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Barnacles


 
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Museum Victoria is developing a series of field guides on marine environments. The latest guide, called “Barnacles”, was written by Drs Gary Poore and Anna Syme.     
Many people think barnacles are molluscs, similar to limpets or snails, but they are actually crustaceans and are thus related to crabs, shrimps and the like.
Many barnacles live permanently under water but those that don’t rely on the tides. When the tide comes in the barnacles open the hard plates on top and legs emerge, which they use to filter food from the water. This activity can be seen when snorkelling or using a mask to look under water.

Barnacles frequently live in clumps of individuals, which helps the larvae (which swim in the water) know where to settle when they go back to shore. While not having true eyes, barnacles have pigments that are photosensitive. Faced with shadows in the water, barnacles quickly close their valves, showing them to be light sensitive.
They also have chemosensors which act like a sense of smell. This allows the larvae to detect chemicals in the water, helping them to track down places environments.
Barnacles also respond to wave action and the roughness of the rock, all of which may help larvae detect the right barnacle clump.

 
Barnacles are hermaphrodites, meaning the carry both sexes but as they don’t fertilise themselves clumping aids reproduction. So too does their possession of what is regarded as the longest penis relative to body size of any animal.

There are three kinds of barnacles.
The acorn barnacles, which don’t in truth look much like acorns, are conical and commonly seen on the intertidal rock platform.
Goose barnacles, also called stalked barnacles, have a soft stalk and a 'head' that contains the feeding legs. Most goose barnacles attach themselves to floating material like logs. Being ocean dwellers they are usually only seen when washed ashore.
The final group is one where the adults do not look like barnacles, are parasitic, finding crabs or lobsters as their hosts after the larvae has inserted into the host’s tissue.
There are some several hundred species of barnacles, all with highly specific habitats. Some live only on turtles, while others are buried in coral. Some species live only on whales while others live solely on the top of jellyfish.

 
Barnacles live for around five years, depending on size and habitat, and while they may look secure even the adults do have predators. Shorebirds will attack them and predatory snails like whelks will take them, as will fish. Most predation, however, occurs in the larval stage, which is free swimming among other planktonic organisms.

 

Above: Dr Gary Poore, Principal Curator (Marine Biology) at Museum Victoria

Left: Goose barnacles with their feeding cirri extended.
These revealing images (Barnacles, page 50, by Kevin Aitken) are typical of the high quality of the range of illustrations included in the field guide.

 
Ships' hulls provide a suitable habitat for many barnacles that live permanently under water on firm substrates. There is a large marine industry in trying to stop barnacles attaching to boats because of their drag and increased fuel cost. This habitat choice can also lead to a biosecurity problem where foreign barnacles are transported to other places around the globe. The problem is most acute in harbours.

Those barnacles that are intertidal can suffer if extreme heat conditions coincide with low tide, as they try to weather the heat before high tide. Oil spills are a particular problem for intertidal barnacles as the spills often end up in this environment, smothering the barnacles and wiping them out.

“Barnacles” and the other field guides are available from Museum Victoria as well as bookstores. They are a great insight into marine habitats along our coastline.

Text: V.B. November 2009
Images from Museum Victoria


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