Know Your Barnacles

Barnacles1You’re walking along the rocky Northwest ocean shore line and you see then by the billions and gazillions.  Barnacles, barnacles, and more barnacles.  The cling to every rock, pylon, piece of drift wood, shell, and just about anything else touched by the sea.  These non-descript bits of life and their protective shells seem to encrust everything.  But what they are and how they do it is a reveals just how fascinating and complex the simplest organism can be.

Barnacles are Crustaceans, a primarily aquatic class of animals grouped under the gigantic order Arthropods.  Like all arthropods the crustacean barnacles prominently display an external skeleton.  Barnacles, however, have a unique feature that sets them apart from other crustaceans and hence puts them into a subclass called the Cirripedia.  In zoological terms cirri (plural of cirrus) are “threadlike appendages that serve as an organ of touch.”  Pedia refers, of course, to the Latin for foot.  Hence the subclass we know as barnacles are called cirripedia, roughly translated as Threadfoot.  And this is a pretty good description for anyone who has seen these little guys submerged in their aquatic homes gently waving their cirri back and forth for a planktonic lunch.

Anyone who lives near or has visited an ocean coastline would hav

Rick Harbo’s Field Guide : “Whelks to Whales” claims that there are 900 species of barnacles throughout the world and that 65 of them are found in the wildlife of the Northwest.  Our barnacles fall into two broad categories and the most common are listed below:

Barnacles Without a Stalk:

Giant Barnacle, Balanus nubilus  As the name implies this is the largest barnacle in our area and all along the West Coast.  Reportedly this four inch cirriped was roasted and eaten by native Americans.

Acorn Barnacles  About 3/4 of an inch wide these guys look similar to a tiny volcano when viewed from above.  The acorn barnacles come in two varieties:

  • Common Acorn Barnacle, Balanus glandula  Whitish in color, the closed aperture or “mouth” of the barnacle looks like a wavy line.
  • Small Acorn Barnacle, also called the Brown Barnacle, Chthamalus dalli  Smaller than the Common Barnacle, it is also distinguished by the cross design displayed when the aperture is closed.

 Thatched Barnacle, Semibalanus cariosus  About a half an inch wide Thatched Barnacles are named for the thatched appearance of their shells.

Barnacles With a Stalk:

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Goose (Stalked) Barnacles   These rather large barnacles have a goose head shaped shell at the end of the substrate attached stalk called the peduncle.  Stalked barnacles are considered more primitive than their non-stalked cousins.  Note:  Goose Barnacles and sometimes the Leaf Barnacle is called the Goose Barnacle and sometimes the Leaf Barnacle is labeled the Pelagic Goose Barnacle.  Yes, it’s confusing.  So if you decide to look up some of the literature on these guys use the scientific name as a guide.

  • Goose Barnacle, Lepas anatifera  This cirriped has a stalk six to eight inches long.  The shell is composed of large plates that extend to the top of the barnacle stalk.  Generally likes to attach to driftwood or other articles that have been floating about the sea.
  • Leaf Barnacle, Pollicipes polymerus  Very similar to L. anatifera.  This barnacle prefers rocky shores and is mainly distinguished from  L. anatifera by the series of tinier shell plates where the base of the shell head meets the stalk.

e to be blind or incredibly unobservant not to notice barnacles.  As previously mentioned barnacles are found hanging on to nearly every surface that is at one point during a twenty four hour period submerged.  These animals have no problem, of course, living underwater and it is obvious that they must be able to live for extended periods out of water as well.  But barnacles do it so well that they are able to withstand conditions lethal to their molluscan cousins.  That invader from the Golden State called the California Mussel may appear in its thousands, but above this dark band of mussels are the crowded millions of cirripeds who are able to survive more extreme conditions of heat and dryness.  That is why barnacles are so predominant on the shoreline landscape – they are inevitably found on top!


From top to bottom:  Fine example of extended cirri from which the barnacles get their name and the distinctive peduncle of a goose barnacle.  Leaf Barnacle, P. polymerus.  Common Acorn and Small Acorn barnacles grouped together on a shell.  Close inspection of the Small Acorn Barnacle reveals the cross shape of Barnaclethe closed aperture (mouth).  Thatched barnacles cemented to a rock. 


Most barnacles are shallow water or intertidal inhabitants that remain attached to a surface through cement produced by a gland at their base or, as in the case of the stalked barnacles by the stalk itself.  But barnacles have not limited themselves just this narrow environment.  Some are inhabitants of the ocean depths.  Barnacle species have adapted to life on surfaces as varied as whales, sea turtles, sea grass, and coral.  One particular barnacle lives inside snail shells – but only if the snail is inhabited by a hermit crab!  There are even parasitic barnacles that attach and feed on other crustaceans and sea stars.

Parasitic barnacles, of course, feed directly on their host with mouths adapted for sucking.  Our Northwest example feed through the use of their cirri by either active or passive methods.  Passive feeders simply open up their cirri and allow the ocean current to randomly push food particles, plankton, and micro plankton into their threadlike appendages.  Others actively sweep up and down in order to pull in food.

Most barnacles are hermaphroditic, they contain both male and female reproductive organs.  On one side of the animal is an ovary and on the other, believe or not, is a tiny penis.  Since barnacles live in large, close populations, however, cross fertilization is more common.  Fertilized eggs brood inside the mantle cavity until they reach a larval state called a naupilus, a triangular shaped, plankton sized creature.  Pregnant barnacles may hatch as many as 13,000 larvae.  The miniscule animal then develops into a free swimming organism followed by several other stages that result in a non-feeding form called a cypris larva.  It is this form of the larva that settles onto an appropriate surface, secretes its biological “super glue,” and blossoms into the adult form of the barnacle where it will spend its surprisingly long life of five to fifteen years and longer.

Like all forms of wildlife the unprepossessing barnacle is actually a miracle of creation.  This miniature animal that seems so commonly simple at first glance has an astonishing internal design of sophisticated organs and a complex life cycle.  They are important for more than just scientific curiosity as well as any boat owner will tell you.  The little barnacle is, like all wildlife, a marvelous thing to behold.



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