Barnacles are marine invertebrates belonging to the subclass Cirripedia within the phylum Arthropoda, related to crabs, lobsters, and shrimp. Unlike their mobile crustacean relatives, adult barnacles adopt a sessile, or stationary, existence, permanently cementing themselves to a hard surface. This fixed lifestyle has driven the evolution of a unique anatomy, modifying the standard crustacean body plan to suit life within a protective shell. Their specialized physical structure moves inward from a durable external shield to an intricate feeding system and adapted internal organs.
The External Fortress: Protection and Anchoring
The most distinguishing feature of the adult barnacle is its external shell, a rigid, multi-plated structure secreted from calcium carbonate. This structure provides robust defense against predators and desiccation in the intertidal zone. In common acorn barnacles, the shell is typically a conical structure composed of six interlocking parietal plates that form the wall.
The wall plates grow by adding material, expanding the barnacle’s living space as the internal body grows. The top of the shell has an opening covered by an operculum, a hinged “door” made of two pairs of movable plates called the scuta and terga. When the tide recedes or the animal senses danger, strong muscles pull the scuta and terga plates shut, sealing the barnacle inside to retain moisture and offer protection.
Barnacles achieve permanent attachment using a specialized, fast-curing biological adhesive secreted by a cement gland. This gland is located near the vestigial antennae at the animal’s head, meaning the barnacle is fixed to the substrate head-first. The resulting cement is one of the strongest natural glues known, allowing the barnacle to resist the immense forces of crashing waves and currents.
The exact form of attachment differs between the two main types of barnacles. Acorn barnacles are unstalked, directly cementing their calcareous base plate to the substrate. In contrast, goose-necked barnacles are stalked, attaching to surfaces by a flexible, muscular stalk, or peduncle, which is tipped with the cement gland.
The Filter-Feeding Apparatus and Digestive Pathway
Once anchored and submerged, the barnacle opens its opercular plates to extend the highly modified thoracic appendages used for feeding. These six pairs of limbs are known as cirri, which are feathery, jointed legs swept through the water column. The Latin name for barnacles, Cirripedia, translates to “curl-footed,” a direct reference to these feeding appendages.
Each cirrus is biramous (branched) and lined with numerous setae, which are fine bristles that create a net for capturing suspended food particles. The barnacle uses the cirri in a sweeping motion to filter plankton, detritus, and other organic matter from the water. This feeding behavior can be active, where the cirri are rhythmically beaten to create a current, or passive, where they are held extended into a strong water flow.
As a non-selective suspension feeder, the barnacle captures particles caught in its cirral net and transfers them toward its mouth. The captured food is manipulated by smaller mouthparts before entering the digestive tract. This pathway includes a stomach for initial processing and a midgut for nutrient absorption, with waste products exiting through an anus located within the mantle cavity.
Core Internal Systems: Reproduction and Sensation
Barnacles possess a unique reproductive strategy, as most species are simultaneous hermaphrodites, meaning each individual has both male and female sex organs. Cross-fertilization with a neighbor is the common method of reproduction, overcoming the challenge of their sessile adult lifestyle. To achieve this, barnacles extend a long, flexible penis to reach nearby individuals. Fertilized eggs are brooded internally within the mantle cavity until they hatch into free-swimming nauplius larvae.
The adult barnacle’s nervous system consists of ganglia, or nerve bundles, concentrated in the head and thorax regions. Sensory hairs on the cirri detect water movement and food particles, triggering the feeding response. The internal body is encased within the mantle, which creates a cavity where gas exchange occurs. Barnacles do not possess gills or a true heart, relying instead on the inner membrane of the mantle and the surface of the cirri for respiration. Circulation of nutrients and gases is accomplished through simple diffusion and the movement of hemolymph within the body cavity.