Zebra mussels can be a real drag
Biofouling is ubiquitous in the marine environment. There are two main categories of fouling: 

• noncalcareous (soft)
• calcareous (hard)

Algae, slime and hydroids exemplify noncalcareous foulers. Examples of calcareous foulers, which form shells comprised of calcium carbonate, are barnacles, encrusting bryozoans, mollusks, tubeworms and zebra mussels.

Zebra mussels colonize pipes and constrict flow, therefore reducing the intake in heat exchangers, condensers, fire-fighting equipment, and air conditioning and cooling systems. Due to zebra mussel densities, the diameters of pipes have been reduced by two-thirds at water treatment facilities. Small mussels can get into engine cooling systems and cause overheating and damage. Navigational buoys have sunk under the weight of attached zebra mussels. Dock piling deterioration increases when encrusted with zebra mussels. Corrosion of steel and structural integrity of concrete can be affected by continued attachment of zebra mussels.
The buildup of a heavy layer of zebra mussels on a surface can produce anaerobic conditions at the substrate. Microbiologically induced corrosion (MIC) is caused by many types of anaerobic bacteria. Stainless steel weldments are particularly susceptible to MIC. Oxygen concentration cells may be caused by zebra mussel infestations. This effect will establish corrosion cells, which may accelerate corrosion of both coated and bare ferrous substrates.

(Fouling and Biofouling: Biofouling of ships increases fuel consumption, increases drag resistance, decreases maximum attainable speed and promotes corrosion. Fouling of power plant intake bays necessitates frequent shut downs and measures such as chlorination. Fouling by calcareous organisms contributes the greatest penalty because of their profile, and their tenacious adhesion to surfaces. Each of the calcareous organisms attaches in a slightly different way using different glues.

Ships were historically constructed by wood. The decay from bacteriological and animal attacks was in general mitigated by using hard tight wood, and by treating the wood with “poisoned” tar or oil paint. Later the ships was constructed from steel other forms of “decay” became dominating, and other solutions to prevent such decay (rust) was employed.)