The shells' secret can benefit the cod

27 June, 2023

On the rocks in Bohuslän live small, discreet shells with special properties. Snail-collecting scientists have now received clues about what is crucial for species’ survival.

Tjärnö in northern Bohuslän is one of Sweden’s most species-rich marine areas. Here, researcher Kerstin Johannesson has been collecting shells for over 40 years. Photo: Björn Larsson Rosvall/TT

The crab shell is small, has a hard shell that is impossible to crush between the thumb and forefinger. The wave shell is tiny and easily crumbles into crumbs at the slightest pressure. The hard one is adapted to cope with threats from crabs, the other remains on the edge of the cliff despite strong waves.

-At first it was thought that they were two different species because they are so different. But it turned out that they can mate with each other and have functional offspring, which is usually the definition of being the same, says Kerstin Johannesson.

She has been collecting shells on the rocks at Tjärnö in the Bohuslän coast for over 40 years. Kerstin Johannesson is professor of marine ecology and director of the Tjärnö marine laboratory, which is part of the University of Gothenburg. With a few meters to docks and rocks, the laboratory attracts researchers from all over the world who want to study the unique marine life on the West Coast.

The crab snails on the left and the small wave snails on the right belong to the same species but have developed large individual differences to cope in different environments along the coast. Photo: Björn Larsson Rosvall/TT

Cherish variety

Sea cucumbers, barnacles, lobsters and mussels are among those studied. And shells. With the advanced technology available today to map genetics and different organisms, researchers have greatly increased their knowledge.

-The knowledge we get from the snails and their genetics provides knowledge about how other organisms work, says Kerstin Johannesson.

When groups within the same species develop certain characteristics that favor them in a certain environment, genetically distinct populations are formed. These have been shown to have a decisive importance for the ecosystems and to be at least as important as the species.

In order to preserve biological diversity, it is therefore fundamental to protect the genetic variation within the species. The greater the variation, the greater the chance that there are variants that are adapted to cope with changes in the environment such as climate change.

-Unfortunately, the administration and authorities have not kept up with this, they have focused mainly on species. But that is starting to change, says Kerstin Johannesson.

The tiny barnacles can stay on the rock walls even though the waves are high. Photo: Björn Larsson Rosvall/TT

And the more we know about the different variations that exist within one and the same species, the better conditions we have to protect it. A couple of years ago, Kerstin Johannesson, together with colleagues at Stockholm University, was commissioned by the Norwegian Sea and Water Authority to start an environmental monitoring of the genetic diversity within a number of aquatic species – eelgrass, bluefish, cod, herring, salmon and trout.

The proximity to the sea means that the laboratory on Tjärnö is suitable for studying different species. Approximately 50 researchers are active at the station year-round on topics such as evolution and speciation and restoration of cold-water coral reefs. Photo: Björn Larsson Rosvall/TT

-Sweden is a relatively species-poor country, and then the genetic variation within a species becomes even more important. Knowledge of the variations is, for example, important for fishing to know if you are about to fish out a sensitive population, says Anna Hasslow, investigator in the department for environmental analysis at the Norwegian Sea and Water Authority.

More knowledge is needed

When the government’s funding for environmental monitoring was drastically reduced at the beginning of the year in the budget, the Marine and Water Authority’s project of monitoring genetic inland variation was one of those that was hit hard.

Now they are working on analyzing already existing data, building up databases and developing guidelines for how the information should be stored.

-More than ever, we need better knowledge about the genetic variation within species that can help slow down, for example, negative effects of ongoing climate change, says Anna Hasslow.

On the walk from the research station to the rocks, Kerstin cheers on a young woman with a bucket in her hand. A PhD student from Germany who will find out why sea snails have both short and long sperm.

Along the cliffs on Tjärnö, nature’s adaptability has shown itself on a very small surface. The crabs and barnacles live only tens of meters apart but would not survive for long if you moved them to the other environment. But that’s not enough. In between, hybrids have formed, a mixture that does not cope with either environment very well. And what happens among the snails also happens in other species tens of kilometers away.

The crab shells have a rock-hard shell that resists the crabs’ claws. Photo: Björn Larsson Rosvall/TT

An example is the herring. Previously, it was not believed that there was any major genetic variation in the herring caught in Swedish waters. They are still managed as if they were only three different stocks – the Baltic Sea, the West Coast and the Bothnian Sea.

“Important to establish quotas”

But thanks to genetic analyses, the researchers now know that there are big differences, and it is about at least 20 different populations where the herring has adapted to living in different water environments. The greater genetic variation there is within a species or within a population of a species, the greater the resilience of the species to, for example, climate change and disease.

“It is more important that we preserve the stocks of our common species such as cod, than the rare ones,” says Kerstin Johannesson who is professor of marine ecology and researcher at Tjärnö marine laboratory. Photo: Björn Larsson Rosvall/TT

Recently, a study came out from the University of Gothenburg which shows that a high proportion of the cod fry in the fjords and near the coast in Bohuslän are coastal cod. In the past, it was believed that the coastal cod in particular was more or less fished out. But this does not mean that cod stocks are on the way to recovery. On the other hand, it highlights the importance of taking into account that there are different types of cod that differ genetically and geographically if one is to try to rebuild the stock on the west coast.

Kerstin Johannesson believes that it is important that those who establish fishing quotas and other types of rules take into account that within most species there are local stocks. And that we don’t just red list and protect rare species.

It is more important that we preserve the stocks of our common species – such as cod. Which is both an important food and has great importance for other parts of the ecosystem. The best way to save the rare species is to save the biodiversity within the common ones.

Within a species, populations can develop traits that allow them to do well in a particular environment.

Traits are inherited and passed on when individuals who have a favorable trait survive and reproduce. The beach conch is an example with the crab conch and wave conch ecotypes.

The beach snails have hard shells that resist crab claws and hide in their shells when approached. The conch shells are small and fragile and can remain on a rock despite strong waves. They often come out of their shell and don’t want to let go and retreat because then they lose their footing.

Source: Kerstin Johannesson, Gothenburg University

Text: Petra Hedbom/TT
Photo: Björn Larsson Rosvall

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