Gills Reduce Drag

close up of an atlantic surf clam with part of the clam sticking out from the shell in grainy sand and blurry background

Biological Strategy

Atlantic surf clam

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Image: Ashley Delvento / Wikimedia Commons / CC BY - Creative Commons Attribution alone

Move in/on Liquids

Water is not only the most abundant liquid on earth, but it’s vital to life–so it’s no surprise that the majority of life has evolved to thrive on and under its surface. Moving efficiently in and on this dense and dynamic substance presents unique challenges and opportunities for living systems. As a result, they have evolved countless solutions to optimize drag, utilize surface tension, fine tune buoyancy, and take advantage of various types of currents and fluid dynamics. For example, sharks can slide through water by reducing drag due to their streamlined shape and specially shaped features on their skin.

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Manage Mechanical Wear

A living system is subject to mechanical wear when two parts rub against each other or when the living system comes in contact with abrasive components in its environment, such as sand or coral. Some abrasive components are a constant force, such as finger joints moving, while others occur infrequently, such as a sand storm moving across a desert. Living systems protect from mechanical wear using strategies appropriate to the level and frequency of the source, such as having abrasion-resistant surfaces, replaceable parts, or lubricants. For example, human joints like shoulders and knees move against each other all day, every day. To protect from mechanical wear, a lubricant reduces friction between the cartilage and the joint.

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Manage Turbulence

A turbulent force occurs when air or water creates a chaotic or irregular motion. The source can be such things as wind, waves, and eddies caused by obstructions to air or water flow (such as that created by a rock in a stream). Because the force is irregular, it acts in unpredictable ways on multiple parts of a living system at any given time, decreasing the living system’s efficiency. Strategies used to manage turbulence include dampening the amount of turbulence, having flexibility to handle sudden changes, and making quick adjustments. An example is the mucus on aquatic organisms, such as barracuda sharks, that can reduce turbulent friction of seawater by 66%. In doing so, it decreases drag and increases the sharks’ swimming efficiency.

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Bivalves

Class Bivalvia (“two-valved”): Clams, mussels, oysters

The bivalve name says it all: these animals are characterized by a two-sided shell that is connected at the hinge with a muscle. The 15,000 species in 90 families have flattened bodies, no head, and generally live a sedentary lifestyle. Instead of having a circular band of teeth like other molluscs, many are filter feeders. They use cilia, or hair-like extensions, to move water into their bodies and pluck out plankton to eat. Consequently, there are no terrestrial bivalves, and only 15% live in freshwater ecosystems. The rest live in marine environments.

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The abfrontal surface of Atlantic surf clam gills may reduce frictional drag using lubricating mucus.

“Mucus in molluscs serves many purposes, one of which is lubrication (Prezant 1985; Simkiss 1988; Davies and Hawkins 1998). In the context of bivalve gills, the secretion of mucus on the abfrontal surface might be adaptive if it reduces friction between water and the epithelium, in a manner similar to the drag reduction produced by fish mucus (Hoyt 1975; Daniel 1981). Abfrontal surface lubrication could be of particular importance in eulamellibranchs because water pumped through their gills is directed into abfrontal chambers of reduced volume, where frictional drag between water and the epithelium is probably important (Beninger et al. 1997a).

“In the species studied, abfrontal mucocyte density was greatest in the eulamellibranch Spisula solidissima…The presence of residual mucus shows that AMPS are secreted on the abfrontal surface of the gills of these eulamellibranchs. Acidic mucus is highly viscous and is a good lubricant because it is not easily hydrated or removed from the epithelium (Hunt 1970; Faillard and Schauer 1972).

“It is therefore possible that mucus on the abfrontal surface of eulamellibranch gills is important in lubrication.” (Dufour and Beninger 2001:303-305)

Last Updated August 18, 2016

References

Journal article

A functional interpretation of cilia and mucocyte distributions on the abfrontal surface of bivalve gills

Marine Biology | 25/08/2002 | Suzanne C. Dufour, Peter G. Beninger

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