Mytilopsis sallei is a widely distributed species living on hard substrata in the Ciénaga Grande de Santa Marta, where it is particularly abundant on mangrove oyster (Crassostrea rhizophorae) reefs reaching densities of 5000 individuals/m2 . Even though it has wide salinity and temperature tolerances, it prefers those places such as mouths of rivers, where fresh-water supply is constant. It can also withstand turbid waters with a high sediment load. There is no competition between M.sallei and the oyster C. rhizophorae, even in heavily-exploited reefs, because of the environmental preferences of each species and the high productivity of Ciénaga's waters assuring a constant food offer for filtering species. M. sallei has separate sexes; males dominate the population. Ripe individuals are found all year-long, but two periods per year of increased reproductive activity and settlement occur, associated to drastic salinity changes: a major one from October to November {rainy season) and a second of lesser importance from March to April (dry season). Juveniles grow steadily; as they attain greater size growth is minimal. Growth and development of these populations seem to be controlled by the influx of fresh water, the high predation rates and relatively low availability of hard substrata.