Balanidae Family of Acorn Barnacles

One Acorn Barnacle of the Balanidae Family can be found in this website:

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

Phylogeny:  Acorn Barnacles of the Balanidae Family are in the phylum Arthropoda, meaning that they have jointed legs. They are in the subphylum Crustacea, along with crabs and lobsters. They are in the class Theocostraca and the order Balanomorpha. The superfamily Balanoidea is one of four superfamilies in this order. The Balanidae Family is one of two families in superfamily Balanoidea. The Balanidae Family is very large, with ten subfamilies, forty-one genera, and nine hundred species.

Distribution: Acorn Barnacles are found attached to hard substrates such as rocks, pilings, sea walls, ship hulls, sea turtle shells, and sea shells. 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). Acorn Barnacles are found worldwide in tropical to polar seas. Twenty-five species of the the Balanidae Family are found Mexican waters. 

Morphology: Unlike other barnacles, Acorn Barnacles do not have a stalk and instead attach to the substrate with their shell. The cone-shaped shell is composed of chitin, the same material found in crab and lobster shells. The 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 cirri (feathery feet). These animals lack gills and absorb oxygen through their cirri. Some Acorn Barnacles inhabit the intertidal zone and must adapt to changing tides. When there is no water available, the shell closes tightly to retain moisture.  They have a gland that secretes the glue that holds them to the substrate. The Acorn Barnacles range in diameter from 2.0 cm (0.8 inches) to 10.2 cm (4.0 inches). 

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. The fertilized eggs are brooded within the shell. The eggs hatch into planktonic, shrimp-like larvae. This phase is brief and they eventually attach themselves to a hard substrate.  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.

Ecosystems Role: Acorn Barnacles are suspension feeders.  When immersed in water their the cirri (thoracic limbs) emerge from the shell to locate detritus and plankton. In turn Acorn Barnacles are preyed upon by crabs, fish, gastropods, and starfish.