Irish moss responds to the speed of flowing water by changing its 

 configuration, as seen from the side (top row) and from above (second row). In 

 calm water it stands upright (left). Mild currents tug the alga into a cone- 

 shaped position, which still allows for photosynthesis but minimizes drag 

 (middle). In extreme conditions, the alga bends down and compacts even 

 further into a streamlined form (right) that may prevent it from becoming 

 detached or torn apart. 



changes in their own shape that si- 

 multaneously reduce their area and 

 their coefficient of drag. 



Boiler and Carrington worked 

 with a common alga of New Eng- 

 land and European surf zones, Irish 

 moss (Chondrus crispus). Though 

 called a red alga, Irish moss can range 

 from dark purple to yellowish green, 

 and it is shaped like a miniature tree 

 about eight inches high [see illustra- 

 tions above]. (It is also a source ot 

 carrageenan, a thickener used in ice 

 cream — and anything to do with ice 

 cream is relevant to my kind of bio- 

 mechanics.) They collected Irish 

 moss samples of various size and 

 shape, and glued their bases — appro- 

 priately called holdfasts — to a plat- 

 form that could measure drag force. 

 Then they submerged the platform 

 in a flume, the aquatic equivalent of 

 a treadmill, and measured the drag 

 force as they changed the flow speed 

 from a gentle lapping to a punishing 

 postgale surge. As the flow speed 

 increased, the algae morphed into 

 lower-drag shapes. 



In still water the algae stood tall 

 and bushy with lots of area for the 

 sun to stimulate the chloroplasts. In 

 moving water the algae took two dif- 

 ferent positions, depending on the 

 strength of the flow. In languid wash- 

 es, each stipe — an algal analog to the 

 trunk ot a tree — bent over, so the al- 

 gae's foliage brushed the bottom of 

 the flume. The change decreased the 

 area presented to the flow and re- 

 shaped the algae from upright tree to 

 pointed cone. At that stage, because 

 drag forces were relatively low and 

 the algae's canopies were still largely 

 exposed, it's likely a good deal of 

 photosynthesis could still take place. 



At faster flows, the algae morphed 

 even more. As the flow speed rose, 

 the canopies of the Irish moss became 

 increasingly compact — each narrow- 

 ing into a cone with an area less than 

 half of its shape in still water. Further- 

 more, the shape changes enabled the 

 algae to hide their canopies in their 

 own flow shadows and thereby slight- 

 ly lower the drag coefficient. No 

 question, the drag force on the algae 



increased with the speed of the flow, 

 but not as fast as it would have with- 

 out the change in shape. Above a cer- 

 tain velocity, however, the algae reach 

 a point where they cannot get any 

 smaller. Carrington and Boiler aren't 

 sure how much the final, squeezed 

 shape affects photosynthesis; pro- 

 longed exposure to fast water can't be 

 good, but the organism can certainly 

 weather the occasional storm. 



No amount of reconfiguration 

 on the part of the Irish moss 

 can keep up with a force change that 

 depends on the square of water veloc- 

 ity — the drag continues to increase 

 with water speed until the current 

 finally washes the algae away. But by 

 allowing the stipe and canopy to go 

 with the flow, the holdfast is usually 

 saved from being ripped from the 

 rock. It brings to mind blustery days 

 I've spent at the rocky shore. Consid- 

 ering my size, my area, and my high 

 drag coefficient I would have done 

 better positioning my posterior to the 

 wind, and taken shelter in my own 

 not inconsiderable bulk. 



Adam Si m.murs (asummers@uci.edu) is an 

 assistant professor ofbioengineering ami of ecol- 

 ogy and evolutionary biology ai the I University 

 of California, Irvine. 



July/August 2006 NATURAL HISTORY 



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