by Peter Jiang


Chthamalus Stellatus, a species of barnacle

external image Balanus_glandula2DLC2005.jpg

Balanus glandula barnacles attached to a cobble

Classification/Diagnostic Characteristics:

A barnacle is classified as a crustacean, which are in turn a part of the phylum arthropoda. Athropods are made of invertebrate organisms that have hard exoskeletons protecting the majority of their bodies, segmented bodies, and jointed appendages. Crustaceans are the dominant marine athropods. This means that they are almost always found in or near water and wet areas. In addition to barnacles, the crustaceans include organisms such as shrimp, lobster, and crabs. [1]

Barnacles share most of its crucial traits with the other crustaceans, especially the hard, protective exoskeleton. However, barnacles are also unusual, as they are the only members the of crustaceans that are sessile, or inactive, as adults. They are completely stationary. This is because they are permanently attached to a solid substrate. This makes the barnacle a highly unusual crustacean since it is the only crustacean that are sessile and not not move freely on their own, being firmly and permanently attached to a solid surface, unlike shrimps and lobsters.[1]

Barnacles are found in saltwater environments and can be identified by their small size and a protective shell-like covering. Many species of barnacles can be seen attached to the hard surfaces of rocks, ships, other structures in long-term contact with saltwater, and even some marine animals. The [[#|appearance]], colors, and sizes of barnacles differ based on their species.[6]

Barnacles on a boat
Barnacles on a boat
Barnacles on a whale
Barnacles on a whale

Barnacles are positioned upside down on a substrate and have feathery appendages known as cirra protruding outwards. Barnacles can have stalks, like the goose barnacle, named for its stalk shaped like a short [[#|goose neck]], or are stalkless and completely protected by a wall of plates, such as the Chthamalus Stellatus, or acorn barnacles shown above.

Relationship to Humans

Barnacles attach to solid substrates. This by extension means that they will attach to solid man-made objects and structures in water.
Some barnacles have also been considered edible by humans (for example goose barnacles are a specialty in Spain and Portugal).
(J. Molares & J. Freire. "Fisheries and management of the goose barnacle //Pollicipes pollicipes// of Galicia (NW Spain)")

Barnacles attach themselves permanently to ships, rocks, and other marine animals. Barnacles on the hull of a [[#|[[#|[[#|ship]]]]]] increase friction and can reduce the vessel's speed. The ship must then be put in dry dock to have the bottom [[#|scraped]]. To prevent barnacles from clinging to ships, the hulls are either treated with toxic paint containing tin or copper, or are coated with plastic.[4]

Barnacles attached to ships are incredibly detrimental to their function, costing up to 125 million dollars for ship owners. Not only is removal costly and difficult, but in just six months times they can grow enough to weigh down the ship to the point of requiring 40-45% more fuel to maintiain cruising speed.

Habitat and Niche

Barnacles live exclusively in wet areas, mostly oceans. They do not actively move throughout the water their whole lives, instead permanently attaching themselves to solid objects in the water before growing into adults. They primarily live in a shallow level of water, mostly the intertidal zone.

Different barnacle populations can occupy specific areas even in close proximity to each other. For example, rock barnacles overtake stellate barnacles in competition due to far faster growth, but physically cannot go higher than a certain height in the water, potentially creating two very different barnacle populations, one living literally just above the other.
external image J1.PrHuYlU9y17XM0SfnVw_m.png
Two different barnacle populations next to each other.[1]

Barnacles will also live on marine life such as whales, on which they attach themselves and gather food as they travel through the ocean. This is a case of commensalism, as the presence of barnacles causes no harm to the marine life.

In part their poor skeletal preservation is due to their restriction to high-energy environments, which tend to be erosional – therefore it is more common for their shells to be ground up by wave action than for them to reach a depositional setting.[5]

Predator Avoidance

Barnacle larvae, after being dispersed, will find a variety of areas with a solid space to attach to, in an effort to find some areas that are suitable for colonization, such as areas with few predators.[1]

Barnacles hide in groups as a predator avoidance mechanism because animals living in groups are more likely to survive than solitary animals. Group behavior can affect the [[#|psychology]] of a barnacle in good ways and bad ways. For example, barnacles living in groups spend less time hiding when faced with a perceived threat than do solitary barnacles. A group is able to spend less time hiding because as a unit, they may be able to fight off the intruder or escape from it. One barncale does not have the strength to fight off a predator such as a mussell or a whelk. So, it may either starve due to lack of foraging or get eaten by the predator. This is how being a member of a group helps. But, they have a flaw too. Barnacles raised in groups may be less tentative of avoiding predators if they encounter one alone. Predators will avoid the barnacles more when the barnacles are in a group because they can lose a lot of valuable energy fighting them. They want to find prey by using as little energy as possible, so they will go off to hunt for something else. Barnacles may think that a predator is actually afraid of them, so when they are alone, they have this same mindset. Of course, the result is pretty ugly.

Nutrient Acquisition

Barnacles use a process of filtering to acquire nutrients from the surrounding ocean currents. Equipped with limbs that are fringed with cirri, barnacles sweep the water to find nutrients, which they then drag towards their mouths. Specialized appendages near the mouth wipe off and sort any particles the barnacles locate, such as organisms up to one millimeter in length and miniscule (1/500 millimeters in length) single-celled plants or bacteria. Barnacles use feeding currents to their advantage while acquiring food and capture nourishment in proportion to their size, with larger barnacles eating larger organisms.

Reproduction and Life Cycle

Crustaceans typically begin life as eggs that have been fertilized and attached to the outside of an adult's body. For barnacles, after developing into the larval stage, they are dispersed by the adult barnacles. The larvae are similar in form to the adults, but in a very small state. The larvae swim until finding a solid area to colonize, where they then attach themselves, where they grow into adults.[1]

Unlike most crustaceans, barnacles are sessile as adults, remaining in fixed positions indefinitely.[1] Because of this, Barnacles have developed a special way to produce offspring without a mate; they are hermaphrodites, which means that they have both male and female reproductive organs. They can produce both sperm and eggs.[2]

The barnacle goes through a mobile larvae stage and an immobile adult stage. When they first hatch from their eggs, the nauplius (the larvae stage) swims through the water. As the larvae grows in size, they will shed their shell and reach the cyprid stage (final larvae stage). The cyprid then looks for a rocky environment, where they will attach to a rock near other adult barnacles. After settling into their home, the cyprid will then shed one last time, becoming an adult.[3]

lt it is also important to note that barnacles are hermaphroditic animals, meaning each individual has both a male and female reproductive organ. Also, they do not fertilize by sending their respective gametes straight into the ocean, they do it by actual fertilization and physical reproduction. Because they reproduce through sexual reproduction, barnacles have extremely long penises due to their inability to move from their fixed position.

external image ESCI296CRUSTA002.jpg
Barnacle reproductive cycle

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Anatomy of an adult barnacle

Growth and Development

A barnacle begins life as a larvae sent out from adult barnacles, which are already somewhat similar in form to the adult barnacles themselves. After the planktonic larvae settle on a surface in the intertidal zone, they start to grow into adult barnacles.[1]

Like most crustaceans, barnacles have a head, thorax, and abdomen.[1]

Young barnacles hatch from eggs and develop into free-swimming larvae that resemble tiny crabs. The young barnacles secrete a gluelike substance which they use to attach themselves to suitable surfaces. They slowly change into jellylike creatures the size of small marbles. The barnacle's legs stick out like tentacles from an opening in the top of the shell. At their tips are cirri, hairlike strands that sweep plankton and other food into the mouth.[4]


Similar to all crustaceans, barnacles are protected by a hard exoskeleton, often called a "carapace," protecting the head and thorax.[1]

Barnacles can be white, pink, grey, or brown, depending on the species or stage of life.


After settling onto a surface, barnacles are sessile, meaning they have taken a fixed position. Afterwards, the barnacles are largely passive in movement. There are discreet movements that it may do in response to presence of food particles. They may extend specialized appendages for specific purposes, such as filtering the food particles from the water as it flows by and moving the particles towards their mouths.[1]

Sensing the Environment

Barnacles are lined with hairs on their exteriors that give it a very high sense of touch, allowing them to react accordingly to its environment. Barnacles, when feeling a sense of urgency and predation, will turn themselves over to their other side and create a water system against the ground for themselves. They adapt to the environment they are given.

Gas Exchange

Barnacles facilitate gas exchange through specialized appendages. This exchange primarily happens through the cirri and cell membrane because they lack cell walls.

Waste Removal

Crustaceans get rid of their waste by excreting through areas called nephridia.
Nephridia are similar to kidneys and are very common in invertebrates.

Environmental Physiology (Temperature, Water, and Salt Regulation)

external image barnacle.gif

Internal Circulation

Barnacles do have mouths, which they use to take in food captured and filtered from the water by cirra. They have organ systems used for breaking down and distributing the food.

"The barnacle has no heart or circulating system as found in higher animals. Body fluids flow through passages among the muscles and other organs in a 'lucunar' circulation."

A barnacle has a simple open circulatory system seen in various crustaceans. The blood fills the entire animal and is pumped by a heart around the body. (BHu)

The circulation of blood around the barnacle is not a closed system. The blood fills the entire animal and is pumped around the body.

Chemical Control (Endocrine System)

Barnacles have cirri that feeds the food it captures into its mouth. The food then travels down the mouth to a short esophogous.
Eventually, as a waste product, the barnacle disperses water and oxygen to the outside environment.

Review questions

1. How do barnacles reproduce?

2. What type of relationship is illustrated by the interaction between the barnacle and a whale?

3. What are the several functions of a barnacle's cirri?


[1]. Hillis, David M., David Sadava, H. C. Heller, and Mary V. Price. Principles of Life High School Edition. Sunderland, MA: Sinauer Associates, 2012. Print.
[5]. William Thomas Calman (1911). "Barnacle". In Chisholm, Hugh. Encyclopædia Britannica (11th ed.). Cambridge University Press.