Charles Darwin is generally famous for his theory of evolution by natural selection based on his study of finches in the Galapagos Islands. However, Darwinophiles know it was his work on the humble barnacle that, um, cemented the best evidence for the theory. Why the barnacle?
Darwin knew he needed to make himself an expert on species variation if he wanted his theory to be taken seriously. Darwin chose barnacles because he thought they would be easy subjects to demonstrate similarities within the group due to a shared ancestry. Instead, he ended up spending more than eight years examining thousands of barnacle specimens. He wrote, “The [barnacles] form a highly varying and difficult group of species to class “¦ my work was of considerable use to me, when I had to discuss in the Origin of Species the principles of a natural classification.” In his landmark book, he explained how new species are derived or evolved from previous ones.
Anyone living near a rocky coast can easily find barnacles. I went tidepooling at the Cove during a recent low tide to observe these invertebrates more closely. One prominent barnacle species is the acorn (Balanus pacificus). To the uninitiated, it may be confused for a mollusk because its identity is hidden in a snail-like shell and because it shares rock space with mollusk species. A barnacle shell looks like a miniature volcano that has previously erupted. The sides are made up of overlapping limestone plates, and the area where the top looks blown off harbors a door, the operculum, which can be opened for feeding and closed against danger or to lock in moisture during exposure to air.
Inside, the animal reveals its true crustacean self. It has a segmented body with multiple pairs of segmented legs, which it uses for filter feeding. When a blast of water washes over a barnacle, the door opens, and the barnacle sticks out its hairy legs to comb the water for tiny plankton.
An apt description of a barnacle may be that it is a crustacean that glues its head to a substrate and spends its life kicking food into its mouth with its legs.
Darwin wrote: “I “¦ discovered the cementing apparatus “¦” Because a barnacle lives in a turbulent environment, it cannot afford to make some flimsy cement. Our superglues are super on dry land but submerged, they pale in comparison with the barnacle’s formulation, which will not dissolve in acid or alkali. Historical records document the squatters plaguing navigators since Roman times by fouling ship hulls and slowing travel considerably.
Barnacles exert their will today as ten tons can attach to a tanker in fewer than two years. Such crustacean encrustation increases enough drag to reduce fuel consumption by 40 percent.
Although research has determined cement glands in the antennae produce the glue, reproducing the recipe for the secret formula continues to elude scientists.
Barnacles are found worldwide and not only on rocks, pier pilings and ships. They congregate on almost any nonmoving or slow moving object such as whale skin. Cryptolepas rhachianecti is particular to a gray whale. Unlike barnacle species attached to hard substrate, C. rhachianecti grows deeply embedded in the skin. The outer shell surface eventually erodes away until it is virtually flush with the skin. This valuable adaptation likely evolved to thwart a gray’s strategy of dislodging the spongers by rubbing itself against objects on the seafloor.
Over the years Darwin spent with his barnacles, he inadvertently documented a love-hate relationship with them. He said variously, ¦ my beloved barnacles “¦ my confounded [barnacles] “¦ I hate a barnacle as no man ever did before, not even a sailor in a slow-moving sailing ship “¦ All nature is perverse and will not do as I wish; and just at present I wish I had my old barnacles to work at “¦” As Darwin experienced, barnacles are creatures to be reckoned with but they are not uninteresting.
” Judith Lea Garfield, biologist and underwater photographer, has authored two natural history books about the underwater park off La Jolla Cove and La Jolla Shores. www.judith.garfield.org. Questions, comments or suggestions? Email [email protected].
