Acorn Barnacles

Barnacles are often thought of as crusty little creatures that wreak havoc on boat and ships. While that is true, there is so much more to the story. Of course there is. This should not be surprising to me. Let’s start with the fact that there are more than 1,400 species of barnacles in the world. Acorn barnacles are the most common. All barnacles are crustaceans that live in busy waters, generally intertidal zones and underwater volcanoes. But unlike other crustaceans they are sensile (new word!). They cannot move.

Acorn barnacles attached to a scallop shell.

They cannot move because they are literally cemented to their host, be it a boat, piling, buoy, rock, or other sea creature. Acorn barnacles start life as free-swimming larvae. When it’s time for them to settle, they use their first antennae to glue their heads to a hard surface (and yes, they spend the rest of their lives head down, feet up). These first antennae have cement glands that produce a fast-curing cement. Once attached, they begin their metamorphoses and build cone-shaped shells around themselves. These shells are equipped with trapdoors that open when the barnacles are covered in water and close to conserve moisture when the tide it out.

As sensile creatures, the question becomes, how do barnacles eat and reproduce? To eat, they use feather-like appendages called cirri (another new word!). The cirri quickly extend and retract from the shell; when extended they filter the water for microorganisms. The next part is even more interesting. Acorn barnacles are hermaphroditic, but they cannot self-fertilize. And remember, they cannot move. How do they reproduce then? They have the longest male sexual organ (length relative to body size) in the animal world – up to three inches long, which is six times their body size. They use it to pass and receive sperm from their nearby neighbors; fertilized eggs then brood within the barnacles’ hard shells. Meanwhile the sperm-providing appendages dissolve! They grow back the following year.

One last detail. As anyone who has ever tried to remove a barnacle of any species from a boat or other hard surface knows, they are extremely, extremely difficult to detach. This is because barnacles produce one of the most powerful natural glues ever discovered – 5,000 pounds per square inch. It is so powerful that researchers are currently working to figure out how to use it commercially.