Many science students are familiar with tests referred to as a lab practical.  In this test creatures are placed in stations around the lab – students rotate through and have 1-2 minutes to answer 1-5 questions about the creature.  I have given hundreds of these to first year marine science students and when they approach a barnacle the response is generally the same.

Question 1 – what is this? Answer – a barnacle… no problem.  Question 2-3 – what is the phylum and class?  Problem… what do you mean… it’s a barnacle – enough said.  As they look at the barnacle and work their way into the classification, they are usually drawn to the calcium carbonate shell.  This is very common, and a key characteristic, for the phylum Mollusca.  So, they write down Mollusca.  And… they are wrong.  Barnacles are not what you think.

Yes, they do produce a calcium carbonate shell like snails and clams, but there the similarities end.  The animal inside this shell is not a “blob of snot” as oysters and snails are – but rather a shrimp like creature with jointed legs.  Yes… barnacles are crustaceans.  The famous 19^th century biologist Louis Agassiz described barnacles as “nothing more than a little shrimplike animal, standing on its head in a limestone house kicking food into its mouth.”

Barnacles begin life as plankton but eventually settle to bottom on something hard where they use cement glands near the antenna to attach themselves by their head.  From here they begin the secrete the calcareous plates that form the familiar “shell” we all know.  Some species attach by a stalk like structure – such as the goose neck barnacles found in the Gulf – and the head is attached to this stalk.  There are multiple plates forming a wall around the animal and a couple used as a “door” to close the animal off during low tides.  The jointed leg-like appendages are elongated and extend upward so they can be extended to feed.

The process of feeding involves opening the plates during high tide and extending the jointed appendages into the water column.  There are two rows of these appendages that form a cup similar to you extending your fingers when you place the base of your two palms together.  This basket can then sweep the water for plankton – which is moved to the mouth.  When currents are strong – no sweeping is necessary – the barnacle will hold the appendages out the catch food.

In some species of barnacles there are males and females, but many are hermaphroditic.  During breeding season barnacles acting as males can detect those acting as females and fertilization of the eggs is done internally.  The fertilized eggs are held within the body of the acting female for several months (over winter for some).  The larva hatch as plankton and an individual female can release as many as 13,000 larvae.  Most are consumed by filter feeding animals, but some will find a hard substrate on the bottom – attach by their heads – secrete their shell-like plates – and begin the cycle again.

These animals are all too familiar on pilings and seawalls, but they also find their way to the bottom of our boats and anchor lines.  Much time and money have been spent to remove these fouling organisms from our vessels.

So… there are your barnacles.  They appear to be mollusk… but they are not what you think.

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