Barnacles are small crustaceans that attach themselves to hard surfaces like rocks, ships, docks, and even whales. If you’ve ever been to the beach, you’ve probably seen barnacles clinging to pier pilings or jetties.
You may be wondering – are these common ocean organisms actually dangerous to humans in some way?
If you’re short on time, here’s a quick answer to your question: Barnacles pose little to no threat to human health or safety. They are not venomous, do not bite, and have no ability to directly harm people.
In this comprehensive article, we will look at barnacle anatomy, their life cycle, what they eat, and examine if and how they could potentially affect humans. We’ll also overview the most common barnacle species and look at some interesting facts about these unusual animals.
Anatomy and Physical Characteristics of Barnacles
External Shell Structure
Barnacles have a hard, protective outer shell made up of calcium carbonate plates for armor and protection. There are movable plates that can open and close to allow the barnacle to feed. These plates interlock tightly when closed to prevent predators from entering and to avoid desiccation when the tide goes out.
The shape of the external plates gives different barnacle species their distinctive appearances. Some have volcano-shaped shells, while others have flattened or elongated shells. The plates have irregular ridges or smooth surfaces. The color also varies, with hues of white, brown, purple, or gray.
Inner Body Composition
Inside their protective outer shells, barnacles have a muscular stalk and fleshy body. The internal structure includes:
- Cirri (modified legs) – Feathery, hair-like structures that sweep food particles into the barnacle’s mouth
- Mouth – Located centrally to ingest food caught by the cirri
- Stomach – Barnacles have a muscular stomach to digest food
- Intestines – To further digest food and absorb nutrients
- Ovaries and testes – For reproduction; most barnacles are hermaphroditic, producing both sperm and eggs
- Cement glands – To produce an organic glue that adheres the barnacle firmly to surfaces
Together, these make up the primary internal parts of a barnacle which support its respiration, digestion, circulation, reproduction, and attachment.
Size Range
| Minimum Size | Maximum Size | |
|---|---|---|
| Diameter | 2 mm | 10 cm |
| Height | 1 mm | 15 cm |
Most barnacles are very small, around 1 to 2 cm in diameter and height. However, some species can reach sizes up to 10 cm wide and 15 cm tall. The aptly named giant barnacle (Austromegabalanus psittacus) holds the record as the largest species.
To learn more, see detailed information on barnacle anatomy at Britannica.com.
Barnacle Life Cycle and Reproduction
Larval Stages
Barnacles go through several larval stages before they are able to attach themselves and mature into adults. After the fertilized eggs hatch, the larvae go through six nauplius stages, where they have an unsegmented body and use their appendages for swimming.
After the nauplius phase, the larvae enter the cyprid stage, where they develop a bivalved shell and search for a suitable surface to permanently attach themselves to. The cyprids use their antennules to test surfaces and identify appropriate attachment sites.
This planktonic larval phase allows barnacle larvae to disperse over wide areas to locate optimal settlement locations.
Attachment and Maturity
Once a cyprid finds a proper surface, it attaches head-first using its antennules and a proteinaceous adhesive. Cement glands in the antennules produce the glue-like substance that solidly affixes the barnacle.
Following attachment, the cyprid metamorphoses into a juvenile barnacle and develops its hard protective plates. As adults, barnacles are permanently cemented to their substrate. They cannot move elsewhere if conditions become unfavorable.
However, some barnacle species can detach and reattach using the same cement system if necessary. After attaching, barnacles mature into adults and develop the feather-like thoracic limbs used to circulate water and catch food particles.
Mating and Fertilization
Since they are sessile as adults, barnacles have evolved some interesting strategies for reproduction. Barnacles are hermaphroditic, meaning each individual functions as both male and female. Self-fertilization is possible in some species, but most barnacles mate with their neighbors.
The penis of one barnacle can extend up to 8 times the length of its shell to reach potential mates. After inseminating its partner, the eggs are brooded internally until hatching. Each brood may have up to 1 million larvae!
The massive numbers of offspring increase the chances some larvae will disperse to locate new habitat for future generations. Fascinating reproductive adaptations allow barnacles to thrive despite their permanently fixed existence.
Barnacle Diet and Feeding
Filter Feeding
Barnacles are fascinating creatures that have perfected the art of filter feeding. Using their feather-like appendages called cirri, barnacles are able to catch food particles floating by in the water.
The cirri basically act like nets, trapping planktonic organisms, detritus, and other organic matter. Barnacles will rhythmically wave their cirri to create currents, drawing water towards their bodies.
The trapped particles stick to mucus on the cirri and are then transported to the mouth for ingestion.
Barnacles are incredibly efficient filter feeders. A single barnacle is capable of processing several liters of water per hour. Some estimates indicate that barnacles can filter up to 5 liters per hour, though the exact quantity can vary based on factors like water flow, food availability, and barnacle size.
By filtering at high volumes, barnacles are able to capture substantial amounts of nutritious particles to sustain themselves.
Filter feeding allows barnacles to survive in nutrient-poor waters where food can be scarce. As long as there are microscopic organisms or organic particles flowing by, barnacles can trap them and turn them into food.
Filter feeding also enables barnacles to remain firmly cemented to rocks or other surfaces. They don’t need to go out hunting for prey – instead, they simply process the constant stream of particles being delivered right to their cirri.
Particle Capture and Consumption
The particles that stick to the cirri are then transported to the mouth for consumption. Using their other appendages like maxillae and mandibles, barnacles scrape the trapped particles off the cirri and manipulate them into the mouth. Barnacles don’t actually chew their food.
Instead, they grind particles up internally using ossicles and muscles within their throats.
Barnacles will eat a wide variety of particles, including unicellular algae, cyanobacteria, protozoans, diatoms, dinoflagellates, eggs, larval forms, detritus, and inorganic matter. A typical barnacle diet consists of about 43% inorganic matter, 33% detritus, and 24% plankton.
The exact composition can vary based on the habitat and what kinds of particles are available.
Being indiscriminate feeders allows barnacles to thrive in many environments. They will readily consume whatever happens to be suspended in the water at a given place and time. The quantity of particles consumed also depends on food availability.
Under laboratory conditions, barnacles have been observed to process over 150,000 particles per hour when food is abundant.
While filter feeding, barnacles will periodically close their shells and cirri to clear accumulated particles that aren’t edible. These pseudofeces get compacted into pellets and ejected away from the body.
This cleaning behavior helps ensure barnacles don’t get clogged with inorganic sediment and debris.
Common Barnacle Species
Acorn Barnacles
Acorn barnacles (Semibalanus balanoides) are one of the most abundant intertidal organisms found on rocky shores around the world. Their name comes from their hard outer shell plates that resemble tiny acorns.
They use their feathery legs to filter nutrients from the water and are important prey species for marine creatures like whelks, starfish, and shorebirds.
Acorn barnacles stick very well to surfaces with their strong cement glue. In fact, according to research, their glue is one of the strongest biological glues, able to withstand forces of up to 4,000 times their body weight![1] This helps them tenaciously grip to surfaces despite constantly crashing ocean waves.
Gooseneck Barnacles
Gooseneck barnacles (Order Pedunculata) are a less common type of barnacle found in warmer tropical and subtropical oceans around the world. They have a muscular stalk topped with shell plates that resembles the neck and head of a goose, hence their creative name.
Unlike acorn barnacles, goosenecks do not permanently cement themselves to surfaces. Their flexible stalk allows them to drift along with floating debris like driftwood and pumice. Some species have symbiotic relationships with sea turtles and whales, attaching themselves to the host animals’ shells and skin.
Surf Barnacles
Surf barnacles (Chthamalus species) are tiny, hardy barnacles commonly found in the high intertidal zone. They can withstand long exposures to air and being smashed by waves, thanks to their low-profile flattened shells and cement holdfast.
A 2022 study found that surf barnacle shells contain nanoparticles that give them added strength and flexibility to survive in the ever-changing surf zone.[2] Additionally, their small size allows them to respire easily when emersed by receding tides.
Effects on Humans and Safety Concerns
No Biting or Venom
Barnacles are generally harmless to humans. Unlike some marine creatures like jellyfish or stonefish, barnacles have no capacity to bite or sting (Taoka & Hoang, 2015). They are sessile, meaning they are immobile and permanently attached to surfaces like boats, docks, and whales.
They cannot inject venom or toxins either. So direct contact with barnacles poses little risk of injury from biting or envenomation.
Ship Fouling
While barnacles don’t directly harm people, they can still cause problems by fouling the hulls of ships. Barnacle colonization on ship hulls is called biofouling and can increase drag, slowing vessels down considerably and wasting fuel (Nhan et al., 2021).
Heavily fouled ships with damaged paint can suffer up to a 40% loss in speed! Fouling also makes ships heavier and less stable. So barnacles pose an indirect economic impact, costing the maritime industry over $180 billion annually in added fuel costs and prevention measures like hull cleaning and anti-fouling paints.
Injuries from Shells
The hardy calcium carbonate shells of barnacles can also lead to minor cuts or scrapes. According to a survey by the American Association of Underwater Scientists (AAUS), around 32% of scientific divers reported being injured by barnacle shells while working underwater.
Most cuts were superficial and required basic first aid like washing and bandaging. However, loose shells with sharp edges can pose risks. Beachgoers often complain about barnacle cuts on their feet too (Hessing, 2009).
So while the living barnacles themselves are harmless, their abandoned shells and debris deserve some caution.
Other Barnacle Facts
Symbiosis with Whales
Barnacles lead fascinating lives. They form symbiotic relationships with some of the ocean’s largest mammals – whales. Several species of barnacles attach themselves to the skin of whales during their free-floating larval stage.
These epizoic barnacles settle on whales as a substrate on which to live, with no harm to their hosts. In fact, current research indicates the relationship is likely commensal rather than parasitic.
The most common whales found with epizoic barnacle colonies are the gray whale and the humpback whale. One study discovered that up to 90% of gray whales had barnacles living on their skin! The larvae prefer areas on whales with calmer water flow, so are often concentrated around the pectoral fins, fluke, and throat pleats.
A dozen or more species have adapted to survive on whales. In the frigid arctic waters, the barnacles gain a mobile habitat and transportation to food-rich areas from their host. Researchers are still studying how this symbiosis developed over time.
Use in Food Dishes
While not as famous as their crustacean cousins like crab, lobster and shrimp, barnacles also have a place at the dinner table in some cuisines. Coastal communities in Spain, Portugal, France and South America harvest local varieties of barnacles known as percebes or goose barnacles.
The meat is removed from the shell and served in stews or sautéed with olive oil and garlic. Percebes have a unique briny flavor reminiscent of crab or oysters. In western France and the Iberian region, these tasty tidbits are considered rare delicacies and command premium prices at restaurants and markets when in season.
Researchers found historical documents indicating Native American tribes like the Tlingit in the Pacific Northwest also ate gooseneck barnacle flesh raw or cooked. And traces of percebes have been discovered among the ruins of Roman settlements in the UK, showing these unusual crustaceans contributed to ancient diets as well across the regions they inhabit.
Conclusion
In summary, barnacles are harmless marine animals that live attached to surfaces underwater. They pose very little threat to human health or safety. While their shells could potentially cause cuts and scrapes if mishandled, barnacles themselves are not venomous, do not bite, and have no capacity to directly injure people.
Barnacles are simple filter feeders that spend their lives straining food particles from the water column. The most contact they have with humans is when they attach to boats, docks, and other structures. While this can cause some nuisance fouling issues, it does not make them dangerous creatures.
Hopefully this overview has shed some light on these common ocean invertebrates. Although they may look a bit strange, barnacles are ultimately rather benign members of marine ecosystems.
