How Do Barnacles Attach to Whales?
It’s hard out there for a symbiotic barnacle, but somehow they find a way
For a hungry barnacle, the rim of a baleen whale’s nostril isn’t a terrible place to be. When the whale swims through a cloud of plankton for a meal, the barnacle — which also feeds on the tiny, floating organisms — gets free table service. All it has to do is extend its feathery, filtering arm and wait.
Barnacles regularly colonize the skin of filter-feeding whales, and they often do so in huge numbers — one humpback whale, for instance, can host almost 1,000 pounds of barnacles. (That may sound burdensome, but relative to a humpback’s nearly 80,000-pound body, it’s about as much extra weight as summer clothing on a human being.)
Whale-bound barnacles aren’t just regular barnacles with wanderlust; they’re different species, most of them unique to the brand of whale they piggyback on. The barnacle Coronula diadema lives only on humpback whale skin, for example, while gray whales host one called Cryptolepas rhachianecti.¹
So how does a barnacle get onto a whale in the first place? Like other stationary marine invertebrates, barnacles begin their lives as larvae — tiny, shell-less swimmers that find a place to settle and develop into the sturdy barnacles we know. Easy enough when all you want to stick to is an immobile rock, but a whale?
“We don’t really know how they’re doing it,” said John Zardus, a marine biologist at The Citadel military college in Charleston, South Carolina. For the last six years, Zardus has studied the barnacles that live on various marine animals, including whales. “These microscopic larvae that are swimming around in this huge ocean — how do they find a whale? … It just seems preposterous, actually.”
Research on whale barnacles is scarce, according to Zardus, because they’re not the easiest beasts to get a hold of. The larvae are small and difficult to distinguish from other kinds of barnacle larvae, and the adults are so deeply embedded in the skin of their hosts that they have to be carved out, flesh and all. Zardus only gets samples to study when there’s a dead, stranded whale he can take a chunk from — but if he takes too long to get to it, the barnacles will be dead, too.
Marine biologists speculate that the barnacles reproduce during the whales’ breeding season, when the whales mill around in warm, shallow waters rather than moving through the open ocean. If that’s true, Zardus said, the whales would swim in a thick soup of larvae; each barnacle parent can release anywhere from 10,000 to 20,000 spawn, and they survive for several weeks in the water. When a whale does swim by, research suggests, the drifting larvae pick up a chemical signal that tells them to hop on.²
There’s plenty of space to squat on a whale, but barnacles are picky. They like spots where the flow of water is consistent, Zardus explained, like the head or the fins. So instead of settling wherever they land, the larvae use their front antennae to “walk” around the whale in search of prime real estate. And that’s no easy stroll: if a barnacle larva were the size of a person, a whale would be over 20 miles long. Luckily, the larvae produce a sticky cement that keeps them from falling off into the ocean during their trek.
“They may walk over the whale for a long time until they find the place they’re looking for,” Zardus said. “It’s not random.”
Once they’re satisfied with their location, the barnacles dig in — literally. As they mature into adults, they form tube-shaped cavities in their shells that actually draw in prongs of growing whale skin. The result is an attachment as firmly rooted as the most pernicious weed.
The barnacle-whale relationship is generally considered to be obligate commensalism — a type of symbiosis where one species benefits, and the other isn’t affected either way. Still, it’s possible that too many barnacles could cause drag, Zardus said, or invite infection if they penetrate too deeply into the whale’s flesh. On the other hand, it’s been suggested that for male humpbacks, who fight over females by ramming and slapping at each other, a sharp barnacle coating may be helpful as a set of brass knuckles.
¹ The uniqueness of whale barnacle species means that whaling and whale habitat loss put not only the whales at risk, but also their hitchhiking companions, said Dan Rittschof, a Duke University marine biologist who has studied barnacles for over 30 years.
“If you’re an endangered whale,” Rittschof said, “then everything that lives on you, like a barnacle, is also endangered.”
² In 2005, in one of the rare laboratory experiments on whale barnacles, Japanese environmental scientists Yasuyuki Nogata and Kiyotaka Matsumura placed Coronula diadema larvae in Petri dishes of seawater — some alone, and some with a slice of whale skin. The larvae by themselves floated around until they died, but the ones near the whale skin started settling on the surface of the dish.
“The settlement cue appears to be released [into the water] from the tissue of the host whale,” the researchers wrote in a March 2006 paper in Biology Letters. The exact nature of the chemical cue, however, remains unknown.