By Sharon Gibbons
Acorn barnacles are commonly found around the bay in the marine intertidal zone along rocky shores, oyster reefs and harbor structures such as pilings and buoys. Acorn barnacles are part of the class Crustacea and are relatives to crabs and lobsters, while being “sessile” or immobile. There are about nine hundred species of Acorn barnacles which are found worldwide in temperate and tropical waters and they range in size from .8” to 4”. Acorn barnacles grow their protective shells directly onto the substrate forming a cone-shaped limestone shell with a ring of plates surrounding the barnacle’s body topped with a trap door. In this carapace, the barnacle lies on its back and projects its feet or cirri through the trap door when covered with water to sweep and grab on plankton and detritus to feed on. When the tide recedes, the Acorn barnacle closes the trap door to conserve moisture and for protection.
Acorn barnacles are hermaphroditic and are able to cross-fertilize with their neighbors as they typically live in dense colonies. The breeding season is in the autumn and the fertilized eggs are raised within the barnacles’ shells. Larvae, called Nauplius, are released in the spring months and live in the water columns for several weeks. The Nauplius larvae then become Cyprid larvae which investigate surfaces to attach to and may be attracted to a colony by compounds released in the water by adult Acorn barnacles. Once the Cyprid larva finds a spot, it glues its forehead to the substrate and metamorphoses into an adult barnacle and begins to build its shell. It will reach sexual maturity in about two years.
Acorn barnacles compete with other barnacles for space by sometimes dividing up the intertidal zone. They also compete with limpets and mussels and can be displaced so Acorn barnacles use a competitive strategy called “swamping” where large numbers settle together in hopes that some will survive. Acorn barnacle predators include whelks and starfish, and mussels eat the larvae. Acorn barnacles can be a nuisance to ship bottoms increasing the drag on the ship resulting in the need for more fuel. They have increased their range of travel through larvae being in ship’s water ballast which has led to them becoming exotic and invasive species.
Acorn barnacles are of historical interest, as in the nineteenth century Charles Darwin studied barnacles in depth for nine years. After scientists of his day discovered the stages of barnacle larvae and their metamorphosis, they concluded that barnacles weren’t mollusks and needed to be reclassified. Darwin had completed his voyage on the HMS Beagle and had written up his specimen collections and his findings while making notes about his theories of Natural Selection and evolution. He realized he needed to become an expert in a species and a serious figure in the British zoological community to support his larger theories. After beginning with a barnacle he discovered on his voyage, he launched a nine- year study of living and fossilized barnacles publishing four monographs on barnacles and their classification. This in-depth study helped to support the development of his theories of evolution which he published five years later in The Origin of Species. Today, we see thousands of barnacles populate our oyster reef at Point Pinole as they wave their feet at us feeding on the plankton in the Bay.