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Today on Animal Fact Files we’re discussing barnacles. There are over 1,000 living barnacle species. The class that barnacles are classified in can be split into three different groups: stalked and acorn barnacles, burrowing barnacles, and parasitic barnacles. The majority of this video will be focusing on the stalked and acorn barnacles. Both burrowing and parasitic barnacles lack a shell. Burrowing barnacles burrow into rocks and coral while parasitic barnacles live inside the reproductive tract of crabs and other invertebrates.

While they might look like clams, acorn and stalked barnacles are actually crustaceans and more closely related to lobsters and crabs than they are mollusks. Barnacles can be found worldwide from the tropics to polar regions in marine water systems. Though they can be found more than 6,500 feet (2,000m) below the waves, they are most closely associated with intertidal areas where they experience both water and open air conditions. In order to avoid drying out while exposed to the atmosphere, barnacles use their shell. Moveable plates that protect the barnacle living inside the self-secreted calcareous shell close when the barnacle is not underwater. This helps trap in moisture and keeps the barnacle safe from desiccation.

These plates are like their rear-end as barnacles settle to the sea floor head first. Barnacles are hermaphroditic meaning they have both male and female reproductive organs. They depend on cross fertilization with other nearby barnacles in order to produce fertilized eggs. Often, when choosing a spot to settle down as larvae, barnacles search for places with a few conspecifics already in place. This is because barnacles, on average, do not broadcast spawn and instead rely on internal fertilization. In order to be able to reach their partners, these animals have developed the largest penises in the world in relation to body size. Barnacles may be anywhere from less than an inch to more than 4 inches in length, and their penises can reach up to eight times the length of their bodies. Fertilized eggs are brooded in the barnacle’s body and are then released into the water as free swimming larvae. These larvae eat and grow until their final stage at which point they stop eating and look for a place to settle down. This whole process takes about two weeks on average. Once a location has been selected, the barnacle secretes a cement-like substance that holds the barnacle in place. It will remain there for the rest of its life, which may last upwards of thirty years in larger species!

Many barnacles attach to hard surfaces like rocks, though they may also attach to human made places like ship hulls, or even other animals like whales and sea turtles. Acorn barnacles attach directly to substrate while stalked barnacles have… well… a stalk. These are also known as goose necked barnacle and were once believed to be where the barnacle goose came from. Threats to barnacles include whelks, sea stars, sea turtles, crabs, nudibranchs, and even people who may boil them or eat them raw. In turn, barnacles are filter feeders who subsist on planktonic organisms and debris floating through the water column. Stalked barnacles tend to rely on wave motion for feeding and are more likely to be found in areas with high wave activity while acorn barnacles are often found in places with tidal changes and water currents that bring food to them. They eat by catching particles of food on outstretched limbs, called cirri, which are then pulled into the shell and cleaned of food particles. Since they attach headfirst, we like to think of these limbs as legs kicking about in the water, what do you think?

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