A Mussel-Bound Robot

At a time when robots build cars, perambulate mars, vacuum homes, and serve drinks, one that lies motionless on a shoreline and takes its own temperature may not seem to have much going for it. But a sessile robo-mussel, the creation of University of South Carolina marine biologist Brian Helmuth, has already changed the way science looks at the effects of global warming on the world's intertidal zones, those turbulent places where ocean and land meet.  

Climate change has already been implicated in the increase in ocean "dead zones," areas where decomposing organic material has so depleted the water's dissolved oxygen that neither fish nor invertebrates can survive. Helmuth's research demonstrates that global warming may create an altogether different kind of dead zone, one in which animals whose body temperatures are dependent upon ambient air and water temperatures are heat-stressed beyond levels they can tolerate.   

Intertidal zones are among the earth's harshest environments. Alternately submerged and laid bare by the tides, thrashed by waves and wind, exposed to sun and cold, they are also a vital habitat where terrestrial nutrients wash into the oceans, algae and plankton thrive, young fishes find protection and food, and crabs, seastars, and snails graze among clusters of barnacles, oysters, and mussels. It's a precarious ecological niche, where these species can escape predators only by living at their physiological limits, withstanding extremes of heat and cold that can vary their body temperatures by 36 degrees Fahrenheit in the course of a day.

Helmuth wondered what the effect would be of, say, a one-degree rise in air and water temperature. Since the mussel is ubiquitous in most of the world's intertidal zones, it seemed a good species to monitor. Helmuth embedded temperature sensors inside black epoxy mussel-shaped bodies (and in some cases in real mussel shells) and, along with Sarah Gilman of the Claremont Colleges in Claremont, California, placed the robo-mussels along West Coast intertidal zones from Santa Barbara to British Columbia.

The robo-mussels' sensors warmed and cooled at the same rate as real mussels. Recording their temperature every 10 minutes gave Helmuth and Gilman a thermal diary, a way to correlate sea and air temperatures with the temperature of the mussels.    

Intuition told them that species living in warmer, southern climes would suffer more from warming air and water than those living farther north. The robo-mussels, however, told them otherwise. Mussels in northern Washington State, for instance, had higher daily temperatures than those in California, far to the south.

Mussels in the northern intertidal zones, Helmuth explains, were being left high and dry during summer low tides that occurred during the warmest part of the day. Farther south, despite warmer temperatures, the timing of summer low tides left the mussels exposed to fewer hours of daylight. Helmuth and Gilman also realized that local conditions could have important effects. The prevalence of coastal fog or the splashing of waves on rocky shores might mitigate temperature increases. The presence of polluted runoff or waters low in oxygen might exacerbate them.

Using robo-mussel data, Helmuth hopes to be able to predict exactly how much the temperatures of species in specific intertidal zones will rise. With Helmuth’s model in hand, one of his University of South Carolina colleagues, David Wethey, accurately predicted a 2007 mass die-off of sea urchins in a New Zealand intertidal zone.

The die-off may not always be so sudden. While a body temperature of 96.8 degrees Fahrenheit will kill a mussel, the sublethal effects of high temperatures can wreak irreparable harm on them and on other species in the intertidal zone. As body temperatures approach their physiological limit, heat shock proteins begin to cause cell damage. Increased temperatures slow growth and inhibit reproduction. Seastars, the keystone species of these habitats, lose their ability to move around and feed. Even plants such as spartina and algae, Helmuth says, may be affected.

Using computer predictions of long-term climate change, he expects his models will eventually be able to predict the effects of global warming on species in the world's intertidal zones for the next 50 years. ("After that," he says, "the models get a little iffy.") His hope is that those concerned with protecting these ecosystems will look for ways to reduce stresses on the inhabitants of these critical areas—most importantly, keeping them free of polluted runoff and controlling development.

Once an intertidal population crashes, reestablishing its complex community can be difficult. If the dead zone is small, larvae from still vital areas nearby can move in and recolonize. But, Helmuth says, if the dead zone is longer than 30 to 50 miles, recolonization may be impossible.  

And if an intertidal dead zone meets an oceanic dead zone? Then, Helmuth says, we're in uncharted waters.