Why do wooden jetties and piers suddenly collapse?

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Photo shows a shipworm removed from its protective case. – PNAS.org photo

I HAVE frequently asked myself this question over the years when, suddenly, it is reported that a hardwood pier’s struts gave way leading to the loss of either a historic monument, or a necessary local means of boarding a ship or small boat. Now that I know the answer, I should like to share it amongst landlubbers such as myself.

The answer is simply through the prolonged eating habits of ‘sea termites’, better known as naval shipworms (Teredo navalis). These shipworms are a species of saltwater clam, which may also be found in brackish waters at river estuarine environments, and are nothing more than a bivalve of the family Teredinidae.

Resembling a worm in appearance, it is encased with a shell of two valves allowing it to bore through wood. At its front end there are two triangular calcareous plates, which it uses effectively as a drilling tool to rasp away through the wood. Gradually it enlarges the circular tunnel it creates and lines it with extruded calcareous material.

Once the shipworm has completed its activity within the timber, the wooden pier strut resembles a sponge-like collapsible and crumbling structure. Hence, the term sea termite.

Habitat

Living off floating driftwood, in submerged jetty pilings, and in the hulls of wooden ships and boats, it is found worldwide. It is thought that originally the shipworm came from the Northeast Atlantic Ocean.

Our ocean currents have shifted driftwood all around the world and the larvae of this creature can hatch almost anywhere on the globe.

Devastation

With the annual timber harvests of the softwood pine forests in Sweden and Finland, the logs are floated down the rivers in the spring-thawing of the ice into the Baltic Sea, where they are collected for timber and paper pulp mills. Within 16 weeks of the pine logs submergence in seawater, they are riddled with shipworm tunnels.

Hardwood oak trees take twice as long to be fully attacked. These worms have infested the hulls of oceangoing vessels all over the world for centuries. Eventually, in the 18th century, it dawned upon the British Navy to use sheets of copper on the keels and lower hulls of their ships and this led to other nations following in pursuit.

It is only now that I realise the origin of the expression ‘copper bottomed’ for I can well remember my parents’ pride in replacing a burst iron water tank, which had flooded our house through frost damage in a very severe British winter, with a copper bottomed tank.

The shipworm ‘devils’ have proven to be headaches for wood treatment companies and so much so that the only answer today is by creating an iron or steel jetty or pier. I just wonder how long they take to attack and devastate Sarawak’s belian wood jetties for, in the South China Sea, these shipworms have adapted over time to greater seawater salinity levels.

Wood as a source of sustenance

Feeding on timber through their rasp-like plates, minute algae are taken in through the worm’s gills and with the assistance of bacteria, living in symbiosis with the algae, nitrogen decomposes the cellulose content of the wood and this product forms the worm’s daily meal.

Born hermaphrodites, they start their adult life as males but in warmer seas they change into females, which eventually release larvae that are carried on the sea and tidal currents towards the shore to be dislodged on driftwood and then mature into shipworms. Attached to a log or timber structure, they then become wood-borers for the rest of their lives.

Photo shows the damage caused by shipworms over time.

Giant shipworm

This giant shipworm

(Kuphus polythalamia) measures up to 1.5 metres in length – the size of a baseball bat – and is the king of all shipworm species. Scientists have likened its appearance to that of the entrails of an alien in a horror movie. Quite distinct from other varieties of shipworms both in length and diet, it does, however, have common characteristics in its survival on bacteria in its gills.

This shipworm lives in mud, which emits hydrogen sulphide (smelling of rotten eggs). Its gills absorb this gas and the bacteria converts the gas into carbon that feeds the worm. It is a process not unlike photosynthesis in plant life when the sun’s energy converts atmospheric carbon to feed vegetation.

Recent discoveries

Known to exist for 200 years, it was only through a TV documentary in the Philippines that this giant shipworm was brought to the world’s attention. It was filmed living and feeding on the mud of a shallow lagoon there.

Subsequently researchers, in a joint expedition from the University of Utah and the Marine Sciences Institute of the University of the Philippines, opened the protective case of one shipworm to reveal an almost jet black, glassy surfaced creature with pinkish coloured appendages. The researchers were amazed by its length and body width.

The tube in which it lives is composed of the calcium carbonate secretions of this animal. As the worm grows in bodily length, so does its protective outer casing. It is thought that the mud in which

it lives contains rotting timbers and hence the production of bubbles of hydrogen sulphide. Thus, indirectly, it may be

related to its wood attacking cousins. Certainly it is sulphur powered.

With its capacity to absorb and convert hydrogen sulphide, there are deep oceanic versions of the giant shipworm that have been observed by semi-submersible vessels living around sulphurous vents where Oceanic Plates diverge.

Frankly, very little is actually known about giant shipworms but it appears that these worms don’t need to digest their food for their internal digestive organs have shrunk from lack of use. The researchers likened their find to that of discovering the fossil remains of a dinosaur.

The next time you find yourself in shallow muddy water, do bear in mind the giant shipworm that may lie in the mud beneath your feet. At least it does not attack humans.