Wildlife of the Week!

© Alexa Desautels. A vast congregation of acorn barnacles is spotted along the intertidal zone on Vancouver Island, British Columbia.

This Wildlife of the Week we introduce a fascinating creature that you may not have thought about much before! The acorn barnacles (Order Sessilia) are sessile, rock-like invertebrates. However, these animals are fascinating in their nature regardless. The acorn barnacle is a mainstay icon of many coastal areas, but is arguably overlooked by many due to its subtle appearance. Acorn barnacle is a common name for a variety of different species. Today though, the acorn barnacle receives full attention and we urge you to read on about this marvellous critter! To learn more, read on below!

The acorn barnacle may have a “tough” appearance, with its thick shell and often closed-off interior, but this is what helps the animal survive in its environment. The hard shell is calcareous in make, and is typically cone-shaped, encasing the vulnerable creature inside. Depending on the species involved, acorn barnacles can reach sizes of a few to many cm (10+). The exterior of the barnacle is usually off-white/grey, with some yellow coloration towards the top. The conical shape is usually composed of six overlapping shell parts. 

At the top of the animal, and within the inside of the shell plates, exists what looks like a “trap door”. This is a movable, protective covering over the interior body. The barnacle can adhere well to its surface due to the presence of specialized “cement glands”. This often allows the shell remnants to exist long after the creature has died.

Some acorn barnacles live in tropical seas, but many are found within temperate coastal zones such as the North Pacific and North Atlantic. You can commonly find acorn barnacles distributed along rocky shores within the mid to upper intertidal zone (area where the tide flows in and out). That being said, barnacles must then deal with an array of stressors that come with living in an inconsistent environment. This includes things like immense heat and moisture changes, land and ocean predators and other challenges. Fortunately, the acorn barnacle can collect oxygen when both underwater and in air using appendages (to be mentioned below) or its body walls.

The acorn barnacle has one of the most fascinating feeding strategies of the animal kingdom. The “trap door” mechanism at its top surface can open and close, usually opening when water is present. The trap door may also close under shadows, which mimic a predator’s presence. 

 This mechanism allows small appendages to extend into the water and grasp any floating food; these extensions are feathery and efficient at wafting through the water. They are also called cirri and are actually feet/ legs! This is another factor that proves that acorn barnacles are a crustacean, and related to other segmented creatures such as shrimp and lobsters. 

Although it may appear similar to other intertidal visitors like chitons or urchins, the barnacle’s internal body plan proves its evolutionary descent. The orientation of the animal has also proven that, evidently, the barnacle stands on its head and feeds upside-down. What a cool animal! 

The acorn barnacle’s life history and reproductive strategy is effective. The acorn barnacle is hermaphroditic, meaning each individual has both male and female reproductive abilities. To mate, barnacles in close proximity find a mate and complete internal fertilization, without needing to move from their area (remember, they cannot)! The fertilized eggs are then brooded within the body. The next stage in life involves a free-swimming larval stage when the animal is planktonic. These larvae are carried by currents until settlement on a proper substrate. Once this occurs, the animal “glues” its head down via its first antennae and builds its shell features. Larvae can cause significant problems when carried by ballast water of ships, as they can present invasive species challenges. 

The acorn barnacle has several predators it must be wary of, including Ochre Sea Stars and Dog whelks. Some limpets can also pose competition challenges to newly settled barnacles. The acorn barnacle will also need to compete for areas to live against creatures such as anemones and mussels.

To humans, barnacles can pose issues to the shipping and marine industry. They can become attached to ship hulls and dock mechanisms very quickly, causing fouling problems and loss of fuel efficiency for ships. Though conservation concerns may not exist at the moment for many acorn barnacle species, it is important to leave the organisms undisturbed around shorelines when possible. 

To learn more about the acorn barnacle, try out some trivia below! 

  1. True or False? 200/1445 living barnacle species are acorn barnacles. (ANS: False. 900 species are acorn barnacles)!
  2. True or False? Barnacle species show separation by height on their habitat shores (i.e. are separated in a “stacked” fashion). (ANS: True). 
  3. What is another benefit for the barnacle to keeping the “trap door” closed during times without waterflow? (ANS: To seal in moisture and prevent drying out). 


Fill in the blanks. Barnacles take in oxygen using their feet or body walls, so they do not have g_l_s. 


Article Authored By: Alexa D., B.Sc./ Five Star Whale Watching.
To discover more about acorn barnacles, check out our References below!

Monterey Bay Aquarium.


Slater Museum of Natural History: University of Puget Sound.