It’s not just the birds, bees, and educated fleas that do it, the barnacles do it too ….. breed in springtime, that is! The results were there for everyone to see, with millions of miniscule baby barnacles smothering the rocks at Rhossili in Gower in early April.
Each newly settled barnacle measured just a millimetre or so. [It was at the limit of the camera’s capability to focus – so apologies if the images are not as sharp as they could be]. The baby barnacles develop from free-swimming cyprid larvae that are only 500 – 800 µ m long. The cypris is the final stage in the larval development of the acorn barnacle – following six consecutive stages as a nauplius larva.
The cypris looks a bit like a tiny clam or an ostracod with two large shells or valves hinged on the dorsal surface and open on the ventral one. Six pairs of fringed appendages used in swimming hang down from between the valves. The cypris has sense organs to detect suitable surfaces on which to settle. It also has small antennules at the head end which it uses to crawl over the chosen substrate before performing a head-stand and cementing itself into position on its back. Newly settled barnacles are referred to as spat.
The shell of the settled acorn (or sessile) barnacle has six over-lapping calcareous side panels making an approximately cone-shaped wall. The animal lives within this ‘box’. Four more hinged plates create a lid to the box that can be opened and closed. Once the new adult-like shell form is developed, the fringed swimming appendages or natatory cirri of the larva can then be protruded through the hinged lid plates to seive food particles from seawater.
[You can see more detail in the photographs if you click on the images once, or twice. If you look carefully, you should be able to recognise a few cyprid larvae with their smooth, glossy translucent shells, in the process of settling amongst the recently metamorphosed miniature adult forms].
The young barnacles settled on every patch of smooth, bare rock where it was available at the base of Rhossili cliff – but also on top of other living adult barnacles, on the old white calcareous bases left by detatched large Perforatus perforatus barnacles, on limpets, and on mussels. Only the dog whelks with their variously coloured shells seemed mostly free from spat fall as they feasted on the mussels and barnacles.
Revision of a post previously published 18 May 2010
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Jessica's Nature Blog 
Truly fascinating, and a great set of pics to accompany your article, Linda
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Thank you, Linda. It was an amazing sight – sort of hidden in full view, if you know what I mean.
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Very cool Jessica. Your images are so revealing.
Years ago I read or heard that the shells of dog whelks become darker if their diet consists mostly of mussels. Have you heard anything similar?
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I thought I was so lucky to spot these tiny creatures. Whever I see something new like this, it sets me off on a journey of discovery. I love to try and identify things and learn more about the natural world.
I used to believe that the different colours of dog whelk shells were related to their diet. However, as you can see from the photographs on my blog, large groups of dog whelks living in the same place and feeding on the same organisms can exhibit amazing variability in colour and pattern. I think colouration and patterning must be genetic but I’ll have to look it up to be sure.
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