Ever wondered how the blue-rayed limpet got its stripes ? SCUBA ’ s science writer Karen Boswarva explains
OCEANSCIENCE
Riders on the kelp
Ever wondered how the blue-rayed limpet got its stripes ? SCUBA ’ s science writer Karen Boswarva explains
If you ’ re anything like me , your underwater camera becomes your brain during a dive trip . A quick-fire way for remembering all the cool species you want to try and look up later . For this purpose , I use a compact – they ’ re small , easy to carry and handle with other survey equipment . I know they ’ re not awardwinning shots , but so long as I capture useful angles and features I can usually see well enough to identify the animal . I do like taking the odd arty snap though , and there are some animals that simply catch your eye the right way ! You ’ re not leaving until you ’ ve taken a hundred images , or your very patient buddy reminds you of their existence . I was scrolling though pictures from a recent dive trip around Orkney when I came across some striking , blue-rayed limpets . I laughed , reminded of how I must have looked straddling a kelp stipe mid-water as it flapped back and forth in the surge . It was almost impossible to get the little cluster of molluscs in focus .
As I zoomed in and played with the filters , I was transfixed on the contrast between the thin uniform bands of neon blue and the underlying tawny shell . I realised that I ’ d never questioned what these electric blue stripes were made of , but when I did , the answer genuinely blew my mind !
Firstly though , a little bit of background info . Blue-rayed limpets ( Patella pellucida ) are a common species of limpet found throughout the UK , wherever there is kelp . They live and feed on various kelp species growing at the surface down to about 20 m deep . You ’ re looking for something about the size of your little fingernail on a kelp frond ( leaf ) or stipe ( stalk ). Fortunately , they often hang out in groups , and due to their vibrant blue stripes , they can capture the eye of a snorkeller or diver 5m away !
Now for the science . To understand what is going on in the shell we need to
“ I realised that I ’ d never questioned what these electric blue stripes were made of , but when I did , the answer genuinely blew my mind !”
look very closely . How close ? Micron scale , that ’ s one thousandth of a millimetre ( cell sized ). Limpet shell comprises of crystalline calcium carbonate in two forms , calcite and aragonite . As molluscs grow , they lay down bands of this mineralised structure . To find out how the blue-rayed limpet got its stripes , a team of scientists used a scanning electron microscope to compare shell structure inside and outside the blue stripes .
They found a striking difference . Only within the boundary of the stripes , they observed multiple layers of mineralised architecture embedded within the shell . The top layer , seven microns thick ( same as a red blood cell ) consisted of a regular spaced , zig
Blue-rayed limpets are typically 1-2cm in size zagged mesh of 40 – 60 layers with liquid in between . Beneath it , a disordered layer of particles , five microns thick .
Together these two layers interfere with light , creating an entirely mineralised photonic system . Absorbing and reflecting light in the blue and green spectral range which we see as the vivid blue stripes . The reflections coincide with the spectral range of minimal light absorption in sea water , making them highly visible even at deepest depths of their range .
These characteristics are considered unique to blue-rayed limpets , at least until further species are found with the ability to harness photons the same way . The ‘ why ’ remains a puzzle . Visual communication for prey evasion is one theory . If I make myself look like a similar distasteful or toxic animal ( such as a nudibranch ), I ’ ll be left well alone to munch my tasty kelp . I reckon we ’ ve all gotten hangry enough at some point to relate to that . �
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