American Oceans

What Are Barnacles?

a shell with barnacles on it

Barnacles are a fascinating group of crustaceans distinctly adapted to life in marine environments. Unlike their mobile relatives like crabs and lobsters, barnacles are sessile, meaning they permanently attach themselves to a variety of surfaces. These surfaces can range from rocks and ship hulls to other marine creatures, like whales and turtles. Once they’ve established a suitable spot, they build hard calcium carbonate plates around themselves for protection and remain there for the rest of their lives.

The anatomy of a barnacle is quite different from other crustaceans. Most notably, they reside upside-down within their protective shells, using their modified appendages, called cirri, to capture plankton from the water for feeding. The life cycle of a barnacle involves several larval stages; beginning as free-swimming nauplii, they eventually settle down and metamorphose into the familiar adult form.

Biology and Life Cycle of Barnacles

Closeup of Acorn barnacles

Barnacles exhibit a complex life cycle and unique morphology that are fascinating examples of adaptation in marine arthropods. They are crustaceans recognized for their sessile lifestyle and reproductive strategies.

Morphology and Physical Characteristics

Barnacles are encased in a shell composed of calcareous plates which provide protection from environmental stressors and predators. The common acorn barnacle has a shell with plates that form a cone-shaped structure, while the goose barnacle has a stalk (peduncle) connecting the body to the substrate. An operculum on top of the shell opens to allow for feeding and closes to retain moisture during low tide.

Reproductive Strategies and Development

Barnacles are typically hermaphroditic, meaning individuals have both male and female reproductive organs. They can reproduce both by self-fertilization and by cross-fertilization with nearby barnacles. The developmental stages include a nauplius larva which molts several times before transforming into a cyprid larva. The cyprid settles on a suitable substrate where it undergoes metamorphosis to become an adult.

Feeding Mechanisms

As suspension feeders, barnacles utilize their feather-like appendages called cirri to sweep through water, capturing plankton and other minute food particles. When the tide is in, they extend their cirri to feed; during low tide, they remain sealed within their shells.

Habitats and Distribution

cluster of barnacles on a rock

Barnacles are exclusively marine animals and are commonly found in the intertidal zone, attached to various substrates, including rocks, boat hulls, and even other marine animals. Their presence is global, from tropical to polar seas.

Behavior and Adaptations

They are sessile creatures spending their adult life firmly attached to a substrate using a powerful glue secreted from their glands. This stationary lifestyle requires adaptations in feeding and reproduction, which barnacles have developed to thrive in their environments, often forming dense colonies where space is available.


Barnacles belong to the subphylum Crustacea and the subclass Cirripedia, which is further divided into orders including Thoracica, Acrothoracica, and Rhizocephala. These orders differentiate barnacles by their morphology and reproductive strategies, shedding light on the diverse adaptations within their taxonomy.

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