Barnacles are often mistaken for mollusks, like clams or oysters, due to their hard outer shell. In reality, a barnacle is a crustacean belonging to the subclass Cirripedia, making them relatives of crabs and lobsters. They spend their adult lives permanently anchored to a hard surface, encased in calcified plates. While the vast majority of the approximately 1,200 known species are native and play a natural role in their ecosystems, a few specific species have become highly problematic. These species are capable of traveling the globe, making them significant invasive aquatic species (IAS) that cause widespread ecological disruption and immense economic burden.
Barnacles in Natural Ecosystems
Native barnacle species are an important part of the marine environment, particularly in the intertidal zone. They are sessile filter feeders, using feathery appendages called cirri to capture plankton and small particles from the water. This process helps to filter and cleanse the water, contributing to overall water quality.
Barnacles also serve as a foundational food source for various predators, including whelks, sea stars, fish, and seabirds. The dense colonies they form on rocky shores act as “ecosystem engineers,” creating complex habitats that provide shelter and micro-environments for many other organisms. Their unique ability to survive the harsh conditions of the intertidal zone, such as extreme temperature changes and desiccation at low tide, is a testament to their evolutionary success.
How Invasive Species Spread Globally
The primary mechanism for barnacle invasion is biofouling, the accumulation of organisms on submerged man-made structures. Barnacles, specifically the free-swimming cyprid larvae stage, possess an adhesive substance that allows them to permanently anchor to virtually any hard surface. This strong, cement-like glue is a major factor in their ability to survive long-distance transport.
Global shipping is the most significant vector for this spread. Barnacles readily attach to the hulls of commercial vessels, pleasure crafts, and offshore infrastructure like oil rigs. Species like the bay barnacle (Amphibalanus improvisus) and the titan acorn barnacle (Megabalanus coccopoma) are aggressive biofoulers transported across natural biogeographic barriers. A single voyage can carry a population of barnacles from one ocean to an entirely new region, where they can reproduce and establish themselves.
While hull biofouling is the main issue, the uptake and discharge of ballast water also serves as a secondary vector for larval transport. Ships take on ballast water in one port for stability and release it in another, potentially carrying non-native barnacle larvae. This combination of high adhesion strength and the volume of global marine traffic explains how these species have become cosmopolitan, or globally distributed.
Environmental and Economic Consequences
The introduction of invasive barnacles has measurable consequences for both marine ecology and the global economy. Ecologically, these species compete directly with native organisms for space and food, often outcompeting them due to rapid growth and high settlement rates. For instance, the invasive barnacle Austrominius modestus has displaced native barnacle species in certain European estuaries.
The establishment of non-native barnacles can significantly alter the structure of a local marine community, reducing biodiversity. Invasive barnacles form dense colonies that monopolize space on hard substrates, preventing native filter feeders and other organisms from establishing themselves. This displacement can have cascading effects throughout the ecosystem, impacting the local food web.
The economic burden of barnacle biofouling on the shipping industry is substantial, costing billions of dollars annually. The accumulation of hard-shelled barnacles on a ship’s hull drastically increases hydrodynamic drag, the resistance experienced as the vessel moves through water. Even 10% coverage of barnacles on a hull can require a ship to increase its shaft power by 36% to maintain the same speed.
This added drag directly results in higher fuel consumption, increasing operational costs for shipping companies. The excess fuel burn also leads to an increase in greenhouse gas emissions; some estimates attribute at least 110 million tonnes of excess carbon emissions per year to barnacle biofouling across the global commercial fleet. Furthermore, significant resources are spent on anti-fouling coatings and regular hull cleaning, adding millions to maintenance budgets.