Thursday, 11 July 2013

Worm Takeover

The long awaited fine warm weather was bound to attract people to bathe or paddle on the north Wales beaches. At the Llanddona end of Red Wharf Bay, walking down to the sea at low tide a couple of evenings ago, we met conditions that caused some to hesitate before proceeding. The topography and texture of this normally flat sandy beach had changed with a series of hummocks and softer muddy black sediment between.

Closer inspection showed that there were astronomical numbers of small sandy tubes so packed together that the sediment was building up into hummocks. The irregular topography had encouraged fine and organic deposits, probably originating from a plankton bloom, to settle. Under a very thin surface the rate of decay was such that the soft sediment was anoxic and black.

Looking a little more closely, the worm so dominating the Llanddona beach now is the Sand Mason Worm – Lanice conchilega. These worms glue sand and shell grains together to create the long tubes in which they live. On top of the tubes they construct a characteristic fringe and feed by grasping particles from the water with tentacles. Normally you see these scattered fringe tops just protruding from the sand, but when there has been a mass recruitment there can be so many that they modify the topography of the beach or nearshore seabed. I did not have time to explore the full extent of the area covered by the unusual density of tubes, but the feature must have spread over 1.5 Km along the shore by 300 to 400 metres wide.

The muddy sand offshore area in Red Wharf Bay has been sampled on an almost annual basis for over 25 years. Experience shows that it is particularly liable to substantial changes in the species that dominate the fauna and are capable of modifying the conditions for others. Having visited Llanddona beach for many years, I do not recall seeing quite such a mass takeover of the lower shore by quite such a hugely successful recruitment of Sand Mason Worms.

Red Wharf Bay with worms - lots!
This post was written by Ivor Rees, a director of Natur Cymru amongst many other things!

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