Scientists from Russia, Austria, and the USA have discovered virus-like particles in the bacterial symbionts of Bryozoa—a phylum of colonial aquatic invertebrates—filter-feeders dominating in many bottom ecosystems. The research project was planned and supervised by scientists from St Petersburg University. Some of the virus-like particles resemble red blood cells, while others have a sea-urchin-like appearance. Although viruses have never been reported inside symbiotic bacteria in bryozoans, scientists suggest that this 'matryoshka doll' may have a prominent effect on the bacterial hosts.