Why are cephalopods absent from the Baltic Sea?

Cephalopods are exclusively marine animals, and their presence in various environments is largely dictated by water salinity. They are absent from the Baltic Sea primarily because the water there is too fresh to support them.1

The Baltic Sea is nearly landlocked and receives significant freshwater input from rivers and snowmelt, resulting in remarkably low salinity levels compared to the open ocean. While most cephalopods require higher salt concentrations, there are rare exceptions; for instance, the squid Lolliguncula brevis has been observed in coastal bays where salinity is as low as 8.5 parts per thousand. However, the general environmental conditions of the Baltic Sea remain unsuitable for the vast majority of cephalopod species.21