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First published online October 17, 2008
Journal of Experimental Biology 211, 3433-3441 (2008)
Published by The Company of Biologists 2008
doi: 10.1242/jeb.020495

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To break a coralline: mechanical constraints on the size and survival of a wave-swept seaweed

Patrick T. Martone^* and Mark W. Denny

Hopkins Marine Station of Stanford University, Pacific Grove, CA 93950, USA

View larger version (27K): [in this window] [in a new window]   Fig. 1. Bend-to-break tests. (A) Articulated fronds were held between two clamps and (B) weights were hung from the free end until a geniculum broke. (C) Genicula did not rupture abruptly; instead, genicular cells ruptured and frayed sequentially with increasing force. Strain of intact tissue approached, but did not exceed, previously calculated breaking strains.

View larger version (22K): [in this window] [in a new window]   Fig. 2. (A) Calliarthron fronds grow in densely packed clusters. Depicted are several fronds emerging from crustose bases in the field. (B) Using the average diameter of basal intergenicula (2y) and distance between fronds (D), we calculated (C) the maximum bending angle () of central fronds supported by neighbors. Note that frond spacing has been exaggerated for illustration purposes. Scale bar in A, 1 mm.

View larger version (10K): [in this window] [in a new window]   Fig. 3. Diagram of high-speed water flume. Water fell through a 10 cm diameter pipe that extended down the side of the building, creating jets of turbulent flow up to 10 m s–1. Calliarthron fronds were attached to a force transducer in the working section, and drag was measured on fronds exposed to flow.

View larger version (3K): [in this window] [in a new window]   Fig. 4. Correlation between tensile modulus (Et) and breaking strain (break) of individual genicula.

View larger version (10K): [in this window] [in a new window]   Fig. 5. Forces (±s.d.) predicted to break tenth genicula in tension and first genicula in bending for 10 experimental fronds. Without neighbors (filled circles), fronds are more likely to break in bending at first genicula. With neighbors (open circles), fronds may break in tension or in bending.

View larger version (8K): [in this window] [in a new window]   Fig. 6. Effect of frond planform area on drag force. Data are presented for all fronds at six representative velocities.

View larger version (7K): [in this window] [in a new window]   Fig. 7. Speed-specific drag (Fdrag/U2) as a function of water velocity (U). Slopes of the linear regressions are Vogel's E values. Calliarthron data are represented by circles and solid line; all other data (E values and dashed lines) were compiled from studies by Vogel, and Gaylord and colleagues (Vogel, 1994; Gaylord et al., 1994). Data suggest that the drag coefficient of Calliarthron drops faster than that of a streamlined body, but not as fast as that of a typical bladed alga. Because of limitations of re-circulating flumes used in past studies, Vogel's E has not yet been properly characterized for other intertidal algae at high water velocities (6–10 m s–1; dotted lines).

View larger version (9K): [in this window] [in a new window]   Fig. 8. Effect of frond planform area on drag coefficient (Cd). Data are presented for all fronds at six representative velocities. Note that drag coefficient varied with both water velocity and frond planform area.

View larger version (8K): [in this window] [in a new window]   Fig. 9. Drag coefficient decreased with frond Reynolds number (Ref), a function of both velocity and area. Dotted lines represent 95% CI around the fitted model.

View larger version (8K): [in this window] [in a new window]   Fig. 10. Drag force increased with frond Reynolds number. The curve was fitted to data generated in the lab (circles); dotted lines represent 95% CI. Mean, best case and worst case scenarios, based on mean Fbreak ± 1 s.d., indicate the Ref expected to break fronds in the field.

View larger version (7K): [in this window] [in a new window]   Fig. 11. Velocities predicted to break fronds of a given planform area (critical frond Reynolds number, Ref,crit). Dotted lines represent 95% CI around the model prediction, based on mean Fbreak ± s.d. Maximum water velocity measured in the field (circle) successfully predicts the mean maximum size (±s.d.; square) of Calliarthron fronds expected to survive. The size of the largest frond (triangle) suggests that water velocities at the field site may exceed those measured during this experiment (see text).

View larger version (10K): [in this window] [in a new window]   Fig. A1. Method used by bending model to calculate the proportion of cross-sectional area lost in geniculum (shown in black) during rupture. Remaining genicular tissue is shown in yellow; intergenicular tissue is shown in pink. r, genicular radii; y, intergenicular radii.

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