The worm forms large mats that smother other sea life and it competes for food with native species. The dinoflagellates can produce toxins that can cause paralysis and sometimes death in humans who eat affected shellfish. Because of these adverse affects, shellfish harvesting has been closed down several times in areas worldwide. The economic impacts of harmful algal blooms can be significant. While introductions to the east coast of the US are likely the result of ballast water or from crabs clinging to heavily fouled ship hulls, introductions to the west coast of the US are likely the result of ballast water.
Green crabs eat molluscs, crustaceans, polychaetes and green algae.
European, South African and California studies indicate the green crab has the potential to significantly alter the distribution, density and abundance of prey species, and thus profoundly alter the ecology where it has been introduced. A partial list of these and other invasive species introduced via the shipping industry are presented in the attached table , with an emphasis on those species introduced on fouled hulls. Us - What's New Site constructed by Washington Web Site Services, Inc.