Barnacles are sessile marine invertebrates, a type of crustacean related to crabs and lobsters. Encased within a protective shell of calcareous plates, these organisms rely entirely on the surrounding water for survival and nutrition. They employ suspension feeding, actively straining microscopic particles from the water column to sustain themselves. Understanding their diet and feeding methods reveals their adaptation to life in the turbulent intertidal and subtidal zones.
The Primary Components of a Barnacle’s Diet
The barnacle’s diet consists almost entirely of seston, the collective term for particulate matter suspended in the ocean water. The largest component of this food source is plankton, encompassing both microscopic plant-like organisms and tiny animals. Specifically, they consume phytoplankton (single-celled photosynthetic organisms) and zooplankton, such as copepods and various invertebrate larvae.
The size of the food particle is a major factor in what a barnacle can successfully capture. Smaller barnacles tend to consume non-evasive prey, like single-celled algae, due to their smaller filtering structures. As barnacles grow, their filtering apparatus develops, allowing them to capture larger, more evasive prey, such as adult copepods. The diet is supplemented by organic detritus (non-living waste particles), providing a constant source of nutrients.
The Mechanics of Filter Feeding
Barnacles capture their food using six pairs of feathery appendages called cirri. These thoracic limbs are extended through an opening in the shell, known as the operculum, forming a basket-like net or fan to intercept passing particles. The cirri are lined with fine, hair-like structures called setae, which act as the filtering mesh.
The movement of the cirri varies depending on the water conditions and the specific barnacle species, demonstrating a flexible feeding strategy. In calm water, many species perform a rhythmic beating motion, rapidly extending and withdrawing the cirral net to actively create a feeding current that draws water and particles toward them. This active sweeping allows the barnacle to generate its own flow when the surrounding water is still.
When water currents are faster, the barnacle will often switch to a passive strategy, extending its cirral net and holding it rigidly in the current to net passing food particles. This prolonged extension requires less energy than rhythmic beating and is highly effective in environments with abundant water movement.
The cirri themselves are differentiated. The innermost pairs are shorter and used for handling and transferring captured food toward the mouth. The outermost pairs are longer and more robust, forming the primary sweeping and netting structure. Once the food is captured, the cirri flex inward, and the particles are scraped off and passed to the mouthparts for consumption.
Environmental Factors Influencing Feeding Cycles
The timing of barnacle feeding is governed by external environmental conditions, particularly those that affect the delivery of food particles. Tidal cycles are a dominant factor, as barnacles living in the intertidal zone can only feed when they are completely submerged in water. When the tide is out, the operculum remains tightly closed to prevent desiccation, pausing all feeding activity until the water returns.
Water current speed is another control on feeding behavior and success. Moderate water flow increases the flux of food particles past the barnacle, enhancing the capture rate. However, excessively fast flows can decrease the efficiency of the cirral net, as water can leak through the filtering mesh before particles are retained. The speed of the current also dictates whether the barnacle uses its active sweeping or passive netting strategy.
Water temperature also affects the barnacle’s feeding rate by influencing its metabolic activity. Most barnacle species have a thermal optimum for maximum cirral beating rates, often between 10 and 15 degrees Celsius for species like Balanus glandula. At temperatures outside this range, the rate of beating can decrease, reducing the amount of food ingested. Furthermore, some species exhibit nocturnal feeding, which may be a response to the higher abundance of zooplankton that migrate toward the surface at night or to a diminished risk of predation.