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The shy nature of molluscs is hereditary

What do sandy beach molluscs from Bohuslän, fish in volcanic lakes and stick insects have in common? According to Kerstin Johannesson, they help researchers to understand how different species develop.

“Last summer we collected 2,400 molluscs from islands off the Bohuslän coast,” says Kerstin Johannesson, Professor in Marine Ecology, “and in each individual case we took precise measurements of the environment they were found in. We then mapped every possible characteristic they had, from shell thickness to the degree of shyness they displayed, and we are now analysing tens of thousands of their genes.”

The molluscs came from different beach environments and have completely different characteristics. The aim is to understand how species change as they evolve and how they adapt to different habitats. Repeating this mapping process for several different beaches makes it possible to understand which genes are involved and to what extent natural selection and circumstance determine their evolution.

“The species is dividing itself into two new ones: one that is adapted to a strong wave environment and the other to become more crab-like. And now with the help of advanced genetic analysis we can find out what distinguishes these two types and understand how and why a species barrier is being erected. Ever since Darwin presented his theory of evolution, species development has been a mystery, but today we have methods for solving the riddle. And research in species development is taking place as regards a number of other species, including stick insects and cichlids in volcanic lakes.”

In the past it was thought that a further barrier was needed in order for a species to split into two, but no such barrier exists between the different varieties of molluscs. Their adaptation to different beach habitats is sufficient for them to be so different that they almost stop mating with each other. The crab variety is large in size and has a thick shell that can fend off attacks from crabs on beaches that have dense crab populations, while the wave variety is small and has a large foot so as to avoid being washed away on beaches exposed to waves.

“In many cases species development takes hundreds of thousands of years, but these two types of molluscs have developed over a period of a matter of thousands of years since the land rise raised the islands they inhabit out of the sea. To establish whether these types of molluscs had evolved in isolation on each island, we carried out a test whose results were published last spring, and it is the first time that anyone has been able to demonstrate a case of parallel evolution, something that we suspect occurs in many different organisms.”

Now Kerstin Johannesson and her colleagues hope that their detailed genetic mapping will also provide answers as to how genes determine different characteristics. How, for example, is mollusc behaviour coded? The personality of each mollusc is established by carrying out a shyness test.

“The wave variety comes out of its shell almost as soon as it’s disappeared inside it in fright, while the crab variety can wait a long time before coming back out. The less shy behaviour is part of the mollusc’s adaptation to cling to slippery rocks, and the other is to protect itself against crabs. And these characteristics are hereditary – shy molluscs produce shy offspring.”