Life in the intertidal zone is pretty tough. Taking a constant battering from the waves, most creatures either hunker down, or move to a more sheltered spot. But relocation isn't an option for seaweed. They simply have to make the most of the situation where their spores took hold. Mark Denny and Patrick Martone from Stanford University's Hopkins Marine Station are fascinated by the ways that seaweeds withstand the waves. Martone explains that fleshy seaweeds `go with the flow', but calcified seaweeds, such as Calliarthron are relatively inflexible compared with kelp. Curious to know how Calliarthron withstands the constant pounding and how the sea may have shaped this coralline seaweed, Martone and Denny set about developing a mathematical model of the organism().
Martone admits that developing the model coralline was challenging. According to Martone, Calliarthron fronds are built from calcified segments linked by flexible joints consisting of thousands of elongated cable-like cells; so the duo modelled the seaweed as short rigid beads joined by linkers made of thousands of independent flexible cables. Building the model in MatLab, Martone was able to calculate how stresses exerted on the articulated seaweed by the waves altered as he varied the seaweed's physical characteristics. Lengthening the joints, shortening the calcified segments,and shortening the calcified lips (which are found at either end of the calcified segments and are ground down, deform and break when the seaweed is under stress) made fronds more flexible and reduced the stress on the seaweed. Which ties in well with the duo's observation that joints near the base of the fronds are longer to tolerate being tugged by the sea.
Ultimately the team's mathematical seaweed looked very much like the real thing, but Martone and Denny wanted to find exactly how much wave force Calliarthron fronds can take before being smashed to smithereens(). Sticking with the seaweed simulation, Martone allowed individual articulation cables in the articulated joints to fail as he increased the force on the frond and found that an individual Calliarthron frond can withstand wave forces as great as 10 N, rising to 20 N when neighbours in a clump supported the frond.
But how much stress does a real Calliarthron frond experience when at the tide's mercy? Martone had to go gathering the seaweed fronds from deep in the intertidal zone adjacent to the Hopkins Marine Station to find out. `I had about 6 seconds between waves to jump down onto a rock, find a frond, cut it and get back up before being washed away,' says Martone. `It's a real testament to the habitat, and a dangerous place to live,' he adds.
Having survived gathering the seaweed, Martone first tested the seaweed's resilience by attaching weights to the fronds until they snapped and found that when the frond's joints were bent through 90 deg. they could support masses over 1 kg before failing. Martone admits that `the fact that the joints could bend through 90 deg. is impressive,' and adds that the 9.8 N force supported by the seaweed `agreed nicely with the model prediction'. But these were static tests. Martone and Denny needed to see how the seaweed performed in more realistic circumstances.
Knowing that conventional flow tanks only produce flows of 3 m s–1, well below the 25–30 m s–1experienced at the shore during a storm, Denny and Martone built a wave simulator. Attaching a long pipe to the side of the laboratory building, the duo filled the top portion of the tube with water before releasing the `wave'and sending it crashing at 10 m s–1 into the parking lot.`These experiments attract a lot of attention when we run them,' laughs Martone. Attaching a Calliarthron frond to a force transducer in the bottom of the simulator, Martone released the wave and measured the force exerted on the frond by the rushing water. Amazingly the force on a typical large frond was only 5 N, well below the 9.8 N that individual fronds had survived in the lab, and the 20 N predicted by simulations of clumps.
But what does all this mean for a cluster of Calliarthron fronds clinging to the Californian coast? Although the seaweed can tolerate average waves with ease, Martone suspects that large fronds may not survive larger storms. He explains that large wave impacts probably limit the seaweed's ultimate size by tearing out larger fronds. `Water velocity probably sets an upper limit to how large intertidal fronds grow,' explains Martone, and adds that this probably explains why seaweeds never grow as large as Californian redwoods.
