This lovely creature isn’t common and shouldn’t really be here. This living Turban Top Shell, Gibbula magus (Linnaeus), was washed ashore at Studland Bay, Dorset. The books say that this type of gastropod mollusc only lives in western Britain and its range only extends along the English Channel as far east as Swanage – so maybe this one got lost since Swanage is just around the corner.
Turban Top Shells belong to the family Trochidae which are all referred to as top shells because they look like the old fashioned spinning top toys. You could say that they are all roughly a pyramid shape but in the Turban Top Shell the sides of the shell do not rise smoothly to the apex but are stepped. It also has bumps or tubercles around the top of the ledge on each whorl that give it a turreted appearance.
The shell can be up to 30 by 35 mm big. It is described as grey with reddish stripes by some experts, while others say it is white, grey or yellowish, with irregular brown, red, or purple streaks and blotches. Obviously, the markings are quite variable.
In the photograph above, you can see the underside of the shell. The animal is in the process of withdrawing into the shell. Attached to the ‘foot’ of the animal is the round brown translucent operculum with which it will ‘shut the door’ when completely inside. The horny operculum is circular and the design is known as polygyrous. This means that, if you could look at it closely enough, it would have a pattern like a catherine wheel firework (a single continuous line starting right in the centre and winding round and round to the outer edge).
In this kind of gastropod you can think of the animal as living in a long tube which increases in diameter as the animal grows. This tube is spirally wound around a central hollow columella. The hole in the centre of the base of the shell is called the umbilicus (like a belly-button) and leads into the columella.
These animals are vegetarians like common winkles and periwinkles – and unlike the carnivorous and predatory dog whelks, netted whelks and necklace shells. Turban Top Shells eat microscopic algae and plant debris by scraping it up with a ribbon-like radula bearing rows of renewable teeth that is protrudes from the mouth.
Most people think that shells are inorganic like stone. Actually, they are more like bone, built from an organic matrix on which different types of calcium carbonate crystals are arranged. Shells are made up of layers of different kind of shell. It’s mostly the organic matric that holds the shell together. When the animal dies, the matrix very, very slowly begins to break down.
In the above picture there is an empty shell that has been rolling around on the sea bed and beach for some time. The outer, duller, layer has begun to fall away revealing the lovely inner nacreous layer – the mother-of-pearl interior. Sometimes it’s possible to see this pearly lining exposed in living top shells, especially at the apex where the shell gets a lot of wear.
The colours in the outer layers of shell are mostly created by the inclusion of pigments and are liable to fade when the animal dies and the shell dries out. Sea shells always seem to look more spectacular on the beach when you first find them fresh and wet. However, the wonderful iridescent colours of the innermost layer are due to the physical properties of the particular type of calcium carbonate crystal used in its creation. These physical characteristics persist indefinitely.
In the final photograph, shown below, is a Turban Top Shell that has been around a long time. The outer dull shell layer is completely absent. Mother-of-pearl is all that remains. The shell is broken by mechanical abrasion so that it is possible to see the internal design of the shell. The final phase of break down would be the reduction to small fragments that become incorporated into the sandy sediments of the seashore.
Revision of a post first published 30 May 2009
COPYRIGHT JESSICA WINDER 2011
All rights reserved