Density-dependent effects of a new invasive false mussel, Mytilopsis trautwineana (Tryon 1866), on shrimp, Litopenaeus vannamei (Boone 1931), aquaculture in Colombia

Dreissenid and mytilid bivalve molluscs have become some of the most important invasive pests in the world's marine, brackish and fresh water ecosystems. We report on a newly invasive dressenid false mussel, Mytilopsis trautwineana, within a shrimp farm on the Caribbean coast of Colombia. The impacts of the mussel and the response of the shrimps can be explained through density-dependent interactions. A low density of filter feeding mussels has a positive effect on aquaculture through the removal of suspended particulate matter. However, at high densities mussels reduce shrimp survival from 76.0 ± 2.1% to 61.2 ± 2.3%. The economic impact of reduced shrimp survival is somewhat mediated through density-dependent responses in the form of compensatory growth, with shrimps attaining a higher mean weight and higher food conversion ratio. Despite these responses, it is estimated that fouling by M. trautwineana costs US$9000 ha− 1 yr−  1.The causes of reduced shrimp productivity are both direct and indirect. Mussels displace the shrimps from the food-rich pond bed. Mussel filtration increases water clarity, which in turn exposes shrimps to enhanced predation from birds. Shrimps respond to clearer water by reducing pigmentation, which reduces their commercial value. Mussels also foul aeration devices, significantly reducing their ability to maintain favourable DO2 levels. The apparent salinity tolerance of M. trautwineana suggests the species will become a more widespread biofouling nuisance. Possible control strategies include changes to management procedures, chemical treatments, microencapsulated BioBullets, polyculture with omnivorous fishes, or encouragement of indigenous mangrove-dwelling grapsid crabs.