Unraveling evolutionary mysteries: The saga of Amphipod Crustaceans

Although we can see the outcome of evolution all around us every day, it is not a simple task to find evidence for what has influenced evolution in the past and why organisms today look the way they do.

In their recent publication researchers at the University of Helsinki have shown that an amphipod crustacean living in both lakes and springs in Finland has a unique evolutionary past that helps unravel mysteries about their morphology.  The 2-cm long shrimp-like crustacean gets its species name P. quadrispinosa from the four sharp spines on its back. However, researchers found that the spines that are well developed in lake populations are reduced or even completely absent in the amphipods living in springs.

“A spring population without spines was discovered back in the 1950’s, and since then three more springs were found where amphipods have no spines or only small ones. This was already a really exciting pattern to discover because it’s basically a natural experiment where the conditions to learn more about spine evolution are provided by nature. But we wanted to make sure that it was indeed evolution at work, and not just a non-genetic outcome of living in springs”  says John Loehr from University of Helsinki’s Lammi Biological Station.

How was the field research conducted?

Researchers set up an experiment at Lammi Biological Station to see whether having spines or no spines is passed down from parents to offspring. After letting the offspring grow in the lab for a year, the outcome was clear: spines are inherited from parents. Using DNA to trace the deeper relationships of the lake and spring populations, Risto Väinölä of the Finnish Museum of Natural History then confirmed that each of the spring populations had lost their spines independently, since the crustacean initially invaded Finland after the last ice age, around 12 000 years ago.

“That spines repeatedly became reduced in spring populations suggests that there is a common evolutionary force behind the change. Selection could favour spineless amphipods, but the effect can be indirect. There may be an advantage in having spines in the regular lake populations, but not in the small springs. If growing spines is costly to the amphipods but does not provide any more advantage, natural selection would favour a loss of the spines in such an environment”, Väinölä comments.

The exact mechanism behind the maintenance and loss of spines remains unknown, but one piece of the puzzle did emerge during fieldwork that helped understand why some springs have individuals with more developed spines.

“We found that two of the springs have fish in them, and those are the same springs where the spines are reduced but not completely lost. This was a very fortunate observation because it helps separate the effect of fish predation from other potential effects of the different lake and spring environments. The spines indeed seem to be an important defence for the amhipods to avoid becoming a snack for fish. Similar natural experiments have been found in other organisms where a species has colonized different habitat types, but I don’t know of one that also has a second important factor like fish predation that varies. It really helps round out the whole evolutionary picture. There’s still lots to learn about the amphipods, but this study lays some great groundwork for much interesting research to come” adds Loehr.

Loehr, J., Sundell, J., Immonen, M., & Väinölä, R. (2023). Patterns in antipredator armature reduction and maintenance in isolated spring populations of an amphipod crustacean. Ecology and Evolution, 13(8), e10423.

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