From a distance, I could see large white blotches on the wet rocks as the tide uncovered Worms Head Causeway. On closer inspection they looked like great lengths of coiled tape-worms. It wasn’t until I actually noticed the small brown creatures congregating around these glistening masses that I realised they were egg masses – which were, at that very moment, being created by Sea Slugs (Nudibranchs – marine molluscs without obvious outer shells). This was something entirely new to my experience of exploring seashores.
Apparently, each Sea Slug is hermaphrodite (both male and female) and so is capable of being fertilised at the same time as fertilising any other of the same species. The millions of microscopic white eggs being produced from the pairings of the sea slugs are coated with a clear mucilaginous coating to form long white slimy ribbons arranged in loose spirals.
The Sea Lemon, Archidoris pseudoargus, is said to be the most common variety of sea slug that you are likely to encounter on the Worms Head Causeway. However, these photographs of mine show groups of much smaller and flatter animals than might be expected if these specimens were Sea Lemons – which are more rounded and can grow to 120 mm in length. These, obviously mature, mating congregations of individuals that I was witnessing seemed to be about 20 – 30 mm long with a warty, patchily-brown surface. The upper surface of the creature is the mantle (which in gastropod and bivalve molluscs would produce the hard outer shell). Here the mantle is covered with large rounded clear unpigmented tubercles.
I think the specimens of sea slug I saw belong either to the Family Onchidorididae. I have consulted the key – British Opisthobranch Molluscs by T.E.Thompson and Gregory H. Brown in The Synopses of the British Fauna (New Series) No. 8, Published for the Linnaen Society of London by Academic Press, ISBN 0-12-689350-0.
Using just my photographs for the identification, it doesn’t seem possible to make out all the necessary diagnostic features but I think my specimens resemble Onchidoris bilamellata (L.). This species may reach 40 mm in length, has blotchy brown markings on the dorsum with abundant club-like tubercles of various sizes but without pigment. It feeds mostly on acorn barnacles (there were plenty around where the sea slugs were mating). It has also been recorded all around the British Isles. I am unable to find a reference to this particular species laying eggs in coiled ribbons but it is a mentioned feature in many other species in this group..
I could well be wrong about this identification. It is a tentative one. So if you know better, please do get in touch to tell me what you think.
For a wonderful collection of underwater photographs of marine invertebrates, including a variety of Sea Slugs species, pop over and visit Jason Gregory’s site at British Marine Life Pictures.
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Jessica's Nature Blog 
Wow, what a find, I’ve never seen them before on any beach, anywhere! Was that a recent find? I’m just wondering if the egg laying happens only at a certain time of year? I’d also dearly love to find a sea hare, or any kind of sea slug, some of them are so wildly coloured. Congratulations!
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oh yes, and how do you pronounce nudibranch – I’m sure I’ve heard some people drop the last letter, so it sounds like “branc”???
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Hi, Chloe. I think there is a particular season for breeding and egg laying. Lots of seashore creatures seem to spawn in spring. I took these photos on 20th March last year. It is the first time I have seen sea slugs in the wild but I have seen some really beautiful ones in a marine aquarium.
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You’re right. It is generally pronounced to rhyme with ‘bank’ rather than “branch”.
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Facinating find, i also have never seen sea slugs, ( great photographs ) but did come across a sea mouse a few years ago during a very low tide.
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Hi, Michael. Thanks for the comment. Sea slugs are great but a bit hard to find by the average beachcomber, aren’t they? I found a Sea Mouse once, too. They have to be the strangest worms anyone is ever likely to find. I must look for my photos of the creature and write a new post.
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I read an article somewhere the the spines of a sea mouse give zero light loss which is better than the fibres i splice at work, they are studying them to see how they do it.It just goes to show how amazing nature is.
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It is wonderful how often Nature provides the inspiration for modern technologies. The spines and bristles of the Sea Mouse are certainly special. I just love the beautiful iridescence of some of them. I found my pictures from a long time ago but they don’t do the creature justice.
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I’m sure onchidoris bilamellata is right; as there’s no other uk seaslug could look like this (eg in habitas.org.uk/marinelife the several similar onchidoris sp’s are 9mm, 12mm,5mm,9mm,8mm). I’ve seen many in Rhossili pools, & in numbers; also the eggs, but not both together. However all these manage to look different from my photos & any others I know, eg at seaslugforum.net ;but maybe that’s because yours are out of water, & maybe they’re taking a teabreak till the tide comes back.But see http://www.seaslugforum.net/showall/onchbila with many pics including them with eggs, & descriptions of mass spawning.
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Thank you very much indeed for this feedback, Chris. It is good to have some corroboration for the identification. Having looked at the site you recommend at http://www.seaslugforum.net/showall/onchbila, I feel confident that I have the correct identification. I guess that I should now get myself a camera capable of taking images underwater so that I can see these sea slugs in action – instead of when they are taking a ‘tea-break’.
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