Amphibalanus improvisus has a smooth white or pale grey conical calcareous shell composed of six fused plates.

[3] It was one of the first recorded introductions to the Baltic Sea, having been found in Sweden and Lithuania in 1844, the Elbe estuary in 1854 and Great Britain in the 1880s.

[5] Amphibalanus improvisus is found, sometimes in vast numbers, down to a depth of about 6 metres (20 ft) on rocks, man made structures, buoys, ships' hulls, the shells of crabs and molluscs, and certain seaweeds.

As an invasive species it competes with native organisms and it is an unwanted coloniser of the shells of cultivated oysters and mussels and aquaculture cages.

It extends its six pairs of modified legs called cirri to catch plankton and other organic material floating past.