Photo Guido Zsilavecz
The shell on the left in the photo is a normal C. coronata - a cream shell with mottled light brown patches. The shell on the right is clearly a light green all over. This fascinated Hans Krop, who believed it might be a new species. After many years he finally managed to get an answer to his query as to whether it is. Unfortunately for him, it is not, and that instead the green colour is because of cyanobacteria. These bacteria are often referred to as blue-green algae, although they are actually not algae at all. They can be microscopic and unicelular, or can form large colonies, and are widespread in fresh and salt waters, and can be found in the harshest environments, including hot springs.
How the cyanobacteria infect the shell is not known, but it may be because at some stage the shell was damaged and the bacteria managed to penetrate it. It is interesting to see that the whole shell is a light green - outside and inside, and the colour certainly cannot be rubbed off.
C. coronata itself has an interesting history. Discribed from a single species found on a beach, by FA Schilder, the shell was lost during World War 2 when the Hamburg museum where it was kept was destroyed. The species was forgotten until 1979 when William Liltved collected a live specimen off Cape Town, and identified it correctly. Since then it has been found more regularly of the western Cape, but it is never common.
Thanks to Hans Krop and William Liltved for information.
Cowries of the world, CM Burgess,1985