Balanidae Family of Acorn Barnacles

California Acorn Barnacle, Semibalanus balanoides. A representative of the Balanidae Family of Acorn Barnacles.

Phylogeny:  The Acorn Barnacles are a member of the Balanidae Family of Barnacles and the Phylum Arthropoda and Subphylum Crustacea. There are nine hundred species in the Balanidae Family of which twenty-five are found along the Pacific Coast of Mexico. They are in the Class Theocostraca and the Order Balanomorpha.

Distribution: Balanidae are found in all oceans, in tropical, temperate, and polar waters. They are found along exposed coasts in bays, in brackish water estuaries and on rocky shorelines in the mid-upper intertidal zone where they are submerged for only part of each day. Those attached to boat hulls, turtle shells and other marine life may spend their entire life submerged. While the great majority of the animals live in shallow water, some species are found as deep as 2,000 m (6,560 feet). 

Morphology: The Balanidae they have jointed legs. They are classified as crustaceans, a very large group that includes crabs and lobsters. They reside in a cone-shaped shell, attached to rock, that makes one believe that they are a type of shell bearing Mollusk. They then secrete a shell composed of chitin, the same material found in crab and lobster shells, around them. Unlike other barnacles, this species does not have a stalk and instead attaches to the substrate with its shell. This shell consists of six plates fused together, in the shape of a volcano. In the center of the volcano are four more plates that are fused into two pairs of movable doors. When the animal is submerged, it opens these doors and extends its feathery feet. Since Balanidae inhabit the intertidal zone, they must adapt to changing tides. When there is no water available, the shell closes tightly to retain moisture. When immersed in water, the cirri (thoracic limbs) emerge from the shell to locate plankton and detritus. In turn they are preyed upon by Acorn Barnacles are preyed upon by crabs, fish, gastropods, and starfish. They range in diameter from 2.0 cm (0.8 inches) to 10.2 cm (4.0 inches).

Ecosystems Role: Balanidaee begins life as a planktonic, shrimp-like larvae. This phase is brief and they eventually attach themselves to a hard substrate as an adult such as boats, crabs, pier pilings, rocks, and turtles.  Once they attach to a hard surface, these animals are unable to change locations, and they remain in the same location for life. There may be up to 70,000 larvae per square meters spending days trying to find just the right spot as they compete with algae, limpets, and mussels for the same locations. They begin life as planktonic, shrimp-like larvae. This phase is brief and they eventually attach themselves to a hard substrate as an adult such as crabs, pier pilings, rocks, boats, and turtles.  Once they attach to a hard surface, these animals are unable to change locations, and they remain in the same location for life. There may be up to 70,000 larvae per square meters spending days trying to find just the right spot as they compete with algae, limpets, and mussels for the same locations. 

Reproduction: The Balanidae are hermaphroditic but cannot self-fertilize. When they reproduce with another individual, it is through internal fertilization. Because they are sessile organisms, they can only reproduce with individuals that are anchored nearby.