A gluttonous American pseudo-jellyfish, giant Japanese oysters, and an unidentified virus killing seals: strange intrusions are threatening Sweden’s seas and fishermen are concerned.

The biggest threat is called mnemiopsis, an animal that measures about 10 centimetres (four inches) and is not technically a jellyfish but has a gelatinous and translucent appearance. It is not harmful to humans.

…Originally from the United States, it has already shown just how much damage it can do in the Black Sea where it spread at the end of the 1980s, likely dumped there by ships emptying the water in their holds while in port.