Sunday, 7 February 2010
An Introduction To Dorset Foraminifera
Recent Foraminifera of the Dorset Shores.
These delightful microscopic shells grace the tidal waters of Dorset. They are not true shells however but are more closely related to the antediluvian Amoeba of our school text books. In order to protect themselves from the multitude of hungry creatures in the sea, they have simply developed a spiral shell like structure called a test, though the smaller ones prefer the simplicity of an urn shape. In the open ocean, the more globular species float with the plankton and it is these free spirits that form deep deposits on the sea bed. Given a few million years of pressure they will become chalk, as at Ballard Down. They are mainly constructed from some form of crystalline chalk and others look externally almost like porcelain.
In the great lagoons of Poole Harbour they enjoy the muddy less-salty water by means of an adaptation. With a shortage of the minerals of the open sea, they create a bag of chitin-like material and bond silt and sand particles onto this to make their tests. This can be seen in the illustrations on the top row, these are called arenaceous tests and are often very beautiful indeed.
Each species has a preferred level of saltiness and temperature, so seen together they are useful environmental indicators. At Swanage and Weymouth the mortalities are washed up at the most westerly edge of the bays with the sea-coal fragments. This alternate black and white banding is a good clue to their location. In the harbours and lagoons like The Fleet they are to be found around the edges near the high tide marks. as well as embedded in the mud itself.
The Foraminifera of warmer waters can be larger than one millimetre but most of the Dorset ones are from a tenth to a half of a millimetre in diameter and provided they are well illuminated they can be seen with a good hand lens or a simple microscope.
This phylum of single celled members of the Protista are just a tip of the iceberg sample of what exists in the seas around our world, when examined with a microscope B.D.