Barnacles are sessile marine crustaceans closely linked to crabs, lobsters, and shrimp. They are part of the subclass Cirripedia, which falls under the larger subphylum Crustacea. Encased in a cone-shaped shell composed of six calcite plates and a hinged lid made from four additional plates, barnacles are well-adapted to life in their chosen habitats.
These arthropods are mostly encrusters and firmly anchor themselves to solid surfaces like rocks, coral reefs, seashells, or even the bodies of whales. Feeding primarily on plankton, they use specialized appendages called cirri—feathery thoracic legs—to filter food particles from the surrounding water. By attaching themselves to mobile marine animals such as turtles and whales, barnacles gain access to a consistent flow of nutrient-rich water, which enhances their ability to feed.
Scientists have identified around 2,100 barnacle species, which are divided into three major infraclasses: Acrothoracica, Rhizocephala, and Thoracica.
They typically range in size from about 0.5 in (1.27 cm) to nearly 3 in (7.6 cm) in diameter, with the largest species, the giant acorn barnacle (Balanus nubilus), reaching up to 6 in (15 cm) in diameter and 12 in (30 cm) in height.
Barnacles are enclosed in a hard carapace made of six calcite plates and a lid-like operculum of two additional plates, forming a tough barrier against predators.
The body is divided into a head and thorax, with little to no abdomen. The head has a vestigial pair of antennae (not shown in the diagram), while the thorax bears eight pairs of long, feathery appendages called cirri, primarily used in filter-feeding. Sensory hairs on the cirri also help to detect water movement and particulates.
Acorn barnacles, a group of sedentary barnacles, have cement glands at the base of the antennae that anchor them firmly to surfaces.
Barnacles lack a true heart. Their blood circulation is managed by a sinus near the esophagus that uses surrounding muscles to move blood through the body. Barnacles also lack gills. Instead, the gas exchange occurs through the body surface and the cirri.
The ovaries lie at the base of the body, nestled near the mantle cavity, while the testes extend into the thoracic region, positioned close to the stomach and intestine, just beneath the scutum and tergum. A prominent feature of barnacle anatomy is the penis, which is both retractable and capable of growing up to eight times the body length (the highest penis-to-body ratio in the animal kingdom). On wave-exposed coastlines, however, it tends to be shorter and thicker to withstand strong currents.
Adults have three simple eyes or ocelli (one median and two lateral) for photoreception. These light-sensitive structures help the barnacle respond to changes in brightness, prompting it to close its opercular plates when shadows fall, an instinctive reaction known as the shadow reflex.
The name of the subclass, Cirripedia, derives from the Latin words cirritus, meaning curly, and pedis, meaning foot. When combined, these words mean curly-footed, describing their curved thoracic legs used for filter-feeding.
The term barnacle was first recorded in the early 13th century from the Middle English word bernekke or bernake. This word is similar to the old French word Bernaque and the medieval Latin word bernacae or berneka, both referring to a species of goose, the barnacle goose (Branta leucopsis). Such a taxonomy was based on ancient folklore, which believed that geese spawned from barnacles.
Initially, Carl Linnaeus and Georges Cuvier classified barnacles under the phylum Mollusca. However, in 1830, John Vaughan Thompson stated that the nauplius and cypris larvae of barnacles were similar to those of crustacean larvae. Finally, in 1834, Hermann Burmeister moved barnacles from Mollusca to Articulata, a group containing annelids and arthropods. Currently, they are classified under the phylum Arthropoda. In 2001, Martin and Davis considered Cirripedia as an infraclass under the class Thecostraca and divided it into 6 orders (under 3 superorders).
However, in 2021, Chan et al. upgraded Cirripedia to the status of a subclass, while the three superorders, Acrothoracica, Rhizocephala, and Thoracica, were stated as infraclasses. Here is the current classification approved by the World Register of Marine Species.
The earliest undisputed fossil of a barnacle is that of a member of the genus Praelepas from the Mid-Carboniferous Period (around 330 to 320 million years ago). Members of the genus Rhamphoverritor unearthed from the Coalbrookdale Formation of England date back to the Silurian Period and are believed to be stem-group representatives.
This group started to radiate and diversify during the Late Cretaceous Period (around 100 to 66 million years ago). They diversified during the Neogene Period (about 23 to 2.5 million years ago), which continues even today.
Barnacles are exclusively marine organisms, preferring the intertidal zones of oceans and underwater volcanoes. Around 75% of barnacle species are found in shallow waters at depths of less than 300 ft (100 m), while the remaining 25% inhabit intertidal zones. Some species, however, have been observed at depths as great as 2,000 ft (600 m).
These arthropods typically attach themselves to hard surfaces, including rocks, coral, mollusk shells, turtle shells, pilings, buoys, and the hulls of ships. Whale barnacles, belonging to the family Coronulidae, specifically anchor themselves to the bodies of whales in a commensal relationship.
These filter feeders primarily consume both zooplankton and phytoplankton, along with detritus.
Most barnacles have a lifespan of 5 to 10 years, although some larger species can live for as long as 20 years.
Most barnacles are hermaphrodites, possessing ovaries and testes in a single individual. In acorn barnacles, reproduction occurs through pseudocopulation, where a long retractable penis transfers sperm to a nearby barnacle.
The fertilized eggs are brooded internally until they hatch into free-swimming early nauplius larvae.
These larvae have a single eye, head, telson, and three pairs of limbs. They undergo further molting to the late nauplius stage.
They further transform into non-feeding cypris larvae, having a bivalved carapace and antennae. Using their antennae, they explore the substrate and attach head-first by secreting a glue-like cement. Once settled, they undergo metamorphosis into juveniles, eventually becoming mature adults.
The most common predators of barnacles are carnivorous snails called whelks, which grind through the calcite plates of their bodies. Adult barnacles are also preyed upon by starfish species, like purple sea star (Pisaster ochraceus), while their larvae are consumed by mussels (like Mytilus edulis) and sea squirts (like Styela gibbsi). In some parts of the world, like Spain and Portugal, humans eat barnacles and consider them a delicacy.
These animals are parasitized by turbellarian flatworms, like Stylochus, certain protozoans, trematodes, and isopods, such as Hemioniscus balani.
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