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No Fouling Around

Boat Propeller Fouled With Barnacles

Home is where the hull is: Since the dawn of seafaring, humankind has had to deal with pesky creatures, such as barnacles, that “foul” ship hulls and boat propellers like this one.


Flickr User Alison Domzalski (Creative Commons)

Since the dawn of seafaring, humankind has had to deal with the pesky creatures that settle on ships—seaweeds, barnacles, and others that take advantage of the empty real estate provided by a clean hull. Fouled hulls make for slower speeds and for powerboats, higher fuel costs (drag is a drag). Boat owners have a handful of strategies to cope with this problem, such as pulling boats out of water when they are not in use, scraping hulls clean, and coating hulls with paints that repel these unwanted settlers. Not surprisingly, marine organisms have evolved some solutions of their own.

Some seasnakes have a problem with seaweeds. When algae attach to and grow on a snake’s smooth scaly skin, swimming speed can slow down by 20 percent. Like their landlubber relatives, seasnakes shed their skin to grow, and with it the seaweeds as well. But between sheddings, at least one species uses color (or rather the lack of it) to make its skin less attractive to baby seaweeds and other settlers, which are typically attracted to dark colors. Snakes that are banded black and white suffer less from fouling than do uniformly dark snakes, and banded snakes are more common in areas where nutrient levels allow seaweeds to grow with abandon. Of course, the costs and benefits of changing one’s appearance can be complicated, but at least for this groups of snakes, it seems that benefits of a sleek scaly skin has made bands the new black.

Unlike ocean-going mammals, birds, and reptiles, fish are slimy. Their slippery surface makes it hard for seaweeds or barnacles to attach, so they don’t have to worry about fouling organisms slowing them down. But blood-sucking shrimp-like parasites, the mosquitoes of the sea, are another story altogether. During the day, cleaner fish and shrimp can help remove these pests, but most cleaners close up shop at night, leaving sleeping fish especially vulnerable. Window screens and bed nets protect people around the world from nighttime biting insects, and remarkably, some coral reef fishes have a similar solution – the only difference is that their nets are made of mucus (a.k.a. “snot”). Some species of parrotfish are famous for the large mucous cocoons in which they sleep. Remove the cocoon from a fish, and the number of parasites increases nine-fold. These nighttime pests can even transmit blood parasites to the parrotfish like mosquitoes transmit malaria to us. So the energy spent every night in making the cocoon, about 2.5 percent of the fish’s daily energy budget, seems to be well worth it. Fish that don’t use the mucus-mosquito-net strategy often have other tricks, for example, toxic skin.

So ship builders are Johnny-come-latelies when it comes to the challenge of keeping hulls healthy. White paint and toxic paint (some recently banned because of the environmental costs) have been commonly used, but could ships in the future use slime to deter hangers-on? It may seem farfetched to some, but not to the U.S. Department of Defense, which is supporting research into ooze that can keep ships hulls clean!

Editors Note: This post was co-written with Amanda Feuerstein, program coordinator in the office of the Sant Chair for Marine Science. Dr. Nancy Knowlton is the Sant Chair for Marine Science at Smithsonian’s National Museum of Natural History. Their blog series is based on Dr. Knowlton's book Citizens of the Sea, which celebrates “the Wondrous Creatures from the Census of Marine Life.”