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November 2, 2011

Wednesday Wisdom: Predator, scavenger and a cannibal from the ice age

Saduria entomon, Bothnian Sea, Sweden © OCEANA / Carlos Minguell


The Saduria entomon is a true survivor. Take a look at this picture, and there will be no doubt that you are looking into the eyes of a predator. It eats amphipods such as monoporeia affinis mussels, freshwater lice and larvae. Even more macabre is that it’s also an omnivorous scavenger feeding on dead fish and other dead organisms it finds at the bottom of the sea. In its quest to survive it even turns into a cannibal and consumes others of its kind – so let’s be glad that the largest male animal found “only” measures 8.8 cm, which makes this species the largest crustacean in the Baltic Sea. Though it mainly lives at the sea bottom this isopod is a known to be a solid swimmer.

The Saduria entomon lives in most areas of the Baltic, apart from the Kattegat and the Belt Sea. During Oceana’s expedition in spring 2011, we found this isopod at 100 meters depth in the Gulf of Bothnia. This is pretty extraordinary, as we did not see any others organism in the area due to the low level of oxygen. But this crustacean actually prefers deeper areas as it depends on cold water. This is because it is a glacial relict.

More than 7000 years ago, the Saduria entomon walked to the area which is now the Baltic Sea across the ice from the Arctic Ocean. When the ice sheets withdrew from the area, the isopod became trapped in this brackish water and is now endemic to the Baltic Sea. Some became isolated in lakes and have survived since the ice age in places like the Lake Lagoda in Russia. It is also found in the deep waters of Vänern and Vättern in Sweden and in six other Swedish lakes where Emil in Lönneberga and other fictitious characters from the universe of Astrid Lindgren catch them in large quantities. The species is still found in the Arctic Ocean. It was later it introduced into the Black Sea.

As it is relatively stationary Saduria entomon is used as an indicator of the state of life in the Gulf of Botnia. Even though Oceana came across it in waters with relatively low oxygen, the lack of oxygen is a threat as it depends on finding other organisms in the bottom area to eat. Nutrient loading in the Baltic Sea, which causes a decrease in the level of oxygen will therefore cause a decline in the population.

HELCOM has red listed the species as vulnerable in the German part of the Baltic Sea. The critical situation ofthe Saduria entomon is a serious problem, not only for this isopod but also for the cod, since Saduria entomon is an important food source for cod and plays an important role in the entire ecosystem.