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.