Barnacles are common marine animals found attached to various surfaces, from rocks to ship hulls. Though they appear simple, these crustaceans possess intricate internal structures that allow them to thrive in harsh environments. Understanding their inner workings reveals how these creatures feed, reproduce, and maintain their unique existence.
The Outer Shell and Inner Body
Barnacles are encased within a robust, multi-plated shell composed of calcium carbonate, providing a formidable defense against predators and environmental stresses. Most barnacles, like the common acorn barnacles, develop six hard calcareous plates that form a conical structure, with four additional plates forming an operculum, or “door,” at the top. This operculum can open to allow feeding appendages to emerge or close tightly to protect the soft body inside, especially during low tide to prevent desiccation. Some species may have variations in the number of these parietal (wall) plates or the presence of a mineralized basal plate that adheres to the substrate.
Inside this protective shell, the barnacle’s soft body is positioned in a unique, inverted manner. The animal lies on its “back” or “stomach,” with its appendages oriented upwards. A thin layer of tissue called the mantle lines the interior of the shell, enclosing the barnacle’s main body. This mantle secretes the calcareous shell plates, allowing the barnacle to grow by adding new material, and forms a mantle cavity, a significant space within the shell where many vital functions occur.
Internal Anatomy and Function
Within its protective shell, the barnacle houses a range of specialized organ systems that support its sessile lifestyle. Its primary method of obtaining food involves filter feeding, utilizing specialized appendages called cirri. These six pairs of feathery, multi-jointed thoracic limbs extend rhythmically through the opercular opening into the water column. The cirri act like a net, sweeping through the water to capture plankton and other tiny organic particles, which are then guided towards the mouth. Each cirrus is lined with small, hair-like structures called setae that aid in trapping these minute food particles.
Once food particles are captured by the cirri, they are processed by the barnacle’s digestive system. The mouthparts, including mandibles and maxillae, manipulate the food before it enters the gut. The digestive tract includes a stomach and associated digestive glands, where enzymes break down the ingested material for nutrient absorption. The barnacle possesses an open circulatory system, meaning its blood, or hemolymph, circulates within a body cavity called a hemocoel rather than being fully contained within vessels. While barnacles lack a true heart, muscles surrounding a sinus near the esophagus help to pump hemolymph throughout the body.
Barnacles are unique in their reproductive biology, with most species being hermaphroditic, meaning they possess both male and female reproductive organs. The ovaries are located in the base or stalk of the barnacle, potentially extending into the mantle, while the testes are situated towards the back of the head. Although self-fertilization is possible, it is rare; barnacles generally reproduce through cross-fertilization with neighboring individuals. This is often achieved by extending an exceptionally long penis to transfer sperm into the mantle cavity of an adjacent barnacle. Gas exchange, or respiration, primarily occurs across the surface of the mantle cavity and the cirri, where oxygen is absorbed directly from the surrounding water. The nervous system consists of ganglia and nerve cords, with a central nervous system comprising a brain and a posterior ganglion. The main sense for barnacles appears to be touch, with the hairs on their limbs being particularly sensitive.
Life Cycle and Attachment
The life cycle of a barnacle involves a complete transformation from a free-swimming larval form to the sessile adult. It begins with eggs hatching into the first larval stage, known as the nauplius larva. This microscopic, one-eyed larva has a head region and three pairs of limbs, which it uses for swimming. The nauplius undergoes several molts, shedding its exoskeleton as it grows, and actively feeds on plankton in the water column.
After passing through multiple naupliar stages, the larva transforms into the cyprid larva, the second and final free-swimming stage. Unlike the nauplius, the cyprid larva does not feed; its primary purpose is to locate a suitable surface for permanent attachment. The cyprid actively explores potential surfaces using modified antennules, which possess adhesive discs, allowing it to “walk” over submerged substrates. During this exploration, the cyprid assesses various factors like surface texture, chemistry, and the presence of biofilms or other barnacles, which can act as cues for settlement.
Once an appropriate site is selected, the cyprid larva attaches itself head-first to the surface using specialized cement glands located at the base of its antennules. These glands secrete a powerful, protein-based adhesive that creates a permanent bond to the substrate. This natural quick-setting cement is remarkably strong, capable of holding significant weight. Following this irreversible attachment, the cyprid undergoes a dramatic metamorphosis into the juvenile barnacle. During this process, the larval body reorganizes, the cyprid’s carapace is shed, and the characteristic shell plates and internal adult structures begin to form, cementing its lifelong, stationary existence. While typically requiring attachment, some cyprids can metamorphose into juveniles while still in the water column.