Dale Fort Blog Number 25

10 01 2014

Barnacles 

Barnacles might well be the best known of all seashore creatures.  Along with limpets (see Blog Number 11 ) they are responsible for the distinctive grey band that is easily seen at low tide in the middle zone of most rocky shores.

Until the early part of the 19th Century most people thought of barnacles as little snails (like limpets).  This was because they hadn’t looked at them properly.  William Vaughan Thompson  was as an Army Medical Officer but he’s remembered today for being the first person to describe the life cycle of a barnacle.  In 1832 Thompson’s  observations of the larval stages of barnacles placed them firmly in the Crustacea  (animals with jointed limbs like crabs, shrimps and lobsters).

In 1854 Charles Darwin said this:

I hate a barnacle as no man ever did before

He had just completed 8 years of investigations which described every barnacle species, both living and fossil.  The stress of examining microscopically around  a thousand  barnacle species  and describing them all had taken its toll on his health.  To produce this work Darwin had to write to scientists throughout the world requesting specimens, information and opinions.  The result was a large network of informed people who admired and respected his views.  Darwin was also probably the first person to describe a whole class of animals in terms of the species relationships with each other.  There is no doubt that the barnacle work was what earned Darwin his scientific credibility.  Without this, his much more famous book  On The Origin of Species  (1859) would not have had the massive impact that it retains to this day.

As Darwin discovered to his cost and benefit, there are many different kinds of barnacles.  Happily for us we are likely to find only about 6 seashore species.  Think of them as being like a tiny shrimp, with its head end glued to the rock, surrounded by armour plating.  When the tide comes in the armour plating opens up at the top and the animal sticks its legs out and sweeps them through the water (something akin to the motions of fathers arms when dad-dancing at wedding receptions).  Few people have ever seen this and it is well worth a look if you are on a suitable seashore (or wedding reception).  Find a rock with some barnacles on it.  Find a rock pool and stir it up a bit to liberate some food particles into the water.  Put the rock in the pool,  watch closely,  (it might take a few minutes),  be truly amazed and consider just how much water is filtered twice a day by the trillions of barnacles on seashores everywhere.  These legs (the technical term is cirri) sweep water containing food down into the opening and into the mouth.  Food is plankton (both animals like copepods and algae like diatoms) and dead stuff (detritus).  The legs are bristly and can trap prey directly, some large barnacles can even catch fish.

Barnacles have interesting and very variable life cycles.  The most common species on UK shores begin as planktonic larvae (floating about in the sea).

NaupliusAn early stage barnacle larva (nauplius = Latin for shellfish).  It spends a few weeks in the open sea feeding on other plankton.

Cyprid  It then turns into a cypris (= Latin for Venus) larva, which settles out on the rocks and wanders about on its stumpy appendages looking for a place to settle.  There’s a strong chemical attraction to adult barnacles.  That little black dot is a light sensitive eye spot.  If it wanders into dark crevice and/or detects other barnacles it gets very excited (in a special barnacley way) and ceases exploring.  It then rocks back and forth and bashes that pointy lump on its front end (on bottom the left had side in the picture) on the rock.  The little lump is a gland which eventually ruptures and releases barnacle superglue which glues the creature down.

Adult barnacleThe animal then sticks to that spot for the rest of its life.  It grows armour plating (a shell or carapace) and becomes a mature adult after one to two years.

Barnacles have separate sexes and have to copulate to reproduce.  This can be difficult if you are glued to the rocks surrounded by armour plating.  How does a boy barnacle ask a girl barnacle out (as it were) given these circumstances?

The solution is spectacular.  In Autumn, cooling sea temperature and rough waters cause the male barnacle to become aroused.  He unleashes his mighty penis which is 8 or 9 times longer than his own body (proportionately the longest in the animal kingdom) and waves it around suggestively.  It has sensory hairs on it which can detect female barnacles.  When he finds a female (maybe as much as 5cm distant) he inserts it into her shell and fertilizes her eggs (up to 8000).  She retains the fertilized eggs for the winter months before releasing them into the sea.  Here they feed on the Spring bloom of plankton before settling and developing into adults (see above).  As if that wasn’t thrilling enough, the Bohemian world of barnacles offers still more.  The once proud organ withers and drops off and the male turns into a female for next year.  If we multiplied the barnacle’s  gentleman’s  equipment up to human proportions it would be about 22 metres.

The technical expression for this type of reproduction is cross fertilizing  protandric hermaphrodity.  You might think this is an unnecessarily tortured phrase but it does describe the whole thing in only four rather obscure words and renders it all much less amusing.  This is important because barnacle specialists get very annoyed when real people fall about laughing as soon as they hear about barnacles willies.

Four Common Species

Semibalanus balanoides

Semibalanus balanoides photo

Chthamalus stellatus

Chthamalus stellatus photo

Chthamalus montagui

Chthamalus montagui photo

Austrominius modestus

Austrominius modestus photo

blog 26

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