ARTICLE 129    January 2014

CROWDED RAFTS 

by Jenny Edwards

 

You may have noticed lots of floating rocks on our beaches and in the water.  This pumice, a type of aerated glass produced in volcanic eruptions, has travelled thousands of kilometres and carried many marine hitch-hikers.  If the pumice is white inside it has come from Havre seamount, an undersea volcano about 1000 km north of the North Island of New Zealand. 

Until 2012 the Havre seamount in the Kermadec Island chain was thought to be inactive.  Then in August 2012 a huge raft of pumice, covering an area of over 20,000 square kilometres, was spotted by a woman travelling on a commercial flight from Samoa to Auckland.  The raft gradually broke up and pieces of pumice drifted in the currents, some reaching the coast of Queensland by March 2013.  Since then the East Australian current has been carrying it south and all the time algae and the planktonic larval stages of numerous marine species have been attaching themselves to these mini floats.  Some of the hitch-hikers washed up on our shores may have come all the way from tropical waters.

 

The pumice lumps can be quite large, up to the size of a coconut.  Others are worn to small pebble size.  The one in the photo, found at North Broulee, was only 4cm on its longest side.  If you see a freshly stranded lump that looks dark or slimy it is worth a closer look.

The upper surface is often dark, greenish black, caused by a film of blue-green algae (cyanobacteria).  Goose barnacles of several species and sizes attach themselves with flexible “necks”.  These crustaceans, like the rock barnacles of our shores, capture food by extending and retracting their fringed legs.  Polychaete worms, ones with short bristly legs on each segment, prey on the goose barnacles.  One piece of pumice found at North Broulee had six short, black polychaetes clinging to its upper surface.

 

Sea anemones are also common passengers but may be hard to recognise when they are just retracted blobs on the surface of pumice that has been washed up on the sand.  If they are still alive they may expand to their full size in a container of sea water.

 

If there is one organism on the pumice then it is likely that every surface will have something attached although many will be almost microscopic.  Hydroids are relatives of the anemones, but so small you will probably need a magnifying glass to see them. These semi-transparent colonies are like fine branched stalks, often growing on the barnacle shells, with what look like tiny anemones on the stalks.  These are the feeding structures.

 

Cockles

Tropical Hitchikers

 

Photo - Jenny Edwards