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In this picture the arms make a clear star. In most examples of A.gibbosa I have seen the animal has much shorter arms and a much more pentagonal shape. See the illustration above (except the first one).
These were found (from notes) onshore 1/2 after L.T. &2day bef lowest spring so the only options (fr guidebks, marLIN etc) seem to be A. gibbosa & A.phylactica; & I'm sure they w. >1.5cm diam, as the 'goodly sized' cowrie indicates (haywards collins guide gives shell as to1.2cm) so ab.1cm. (A. g. up to 5 cm).
The only other known to me is red cushion star Porania pulvillus but it's fr 10m. downwds & marLINs photos (marlin.ac.uk) & description dont fit at all (esp the papillae on surface). I noticed marLIN's asterina gibbosa is very close to my orange one. (& i have a theory, wacky or not, that maybe they can flex accdg to surface they're on)
We don't have many cushion stars in UK waters and we dont have many "asteroid" stars that aways have short arms so the ID given is likely to be correct.
I think the point I wanted to make was that although Asterinidae can be defined as "with very short arms giving an almost pentagonal shape to the flattened body" some individuals appear not to meet the description and hence would not be identified using a standard key.
The cushion stars of Pembroke are particularly well known scientifically following the work of Emson. So, ideally I'd like to check definitions with the primary literature but don't have that at home. Sorry.
Thanks for your input. I readthe "Nat'l hist .. of 2 Brit sp of asterina, 1983 Crump (&Empson) in pdf when I googled Asterina phylactica. It gave the max diam of A.p.as "15mm diam ..." (Haywards shore guide also 15mm diam) & even a formula for A. g (R=1.2-2r), & it does look like mine are a bit over 2. It also described A.phylactica as green w. chocolate "substar", so when I found tiny cushionstars with red substars I(at this locn in pembroke where they're still studying it, & later on in the Gower) I'd assumed them young A.g's. But I now find most photos of A.p on the net with red substars, & thankfully also the site theseashore.org, from the Dale fort field centre shows A.p with a red substar. NB I've posted my Gower & Pembs A.p's on Ispot also.
Lat/Lng: 51.687, -5.0571
OS grid ref: SM887031
Onshoe on a low spring tide.