"It has lived in our region for almost 75 years”: Kant University biologist on crabs of the Baltic coast

The Harris mud crab (Rhithropanopeus harrisii) is a small species, with a carapace diameter not exceeding 3 centimeters. Originally, it inhabited the Atlantic coastal waters of North America, but since the late 19th century it has spread to other continents and seas. The species became widely distributed as adult crabs attached to ship hulls (biofouling) and as larval stages—zoea and megalopa—carried in ballast water. It was first recorded in the Baltic Sea in 1936, and has been observed in our region (the southeastern part of the Baltic) since the 1950s.

This crab prefers sandy or sandy–muddy substrates. During periods of mass development, the species can cause problems for fisheries (by damaging nets) and undermine sandy dikes and coastal structures while burrowing. Moreover, it is a carrier of baculoviruses, which can infect shrimp and other crab species.

On the Kaliningrad coast, the Harris mud crab is found regularly but in small numbers. Its larval stages (zoea and megalopa) are also detected in plankton samples, indicating that the species reproduces in local waters. Some benthic fish species feed on these crabs. Considering that R. harrisii has inhabited our region for nearly 75 years, it has not had a significant negative impact on the coastal marine ecosystem. However, given the potential risks associated with large population outbreaks, further studies on its occurrence and ecological effects—particularly in bays and nearshore waters—are needed,