will be an important piece in the puzzle for their future management and conservation.
- The key is to manage the watercourses in a way that conserves the right mussel with the right species of fish. We need a common management for fish and mussels, Larsen concludes.
Contact:
Sten Karlsson
Bjørn Mejdell Larsen
Publications:
Karlsson, S., Larsen, B.M. & Hindar, K. 2014. Host-dependent genetic variation in freshwater pearl mussel (Margaritifera margaritifera L.). Hydrobiologia 735: 179-190.
Karsson, S. & Larsen, B.M. 2013. Genetiske analyser av elvemusling Margaritifera margaritifera (L.) - et nødvendig verktøy for riktig forvaltning av arten. NINA Rapport 926. 44s.
Karlsson, S., Larsen, B.M., Eriksen, L. og Hagen, M. 2013. Four methods of nondestructive DNA sampling from freshwater pearl mussels Margaritifera. margaritifera L. (Bivalvia:Unionoida). Freshwater science 32(2): 525-530.