Biomechanical properties of Udotea (Halimedales, Chlorophyta) in a Mexican reef lagoon

Udotea flabellum (Ellis et Solander) Lamouroux (Halimedales, Chlorophyta) is a coenocytic alga, common in many tropical marine waters, growing anchored in sand or attached to dead coral. To date, there have been no biomechanical studies of tropical sand-dwelling algae. We report the results of a biomechanical investigation on the forces necessary to detach U. flabellum, whether these forces exceed those required to break this seaweed, and what water velocities are necessary to either break or detach sand-dwelling individuals. When pulled in situ, 94% of thalli of Udotea flabellum from the lagoon came out of the sand as entire plants; this required a force of 8.6 +/- 0.81 Newtons (N) (mean +/- SE). In the laboratory, 19.2 +/- 1.10 N are required to break these individuals. We calculated the coefficient of drag (= C-d) as 0.2 at water velocities of 1.5 m s(-1), quickly dropping to 0.02 at 3 m s(-1). Calculations show that for a blade of average surface area (0.00108 m(2)) a C-d of 0.20 requires a water velocity of 9-10 m s(-1) to remove U.flabellum from the substratum; with a C-d of 0.02, water velocities in excess of 15 m s(-1) are required. Thus, assuming drag force is the primary hydrodynamic force experienced by U. flabellum, changes in C-d ensure that water-generated forces sufficient to remove individuals of Udotea from the substratum are rarely encountered. However, such forces might be present during tropical storms and hurricanes.