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Giant Clam Shells, the Intermediate Disturbance Hypothesis, and a Big Box of Markers
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The Living World

Here comes Venus, riding in on her giant seashell. The biggest clam in the world is, of course, Tridacna gigas from the South Pacific. Why does it look like that? It's fluted, weighs hundreds of kilograms, and when alive has a fat mantle that's extruded and colored. This monster depends on symbionts, dinoflagellate algae, that live in the mantle and supplement its filter feeding with photosynthetic products. This supplementary nutrition helps explain the huge fluted shell (maximizing surface area for the algae) and the enormous size, impossible to reach by siphon generated food alone. Way to go, Venus!

It is mollusk structure and function on hyperdrive. For all members of this phylum, the shape of the shell is driven by the evolutionary pressures of living in the sea. Scallops are thin, light and have shell pleats for strength. But scallops swim through the water column; they don't burrow in the sand. Razor clams, long and thin like sabers, have vertical burrows and migrate up and down with the tide to facilitate their filter feeding. Many bivalves, such as cockles and ark shells, have deep ridges on the shells that help anchor them in the sediment, brakes against the pulling force of the tides and predators. Oysters settle on each other like a random mob of acrobats, forming reefs that facilitate their food gathering in shallow waters.

These shellfish with their complex morphology and functioning modify water quality and substrate topography and form a critical link in our coastal food webs. Even people eat them. They are so important that modern city planners are interested in reintroducing these creatures into the urban coastal world despite the insults mollusks have suffered from poor water quality and overharvesting. "She sells a surfeit of seashells and soon there won't be a seashore." That should be the tongue twister.

Can this be possible? What team of workers can bring back mollusks to protect our coastal zones? What skills are needed to move forward?

The Theoretical World

There is a tight interplay between the living and abiotic components of our environment. For each organism there is checklist of physical and chemical properties that must be satisfied for a population to be initiated or to persist. The organisms are not passive residents just using their piece of the world. Their physiology and behavioral needs modify the living space, sometimes severely. Some organisms change the structure of their environment so profoundly that they are termed ecosystem engineers. Beyond changing the gas concentrations around their bodies or the membership of the living community around them, these organisms re-configure their spatial environment and create a new community trajectory. Many mollusks, including the giant clam and diminutive oyster, can act in this way. Their calcareous shells create new three-dimensional space over time modifying water flow dynamics, creating friction that slows down tidal surges, and slowing down particulate movement in the water column.

The submarine dance between mollusks and salt water never ends. The dance takes time, as the slow interplay sequentially changes the physical and chemical environment and the mollusk community expands. The dance is not always in one direction into the ecological future. Very often disturbances of different intensities and durations push the molluscan dancers back and the interplay starts anew. The timing and intensity of disturbances themselves act as metronomes for this dance. Lack of disturbances does not yield the richest community, as the more powerful members co-opt resources from less aggressive species. Too frequent disturbances eliminate populations before growth and reproduction allow them to persist in the area. Ecological theory, the intermediate disturbance hypothesis, has been shown in many systems. The highest biodiversity is often achieved when disturbances are neither too few nor too many. Over time, a large community of organisms can develop and cause a major structuring of the spatial world.

As urban planners look towards the natural world for components of our future infrastructure, one of the most difficult things to negotiate is the level of disturbance to create the most useful bioengineering features. In civil engineering, using steel and concrete, stasis and long-term persistence is assumed if the design...

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