Biomechanics: July-August 2006

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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.

Tom Moore

Now is a lovely time of year to head down to the beach. But forgo the warm sand in favor of the more interesting rocky headland down the shore. Then, as the waves pound in, consider what it must be like to live here.

At low tide, the sun beats down and heats what little water remains in scattered pools, where oxygen levels madly fluctuate and salinity increases with each evaporative minute. When the tides change, the physiological insults ease, only to be replaced by the physical battering of the waves, which try alternately to shove things higher onto the shore and to suck them into the deep. The habitat might as well be called intertidal hell.

Yet organisms from anemones to zooxanthellae are perfectly happy here. How surf-zone denizens manage to survive the wave-swept environment could fill entire books on biomechanics. But the story of one alga’s fight to hang on caught my attention.

Moving water exerts drag; no doubt you’ve felt it—the force that knocks you off your feet as you stand in the surf. It’s the same force that scours rocks on the seashore clean of encrusting critters. In this context the drag on an organism changes with three factors: the speed of the water and the shape and the size of the organism itself. Faster water exerts a lot more force; in fact, drag varies directly with the square of velocity, so, for instance, doubling the water velocity bumps up drag fourfold. Size, or more properly cross-sectional area, also matters because the bulkier an object, the more rushing water slams into it. (A kayak pinned broadside to the current against a submerged rock is much harder to move than a kayak lodged against the rock head-on.) The way shape influences drag force is captured by the drag coefficient, a term automakers invoke so often that you might think it applies only to cars.

Creatures trying to make a living in the surf zone deal with the push and pull of waves by manipulating all three of these determinants of drag in their efforts to stay in the same general spot. Many animals hide in crevices or in the lee of a rock, where flow speeds are lower. Others take an engineer’s approach to the problem: they assume the minimum size that will enclose their feeding and reproductive organs—and never budge in size or position. Thus barnacles, limpets, and chitons that fall into the fixed-shape category are stuck with the same shape all the time.

Other creatures, however, have opted for more flexibility. Two biologists, Michael L. Boller at Stanford University’s Hopkins Marine Station in Pacific Grove and Emily Carrington at the University of Washington’s Friday Harbor Labs on San Juan Island, are looking at how algae manage to stay attached to their rocky homes despite battering waves. Algae can’t take the fixed-shape path, at least not without sacrificing an awful lot of area needed for photosynthesis. They can’t trot off to the far side of the rock, either, to hide from the incoming waves. The solution, at least for some macroalgae, appears to involve tricky contortionism; they co-opt the force of the water to produce changes in their own shape that simultaneously reduce their area and their coefficient of drag.

Boller and Carrington worked with a common alga of New England 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 illustrations below]. (It is also a source of carrageenan, a thickener used in ice cream—and anything to do with ice cream is relevant to my kind of biomechanics.) They collected Irish moss samples of various size and shape, and glued their bases—appropriately called holdfasts—to a platform 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 different positions, depending on the strength of the flow. In languid washes, each stipe—an algal analog to the trunk of a tree—bent over, so the algae’s foliage brushed the bottom of the flume. The change decreased the area presented to the flow and reshaped 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 narrowing into a cone with an area less than half of its shape in still water. Furthermore, the shape changes enabled the algae to hide their canopies in their own flow shadows and thereby slightly 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 without the change in shape. Above a certain velocity, however, the algae reach a point where they cannot get any smaller. Carrington and Boller aren’t sure how much the final, squeezed shape affects photosynthesis; prolonged 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 velocity—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. Considering 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.