Giant mobile coralliths from the Florida Keys, USA

Coralliths are spherical, free-living (motile), scleractinian colonies inhabiting present day and ancient coral reefs. They form by the coral rolling on the seabed which can occur through biological and/or physical processes. While diving and snorkeling in nearshore environments in the upper Florida Keys, we observed hundreds of coralliths of varying sizes and species. This included colonies of Porites astreoides, Siderastrea radians, and Solenastrea bournoni . The largest coralliths we observed were all S. bournoni and ranged between 0.5 and 1.4 m in diameter. The exceptionally large colonies identified (>1 m) may be the largest and oldest spherical coralliths described to date. Relatively frequent movement of a corallith is key to their formation as all colony surfaces must be exposed to sunlight and currents periodically to maintain their spherical shape. Intuitively, the larger a corallith grows, the stronger the current energy must be to initiate movement. Considering the shallow depth of the corallith habitat and the high frequency of tropical storms and hurricanes impacting south Florida, we hypothesized that the forces generated by shoaling, breaking storm waves would be sufficient to cause the formation of these giant coralliths. Calculations based on hydrodynamic forces and field observations after the passage of a storm support the efficacy of storm-generated waves and surge to move and roll these giant coralliths. The tropical storm/hurricane return time where we found these large colonies in the Florida Keys is one in every seven years. This return time is apparently frequent enough to maintain their spherical morphology and assist in the formation of these giant coralliths.