No sea creature wreaks more havoc for the mariner than the barnacle. They quickly attach themselves to boat hulls, keels, rudders, prop shafts, propsevery underwater surface possible. The age-old struggle against these creatures seemed to have been close to victory a few decades ago with the advent of tin antifouling paints. But dangerous environmental side-effects forced mariners to seek yet other powerful weapons against the barnacle.
By latching in prodigious numbers onto a boat’s undersides, barnacles affect vessels by causing considerable drag. A vessel’s speed diminishes by as much as several knots, and fuel efficiency of power boats plummets some 40%. Not satisfied to stop there, barnacles can also induce nasty corrosion of metal surfaces and will roost in the most inopportune cranniesplaces such as water intakes, which can cause mechanical malfunction when fouled.
All in all, it’s a mariner’s nightmare: Each year, barnacles inflict worldwide costs of an estimated $320 million. Not bad for an animal that begins life smaller than the period at the end of this sentence and often grows no larger than a quarter-inch.
Even though they are roundly disliked by boat owners, barnacles should warrant at least grudging respect. After all, they are spectacularly successful at gluing themselves on to underwater surfaces. Fortunately, key aspects of barnacle biology can be turned against them, and researchers around the world are devising an arsenal of anti-barnacle weaponry.
Think "barnacle," and what comes to mind, if anything, is probably an acorn barnacle: a hard, rivet-like, volcano-shaped thing attached to the substrate so firmly that it may as well be a part of it. A fleshy, pliable stalk resembling a neck and extending between the substrate and the animal’s body distinguishes the less-familiar goose barnacles from the more common acorn barnacles.
Of the 800 or so barnacle species, some 118 are known to foul ships’ hulls. Historically, goose barnacles, more so than acorn barnacles, caused most ship fouling. Sailing vessels becalmed during sea voyages made perfect homes for oceanic, surface-living goose barnacles, known as lepadomorphs, partial to inhabiting floating wood. With the eventual transition to steam power, however, ships began traveling at high speed for entire passages, and lepadomorphs could no longer successfully colonize. The coastal-living acorn barnacle, latching on in port, took over as the dominant fouling barnacle. Then in the 1970s, as skyrocketing fuel costs motivated ships to cruise more slowly, oceanic goose barnacles could again grasp footholds (or "headholds"); they rebounded as fouling organisms, especially Conchoderma, otherwise found on whales and fish. For like reasons, most recreational boats are plagued by acorn, not goose, barnacles unless you spend days or weeks becalmed in oceanic waters, offering lepadomorphs a chance to settle.
Barnacle behavior
Watch a barnacle feed underwater and you’ll see its wispy feet, shaped as food-snagging devices called cirri, wave outside the shell. Cirri of mid-sized barnacles capture small zooplankton like copepods and the larvae of sea stars, snails, and sea squirts, while smaller barnacles catch even tinier food: phytoplankton. Conversely, some of the largest barnaclesLepas anatifera, L. anser-ifera, and Dosima fascicularisgrasp sizable prey, even small fish. Luckily, there are no known reports of large prey (e.g., humans) being chomped by barnacles. Cannibalism, however, is common, as adult barnacles readily snack on hapless barnacle larvae who drift too close.
This brings us to one of the more astounding aspects of barnacle biology: reproduction. Surprisingly, barnaclesunlike numerous other marine invertebratesdon’t just spew eggs and sperm into the water and rely on chance fertilization. Rather, they copulate. A difficult proposition given the absolute immobility of these animals as adults. What to do when you can’t move to find a mate? Well, to begin with, most barnacles increase the odds of successfully scoring by being hermaphrodites, equipped with both male and female organs. But what if there’s nobody at all right next door? In a stunning and humbling display of natural selection, barnacle penises reportedly can extend as far as seven shell diameters away. That’s a penis-length-to-body-width ratio of roughly seven.
The eventual product of all this reproductive finagling is a huge batch of nauplii, tiny larvae released from the parent’s shell to drift with the currents. In the tropics a barnacle may spawn 10,000 young three or four times a year. On England’s Isle of Man, barnacles crowding a half-mile stretch of shore produce, says one expert, perhaps a trillion offspring a year. And some researchers estimate that, on average, 2,000 barnacle larvae crowd each cubic meter of sea water at any given time. Adult barnacles generate these copious babies as insurance against predators and fickle natural forces that dramatically slash the chances of any one larva surviving to adulthood.
Over a period of weeks, a nauplius journeys as much as several hundred milesbut nobody knows for sure how far; marine biologists can’t track the minuscule larvae. Along the way, the nauplius radically metamorphoses into a cyprid, a non-feeding being with a sole mission: to find a suitable, hard substrate to attach to and develop into adulthood.
Beginnings of fouling
The cyprid faces a crucial, irrevocable choice. If it should glue itself too high on a rock, for example, at low tide it will become exposed to harsh heat and sunlight, perhaps leading to death. On the other hand, if it selects a spot well below the low tide mark, it may risk high predation rates and competition from other species. To make the choice, the cyprid "walks" on its antennules on any hard surface it may encounter, checking out the site. Chemical "scents" appear to play a big role in this decision. If the cyprid’s antennules whiff the chemical scent of other barnacles of the same species, for instance, the site is likely a good one.
During this exploration phase, temporary attachment of the cyprid fares best on "sticky" surfaces, ones with high free energy from molecular polar bonds, a fact exploited by some of the most promising new antifouling technologies. Then, upon selecting a settlement site, the cyprid oozes glue from its antennules; within hours the adhesion strengthens, and soon the animal transforms into a juvenile, and finally an adult, constructing a sturdy, calcareous shell, its home for the rest of its lifeand the beginning of a drag-inducing nub on your hull.
When a barnacle settles, it does everything it can to stay permanently. Barnacle cement can persist 15 to 20 million years, endure shear strengths twice as strong as those faced by spacecraft epoxies, and soften only slightly at 662° Fthat’s 42° F higher than the melting point of lead. Needless to say, even when the animal within dies, the shell often remains glued, unless you remove it.
Of course, a host of non-barnacloid organisms also fouls boats: bryozoans, mussels, green and brown algae, hydroids, tube-worms, tunicates, bacteria, and more. But in research by the Woods Hole Oceanographic Institute, barnacles prevailed as the dominant fouling organism in nearly a hundred years of data. Likewise, a study of 600 vessels painted during 1980 and 1981 showed that barnacles caused 86 percent of all cases of animal fouling.
"Barnacles are extremely important foulers," said Judith Stein, a chemist at GE Corporate Research and Development, one of several companies creating new antifouling technologies. "They’re tenacious bonders, and they really increase drag because of their shape. We are targeting barnacles in the marine environment."
Anti-barnacle warfare
The mariner’s battle against barnacles carries an age-old pedigree. At the time of Christ sailors matched wits against barnacles by sheathing hulls in lead and smearing the metal with a brew of oil, arsenic, and sulfur. Much later, in 1625, someone in England sought a patent for an antifoulant potion of copper, arsenic, and gunpowder. And until the mid-1800s, shipbuilders encased wooden hulls in copper to counter fouling. Yet, when iron replaced wooden hulls, so too by necessity did antifouling paints supersede copper sheathing, since copper and iron form a galvanic reaction in sea water.
For many years, antifouling paints of choice contained cuprous oxide. Reasonably potent when first applied, the concoctions suffer over a period of months the reaction of the copper with sea water, causing a green layer of insoluble cupric salts to build on the paint surface. This layer traps remaining copper in the paint, rendering the antifoulant ineffective. Scrubbing removes the green layer, but eventually even this is not enough; not the ideal solution.
The panacea, or so it seemed, emerged in the 1960s and 1970s as industry introduced paints spiked with tributyltins (TBT), which are organotins (organic compounds that contain tin) highly effective at eradicating fouling. At about the same time, two new paint formulas were devised that don’t rely on simple diffusion for release of the toxicant, unlike traditional, "free association," cuprous oxide paints. The first new approach, ablative paints, enable the paint surface to slough off as the vessel travels through the water, exposing a new layer of toxicant and refreshing the surface’s antifoulant ability. The other option, self-polishing paints, achieve a comparable effect by chemically binding the toxicant and releasing it by hydrolysis when in contact with the slightly alkaline sea water. Potent, with a long service life of some five years, TBT self-polishing paints quickly established themselves as the best antifouling weapon, and were widely adopted.
By the early 1980s, however, deformities started plaguing marine life near marinas and shipping ports worldwide. Shells of some oysters, including economically important species, abnormally thickened, and their edible muscles shrank. The oyster fishery in one French bay alone suffered $150 million in losses from TBT poisoning. Simultaneously, female dog whelks in TBT-poisoned waters began developing penises.
Recognizing the danger, the United States and other nations restricted TBT-based antifouling paints. The U.S. Organotin Antifouling Paint Control Act of 1988 (OAPCA), for example, bans application of TBT antifoulants to non-aluminum-hulled boats less than 25 meters in length, with exceptions for certain aluminum parts of otherwise restricted vessels. Additionally, some states established criteria for maximum acceptable levels of TBT in sea water. Virginia’s criterion, at just one part per trillion, ranks among the most restrictive.
Nonetheless, the Environmental Protection Agency (EPA) reported in recent months that even now, almost 10 years after OAPCA, TBT levels in coastal waters of the U.S. persist threateningly high. Worldwide, TBT continues to pose a threat to marine life. A Japanese scientist, Hisato Iwata of Ehime University, even discovered unexpectedly high levels of TBT in tissues and organs of porpoises found dead off Japan’s coast. Although causal links remain undocumented, he speculates that TBT may be responsible for the mortalities.
Alternatives to TBT
OAPCA sent small-boat sailors scurrying back to the old antifoulant standbys, and copper-based paints have rebounded as the most widely used antifoulants on recreational vessels. Tracking the reawakened demand for cuprous oxide, some 190 cuprous oxide paintsin both free association and ablative formulationsare now registered in the U.S. Cuprous oxide, of course, still carries the same disadvantages it had before being temporarily supplanted by the seemingly miraculous TBT.
Primarily intended for fiberglass, wooden, and steel hulls, cuprous oxide can only be used on aluminum hulls if an anti-corrosion primer is first applied, and even then with a risk of corrosion developing at those places on the hull that are nicked or scraped. Free association cuprous oxide paints suffer from build-up of the green layer, don’t subdue biofilms and grasses, and lose antifoulant strength quickly, within a year or two. Ablative cuprous oxide paints are more effective, but only if the boat is usually in motionand many recreational vessels stay in port for a majority of their lives.
Moreover, cuprous oxide is toxic, though apparently less so than TBT, and accordingly is only slightly preferable, environmentally speaking. The impact of cuprous oxide on the environment is now being assessed to determine if this compound needs to join TBT on the list of restricted antifoulants. TBT restrictions and likely future regulations on copper have sent researchers back to the lab, searching for a better, safer antifoulant.
Even though some 70% of the global market for antifoulant paints still relies on TBT (it’s the first choice of the shipping industry), incentives to develop a superior solution are substantial. As one example, the U.S. Navywhich opted to avoid TBT’s environmental harm by reverting to painting its vessels with cuprous oxideestimates that if an effective and durable system with a five-year operational lifetime emerges, they’ll benefit from huge reductions in costs of fuel (savings of $100 to $150 million per year), underwater hull cleanings ($2.4 million), scheduled dockings ($80 million), and hazardous waste disposal associated with paint removal ($2 to 5 million), for total annual savings of some $200 million.
These benefits could be realized with development of the "ideal" antifoulant. Currently, promising alternatives to both TBT and traditional cuprous oxide paints are becoming available, with potential future options under development in government, industrial, and academic labs.
Now, antifoulant options range from the traditionally based (but still innovative) to the truly futuristic. Hearkening back to ships of the 1800s, for example, some products coat the hull in metallic copper, either encapsulated in an epoxy resin or heated and sprayed onto the hull in a molten form. Alternatively, recently developed acoustical antifouling systems use resonators attached to the inner surface of the hull to produce sound waves that vibrate through the boat, causing the hull boundary layer of water to constantly move at the equivalent of a two- or three-knot current; barnacles purportedly struggle to attach in these conditions.
Improving copper-based paints
Other alternatives aim to enhance the effectiveness of copper-containing antifouling paints: Tetracycline, for example, when used as a paint additive, may inhibit barnacle settlement by interfering with prior bacterial growth on the surface. In the realm of new paints, zinc oxide replaces cuprous oxide and TBT as the active ingredient in some coatings. Given exposure to light and immersion in water, the zinc oxides generate peroxides, stymieing barnacle settlement. Someday certain antifouling paints may even be stamped "green" or "environmentally friendly." In their daily lives, corals, sponges, and other sedentary sea creatures wage a constant battle against fouling, and researchers suspect these creatures’ natural chemical defenses could be co-opted for the human fight against hull fouling. Already, scientists have extracted hundreds of substances, and some appear effective at deterring barnacle settlement.
"The advantage of the natural toxicants," explained Richard Zimmer-man, research scientist at Moss Landing Marine Laboratory in California, "is that they generally are degradable in the environment." Released by paint into the water, these toxicants ideally would quickly become benign as they diffuse away from the boat. Correspondingly, "if TBT did its job at the hull surface and had a half-life of a week or a month," said Zimmerman, "there would be a lot less concern." Instead, it accumulates in the environment without rapid losses of toxicity. Safer compounds from extracts of corals, sponges, or sea grasses may well someday wind up as active ingredients in environmentally friendly antifouling paints, but major technical hurdles remain.
Despite this smorgasbord of alternatives, some experts suggest that the future of antifouling technology lies in a different direction, one that you’ll appreciate if you like non-stick pots in your galley. It’s a new strategy of so-called "easy-release" surfaces, designed, according to GE’s Stein, so that a boat moving about 15 knots readily sloughs barnacles and other foulants from the hull with no human labor required, or in port "you can clean them off with a sponge or just a high-pressure stream of water."
"These are silicone-based materials," said Stein, "and they work by a different mechanism than traditional antifouling paints. They don’t kill the organisms but instead provide a surface to which fouling organisms can’t firmly attach, a low free-energy surface. It’s sort of like Teflon, although silicones are better than Teflon in this application because silicones have a more fluid surface." Although GE currently does not market its easy-release product to recreational boaters, according to Stein, largely because of its high cost, "there are people who have asked us to do their yachts, so I think it could be obtained." It’s a hugely attractive option, as easy release coatings act through passive physical characteristics rather than relying on release of toxins.
"I think the best strategy is to understand the environmental impacts of the coating that you’re using and use an appropriate coating to minimize those impacts," said Harold Guard, who oversees the antifoulant program at the Office of Naval Research. "For ships in sensitive areas, like pleasure craft that stay in the Chesapeake Bay for a lot of time, I think they should look seriously into using non-toxic antifoulant coatings, as the Navy is."
Who knows, maybe next time a barnacle cyprid encounters a previously inviting hull it will scramble unsuccessfully on a surface as slippery as a non-stick frying pan, the latest high-tech brush-off in the ancient feud between barnacle and mariner.
Trained as a marine biologist, Pete Taylor is now an editor at Islands, an international travel magazine based in Santa Barbara, Calif.
