Barnacles are unique marine invertebrates often mistaken for mollusks due to their hard, cone-shaped shells. These fascinating creatures are actually crustaceans, sharing a lineage with crabs, lobsters, and shrimp. Adult barnacles lead a stationary existence, firmly attaching themselves to various hard surfaces in marine environments, such as rocks, boat hulls, or even the skin of whales. Despite their fixed position, they actively gather sustenance from the surrounding water.
Primary Food Sources
Barnacles are omnivorous organisms, primarily consuming microscopic particles suspended in the water column. Their diet largely consists of plankton, which encompasses both phytoplankton and zooplankton. Phytoplankton are tiny, plant-like organisms that drift in the upper layers of the ocean, performing photosynthesis. Zooplankton includes small, animal-like organisms, such as copepods, larval stages of other marine invertebrates, and even small fish larvae. Beyond living plankton, barnacles also ingest detritus, which is decomposing organic matter. This detritus includes fragments of dead algae, decaying animal remains, and other organic debris. By consuming these particles, barnacles act as filter feeders, processing organic material within their marine habitats.
The Mechanism of Filter Feeding
Barnacles capture food using specialized appendages known as cirri. These are feathery, leg-like structures, modified thoracic limbs that extend from the barnacle’s shell opening. When submerged, a barnacle opens the protective plates of its shell, exposing these cirri. The cirri then extend into the water, performing a rhythmic, sweeping or “kicking” motion. This action acts like a net, efficiently trapping microscopic food particles from the passing water. After capturing the particles, the cirri retract into the shell, scraping the collected food towards the barnacle’s mouth.
Environmental Influences on Feeding
A barnacle’s feeding success is shaped by its environment, particularly water movement and temperature. Water current plays a dual role; insufficient flow means fewer food particles are delivered, limiting its intake. Conversely, strong currents can impede the barnacle’s ability to extend its cirri or reduce capture efficiency. Research on Balanus glandula indicates that particle capture rates may peak at moderate water velocities (7.5 to 20 cm per second), but overall capture efficiency is highest at slower flows (around 1 cm per second). A barnacle’s location, whether in a nutrient-rich estuary or the open ocean, also dictates the density and type of available food.
Water temperature affects the barnacle’s metabolic rate and the seasonal abundance of its planktonic food sources. Studies show that Balanus glandula exhibits an optimal feeding temperature range, typically between 10 and 15 degrees Celsius. At higher temperatures (e.g., 25 degrees Celsius), feeding activity decreases due to changes in cirral beating. Similarly, at lower temperatures (e.g., 5 degrees Celsius), a slower cirri beating rate reduces food capture.