Barnacles are marine crustaceans, distantly related to crabs and lobsters, belonging to the subclass Cirripedia. Unlike most of their mobile relatives, adult barnacles adopt a unique sessile lifestyle. This adaptation allows them to thrive in diverse marine environments, from intertidal zones to deeper waters, often forming dense colonies. Their peculiar appearance, encased in a hard shell, often leads to misconceptions about their true animal nature.
The Protective Outer Shell
The external structure of a typical barnacle, particularly acorn barnacles, is a robust, calcified shell. This shell primarily consists of several interlocking plates made largely of calcite, a form of calcium carbonate. Acorn barnacles typically have six main wall plates, known as parietal plates, that form the conical or barrel-like body of the shell. These plates are secreted by the barnacle’s mantle and grow by adding new material.
At the apex of the shell, a movable “lid” called the operculum provides further defense. This operculum is composed of two pairs of hinged plates: the scutum and the tergum. These opercular plates can open to allow the barnacle to feed and respire, but they can also close tightly when exposed to air or threats, effectively sealing the organism inside. The base of the barnacle, which adheres to a substrate like a rock or a ship’s hull, is often a calcified plate or a membranous structure, secured by a strong natural cement produced by the barnacle itself.
The Living Organism Within
Inside its protective shell resides the soft-bodied barnacle, a complex arthropod. The animal lies on its back within the shell, with its head attached to the substrate by cement glands. The body is largely divided into a head and thorax, with a minimal or absent abdomen, and its segmentation is often indistinct.
The mantle cavity is a space enclosed by the mantle, a double fold of the body wall. Within this cavity, the barnacle extends its feathery feeding appendages, known as cirri. These are modified thoracic limbs which protrude through the opening of the operculum. The rudimentary head possesses a vestigial pair of antennae and mouthparts positioned to receive food captured by the cirri. Internal organ systems, including a digestive tract, nervous system, and reproductive organs, are all contained within this compact body plan.
Life’s Processes Inside
The cirri function as filter-feeding mechanisms. When submerged, the opercular plates open, and the barnacle rhythmically extends and retracts its cirri into the water column. These feathery appendages, equipped with fine hairs called setae, act like a net, sweeping through the water to capture plankton and other small organic particles. The captured food particles are then drawn towards the mouth by the coordinated beating of the cirri, allowing the barnacle to ingest them.
Respiration in barnacles occurs directly through the surface of their bodies and the cirri, as they lack gills. Oxygen is absorbed from the water passing over these surfaces within the mantle cavity. The flow of water generated by the cirri during feeding also facilitates this gas exchange. Barnacles are hermaphroditic, possessing both male and female reproductive organs, an advantage for a sessile organism. Fertilization involves cross-fertilization, where one barnacle extends a long, flexible penis to transfer sperm to a nearby individual. The eggs are then brooded within the mantle cavity until they hatch into free-swimming larvae, dispersing to find new surfaces for attachment and growth.