Like its crustacean relatives, crabs and lobsters, barnacles have a hard exoskeleton and multiple segmented legs. These legs, however, no longer function as legs but have become specialized feeding and respiratory appendages. In addition to the exoskeleton, barnacles grow a hard outer shell; this outer shell is what is visible. The animals also have an opening at the top where the feeding and respiratory appendages are exposed to water. Barnacles are mostly hermaphroditic, and they cross-fertilize with their next-door neighbors.
Encrusted on ships, barnacles can cause enough drag to increase fuel consumption by 40 percent. Cement glands within the antennae produce the brown glue that fastens a barnacle to a hard surface. Acids and alkalis do not dissolve this incredibly strong glue that can hold the base of the shell to a surface long after the barnacle is dead. Dentists interested in the adhesive power of this glue have been trying to determine its composition.
Here are some of the barnacles you may see living at Fitzgerald Marine Reserve.
Common Acorn barnacle: Balanus glandula
This species is the most common intertidal barnacle in California, and it occurs in large patches, usually in the mid to upper tidal zone. Acorn barnacles begin life as free swimming larvae. When the time comes to settle, the larvae “glue” their heads to hard surfaces, such as pilings, wharfs, ships, rocks or other hard-shelled animals. Barnacles spend the rest of their lives in this position—head down and feet up. The Acorn barnacle shell width ranges from 1/4 to 1/2 inch, and their color varies from white to light brown. They are volcano-shaped; that is, the shell has a larger base that tapers at the top. Plates can seal off the animal when it is exposed to air by low tides, preventing desiccation. The top opening is diamond shaped and sometimes when the plates are closed, a line appears down the middle of the top, at the juncture of the plates.
Buckshot barnacle or Small Acorn barnacle: Chthamalus dalli and Chthamalus fissus
This barnacle closely resembles the Acorn except that it is much smaller (shell sizes range from 1/8 to 1/4 inch wide for adults), and it has an oval rather than a diamond-shaped opening. It can live on almost any surface that it can attach to: this includes many other animals such as limpets, snails, mussels and even other barnacles. When its inner plates are closed, sometimes cross-shaped lines appear on the top. Buckshot barnacles can settle in very high densities that can cover entire areas of rock.
Thatched barnacle: Semibalanus cariosus
Thatched barnacles are usually found under ledges and on vertical surfaces. The volcano-shaped shell can be up to 2.4 inches in diameter, brownish green or greyish in color, with a rough or corrugated outer shell that has narrow ridges from the top to the base. The wall plates are composed of vertical tubelike ribs which become downward pointing, finger-like (“thatch-like”) projections. They are found in the middle to shallow intertidal.
Volcano or pink barnacle: Tetraclita rubescens
The Volcano barnacle shell has an average diameter of 1.5 inches, rarely 2. Its exoskeletal wall consists of four plates with no basal plate (all other local Acorn barnacles have six plates). The shells of adults are pink to reddish and appear thatched, while the shells of (uneroded) juveniles are white. They are common in middle to low intertidal zones on rocks exposed to strong surf. Their range appears to be moving farther into Northern California than previously noted.
Gooseneck barnacle: Pollicipes polymerus
Gooseneck barnacles are often seen in the exposed areas of the middle intertidal. They can be identified by several light-colored plates that form the top of a cone that sits on top of a dark stalk. These barnacles are typically found in large clumps and often in the midst of mussels. Gooseneck barnacles can grow several inches long depending on the conditions. Despite their strange appearance people like to eat them, especially in Europe, where they are farmed for consumption.
A note from Martie (designer): Sasha sent another Acorn barnacle photo (shown at far left), and when I looked at it I thought there was a flowerlooking object stuck in it. No! It is feeding and those are its beautiful, feathery modified legs filter-feeding small plankton and other particles from the water. It’s gorgeous. At near left is the usual photo one sees of an Acorn barnacle, the kind I always thought were just dead shells on the bottom of a boat that need to be scraped off (these just have their feeding legs closed in). To see the action of the feeding, check out one of these amazing videos: https://www.arkive.org/acorn-barnacle/semibalanus-balanoides/video-00.html https://www.youtube.com/watch?v=9kQyZTid3nM https://www.youtube.com/watch?v=609ChAyR3yM