The marine grass Thalassia testudinum Konig is distributed throughout the West Indian region and the island of Bermuda. Its distribution is controlled by temperature, salinity, turbulence, and depth. It supplies a substrate for many organisms including benthonic Foraminifera. Sixty-six benthonic foraminiferal species were found living on Thalassia in a relatively small area in the Florida Keys. Of these only 18 species were abundant and these same species were noted living on the marine grass wherever it was examined throughout the area of its distribution. The distribution and abundance of these Foraminifera are controlled by competition with other organisms living in the same environment as well as by interspecific competition. Tropical and subtropical shallow-water ben honic foraminiferal faunas are composed essentially of the same species throughout the West Indian region. The Thalassia blades provide a means of dispersal for the benthonic species. When the tops of the blades die, or when complete plants are broken off by storms or strong wave action, they float and can be transported great distances by currents. The organisms living on the grass blades are carried to different areas where they can survive and reproduce if environmental conditions are favorable. Even sediment-preferring species can be transported by this means, because juvenile specimens, and even some adult specimens, are usually of such low specific gravity that they can be thrown into suspension by storm waves and may settle onto blades of Thalassia. In this manner a fauna of organ sms as a minute as the Foraminifera, which otherwise might be restricted to microenvironments, can become cosmopolitan.

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