Hunting Patella pellucida

30th October 2020

They were there, on the first piece of kelp we looked at. On the side facing down into the sea, just where the fronds join the stipe. A little cluster of five, each no bigger than a fingernail.

It had almost been too easy. But, looking back the clues had been there a few weeks before when we had found, amongst the usual limpets and topshells, some small soft-brown domes with barely visible pale stripes radiating from one end. Apparently a type of limpet, but not looking like any of the textbook illustrations. We had wondered then if they were old and worn blue-rayed limpet shells.

Checking online and in our favourite textbook¹ it appeared that blue-rayed limpets often feed on a species of kelp known as Laminaria digitata; a large brown fingered seaweed which grows on rocky shores and is usually only accessible at the lowest of low tides. I knew from sailing that these tides, ‘spring tides’ typically occur twice a month, near full and new moons, when the alignment of the earth moon and sun produces maximum gravitational pull. Looking at our local tide tables, we noted the date and time of the next such tide.  

But where to look? Laminaria digitata grows by clinging to rocks. Much of the north coast of Pembrokeshire is rocky, but as I had recently found this seaweed washed up on a beach near Dinas Island, while doing a shore search for Living Seas Wales (https://livingseas.wales/), we chose this as our hunting ground.

Arriving shortly before low water and climbing over rocks still wet from the receding tide, we made our way to the sea and turned over the first piece of kelp we saw. And there they were, smaller than expected, but unmistakable.  Tiny translucent fawn-green domes with piecing radiating blue-green lines; the living version of the pale brown shells we had found previously.  The tide was already on the turn and balancing on slippery rocks we took what photographs we could and retreated.

Looking at the photographs now the lines are striking. From amongst the dark softness of the kelp, where all is organic asymmetry, these draftsmen-drawn lines shine out like decals on tiny spaceships. Their iridescence arcs across the shell domes, with apparently little regard for the assumed concentric or radial growth patterns and symmetries. They are the colour of kingfishers caught in slanting light against the greens and browns of a river.  

References:

1. A Student’s Guide to the Seashore. J. D. Fish & S. Fish (Third Edition)

Notes:

1. Patella pellucida is the Latin name of the blue-rayed limpet. ‘Patella’ and ‘Pellucid’ are Latin words meaning ‘little dish’ and ‘lucid or clear’ or ‘to shine’ respectively.
2. The iridescence of these limpets is an example of ‘structured light’, in which colour is caused, not by the usual method of pigmentation, but by geometric arrangements of microscopic structural elements that cause interference and filtering of certain wavelengths of light.  The example of blue-rayed limpets is unusual because this effect is created by mineral rather than organic compounds as is the case in creatures such as kingfishers or butterflies.
3. There is a scientific study into the Blue-rayed limpet by the Massachusetts Institute of Technology (MIT): The paper is here: (http://web.mit.edu/cortiz/www/LingLiPhotonic.pdf) and a less technical article here: https://news.mit.edu/2015/optical-structures-in-limpet-shell-0226. Some of the more accessible points are:
   1. The iridescence is produced by layers within the thickness of the shell rather than on the surface.
   2. The blue of the rays is the optimum shade for penetrating water.
   3. The changes in shell structure needed to cause iridescence do not weaken the shell.
   4. It is thought that the evolutionary advantage of the iridescence of blue-rayed limpets is that it mimics colourful species of toxic sea slugs (nudibranchs), such as Polycera elegans.  

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