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 (, 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.

Blue-rayed limpets (Patella pellucida) feeding on kelp.

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.  


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


  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: ( and a less technical article here: 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.  

Additional Figures:

The Preseli Hills

7th October 2020

A lovely place, this hare’s scrape of a lay-by below Carn Enoch on the hills above Dinas Island. To the north and west is mainly sea, and to the south and east rural Wales. There are no large towns for many miles and the darkness runs clear and deep.

I arrive at twilight, and sit as the world-clamour is replaced by a sheep-punctuated silence: the detailed landscape reduced to a lumpy skyline of soft dark hills.   The sense of peace is transformational, inducing a feeling of calm and opening.

Then, as twilight deepens the lights around Newport appear, followed by the first stars. It is as if the town lights have lured the wild stars into visibility, as plastic decoys lure wild geese. But the timescale over which the stars appear is awkward; too long, unless you have an astronomer’s patience, to watch continuously, but short enough so that, having looked away, and maybe become momentarily distracted, you find on looking back, the sky quite changed.  It reminds me of what happens in spring, when, one moment there are a few leaves against bare branches and the next a cathedral of green. Both transformations are at once gradual and sudden.

But this place is not only beautiful by night. To the north lies the stretch of coast and seascape around Newport Bay, to the south the Preseli hills. The Preselis are not high, only 536m at their highest point, but they are heath and moor and possess an unexpected wildness. There are no lofty peaks, but the open landscape induces a sort of walking meditation in which, because of this very openness, attention becomes focused on nearby details rather than distant vistas; the quick mid-stride gurgle of a stream running invisibly in the channel it has cut in the peat; patterns of rock and lichen; changes in vegetation; a momentary flash of sky in water collected around the base of a boulder. In the summer there is the constant background of the skylark’s song; a stream of summer consciousness that mingles with the soft scents and changing airs of the moor.  Walking up here clears the mind and spirit leaving space to think and feel.

One of the more unique characteristics of this landscape is the large outcrops of rock that are dotted across it. The nearest to me here, Carn Enoch, lies just a few hundred yards up the hill from where I park.  It comprises large grey boulders, closely fitting in places, a looser tumble in others, and it rises so abruptly from the surrounding grass that you might think it had been placed by hand, inviting creation myths of giants and magic. These rocks are the haunt of wheatears and pipits and sometimes, when the evening sun is setting in a clear sky towards Ireland, of a beautiful warm golden light.   The first time I saw these rocks it was early spring and their soft grey colour, their creased and lined surface, and the way they rose out of the then brown grass, reminded me of a group of elephants kneeling or lying on an African savannah. It is a feeling that has never quiet left me; that these outcrops are warm, wholesome and somehow animate.

Carn Enoch on Mynydd Dinas

I could spend a lot of time up here, parked in this spot, in this tiny camper van, tea and coffee to hand, wine for the evenings and a gas ring for cooking.  I would alternate between walking, reading, thinking, and trying to write, and life would be simple and good. What thoughts might one have, slowly sinking into the peace of this place?

Mynydd Dinas
An early frost;
white and sharp under a blue sky.
The air full of larks and the scent of sheep. 
Grey rocks rise abruptly from winter-brown grass,
their skin lined and textured like elephant hide,
and with the same feeling of mass and warmth.
To have spent the night up here.
A clear dark sky, sliver moon,
pin sharp milky way and Orion dipping toward the sea.
A night to have felt part of the universe
and to have remembered.
Presili night

Nothing to hear,
but a few sheep tearing the grass.
Nothing to see,
but a thousand suns,
blazing above the hills.

Copyright (c) Jon James 2020

Outer Hebrides

7th July 2020

The Outer Hebrides are a chain of islands about two hundred miles long, thrown up, seemingly, like an arm, to protect Scotland from the casual violence of the Atlantic Ocean. 

The beaches lying along the ocean facing west coast are extraordinarily beautiful, possessing, for me, an otherworldly quality.  The long journey from the south of England maybe contributes to this impression; wonders are passed and, as with many mythic journeys, things get stranger as you progress; brooding mountain ranges with snail-trail waterfalls high in their crags, wild moors and stony burns, gannet-mobbed ferry crossings and sinuous causeways linking island-dotted-sea to lochen-strewn-land. Gradually, it seems, the familiar world is left behind, and the imagination left vulnerable to subtlety and impression. 

Some of my excitement is a remnant of childhood visits to Scotland.  A key element then was the inherent uncertainty around the means of travel “we’re going to Scotland in daddy’s car – if daddy’s car will get that far” was sung, partly for fun, but also maybe as a charm to ward off the real possibility of brakedown.  The whole venture felt like a game of chance: one wrong turn in labyrinthine Glasgow, for example, could see you sliding down a snake headed back to exactly where you were forty minutes ago. Or worse, you might step on The Gorballs* and not escape at all.  

In those days the goal was the Ardnamurchan peninsula, a final singular paradise, guarded by two small car ferries; throw a double six and you could be through in an hour, but anything less would see you locked for eternity on the wrong side of the water, unless you were brave enough to take a chance and dare the precipitous Kinlochleven road.

Today, the psychological geometry is different. The reliability of modern travel has diminished the physical uncertainty, but the desire to reach the haven of the islands is made both more powerful and more poignant by the need to escape everyday life and by an adult’s knowledge of vulnerability and change. That something will have happened to cause the magic to fail is a powerful subconscious fear.

The effect of the journey, and these feelings, is to create the impression, on finally arriving, of having reached somewhere that is more than special. A place that is simultaneously the edge of the physical world and the centre of an internal world: a singularity where to travel in any direction is to travel away. 

It is the beaches that convey these impressions most strongly, seeming places of essence rather than substance, removed from the material world and characterised less by geology and biology than by some more abstract discipline, theoretical physics or pure mathematics maybe. At times it feels that normality might be suspended, allowing secret truths  to be revealed.

Physical underpinnings of such impressions of otherworldliness can be identified. The coarse shell fragment sand does not hold much water and so is not cloying. Each flake is subtly distinct in form and colour and there is little sign of organic matter – all is clean, sharp and precise. The water too is different; being so clear that it might not to be a material substance at all but an abstraction; like the aether proposed by nineteenth century physicists, something only required to explain the observed properties of light, which, in this case, it diffracts and focusses into bands that flicker and lurch across the sea floor.

But these are places of contrast. If the islands are an arm protecting the mainland from the sea then it is a pugilist’s arm; hard, tough and sinewy, with course marram grass hair and rock bones that were formed three billion years ago when the world was young.  The rocks of these islands, some of the oldest in the world, make the era of the dinosaurs look like recent history. But if the rocks are old and hard then the air seems new and soft. Under the prevailing westerlies these islands are the first land the wind strikes after a fetch of some thousands of miles over a bird-strewn, but otherwise empty ocean.  If you stand on one of the Atlantic facing beaches of the west coast you might think the air was freshly made and that you were the first to take it in.

I was on the island of Barra a few years ago, towards the end of a holiday. The holiday had not quite worked.  It had been cold and windy and the tent I was sleeping in had felt confining rather than liberating. Even in such beautiful surroundings it had been hard to relax. Then, one morning, the weather changed and, walking from the dunes at Eolaigearraidh in the north of the island, up over Beinn Eolaigearraidh Mhor, and dropping down onto Barra Beach in perfect light and with a rainbow for company, spirits started to lift.

Strolling along a beach is often an aid to dreaming and this morning was no exception. As I walked a sort of rolling reverie unfolded; a reverie in which the sand became a close packed star-field, and the scattered pebbles, planets floating against a starry background. The action of a retreating tide had removed sand from around each pebble-planet leaving it sitting in a small hollow – a distortion in the otherwise perfect flat of the beach that reminded me of the illustrations in physics textbooks showing how, in Einstein’s theory of relativity, space-time is distorted by massive bodies such as planets. The crabs even became small spaceships scuttling from oasis to oasis across the desert of space and the strands of beached weed exotic nebula. Finally, wandering down to the shoreline, I arrived at a perfect bounded infinity: a wave lapped edge of the purest translucent blue.

* No offence meant – I knew nothing of the Gorballs at the time, this was merely a child’s response to – I don’t even remember what.

Riemann Rhyming
Walk on Barra Beach
With eyes turned down to better see the sky
And you might observe
As I hope you do
(As indeed did I)
A pristine place
A field of stars
Olber’s paradox in sand
Where scattered there all round about
Disrupting the metric of the plane
Banded pebble-planets lie
In Sea-washed hollows of perfect curved space-time
And Hubble objects
Whorls of weed and jellyfish
May cross your field of view
And at the wave-lapped edge
A singularity of pure Euclidean blue

Copyright (c) Jon James 2020