BARNACLES

'Living on the edge'

By Wim van Egmond

Visiting a tide pool is always a wonderful experience. Living in this zone between land and water is continues struggle against the forces of the ocean. One of the most interesting inhabitants of the tidal area is the barnacle. It lives in the upper zone where the water only comes at high tide. The barnacle's appearance is rather deceptive. At first glance it looks like a mollusk. It has a white shell made of calcium carbonate. But when you observe the larva of the barnacle their true nature becomes visible.
The first stage of the larval development reveals that we are dealing with a small crustacean instead of a mollusk. This so-called Nauplius stage closely resembles the larvae of a Copepod. You can recognize the larva of the barnacle by the two little 'horns'.

After the Nauplius stage has lived some time swimming in the coastal plankton it undergoes a surprising transformation.

 

The Nauplius larva of a barnacle

Cypris larva of a barnacle

The next stage is called the Cypris larva because it resembles a member of another group of crustaceans, Cypris, an Ostracod, (tiny bivalved crustacean).

The remarkable aspect of the Cypris larva is that it does not eat. It is unable to take in food. What could be the advantage of that? It appears that this stage is a far more energy efficient design. One may regard it as a survival capsule.

Extensive research done on the creature has proved that the energy reserves of the Cypris larva enable it to survive up to thirteen days. Within this period it has to find a suitable spot to attach itself. When the thirteen days are exceeded the larva will die.

It seems an impossible task but the larva does find a way to glue itself against a surface in the tidal zone. It uses a sticky substance on one of its appendages. Not only rocks are ideal for settlement but also pieces of wood or other smooth surfaces like the shells of mussels will do.

Then another complete transformation occurs. Within twelve hours after attachment it builds a shell made of several (mostly six) plates. The rest of its life the now adult barnacle will stay positioned with its head down. Inside the shell the barnacle can create a little watery environment to survive the many hours of drought at low tide. At high tide the shell opens and the barnacle begins to feed. Long comb-like legs are swept backwards and forwards to catch little organisms (see top image).

Living fixed on one spot has its disadvantages. It is more difficult to find a partner to mate. The barnacle has overcome the problem in a really spectacular way. From its shell the male barnacle projects a penis so huge it is many times the size of the owner. It is simply breathtaking to see how the organ finds its way between the neighbors searching for the ideal partner.

That the method is successful is proved by the number of barnacles that can be found in many tidal areas. They can be so numerous that they cover every inch of rock. The adults can be observed very easily when you put a small stone with barnacles in a small tank. The larvae can be caught from the plankton with a plankton net.

 

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More oceanic micro organisms can be found in The Virtual Ocean

All Material Copyright: Wim van Egmond

 

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Published in January 1999 Micscape Magazine.

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