Acorn Barnacles Facts

Scientific Name: Balanus

Acorn Barnacles
White acorn barnacles attached to a rock in Larsen Bay, Kodiak Alaska.

EdwardSnow / Getty Images Plus

Acorn barnacles are crustaceans in the Balanidae family and Balanus genus that all share the same common name and can include any stalkless barnacle in the order Sessilia. They are part of class Maxillopoda, and their genus name comes from the Greek word balanos, meaning acorn. Acorn barnacles live along rocky shores and are filter feeders. They begin life as free swimmers like other crustaceans but attach themselves to rocks or bottoms of boats and spend the rest of their lives in this position.

Fast Facts

  • Scientific Name: Balanus
  • Common Names: Acorn barnacle
  • Order: Sessilia
  • Basic Animal Group: Invertebrate
  • Size: from 0.7 inch (balanus glandula) to above 4 inches (balanus nubilus)
  • Life Span: 1 to 7 years
  • Diet: Plankton and edible detritus
  • Habitat: Rocky shores
  • Population: Not evaluated
  • Fun Fact: In just 2 years, as much as 10 tons of acorn barnacles can be attached to ships, causing enough drag to increase fuel consumption by 40%


Acorn Barnacle Shells
Acorn Barnacle Shells.  medveh / Getty Images Plus

Acorn barnacles are crustaceans and not mollusks. They are joint-legged animals that live inside cone-shaped shells, standing on their heads and grabbing food with their legs. Acorn barnacles are also sessile, or fixed in place, and remain in the place they attach themselves to as larvae. Due to their stationary lives, there is no discernible separation between the head and thorax.

Because their legs absorb oxygen, acorn barnacles’ legs are feathery and gill-like. They produce a shell as they reach adulthood, which is made of six fused plates with a hole in the top to allow them to feed and a valve to seal the shell against predators. They also have cement glands that produce brown glue that attaches them to surfaces, an adhesive so strong that not even acids can remove the shell even after they have died.

Common predators of acorn barnacles include starfish and snails. Both have the ability to penetrate their hard shells. Starfish can pull the shells apart while the snails are able to penetrate via the fused plates.

Habitat and Distribution

These creatures live on rocky shores along the Atlantic and Pacific oceans in temperate and tropical regions across the world. They primarily live in tropical, tidal zone, marine environments but can survive in cooler regions. They attach themselves to ship hulls, whales, turtles, and rocks depending on the surface contour, water movement, and light.

Diet and Behavior

Their diet consists of plankton and edible detritus that they filter from the water with their feathery legs. Once attached to a surface, the barnacle’s valve opens, and its legs search the water for plankton. The valve tightly closes when it is threatened by a predator or when the tide becomes low. The door allows them to trap water in their shells and conserve moisture so that they do not dry out.

Acorn barnacles prefer to settle in large groups, which comes in handy during breeding season. Some species, like balanus glandula, can reach population densities of up to 750,000 per square foot. They compete for space with other rock dwellers such as anemones and mussels. Each species adapts to different tidal zones, so different acorn barnacle species can be zoned above or below each other.

Reproduction and Offspring

These barnacles are hermaphroditic, meaning they have both female and male sex organs. Since they can’t fertilize themselves, they rely on fertilizing neighboring individuals. Because acorn barnacles are stationary, they grow long penises, which can be up to 6 times the length of their own bodies at 3 inches. They pass and receive sperm within a 3 inch range, and any barnacles more than this range from any neighbor can not reproduce. At the end of mating season, the penis dissolves only to be grown again the next year.

Each barnacle broods fertilized eggs within their shells. Once hatched, acorn barnacles begin life as free swimming larvae. When they decide to settle, the larvae glue their heads to a hard surface and build their cone-shaped shells of limestone, becoming miniature adults.


Closeup of Balanus balanoides on a stone. HHelene / iStock / Getty Images Plus

Acorn barnacles are any stalkless barnacle species in the genus Balanus, and any barnacle in the order Sessilia can have the same common name. There are approximately 30 different species in the genus Balanus, from the smallest in size, Balanus glandula, to the largest, Balanus nubilus. All Balanus species are hermaphrodites.

Some additional examples of acorn barnacle species are: Balanus crenatus, Balanus eburneus, Balanus perforatus, and Balanus trigonus.

Conservation Status

Most Balanus species have not been evaluated by the the International Union for Conservation of Nature (IUCN).

Balanus aquila has been designated as data deficient. However, their range and prevalence continue to increase as barnacles attach themselves to boats and animals that displace them large distances.


  • "Acorn Barnacle". Monterey Bay Aquarium,
  • "Acorn Barnacle". Oceana,
  • "Acorn Barnacle". Slater Museum Of Natural History,
  • "Balanus Aquila". IUCN Red List Of Threatened Species, 1996,
  • Lott, L. "Semibalanus Balanoides". Animal Diversity Web, 2001,
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Your Citation
Bailey, Regina. "Acorn Barnacles Facts." ThoughtCo, Sep. 13, 2021, Bailey, Regina. (2021, September 13). Acorn Barnacles Facts. Retrieved from Bailey, Regina. "Acorn Barnacles Facts." ThoughtCo. (accessed June 6, 2023).