BIOMECHANICS 



Keep Me 

 Hanging On 



Surviving in the intertidal zone 

 tests the rubbery limits of algae. 



By Adam Summers ~ Illustrations by 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 hap- 

 py 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 four- 

 fold. Size, or more properly cross- 

 sectional area, also matters because 

 the bulkier an object, the more rush- 

 ing 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 cap- 

 tured 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 gener- 

 al spot. Many animals hide in crevices 

 or in the lee of a rock, w T here flow 

 speeds are lower. Others take an en- 

 gineer's approach to the problem: 

 they assume the minimum size that 

 will enclose their feeding and repro- 

 ductive 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 biol- 

 ogists, Michael L. Boiler at Stanford 

 University's Hopkins Marine Station 

 in Pacific Grove and Emily Carring- 

 ton at the University of Washington's 

 Friday Harbor Labs on San Juan Is- 

 land, are looking at how algae man- 

 age 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 in- 

 coming waves. The solution, at least 

 for some macroalgae, appears to in- 

 volve tricky contortionism; they co- 

 opt the force of the water to produce 



