I sent for the original description of Ulva fenestrata. Postels, A. & Ruprecht, F., 1840. It was in a volume of Illustrationes Algarum, Nicolai 1; the text was in Russian.
However there are coloured plates of the algae described and this is that for Ulva fenestrata.
Not exactly what I think of as European Sea lettuce (prev known as Ulva lactuca).
All adds to the evidence that morphological identification of Ulva species is not really viable.
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I suppose Ulva latuca in a non-native species, so is it invasive and how would anyone know?
Russian description. The whole volume, apart from the title page, is in 1840s printed Russian. It covers several seaweeds, and I could not distinguish one from another in the text.
The illustrations were labelled with their latin names, so that’s how I found U. fenestrata. I thought all the illustrations are worth looking at as examples of artistic expertise.
I got it by requesting the pdf from the algaebase website. Ulva fenestrata Postels & Ruprecht :: AlgaeBase
Had a brief look at the paper, looks like the same fun as the Ulva species.
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I am not surprised the land locked universities are asking for samples from Orkney!