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

Udotea flabellum (Ellis et Solander) Lamouroux (Halimedales, Chlorophyta) i s a coenocytic alga, common in many tropical marine waters, growing anchore d in sand or attached to dead coral. To date, there have been no biomechani cal studies of tropical sand-dwelling algae. We report the results of a bio mechanical investigation on the forces necessary to detach U. flabellum, wh ether these forces exceed those required to break this seaweed, and what wa ter velocities are necessary to either break or detach sand-dwelling indivi duals. When pulled in situ, 94% of thalli of Udotea flabellum from the lago on 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 requ ired 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 (0.00 108 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 e xcess 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 du ring tropical storms and hurricanes.