Wednesday, 4 June 2014

The Foraminifera of the Swanage Shores.

These beautiful shell like creatures can only be seen with a good hand lens or better still a simple microscope. Our local species are only a fraction of a millimetre in diameter.

Foraminifera are to be found throughout the world as fossils in the sedimentary rocks of marine origin. They are an important part of the structure of many hills and mountains around the world, like Ballard Down. A small block of chalk can contain millions of tests of Foraminifera.
What is rarely appreciated is that they also flourish today as living creatures in the open sea as part of the plankton and the estuarine areas of river as at Poole.  Foraminifera are actually closely related to the Amoeba of our school text books but they grow a shell like structure called a test to protect themselves from the many hungry creatures of the seas. There are no organs and these animals belong to the Protista: the simplest and earliest of creatures to appear on Earth. The tests are made of various types of chalk (calcite). Some species seem to be almost transparent, being constructed of crystalline calcite, but others seem to be paved with porcelain like chalk but in the river mouths where there is less chalk dissolved in the water, they are protected by particles of sand or debris bonded onto a covering of a chitin-like material. These are called arenaceous species. The Foraminifera are clearly masters of adaptation as well as being climate indicators. Generally the white tests are noteasy to find because they are very small but in deposition areas they can be found en masse, usually mixed with black sea coal from industrial spillage and ship wrecks. This gives these sites a black and white banding which is easy to spot (See above Right).  In Swanage Bay an area can be discovered between the Mowlem building and the Public Hard at low tide. Washing, sieving, drying and floatation in clean, fresh water all help to  improve the sample.
As well as Swanage, Weymouth and  Fleet, there are local deposits within Poole Harbour near the Salt marshes at Arne and Redhorn Quay where they lie almost unseen, amongst the small washed up shells. The ornamental tests of these invertebrates are objects of considerable aesthetic appeal and beauty as well as having a significant scientific interest.    The two illustrations above, clearly show the difference between the Open Sea British Channel fauna of Swanage Bay and the estuarine fauna of Poole Harbour below with a proportion of gritty arenaceous types, which are very ornamental in polarised light. 

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