Barnacle

Quick Take

• Barnacles hold a little-known anatomical world record that makes their reproduction unlike almost any other creature on earth. Discover their unique anatomy →
• For centuries, scientists placed barnacles in the completely wrong animal category, a mistake that is easier to make than you might think. See their classification history →
• Barnacle fossils do something unexpected: they let scientists measure something about the ancient world that has nothing to do with the barnacles themselves. Explore fossil time clues →
• Some barnacles are a sought-after delicacy, though only a specific type. The reason the others aren't edible is more alarming than you'd expect. Find out who eats barnacles →

What is a barnacle? The barnacle is one of the oldest surviving creatures on earth and lives in oceans around the world. It is a small crustacean that attaches to the hard surfaces of rocks, sea walls, boats, debris, land structures, and other marine animals, such as sea turtles, sea snakes, lobsters, crabs, and whales. This arthropod is a member of the subphylum Crustacea, which includes crabs, lobsters, shrimps, prawns, crayfish, krill, and woodlice.

It forms a symbiotic relationship with other marine animals by creating a plate of armor in exchange for being transported to plankton-rich waters, where it uses filter feeding. Many humans consume the edible gooseneck species.

6 Incredible Barnacle Facts

• It is one of the oldest living animals in the world.
• It was once believed to be related to snails, which are mollusks, because of the hard shell that many species have.
• It eats with tiny legs or feet called “cirri” while suspension feeding.
• The rhizocephalan barnacle is a parasite.
• Commensalism refers to the symbiotic relationship a barnacle has with another marine animal, where the barnacle has most of the benefit.
• It is hermaphroditic.

Scientific Name and Types of Barnacles

There is no type species for the barnacle, so there is no one scientific name. Barnacles are members of the marine invertebrate class Maxillopoda. The subclasses are Theocostraca, Tantulocarida, Branchiura, Pentastomida, Mystacocarida, Copepoda, and Cirripedia. From Theocostraca, the infraclass Cirripedia was divided into superorders Thoracica, Acrothoracica, and Rhizocephala, and then into 11 orders.

There are more than 3,000 described species of barnacles, with the most common being the acorn barnacle. However, many species have the common name of acorn barnacle. Here are some of the most common acorn barnacle species:

• Common Acorn Barnacle (Balanus glandula): Very common around North America’s Pacific Coast, these barnacles can draw oxygen from both air and water. Overall, they have a conical shape and can live up to 10 years.
• Giant Acorn Barnacle (Balanus nubilus): As its name suggests, this is the world’s largest barnacle. They can be found on the Pacific coast of North America, from Baja California to Alaska, and at depths of up to 300 feet.
• Acorn Barnacle (Chthamalus antennatus): Also known as the six-plated barnacle, these barnacles are found on the eastern and southern coasts of Australia.
• Titan Acorn Barnacle (Megabalanus coccopoma): This type of barnacle was first written about by none other than Charles Darwin. These can be found in South and Central America on their Pacific coasts. They are large, conical barnacles.
• Titan Acorn Barnacle (Megabalanus tintinnabulum)
• Red-striped Acorn Barnacle (Paraconcavus pacificus): These are a type of balanid barnacle that grow up to just under one and a half inches. They have pink stripes that are striated over their plate growth rings.
• Northern Acorn Barnacle (Semibalanus balanoides): These are also known as the Common Barnacle and Northern Rock Barnacle. They are a very common occurrence and can be found in rocky areas of the western European coasts and also on the western and eastern coasts of North America.

Gooseneck barnacles, also called stalked or goose barnacles, are also common. They are the common name for the members of the infraclass Thoracica and the order Pedunculata and can include the genera Pollicipes and Lepas.

• Goose or leaf barnacle (Pollicipes polymerus)
• Gooseneck Barnacle (Pollicipes pollicipes): These are stalked versions of the barnacle. They are found on the Pacific rocky shores of North America, where they directly compete with other animals for limited living space.
• Smooth Gooseneck Barnacle (Lepas anatifera): Also known as Pelagic Gooseneck Barnacles, these barnacles use their stalks to attach to all types of driftwood, piers, hulls, and flotsam.

History and Evolution

The barnacle is one of the oldest types of animals. They have survived for hundreds of millions of years, and the oldest fossil is around 320–330 million years old! They evolved into something closer to their modern form about 20 million years ago.

Scientists can infer information about prehistoric water depths from barnacle fossils and other evidence. They do this by studying the placement of these fossils and estimating the distances they extend downward and deeper into the ocean.

Appearance

What are a barnacle’s characteristics? A barnacle has specific anatomy depending on its species and whether it is in a larval stage of either the nauplius or cyprid, or a mature adult. The nauplius larva hatches from a fertilized egg and has one eye, a head, and a telson, but no thorax or abdomen. It then goes through several moltings to shed its cuticle, during which it looks like a shrimp until it becomes a juvenile adult.

Adult barnacles measure about 0.5 to nearly 3 inches across, depending on the species. Their color is usually white, cream, yellow, or black, and their shells are bleached white. Barnacles secrete 4 to 8 calcite plates with an average of 6 to protect their soft bodies. Adult barnacles have few internal organs and no appendages except for legs or feet, called “cirri,” which they use for feeding and breathing. They have no heart or gills. Weight data is not available, though most of their mass would come from their shells.

The barnacles that encrust themselves onto boat hulls, buoys, and piers are volcano-shaped, stony, and gray. Gooseneck barnacles have a stalk to attach to hard surfaces, but acorn barnacles do not and are cone-shaped. Acorn barnacles also have an “operculum” or opening at the top, which has a “door” of 2 or 4 additional plates, while gooseneck barnacles have chalky-white, heart-shaped shells with black lines. Each type of barnacle has different anatomy.

Behavior

The barnacle’s behavior during its life cycle — including free-swimming and feeding — is often called intertidal, referring to its activity in sync with the ocean tides. When the tide comes in, it opens its “door” plates and feeds using its cirri, shutting its operculum to save water. Many barnacles hibernate during winter, relying only on their energy reserves.

Habitat

The barnacle larvae free-swim for a short while after hatching from the egg to find a surface to attach to. It tends to settle in sites where there are many other barnacles, meaning those sites are favorable for survival. The surface it settles on must first have a film of algae, seaweed, bacteria, or diatoms to allow it to attach. Once it settles, it becomes sessile and may not move for the rest of its life.

It lives in marine waters around the globe on rocks, boats, and sea creatures such as whales, fish, and crustaceans. Because barnacles tend to encrust boat surfaces in large numbers, create drag, and require pressure washing to remove, they are often called “crusty foulers.” When they accumulate in large numbers, this is called biofouling — meaning they are damaging the surface — and it increases drag.

Rhizocephala barnacles live inside thoracican barnacles, mantis shrimps, and other crustaceans. Some non-parasitic barnacles attach to sites by growing their shells into the surface, while other species use a strong glue and peduncle (stalk) to attach themselves head-first.

Diet

The diet of the barnacle is plankton and detritus (dead organic particles), which it consumes through suspension-feeding and filter-feeding. It uses its cirri, which it extends and retracts to push the food inside, and opens its operculum to allow water in, after which it closes its operculum. The barnacle is omnivorous, eating both plant and animal matter.

Predators and Threats

The barnacle faces many threats, including competition from other barnacles for space. Other predators include mussels, starfish, limpets, whelks, and other sea snails. The barnacle is especially vulnerable to predators during the larval stages of its life cycle. If barnacles are kept as pets, other animals in the aquarium are likely to prey on them.

Barnacles are also considered a delicacy for some humans, such as in Spain, Portugal, and other European countries. It is the Pedunculata (gooseneck) order of barnacles that is edible and have a fleshy stem. The barnacles are scrubbed clean and steamed in broth with herbs and wine, after which the stem is peeled off and eaten with the soup. Acorn barnacles are not edible because they have less flesh on their stems and have high levels of accumulated toxins.

Reproduction, Babies, and Lifespan

Barnacles are hermaphroditic but cannot reproduce by self-fertilization and need to settle with other barnacles to reproduce as well as to be near food sources. When ready to reproduce, one barnacle becomes receptive to the other’s gametes. Barnacles have very long penises that can extend up to eight times their body length and are used to fertilize neighboring barnacles.

The life cycle of a new barnacle starts with the egg, two larval stages, and then juvenile through to full adulthood. The barnacle has a mantle cavity in which it broods hundreds of eggs at a time, producing up to 6 broods per year. The eggs spend winter inside the sac and hatch into baby barnacles, called larvae. The first larval stage is called Nauplius and involves free-swimming for a few weeks, after which the larva transforms into the second larval stage, Cyprid, which lasts anywhere from a few days to a few weeks. During this time, the cyprid searches for a suitable surface to attach to. It then secretes a sticky glue from the cement gland on the bottom of its antennae, begins producing a hard outer shell, and transforms into a juvenile adult. Barnacles are sexually mature at two years.

Most barnacles live from 5 to 10 years, with some larger species living up to 20 years.

Population

Barnacles do not move once they attach to a surface or a host, so they are especially vulnerable to their environment. Certain barnacle species are threatened, endangered, or critically endangered. One threat is coastal pollution from the dumping of garbage, plastics, industrial runoff, chemical spills, sewage, and animal agriculture. Other threats include mangrove forest clearing and deep-sea trawling, which scrape underwater mountain ranges and result in habitat loss.