I've lost my ability to ID polychaetes... 10 years ago I used to be pretty good. There is another worm to go up in a second. Can't ID that one either. I think this is genus Harmothoe but I'm only 10% on that one...
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Scaleworms are very difficult to identify from a photograph. I this case I'm not even sure how many pairs of elytra there are; and thats before I get down for the need for a close look at antennae and chaetae. Sorry I can't do more. For polychaetes of Scilly there is old scientific paper in Journal of Natural History by Tegwyn Harris. I have records of both Harmothoe impar and Lepidonotus clava form the area but these were not from the St Mary'd boulder beach.
Thanks for the comment - I have found it tricky to identify other inverts by photo and to be fair the photo is poor: I seem to have difficulty uploading images larger than 1MB on iSpot. Even then, the source image is not brilliant.
I agree its probably Lepidonotus but the key character separating L clava an L.squamatus according to Susan Chambers and Alex Muir in their key to UK scaleworms (in Linnean Soc Synopses of the British Fauna No 54 (1997) is that the elytra of L.clava are smooth and those of L.squamatus are fringed. On the subject of a clear midline they say "scales overlap in the anterior region usually leaving the mid-line of the body uncovered".
In my reading, the use of "usually" means there can be exceptions. I have no idea how frequent such exceptions might be.
As Paula indicates in her identification, I think an uncoversed mid-line may be a good field indicator of probable identity but confirmation of the ID needs examination of the elytra.
Sorry to be so pedantic.
It's taken me a while to get to this..
I think Paula is right but 'cautious Mike' suggests there is a need to look even closer.
I have a posting here http://www.ispot.org.uk/node/367341 (please check it out!) which shows how even close scrutiny may not get you where you want, though there is little doubt in that case. There are a number of these scaleworms, of differing types, in Scapa Flow and I will spend more time, now I have a low power microscope.
Lat/Lng: 49.9202, -6.3091
OS grid ref: SV908111
Boulder-strewn sheltered rocky shore - niche bonanza...