Imagine an organism that spends its adult life lying on its back and kicking food into its mouth.
By Tom Baird
Imagine an organism that spends its adult life lying on its back and kicking food into its mouth. Now imagine an organism that glues its head to the floor when it grows up. Imagine an organism that secretes a glue so strong that it cannot be dissolved by any acid or alkali. Imagine an organism so powerful in can create enough drag to slow down a supertanker to the point that its fuel consumption is increased by forty percent. Imagine an organism whose life cycle and classification has confounded some of the greatest naturalists in history. Now imagine all of those characteristics belong to a single organism, and that organism lives in St. Joseph Bay. Wouldn’t you want to see that super creature? You can see that strange creature right now by going down to any seawall or boat ramp on the bay.
All those extraordinary characteristics belong to the acorn barnacle, one of several species of the genus Balanus. They are those little white, volcano-shaped encrusting forms growing on wooden pilings, boat hulls, and shells. They can be found attached to any hard surface regularly washed by the tide. On an extremely cold day a few weeks back, I was fortunate to witness teams from the Florida Fish and Wildlife Conservation Commission, the U.S. Fish and Wildlife Service, the University of Florida,and volunteers rescuing cold-stunned green turtles from the bay near Pig Island. I couldn’t help but notice some of the turtles had large living acorn barnacles hitching a ride on their shells. Some species of barnacles will also attach to whales.
Because the adult barnacles encase themselves in a hard calcareous shell, barnacles were once thought to be a form of Mollusk, like clams and oysters, and they do superficially resemble limpets. However, inside that protective shell is a little joint-footed animal, hence they are Arthropods and more closely related to shrimp and crabs. While they are glued to one spot as adults, they begin life as a free-roaming, one-eyed, planktonic larva, called anauplius, which has a head and a telson, like the tail of a horseshoe crab. Looking at a sample of plankton under a microscope you will almost always find a barnacle nauplius larva or two. They will remain in this stage for about six months while feeding and growing. The next stage of development is the cyprid larva and its job is to find a place to attach. The cyprid stage does not eat. Drifting about the cyprid will use its antennules to investigate various surfaces it encounters. Once a suitable surface is found, cement glands in the antennules produce an extremely strong glue that fastens the organism to that surface. The barnacle is then home and will go through a metamorphosis into a tiny adult barnacle.
Since acorn barnacles do not move once cemented in place, the sea must bring food to their doorstep. Lying upside down the barnacle will extend its feathery legs, called cirri, and rhythmically sweep planktonic organisms and detritus from the water. If you have never watched barnacles feed, it is something to see. Most people just see barnacles looking dry and dead on a piling above the waterline, but watch them when submerged, and you will get an entertaining show as the little cirri hungrily sweep the water.
To prevent drying out when exposed, barnacles close the opening with two plates. The legs are withdrawn behind the tightly closing plates as the tide recedes. This will keep the barnacle moist for several hours. If the barnacle is not wetted by the returning tide or wave spray, the barnacle will die. On a vertical piling or sea wall, different species of barnacles will settle based on their ability to withstand drying. Generally, the most common barnacle at the highest levels belongs to the genus Chthamalus. As adults, barnacles have few senses; they can detect light and dark and the cirri have the ability to sense touch.
When in the larval form, barnacles can be preyed upon by any filter feeders in the water, such as clams, oysters, sponges, etc. As adults, enclosed in their hard calcareous cones, they are feed upon by whelks, which can grind through the outer covering, and by some species of starfish.
While small and insignificant as individuals, barnacles are nevertheless formidable in mass. They prefer to attach near other barnacles in order to swamp the environment and prevent other organisms from attaching and thus competing for food. This massive build-up on ships not only adds weight, but also creates drag, disrupting the smooth, laminar flow of water over a ship’s hull, thus slowing it down. Barnacles, as well as other fouling organisms, must periodically be removed from vessels by scraping, which is an additional cost of operating commercial vessels.
Besides our common acorn barnacles, there are goose-necked barnacles or goose barnacles that attach to the substrate by a muscular stalk with the shell-covered body at the top of the stalk. Species of goose-necked barnacles inhabit the Gulf in deeper waters over the continental shelf or adrift far at sea. They attach to pieces of driftwood or floats made of glass or styrofoam. You are unlikely to see goose-necked barnacles unless they are washed ashore attached to driftwood. In an amazing bit of adaptation, there are even parasitic barnacles, which are nothing like the barnacles we know.
So, on a warm day as you are scraping barnacles off your boat and cursing their existence, keep in mind that within that tiny, volcano-looking shell is an organism that is highly successful with some amazing powers of survival. Now if dentists can determine the properties that explain the adhesive power of barnacle glue, we might all celebrate the little barnacle.
Tom Baird has been a fisheries biologist, high school and community college teacher (oceanography and microbiology), director of a science and environmental center, teacher of science and principal in Pinellas County as well as an educational consultant. He retired from the Florida Department of Education and he and his wife divide their time between Tallahassee and Cape San Blas.